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Engineering '"specti°n.
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http://www.archive.org/cletails/cu31924004021394
ENGINEERING INSPECTION
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Engineering Inspection
BY
E. A. ALLCUT, M.Sc.(Eng.)
Mem. Am. Soc. M.E., A. M. Inst. C.E., A. M. I. Mech. E.,
A. F. R. Ae. S.
Associate Professor of Thermodynamics in the University of Toronto
AND
CHAS. J. KING
WITH NUMEROUS DIAGRAMS
AND TABLES IN THE TEXT
NEW YOEK
D. VAN NOSTRAND COMPANY
EIGHT WAREEN STREET
Fa-
I ii;;:
f\S\<\A^%
Printed tn Great Britai.
PREFACE
Much descriptive matter has already been written on the details of
the various inspection methods employed in different engineering works,
but the object of this work is to present to the reader, in a compact and
convenient form, a description ' of the various principles involved in the
inspection of an engineering job from the raw material to the finished
article. It is obviously impossible to go fully into detail in each case, nor
is it necessary to do so. Every engineer and manager has different
problems to face, but if he has a thorough grasp of the principles required
for their solution, it is not usually difficult to work out the necessary
details. The examples given are chosen to be representative (as far as
possible) of general engineering practice and to illustrate the different
principles of inspection and measurement in common use. From
considerations of space, mechanical engineering operations only are
described, as the special apparatus and methods used in electrical and civil
engineering are too numerous and varied to be included in any work of
reasonable size.
It frequently happens that the works inspector touches only one or
two particular processes or operations, and it is highly desirable that he
should have some idea of the work done before the material reaches him,
and to be done after it leaves his hands. He is thus enabled to do his own
work more intelligently, to avoid bad workmanship and unnecessary scrap,
and to fit himself for the higher branches of his profession.
It is hoped also that this work will be useful to engineers and draughts-
men, by indicating what information is necessary on working drawings,
how such information is used in the shops, and by giving assistance in
specifying material or machinery and indicating the defects that are likely
to be encountered in different cases.
All branches of engineering science are to a certain extent inter-
dependent, and acquaintance with any one section is frequently useful to
those who are occupied in others.
E. A. ALLCUT.
C. J. KING.
CONTENTS
CHAP.
I. — Objects of Inspection
II. — The RifcEiPT and Storage of Material
III. — Material Tests and Specifications
IV. — Inspection of Raw Materials
V. — Inspection of Partly Finished Material
VI. — Inspection of Finished Material
VII. — Gauges and Measuring Instruments
VIII. — Machine Shop Inspection
IX. — Fitting and Erecting Shop Inspection
X. — Final Tests
XI. — Repairs, Rectifications and Obsolete Parts
XII. — The Human Element
Appendix. — Physical Test and Acceptance Sheet
Table i. Reduction of Area in Tensile Test Pieces
2. Properties of Metals and Alloys . . . .
3. Brinell Numbers and Approximate Tensile
Strengths for Steel (at 3000 Kg. load)
4. Brinell Hardness Numbers for Pressures less than
3000 Kg. . . . .
5. Hardness Values of Metals on Shore Scleroscope
Scale ........
6. Sheet Gauges . . ...
7. Contraction of Castings
8. Metric Equivalents
PAGE
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182
LIST OF ILLUSTRATIONS
FI<^- PAGE
Diagram of Inspection System ..... front.
I .; — Arrangement of Straining Gear for Testing Machine . 1 1
2. — Diagram of Multiple Lever Tensile Testing Machines . 12
3- — Tensile Testing Machine (electrically driven) with Torsion
Tools Attached. Capacity 25-30 tons . . .14
4. — "Bar" Testing Machine for Transverse Tests and small
Tension Tests . .... . . 15
5. — Types of Tension Grips . . . .16
6. — Broken Tensile Test Specimen ... . 17
7. — Autographic Stress Strain Recorder . 19
8. — Stress Strain Curves for Iron and Steel .... 20
9. — Stress Strain Curves for Non-Ferrous Metals . .21
10. — " Brinell " Hardness Testing Machine ... 23
1 1 . — The Shore Scleroscope . ... . . 24
12. — Diagram showing Principle of Impact Test . 27
13. — Notches for Izod Impact Test Specimens . . 28
14. — Izod Impact Testing Machine ..... 29
15. — Alternative Izod Impact Test Pieces for small Specimens 29
16. — Charpy Impact Test and Test Pieces . . . 30
17.— "Erichsen" Sheet Metal Test . . -31
18. — Characteristic "Erichsen" Bulges . . -32
19. — Workshop Tests for Wrought Iron ..... 39
20. — Defects in Steel Bars . . .48
21. — Bending Test on Mild Steel Bars 48
22. — Tensile and Impact Test Pieces in Case-hardening Steel . 49
23. — Cement Testing Machine (capacity 1200 lbs.) . 53
24. — Cement Testing Apparatus ... . . 54
25. — Tests for Timber . ..... 56
26. — Fabric Testing Machine (capacity 1200 and 240 lbs.) 58
27. — Grips for Fabric Testing Machine . . 59
28. — Diagram of " Schopper " Tensile Test for Rubber 61
29. — Thurston's Oil Testing Machine . . .62
30. — Bench for Hot and Cold Water Pressure Testing 66
31. — Report of Rough Viewing and Marking Out . 69
XIV
LIST OF ILLUSTRATIONS
FIG.
32. — Formation of Laps in Forging down round Sections .
33. — "Flash" of Overheated Stamping ....
34. — Direction of Fibre in Forging Gear Wheels
35. — "Offset" and Eccentric Forgings .
36. — Spinning Table for Eccentric Forgings and Castings
37. — Tests for Copper and Brass Tubes ....
38. — Compression Test on Aeroplane Tubes .
39. — Machine for Testing Coil and Laminated Springs (electrically
driven). Capacity 4 tons
40. — Grips for Continuous Chain Testing
41. — Rope Testing Grips ....
42. — Fracture of Case-hardened Part ....
43. — Effect of Different Thicknesses of Case on Strength qf Gear
Teeth ...... ...
44. — Workshop Tests for Rivets (Steel) ....
45. — Diagram, illustrating Unilateral System of Limit Gauging
46. -Plug Gauges .........
47 . — Use of Plug Gauges in Elliptical Holes .
48. — Large Plug Gauges
49. — Protected Centre .......
50. — Ring Gauge . . ... . .
51. — Snap Gauges . ....
52. — Diagram, illustrating Principle of Taper Gauging
53. — Taper Gauges ......
54.- — Vernier Height Gauge
55. — Profile or Form Gauges
56. — Screw Threads ......
57. — Screw Thread Gauges .....
58. — Curves of Pitch Error . ...
59. — The Measurement of Screw Threads
60. — Combination Angle Gauges ....
61. — Testing Internal Diameter of Ring with Johansson Gauges
62. — Micrometers . . ...
63. — Thread Micrometer
64. — Two-point Inside Micrometer with extension bars
65. — Three-point Internal Micrometer
66. — Method of Reading Vernier Scale .
67. — Vernier Calipers
68. — Gear Tooth Vernier ....
PAGE
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LIST OF ILLUSTRATIONS xv
FIG. PAGE
69. — Multiplying Lever Gauge
— Diagram, shewing Principle of Hirth Minimeter
Diagram of Lever and Segment Dial Indicator ]
Diagram of Rack and Pinion Dial Indicator j
— Inspection Stamps on Connecting Rod .
— Crankshaft ....
— Piston . ....
(Registering Speed Indicator
I Tachometer
— Mahler Bomb Calorimeter
— Diagram of Junker's Gas Calorimeter
— Gauge for Small Pressures
— Testing Pressure Gauge
— Arrangement of Differential Gauge for Measuring Velocities or
Quantities of Gas or Air .
81. — Calibration Curve for Differential Gauge
82. — Indicator ....
83. — Steam Engine — Indicator Diagrams
84. — Rope Brake for Engine Testing .... 160
85. — Froude Water Brake ... . . . 161
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CHAPTER!
OBJECTS OF INSPECTION
The average mechanic and his foreman look upon the inspector and
the inspection department as their natural enemies, and are apt to
imagine that these exist solely for the purpose of annoying them and
their kind.
This idea is quite erroneous. An inspector is only concerned, firstly,
to prevent waste of time or material, which ultimately results in waste
of money, and secondly, to secure the production of a high class article
instead of an imperfect one, even if output is reduced, temporarily, to
obtain this. Too often the machine-shop man classes inspection as one
of the annoyances imposed by a malicious management for the tormenting
of its employees. But, properly organised and handled, an inspection
system can be of very real benefit both to management and workmen.
Any system that tends to increase production and to save waste, is
beneficial to industry generally, provided that it is not over-elaborated.
The extent to which inspection is taken, depends, largely, upon the
character of the work to be inspected. For instance, aeroplane work
demands a very rigid inspection, as weight is reduced to the lowest
possible limit, factors of safety are unusually low, and the product is a
high speed machine. Motor cars and other rolling stock come upon a
lower plane, as machines in which the factors of safety are greater and the
risks of failure less.
Generally, when the consequences of failure involve danger to human
life, inspection must be installed upon a "safety" basis,' but when there
is no such risk, inspection must be organised upon an "economic" basis.
In the former case the system is necessarily elaborate and far-reaching,
and as the price of security must be paid by the consumer, the inevitable
result is that the finished article must be sold at a high figure.
Where inspection can be organised upon an economic basis, its
Money saved by inspection.
efficiency may be expressed by the ratio ^
Cost of inspection.
If this ratio is greater than I'O, the system of inspection is more or less a
success, the degree of success depending on the magnitude of the ratio.
B
2 ENGINEERING INSPECTION
If the ratio is less than I'O, inspection does not pay, and should be
differently arranged or abolished.
The money saved by inspection, however, is sometimes difficult to
assess, as the moral value of inspection is often an important item, and
one difficult to estimate. In a shop making drop forgings, it was found
that large quantities of scrap were being produced, and although these
were detected and thrown out on examination, the loss in material and
labour on the forgings was a serious matter. An inspector was then
put into the shop to watch the production of these forgings, and to impress
his stamp on any that were forged at too high temperatures. The result
was that greater care was exercised by the workmen, and the percentage
of scrap due to this particular fault, diminished practically to nothing.
Casual observers, knowing nothing of what had gone before, noticed that
the inspector had little to do, and objected to the needless expense,
but in point of fact the moral effect of the man's presence in the shop
saved the firm many hundreds of pounds. In this case, the very fact
that the inspector had practically no work to do, was the best argument
for the continuation of his duties.
The objects of inspection are manifold, and vary with the class of
work involved, but the general principles may be summarised as follows : —
(1) To detect and isolate faulty material or work, at the earliest
possible stage in the life of the job concerned, and to decide whether the
fault is capable of rectification or whether the piece must be scrapped. In
the former case, the cost of rectification is sometimes so great that it is
more economical to scrap the piece and totally replace it. This point
is usually decided by the production staff.
(2) To protect the buying firm against payment for faulty or defective
goods. Early inspection after the receipt of goods detects many troubles
that would otherwise not be discovered until a certain amount of machining
or hand-work had been done. This often entails a delay of several weeks,
during which time the goods are lying in the stores and perhaps have
been paid for. In the meantime thousands of similar parts having the
same defects may have been produced by the supplying firm, with conse-
quent loss and further delay before these can be replaced.
(3) To prevent the production of bad work in the shops, by calling
attention to incipient defects before they become serious. In instances
of this kind a slight fault is noticed which gradually becomes worse as
production proceeds, and if allowed to continue would result in a con-
siderable amount of scrap being made.
OBJECTS OF INSPECTION 3
(4) To prevent operators from being paid for faulty or inaccurate
•work, or for work not actually performed. When men are paid by the
piece {i.e., on the number of correct articles produced by them) it is
necessary to see that the number of pieces correctly produced, corresponds
-with the number for which payment is claimed, and also that the work
is properly done. If the pieces produced are not carefully inspected there
is a tendency for quality to be sacrificed to quantity, and money will
liave to be spent at a later stage to rectify or complete the operations
already paid for.
(5) To prevent further work from being done on pieces that are
already faulty. In most cases it is not economical to inspect work after
the completion of each operation, but certain stages of manufacture are
chosen, and the work is examined at these points. If the work is found
to be faulty after, say, a roughing operation, it may frequently be
rectified or specially treated to produce a satisfactory finished article.
If, however, these faults were allowed to remain until the finishing
■operation, the articles concerned would probably be entirely scrapped
■owing to the fault being discovered too late. As an example of this, the
■case of some important steel castings may be taken. These showed
some fine cracks when the surface of the casting was removed by rough
-machining. The castings isolated at this stage were repaired by welding
and a satisfactory job resulted. In cases where the cracks were not
•observed until the finishing stage, it was found impossible to repair in
this way, as the distortion, consequent upon the application of the welding
-temperature, made the castings unusable. They were therefore scrapped,
.and the value of all the expensive machining operations was lost.
(6) To enable large quantities of standardised components to be
produced, with the certainty that they will go together in erection without
•expensive hand or fitting work, and to cheapen the production of
•engineering work by eliminating, as far as possible, selective assembly.
Generally speaking, when large quantities of similar articles have to be
produced, it is economical to perform as many operations as possible in
the machine shop, with the object of reducing hand work to a minimum.
(7) To protect the reputation of the selling firm. It is far cheaper
to detect faults and to rectify them on the manufacturer's premises,
than to allow faulty articles to go out and to have the defects discovered
by the customer. In the latter case, the failure of faulty parts often
involves the destruction of good components that are adjacent to them,
and with goods sold under a guarantee, both have to be replaced. A
4 ENGINEERING INSPECTION
series of such mishaps may be sufficient to ruin the manufacturer's
reputation and business.
(8) To check the work of the designer and draughtsman, and to see-
that the engine, machine, or other unit, gNes the power or performance
required. In many cases the inspection of a finished job reveals faults
and difficulties that were not fofeseen or expected when the drawings-
were made. By inspection and testing both the component parts and
completed article, sources of weakness, trouble, and danger are weeded'
out, and the reliability of the article is improved.
(9) To protect the customer against loss or damage. The economic-
aspect of tjiis point is dealt with under (7), but there is the further con-
sideration that if a mishap occurs, involving loss of life or serious damage
to the customer, the manufacturer is faced with the possibility of legal
action to recover damages, in addition to the probability of loss of
business.
(10) The foregoing points may be summed up in the statement that
the objects of inspection are to reduce costs, to increase production,
and to protect both producer and consumer.
The natural consequence is that co-incident with the installation of
an efficient system of inspection, the cost of manufacture should be
reduced, and the selling price can then be lowered. If this is not the
case, there is something wrong with the system or the people who work
if. If the reduction does materialise, both producer and consumer are
benefited, the former by increasing his output (and consequently his
profits), the latter by obtaining at lower prices the articles he needs,,
or a better class of article for his money.
CHAPTER II
"the receipt a'nd storage of material
The first stage in tfee life of any engineering work is its delineation
on paper in the form of a drawing. This is usually checked in the
drawing ofKce, by the chief draughtsman or one of his senior men, but
the duties of the inspection department do not generally commence until
the drawing or a print of it is issued for manufacturing purposes. The
inspection department, however, can assist the drawing office very
materially by pointing out mistakes or inaccuracies in the drawings
immediately they are discovered in the machine or erecting shops. As
practically all complaints have to be dealt with by the inspectors, they
are in the best position to do this.
Further, certain points in design, such as insufficient radii, uneven
thickness of metal, or difficult machining and erecting operations, are
frequently taken up with the drawing office, by the inspector concerned,
and in many instances the design can be modified to avoid the difficulties
encountered in manufacture.
These points are merely incidental, however, and the main work of
inspection commences with the placing of orders for raw material.
As an order is a legal document that may lead to considerable trouble
and loss, it is advisable to make sure, in the first instance, that all
necessary information and requirements are covered by it. It is therefore
advisable, in the case of large orders, to send a copy to the Inspection
Department so that the latter may check over the technical details. As
it is inadvisable for purchase prices to be generally known in the works,
this copy should be unpriced. The inspector concerned may then check
the sizes given on the order, with the drawings of the finished article,
and satisfy himself that the material on arrival will be of suitable dimen-
sions for completing the job. Many firms also issue their own
specifications for quality of material and tests to be imposed, either at
the supplier's works, or on arrival at the purchaser's premises. These
points must be stated on the order or on a covering letter sent with it,
•so that the supplier will know exactly what quality of material he has to
supply, and what tests or severity of inspection will be imposed. If this
is not done at the outset, the supplier can refuse to accept any stipulations
of this kind that the purchaser wishes to impose at a later date.
5
6 ENGINEERING INSPECTION
There is a further reason for supplying the inspector with a copy
of the order. When the material arrives at the purchaser's works, it
has to be inspected to some standard. The usual standard is the suit-
ability of the material supplied, for making the finished article. This
implies the consideration of processes of manufacture, and necessitates
certain allowances for machining, etc. In some cases, however, the usual
size or quality of material cannot be obtained at the moment, and a
contract is made for an alternative supply. The basis of inspection,
therefore, must always be the terms of the order, as faulty material can
only be returned to the makers for free replacement, if it fails to meet
the conditions under which the order was placed.
Briefly, the order is an agreement between the two firms, and the
inspector is responsible for seeing that the terms of the agreement are
correctly stated and fully observed.
In some orders, it is stipulated by the supplying firm, that faulty
material will not be replaced by them unless it is returned within a
certain period after the date of delivery. It is the business of the
inspector to see that this time is a reasonable one, bearing in mind the
period that" will naturally elapse before the material is drawn from the
stores, and also the additional time taken by the various processes of
manufacture.
On the arrival of material at the purchaser's works, it should be
handed at once by the Receiving Department to the particular section of
inspection concerned with that class of goods. It will then be kept in
"bond" until released by the foreman or head of that section, as satis-
factory for the part it is required to make. Under no circumstances should
material be released from the bond until it has been approved in this way,
as with quantity production, once the material gets into the stores, it is
a very difficult (and sometimes impossible) matter to separate out the
last consignment, should a fault subsequently be discovered. In this way
it can be assumed, with reasonable safety, that the material actually in
the stores is suitable for issue to the various manufacturing departments.
Should a fault be discovered, which is not necessarily fatal, but
entails special care in machining or handling certain parts, the consignment
in question can be specially stamped, painted, or otherwise marked, before
leaving the bond. In this case the departments concerned must be
notified to give special attention to the marked articles when they arrive
in the shops. Neglect of this precaution may entail extensive scrapping,
and waste of money in material and workmanship.
THE RECEIPT AND STORAGE OF MATERIAL 7
When similar articles are being received from more than one source
of supply, it is necessary to stamp the parts in question with identification
marks, so that in case of faults subsequently being discovered during
machining processes, the matter may be taken up with the suppliers, and
in case the latter agree to replace the faulty material, the "rejects" may
be sent to the correct firm. In marking such material, it is advisable to
impress the stamp on a surface that is not to be machined, as otherwise '
it may be removed in the course of manufacture. Where this is impossible,
arrangements should be made to transfer the identification marks between
the machining operations concerned. It is necessary that the transference
should be effected before the orginal mark is effaced, to obviate the risk
of losing the identification mark altogether, or of having the material
wrongly stamped.
If faults are found, the matter should immediately be taken up by the
inspector concerned, with the Buying Office, to prevent further deliveries
of the same kind, and to enable the suppliers to alter their raw material
or methods of manufacture and eliminate the troubles complained of. In
some cases, these faults are capable of rectification by doing extra work,
and spending more money on the material. In this event it is necessary,
before rectification is commenced, to enquire whether the suppliers are
willing to receive the material back at their works for rectification by
themselves, or whether, to save time and carriage, they are willing to
allow credit to the purchasers for the cost of rectification by them. In
some cases, if the material is urgently wanted, a compromise may be
offered and accepted on this point, to enable production to proceed
quickly.
If rectification is impossible (or costs more than the job is worth),
it is usual for the supplying firm to allow credit for the value of the faulty
material, or to replace it free of charge. The value of the work done by
the purchaser on the faulty material is not credited unless special arrange-
ments to this effect are made, when placing the order.
Material rejected by the purchaser's inspector, and subsequently
rectified by the supplier, must be specially dealt with, as in some instances
the process of rectification {e.g., welding of steel castings*) introduces
other faults (such as distortion or cracks), from which the material was
previously free. For this reason, any rejected material should be specially
marked or stamped so that if an attempt is made by the supplier to
return rejected material it will immediately be detected on arrival.
* See Chapter V.
8 ENGINEERING INSPECTION
In some instances attempts have been made by suppliers to get rid of
rejected castings or stampings, by sending them back, a few at a time,
among consignments of new and perfect work, in the hope that they will
escape detection. The rejection mark, therefore, should be such that it
cannot easily be erased. Of course, the easiest way of avoiding this is
to deal only with good firms who have reputations to maintain, but while
there are firms and individuals who will resort to shady tricks of this
kind, it is impossible to be too careful.
A further safeguard is to stamp each article passed, with the
distinctive mark of the inspector or viewer responsible. In this way,
any attempt to remove material from bond without passing inspection,
may be frustrated, as the absence of the inspector's stamp is likely to be
noticed when the material is issued from the stores to the machine-
shop. This procedure is also a check against carelessness on the part
of the viewers. It is also advisable for the Inwards Receiving Note
accompanying each consignment received, to be delivered to the Material
Bond when the goods are sent for inspection. When the goods are
released the Receiving Note is stamped by the Inspector, with particulars
of the number passed, rejected, and to be rectified. The good materials
are then sent on to the stores, and the Buying Office notified of the number
of faulty pieces, so that the matter can be taken up by them with the
suppliers. A sample Inwards Receiving Note is shown below.
INWARDS RECEIVING !
sOTK.
rder No
Date
Suppliers
Part No. Description
or Drawing No.
Weight Quantity
SrACF. FOR Viewer's Stamp.
Date
Remarks
Accepted
Rejected
To be Rectified
Viewer's Signature :
THE RECEIPT AND STORAGE OF MATERIAL 9
In very few cases are all the parts, required to build a complete
engine, machine, or other engineering work, manufactured on the premises
of the supplying firm. With the present tendency towards specialisiation,
some firms buy out all their components, and become merely assembling
shops. It is obvious that firms which specialise upon the production of
one particular component or accessory, become experts in that work,
and can produce it cheaper than anybody else. Sometimes these com-
ponents include trains of gears, important links or levers, and other pieces
that are of vital importance to the durability or efficiency of the complete
machine. As it would be a very costly matter to dissemble, break, or
cut up such components for testing purposes when they arrive at the
purchaser's stores, it is customary to inspect material and workmanship
before, and during manufacture at the supplier's works. In the case of
a large contract, a resident inspector is often employed, who stamps all
the work passed by him, and reports daily or weekly, the quantity of
material passed and forwarded, so that a check may be kept to ensure
that all material is examined before being sent away.
An alternative, in the case of smaller contracts, is for an inspector
to visit the supplying firm at definite or irregular intervals, to inspect
the work produced during the period between his visits, or to check
samples of the work in operation at the time of his visit respectively.
The first method has the advantage that every article is inspected, but
the latter is cheaper, as the same inspector can cover a greater area
when samples only are viewed.
A further method of dealing with work done outside, that is especially
useful in dealing with foreign countries, is to utilise the services of a reputable
agency to inspect the work before dispatch. This may be done by agents
such as Lloyds in this country, or the Bureau Veritas on the Continent.
For continuous working, it is cheaper and handier in the long run,
for the purchasing firm to have its own inspectors on the spot, but for
short (and costly) contracts, such agencies are very convenient. Money
is saved in time and travelling expenses, but if no inspector were
employed, the expense of transport, handling, etc., on faulty material,
would be a very serious item. Also many goods have to be paid for
on receipt of bills of lading, and in any dispute arising after this time,
the purchasing firm is at a decided disadvantage, as the distance between
the disputants makes negotiations very slow and troublesome. In
addition, the fact that such agencies are quite impartial and frequently
have rules and specifications of their own, puts them almost in the position
of arbitrators and tends to prevent disputes from taking place.
CHAPTER III
MATERIAL TESTS AND SPECIFICATIONS
Before any detailed inspection is performed on material arriving at
an engineering works, it is obviously advisable to ascertain whether its
quality conforms to the specification laid down in the order, and samples
must be taken to verify this.
The proportion which these samples bear to the total consignment,
and the manner of their selection in different cases, are detailed under
the several headings in subsequent chapters, but the equipment necessary
for such tests must now be considered.
The first and most important item is a tensile testing machine. This-
is required to measure the tensile strength and ductility of the sample
chosen as representative of the bulk consignment, and the points to be
observed are, the elastic limit or yield point, breaking strength,
elongation and reduction of area of the test specimen.
Although the breaking strength or "ultimate tensile strength" of
the material is often taken as the standard of quality, the most important
point is usually the elastic limit. The latter is the stress at which
Hooke's Law breaks down, or the point at which the strain in the material
ceases to be proportional to the stress. For practical purposes, the yield
point is generally taken, as it is simpler to observe, being indicated by a
sudden drop in the beam of the tensile testing machine while the load
is being applied to the specimen, and marking the point where permanent
elongation commences. Although the subsequent behaviour of the
specimen or test piece is of great interest and value, this point marks
the limit of usefulness of the material to engineers, and upon it factors
of safety should be based.
The elongation of the test piece, or the difference between its length
after and before breaking, is a measure of its ductility, and the ratio
between its sectional area at the point of fracture before and after testing
is also an indication of ductility, being termed "reduction of area."
The tensile testing machine is composed of two parts, a "straining"
mechanism for applying the load to the specimen, and a weighing
apparatus for measuring that load. The former may be applied by
gravity, manual labour, hydraulic, belt or electric power. The gravity
10
MATERIAL TESTS AND SPECIFICATIONS
11
loading, which is appHed by a falling weight through gearing, cani
obviously only be appHed to very small specimens or very weak material,
such as paper, fine filaments, etc. Hand loading is limited by the pulling,
power of a man, and should not be applied to machines of more than
5 tons capacity, as otherwise it becomes very slow and inefficient in view
of the amount of gearing required. Hydraulic power gives a quick and
accurate test, but necessitates pumps and an accumulator for steady
work, and these add to the cost, complication, and space occupied by
the plant, unless the power is used for other purposes on the works. In
addition, the hydraulic pressure usually employed (about 1 ton per square-
inch) is high, and trouble is experienced with leaky joints, leathers,
valves, etc., which gives the test house rather an untidy appearance..
The machines used with belt and electric power are generally similar,
save that the former is driven by belting and the latter, through silent
gearing, by an electric motor. Of the two, electric driving is preferable,
as it eliminates belt slip, gives easier starting and stopping, and (if a-
variable speed motor is used) permits easy variation of speed to suit
different conditions.
The power is usually applied to the specimen through worm gearing,
and one or more screws. Single screw machines are used up to 30 tons^
and above that, two, three or four screws are usually employed (Fig. 1).
T/?rusf'bear/n^
pvotm ff/iee/
/fei/kvai/ to
/fec/uchon Gean'nc
3haff i^ri/e/r
6u motor or
^*^^3fraining
3cretv
fy'a. / Arrangement ofSfraining Gear r'or Teshn^ Macft/ne .
12
ENGINEERING INSPECTION
The weighing or indicating mechanism is usually a lever or levers,
pendulum, diaphragm or gauge. English machines are generally of the
single or multiple lever types (Fig. 2). The load is taken through the
specimen to the top grip holder, and thence through a shackle to a knife
edge of hardened steel resting on a large lever. This lever is supported
on a second knife edge resting on the frame of the machine, and the
Sfee/j/ard
\ ffn/fe Ec/ges
Upper Gr/p.
^o/der
Specimen
J. ower Crip.
Ho/c/er "^
Straining
Ci///nder-
Po/se
/Jiaaram of
5/n^/e Let/er
Ten^/Ze Teshng
Mach/ne
Spec/men
^ Strain ing Scretvs
Lotver Grip Molder
Po/se
J/-ee/i/ard
Mam Lexers
Knife ed^es ine/iecrfe' V l^cntiis^icn III II
ml 7'.
ini^ ;ill:ii.hnii'lH U
r,y. IJil.)
increments of loading. Where this is employed, the total extension is
indicated on the diagram, and measurement of the specimen may be
dispensed with. An arrangement of this apparatus mounted on the testing
machine is shown in Fig. 7. The extension is taken by means of the cord
or wire which is attached to the lower clips on the specimen, passes over
a pulley on the upper clips and round one of the pulleys on the drum
20
ENGINEERING INSPECTION
of the indicator. The scale of this extension may be increased by using the
smaller pulleys on the drum. The hand-wheel which propels the poise is
HOW) ft"^ H3^ 'St~J O{
O Q
10 05 ^ K) n
"_1_
O
%
S
1-
r
u
U
~ u
i.
ID
35
r
u
h
3) «)'V>0't>'^', showing swing ami for tcsling large pieces,
and rrlcrrnce l^ai-s (liai'd and soft).
The speed of pumping, or applying the load, should be reasonable
and uniform, as otherwise, accidental variations are likely to be obtained.
MATERIAL TESTS AND SPECIFICATIONS 25
The surface of the material to be tested should be as smooth as possible,
and horizontal, as otherwise the impressions will be irregular in form
and difficult to measure.
The approximate tensile strengths given in the table are useful for
many purposes, but care should be taken in their application as the ratio
between the tensile strength and hardness is not always constant for the
same material. In the case of castings, particularly, want of uniformity
in the material, the existence of hidden flaws or blowholes and other
accidental circumstances are always liable to affect the results.
The scleroscope (Fig. 11) is the best known dynamic hardness tester,
and consists of a small diamond-faced hammer (1/20 oz.) which is released
by a pneumatic mechanism and falls from a height of 10 inches on to the
smooth surface of the article to be tested. The hammer is surrounded
by a glass tube "002" to 'OO?)" larger in bore than the diameter of the
hammer, and this contains a scale reading from to 140. On reaching
the specimen, the hammer makes a small indentation in the surface and
rebounds up the tube. Obviously, the smaller the indentation {i.e., the
harder the specimen) the greater will be the height of rebound, and the
reading on the scale during the first rebound is taken as a measure of
the hardness of the material. This instrument can be used on surfaces of
any hardness, but for soft materials a magnifier hammer is supplied,
which gives higher readings. These may be translated into standard
readings by means of a scale supplied with the instrument. Steel blocks
are also supplied to check the accuracy of the instrument at hardnesses
of 30 and 100 respectively.
The following are errors that may be expected and remedied : —
1. Low Readings. Due to dirt inside tube or on hammer, clogged
vent holes causing air cushion under hammer, glass tube too small or too
large, diamond split or loose, instrument not vertical, specimen not
horizontal or level, face of diamond not clean.
2. High' Readings. Due to wear on diamond point.
3. Hammer fails to fall when bulb is pressed. Due to catch hooks
being too far apart or close together, edges of hooks out of centre, hooks
not working properly, plunger in pneumatic cylinder not working full
stroke.
4. Hammer does not catch on hooks when sucked up. Due to
hooks are too far apart or close together, spring tension not flexible, suction
too weak (new bulb or rubber tube required).
26 ENGINEERING INSPECTION
5. Hammer does not rise to top of tube. Due to split or leaky bulb
or air connections, wet or oil soaked bulb.
6. Readings erratic. Due to bore of glass tube being too large or
loose diamond.
Scleroscope tests should not be taken closer together than 1/32",
and the specimen must be of reasonable thickness and well supported. A
jig is useful when a large number of similar articles have to be tested.
In all cases the average of a number of readings should be taken, as the
area of the diamond point is small, and isolated readings are liable to be
misleading. Rounded surfaces must be normal to the centre line of fall,
as otherwise a glancing blow is given and low readings are obtained. Dry
grinding should be avoided in preparing test surfaces, as it is apt to
soften the material.
Scleroscope readings of 80 and upwards are obtainable on mild case-
hardened steel surfaces, but for alloy steels (case-hardened) a hardness
of TO is more usual.
The impressions made by the scleroscope test do not seriously affect
finished surfaces, so that this instrument may be used for hardened and
ground work.*
Machining hardness does not always follow the indications of the
penetration test, and therefore a drill test is sometimes used to indicate
this property. A standard drill under constant speed and loading is used
to cut into the surface of the specimen. The depth of penetration in a
given time is an indication of the ease with which the material in question
may be machined.
Abrasion and scratch tests have also been used for the measurement
of hardness, but their use is at present restricted to laboratory work so
that they will not be described here.
Impact.
The value of drop tests for determining the shock-resisting qualities
of railway and other materials, has been known for some considerable
time, but the standardised Impact test only rose to practical importance
during the Great War, when toughened and heat treated materials were
extensively used in large quantities for aeronautical and motor work.
The specimens used in impact testing are usually notched in order to
give greater sensitiveness to the test, which is performed by breaking
* For hardness figures given by Shore Instrument and Manufacturing Co. see Table 5.
MATERIAL TESTS AND SPECIFICATIONS
27
the specimen with a sharp hammer blow. The energy lost by the hammer
during this operation is a measure of the toughness of the material, and
to enable this to be measured easily, the pendulum hammer is used.
Assuming that the hammer of weight W swings freely from a height H
(Fig. 12) and after breaking the specimen rises to a height h on the
CfK/=iov^-ri:a Sti^i-^
F/g. /2_ D/agram jhoiv/ng Pnnc/p/e of Jmpcrcf' TesA
opposite side, the energy absorbed in breaking the specimen is W {H — h)
and this is indicated by a loose pointer which is pushed forward by the
pendulum rod and left in the furthest position reached. The scale is
calibrated in foot lbs. or kilogram metres.
The specimen used in the Izod machine is round or square in section
(Fig. 13) and is struck by a hardened steel knife edge in the pendulum
at a distance of 22 mm. above the centre af the notch. The 120ft. lb.
machine (Fig. 14) has a 60 lb. weight falling through a height of 2ft.
(angle 60°) and the 150ft. lb. machine has the same weight with a fall of
2"5 feet. The round test pieces are notched in a jig with a standard
turning tool carefully cut to the form of the notch, as this, together with
the depth of the notch is very important. The distance between the
bottom of the notch and the back of the test piece should be made to a
limit gauge, and the notch itself to a form gauge.
In some cases the specimen has three notches cut at right angles to
each other so that the average impact strength in three directions may
28
ENGINEERING INSPECTION
be obtained. If the thickness of the part to be tested is insufficient to
give the standard impact piece, smaller specimens may be made to the
dimensions shown on Fig. 15.*
The Charpy Impact machine is not greatly used in this country,
but is prevalent on the Continent. Here the specimen is in the form of a
beam, and has the "Copenhagen" notch shown in Fig. 16. The
specimen is struck by the knife edge directly opposite the centre of the
OR -3Srl.M.
nound 3peamen — Turned A/ofc/i
1'.
/fauna' 3pec//nen — M///ec/ /VoA:/i
Turning Too/
*_
Souare Specimen
/^/5_ No/ches for /zonf /mpccfTesf spec/ n7en's.
notch, but in other respects the machine is similar in principle (though
not in constructional detail) to the Izod machine.
The size of machine most convenient for general engineering work
is 30 kilogramme metres, and the velocity of striking is somewhat higher
than that in the Izod machine, being 5"3 metres per second as against 3'5
metres per second in the latter case.
There is not a great deal to choose between these two machines as
far as accuracy is concerned, but as the Izod machine has been chosen as
* Enj^ineering Standard Specification, No. 131—1920. 2S, Victoria Street, S.W.I.
MATERIAL TESTS AND SPECIFICATIONS
29
Vie. li. — Izod Impact Tcstiin; Madiin-', Capacity 1-20 foot lbs
{Bv peniiissioii of IT. and T . Avery, Ltd.)
6 l— 0\A'
'^-
-<
?
^^(•SMf-
■ 25 i-T^
1
I*
MM
:^-i
1^:
J
1
:
-5-
/v /J, Alternaf/i/e /zod Impacf
Test Pjece^ ^r 5ma// spec
specimens.
30
ENGINEERING INSPECTION
the standard type for this country and most specifications are based upon
it, the Izod machine is the more useful of the two.
Other types of machine are the Guillery and Amsler machines, but
these again are not used to any great extent in this country. In all cases
it is advisable that the Impact machine should be firmly bolted to the floor
of the test house, as otherwise it is liable to move about.
(fad.
•osst
Hntfe. £dq c-
/^/^
11^^ — — ^otj. SmM.
IQO rr\.yt\.
I A 1^ f Y I m fA C-T i SST AHO
Tks-rR
■e.c.-e.S3.
It has been stated in various places that the Impact test is valueless,
being erratic and unreliable, but it is now generally recognised in
authoritative circles that an accuracy of 5 per cent, can be obtained if
reasonable care is taken in making the specimen and performing the test.
The impact test is certainly sensitive to slight variations, and the form
and size of the notch are very important, but though a poor impact test
may occasionally be obtained in good material, a bad material has never
been known to give a good impact test. The impact test certainly shows
variations that are not indicated by tensile or other tests, and there is
no doubt that its use in the future will be more extended than it is at
present.
MATERIAL TESTS AND SPECIFICATIONS
:il
A variety of the impact test that is used on railway axles, tyres, etc.,
is the drop test. In this case the whole article is tested and no notch is
used. Axles are supported at fixed distances apart, depending on their
length, and a weight is dropped upon them from a fixed height. This is
repeated five times, the axles being turned through 180° after the first
and third blows, so that the blows fall on both sides. The axle must stand
ihis treatment without giving way.
Tyres are tested in a similar way, save that they are supported on a
rigid block of metal, not less than 5 tons in weight, and the height of fall
varies from 10 to 20 feet and upwards.
The "Erichsen" machine (Fig. 17) is a handy instrument for testing
sheet metal. The specimen is placed between two flat gripping
I>0lK«OK
.Screw f-o"^
/^ //■- fr/cAse/} Shee^Mefa/ Tesf
■surfaces and clamped into position. A plunger "A" is then moved
forward by a hand-wheel and screw to form a "bulge" in the specimen.
This is increased until a crack is formed, which is immediately observed
in the mirror, and at this point a reading of the height of the bulge is
taken on the machine. This gives an indication of the hardness and
xlrawing qualities of the sheet, but no definite standard can be laid down
for different purposes. The only method of using this machine in
connection with press work is for the manufacturer to test a number of
samples which have proved satisfactory in the presses for different jobs,
and to use the results thus obtained as a standard for future supplies. Before
fixing this standard, it is advisable to test as many as possible of the
sheets that have failed in the presses, so that the margin of safety may be
ascertained with reasonable accuracy.
32 ENGINEERING INSPECTION
Some characteristic "bulges" on Erichsen test specimens are shown
in Fig. 18. The size of test piece required is ■')}y" x 31", or preferably a
strip .'J^, " wide so that a number of tests may be taken. It is better
to take the average of 5 tests. It is not advisable to test sheets thicker
than ~" in this machine.
The question of "fatigue" testing has exercised the minds of
engineers and metallurgists for a number of years, but although interesting
results and information have been obtained by various investigators, this
form of test has not been adopted to any great extent in commercial work.
One of the principal difficulties is the time taken to make these tests,
and another is the want of uniformity in the various forms of fatigue
test. Until some standardised form is agreed upon, the fatigue test cannot
Fn:. IS. — Chararloristic " EricllSfn " bulges.
be generally adopted, and up to the present, the tests described above
have been considered sufficient for practical working.
A suitable testing equipment for a large engineering or manufacturing
works is the following : —
1 — 30 or 50 ton Tensile testing machine (with compression, bending
and torsion tools if necessary).
1 — 10 or 5 ton Tensile testing machine for small \
specimens. '-Alternative.
1 — 3 ton Wire testing machine. /
1 — Brinell testing machine with microscope.
1 — 120ft. lb. Izod Impact testing machine.
1 — Erichsen testing machine (if press work is done).
1 — Scleroscope (or more if case-hardening and grinding is done, as a
standard instrument should be kept in the test-house for
reference).
MATERIAL TESTS AND SPECIFICATIONS 33
Special testing appliances have to be bought or made for any special
work, but the above is a nucleus that will provide all necessary information
in most shops.
Specifications.
The proper specification of materials is an important point as errors
or omissions at this stage may involve the loss of large sums of money to
the purchasing firm. Many firms issue their own specifications, and
others rely on the British standard specifications. As these, or other
standard specifications have to be worked to in Government contracts,
suppliers have become familiar with their provisions, and if they were
more widely adopted would keep stocks in hand to meet the various
standard specifications called for. This would save much waste and over-
lapping, and would frequently avoid disputes. For many purposes, how-
ever, the standard specifications are not suitable or available, and special
specifications have to be drawn up.
Some buyers merely specify the mechanical tests required without
entering into details of analysis, testing procedure, etc., and they justify
this by the contention that an elaborate specification restricts supplies and
tends to raise the price of the articles supplied, so that the degree of
completeness required in the specifications is largely a matter for individual
judgment.
The following points, however, may profitably be borne in mind
when drawing up specifications, and any or all of them included according
to the conditions of supply : —
1. Class of material from which the articles are to be made, and, if
necessary, name of firm or firms supplying that material.
2. Method of manufacture in more or less detail, noting any
special points required.
3. Dimensions and limits of accuracy.
4. Faults to be avoided in material and general quality required.
5. Chemical composition of material with limits if necessary.
G. Mechanical tests required and condition of test pieces (whether
machined, normalised, heat treated, etc.), when tested.
7. Form and sizes of test pieces, with limits.
8. Selection of tests, including proportion of tests taken, procedure
of selection, and method of marking and numbering test pieces.
9. Method of inspection, by whom carried out, and when.
10. Specification of heat treatment, if required.
34 ENGINEERING INSPECTION
11. Return of material previously rejected by inspector, and procedure
to be adopted.
12. Provision for independent tests in case of disagreement between
supplier and purchaser.
13. Arrangements with reference to the proportion of the cost of
preparing test pieces and testing to be borne by supplier and
purchaser.
14. Method of marking or painting accepted and rejected material,
and by whom this is to be done.
15. Stipulations with reference to the repair or patching of faulty
pieces and procedure to be adopted.
16. Procedure for rough machining, pickling, sand blasting or clean-
ing articles before leaving supplier's works so that defects may
be observable.
17. Method of protecting articles from corrosion during storage or
transport, and the degree of such protection to be provided
by supplier.
The descriptions of testing machines given in this chapter are necessarily brief, but
fuller particulars may be obtained by reference to the following works : —
" Testing of Materials," by W. C. Unwin.
" A Handbook of Testing-Materials," by C. A. Smith.
" Materials of Construction " — Popplewell.
" The Application of Materials to Engineering Design " — AUcut and Miller.
Also the following papers : — '
" Use and Equipment of Engineering Laboratories " — Sir A. B. W. Kennedy,
Proc. Inst.C.E., 1887.
" Mechanical Properties of Materials " — W. C. Unwin. Proc. I.Mech.E., Nov., 1918.
■" Researches made possible by the Autographic Load Extension Indicator," Journal
Inst, of Metals— W. E. Dalby. May, 1917.
■" Shock Tests and Determination of Rosilience " — Charpy and Cornu-Thenard. Iron
and Steel Inst., Sept., 1917.
" Single Blow Impact Test on Notched Bars " — Greaves and More. ^ Proc.
" Shock Tests and their Standarisation " — Sir R. Hadfield and Main. - Inst.C.E.
■" Characteristics of Notched-bar Impact Tests" — Stanton and Batson. ) 1920.
" Brinell Method of Testing the Hardness of Metals." Page's Weekly, Jan. 8, 1909.
" Resistance of Metals to Penetration under Impact " — C. A. Edwards. Inst, of
Metals, Sept., 1918.
■" Report of Hardness Tests Research Committee." Proc. I.Mech.E., Nov., 1916.
" Hardness Testing " — A. I". Shore. Proc. Iron and Steel Inst., Sept., 1918.
" Prism Hardness " — B. P. Haigh. ( Proc. Inst.
" Measurement of High Degrees of Hardness " — J. Innes. \ Mech.E., Oct., 1920.
" Notes on the Ball Test "— T. Baker & T. F. Russell. Iron & Steel Inst., May, 1920.
" The Brinell and Scratch Test for Hardness " — W. C. Unwin. Engineering,
Nov. 21, 1919.
" Brinell and Scratch Tests for Hardened Steel " — Hadfield and Main. Proc.
Inst.Mech.E., Oct., 1919.
CHAPTER IV
INSPECTION OF RAW MATERIALS
The natural division of materials into three classes — raw, partly
manufactured, and finished — is a difficult matter in any general treatise,
because the finished material of one factory or trade, is the raw material
•of another. Any division of this nature, therefore, is necessarily arbitrary
and vague, but as this treatise is written from the standpoint of a general
■engineering works, raw material is assumed to be material that is likely
to enter such a works for the purpose of having further machining or other
•engineering work done upon it. It is also assumed that the firm in
•question has its own foundry, smithy, etc., and therefore castings and
forgings are classed as partly manufactured material.
The raw materials used in general mechanical engineering practice
.are divisible into two main classes — metallic and non-metallic.
The former may be divided into Ferrous materials, which are generally
the most numerous and important, and non-Ferrous, and these two main
sub-divisions may be again split up into different sections as in the
following table : —
METALLIC MATERIALS.
I I
Ferrous. Non-Ff.rrous.
I I
I I I I I I I I
I I 1 .^ -I I P R I
rt ,„ rt t/i iXJ -- o <'
^ ten 12 iL> o — '
I ^ 1^ I H ^ <
These classes do not cover all the kinds of material used in
•engineering practice, but are the most important metallic materials that
mechanical engineers have to deal with.
Similarly non-Metallic materials may be classified as follows : —
NON-METALLIC MATERIALS.
I
Stones & Earth:
1
s.
Timber.
1
1
Fabrics.
1
i
c
a
'C
3
Oils &
1
Fuels.
1
Cement; _
Sand-
1
(A
m
1
11
1
.1
en
2
<
2
<
1
1
a
1
g
.9-
(A
1
c
'5
CO
1
u
U
<
u
V
.£3
3
1
c
'o
1
o
ll
—
O
1
o
3
3
JO
1
m 3
c
1
33
3G ENGINEERING INSPECTION
Ferrous Materials.
Cast Iron. — Cast iron is a name given almost indiscriminately to the
raw material fed into the cupola, and to the finished product of the foundry.
The former is more popularly known as "pig-iron," and is the form in
which cast iron is sent out from the blast furnace. It is generally
classified into " grey" and "white" irons, and the various qualities may
be distinguished to some extent by examining the appearance of the
fractured surface when a pig is broken, but more satisfactorily and
accurately by chemical analysis.
No. 1 iron has a coarsely crystalline structure of dark grey colour
and blackens the hands when touched. This is due to the fact that plates
or flakes of graphite (Carbon) are intermingled with the crystals of iron.
Nos. 3 and 4 are of similar formation, but the crystals are much smaller,
and the amount of graphite less. These irons are stronger than Nos. 1
and 2, and are not so easily fusible, the lower numbers being used almost
exclusively for ornamental work and for making special mixtures.
The foundry man mixes his ingredients almost like a doctor's pre-
scription, and has his own pet mixtures for different classes of work.
The various grades and kinds of pig are used by him for producing the
quality of mixture that he has found to be successful in the past. This is
only the case with old foundries and foundry men, as qualities even of
recognised brands vary so much nowadays that most foundries work on
chemical analysis.
White iron contains practically no graphite, and is generally used for
making malleable castings. It is hard and brittle, and the fractured
surface is almost white and silky in texture, showing none of the crystalline
appearance characteristic of grey irons. Mottled iron is a quality midway
between white iron and No. 4 grey, and possesses some of the character-
istics of both.
The best way of inspecting pig or cast iron is to take one or two
sample pigs from each consignment entering the foundry, and after break-
ing, observe the fracture to see that the correct grade has been supplied.
If this is satisfactory, the broken pigs should be sent up to the laboratory
for analysis, and the consignment carefully kept apart from the remainder
of the store until it has been certified correct. Often pigs are kept in
piles labelled with the name of the suppliers, but when analyses are
systematically and regularly taken, classification by chemical composition
is better, as then the buying department is not tied down to any one
particular supplying firm.
INSPECTION OF RAW MATERIALS
37
Average analyses for the different grades (English) of Haematite
cast iron* are as follows : —
Combined
Grapliitic
Manganese
Sulphur
Phosphorus
Silicon
Grade.
Carbon %
Carbon %
%
%
%
%
1
.39
3.89
.21
.01
.07
1.07
Q
.57
3.66
.12
.02
.06
.97
3
1.07
3.02
.20
.05
.06
.59
4
1.07
2.86
.17
.05
.07
.73
Mottled
1.31
2.37
.11
.06
.06
.40
White
2.39
.83
.14
.09
.07
.30
The effect of these various constituents on the properties of cast
iron may be summarised briefly as follows : —
Graphite. — Makes castings soft and easy to machine, diminishes
shrinkage, but makes castings weak and brittle if graphite is present in
the form of flakes or plates.
Combined Carbon. — Makes castings hard and brittle, and tends
towards the production of white iron. If castings are cooled rapidly
(chilled) produces a very hard surface, practically unmachinable.
Manganese. — Partly neutralises the effects of Sulphur, Silicon and
Phosphorus if present in small quantities, but any excess produces a
hardening effect. It increases shrinkage but produces sounder castings.
Phosphorus. — Makes castings fluid, hard and brittle, but diminishes
shrinkage. If strength is a consideration it should not exceed "5 per cent.,
but is suitable for the production of thin and ornamental castings.
Sulphur. — ^Tends to make the iron thick and viscous in pouring. It
produces greater shrinkage and gives hardness and blowholes, so is an
impurity to be avoided. It is generally derived from the coke used in the
cupola.
Silicon. — Produces a soft, grey, fluid iron, and is useful in helping
to eliminate blowholes and in giving sound castings.
The tensile strength of cast iron varies from 6 to 14 tons per square
inch.
Semi-Steel.
This is an attempt to improve the quality of cast iron by adding
a percentage of small pieces of steel to the mixture in the foundry. The
usual quantity added is abouc 20 per cent., the object being to increase
* Turner's " Iron."
38 ENGINEERING INSPECTION
the strength and ductility of the castings without incurring the complica-
tion, expense and delay of making malleable castings. Semi-steel castings
have been obtained with a tensile strength of 28 tons per square inch
and an elongation of 3 per cent, on a length of 2 inches, but this material
should be very carefully watched as frequently it is no better than cast iron.
Semi-steel castings (so called) have been obtained with a tensile strength,
of 6 to 8 tons per square inch and no elongation, but the usual figure is
about 15 to 18 tons per square inch and 2 per cent, elongation.
Malleable Iron.
The greatest objection to iron castings in engineering practice is their
weakness and brittleness. Malleable castings are made by packing white
iron castings in iron oxide (red haematite) and heating for a considerable
period at a temperature of about 900°C. This results in the abstraction of
some of the Carbon from the cast iron which then attains some of the
ductility of wrought iron and is therefore called "malleable."
The period of heating varies with the size of the casting, as the de-
oxidation or " malleabhsing " effect takes longer to penetrate a thick
casting than a thin one. This material should therefore be tested by
fracture, as in some instances it will be found that there is a thin shell of
malleable iron surrounding a hard and brittle core of cast iron. A tensile
test should also be applied, and for important work, an impact test.
There are two kinds of malleable castings — white and black. The
former are generally made by English foundries, and have tensile strengths
of 20 to 30 tons per square inch, but little ductility (generally 2 to 4
per cent, elongation). Black heart malleable castings are usually made by
American foundries,* but there is one foundry in England that specialises
in this product. They are lower in tensile strength than white heart
castings (about 18 to 20 tons per square inch) but have greater
ductility (5 to 10 per cent, elongation). f High tensile strength is often
cited as evidence of good quality in malleable castings, but this is not
necessarily the case. In many castings immunity from breakage or crack-
ing under shock or vibration is of greater importance than high tensile
* These castings are " annealed " at a lower temperature than white heart castings, the
temperature being 750 — 850° C. The de-oxidising effect is much less at this temperature,
but the carbon is retained in the form of " rosettes " instead of flakes, the weakening effect
in thii case being much less than that of the " flaky " carbon of cast iron. Black heart
castings, therefore, are only de-oxidised on the surface, but the interior has practically the
same amount of carbon as the original cast iron. This is shown by its fracture.
+ In a measured lenglh of 2 inches.
INSPECTIOTST OF RAW MATERIALS
39
strength, and as the latter is generally associated with a corresponding
want of ductility, it is often a positive disadvantage.
In no case should malleable castings have a lower breaking strength
than 16 tons per square inch, but where the castings are subject to live or
rapidly fluctuating loads, the other extreme should also be avoided, and
castings having a greater strength than 25 tons per square inch should be
looked upon with suspicion.
Wrought Iron.
This has been largely superseded in recent years by mild steel,
which is cheaper and has very similar characteristics, but it is still used
to some extent for intricate forgings and parts that have to undergo a good
/%?/■ Forging Test
for "BB' /ran
B
Hot Forg/np Tesf
For "B" /ran
Fracf-urs Tesf For iVrou^iht Iron
F/£. /^_ l^orkshoD Tesh for l/Vrou^M //-an
deal of cold work. It has a fibrous structure on account of the slag
entangled in the original bloom, which is rolled out into long threads. If
a bar is cut half way through and bent over, the fibres are at once revealed,
and this constitutes one of the tests for wrought iron. If the iron is heated
and re-rolled several times it improves with each rolling, and thus, from
common puddled iron the superior qualities of best, double best, and treble
best iron are obtained.
40 ENGINEERING INSPECTION
The tensile strength of wrought iron is usually 20 to 26 tons per square
inch, with a yield point of 13 to 15 tons per square inch. The common
varieties have low ductilities as shown by elongations of 5 to 10 per cent.,
but the best Swedish or Yorkshire irons may have elongations up to 60
per cent.
On account of its fibrous nature, the tensile strength of test pieces
taken across the grain is usually from 2 to 4 tons per square inch lower
than that along the grain, but this difference should not be too pronounced.
Wrought iron is the purest commercial form of iron, and is susceptible to
the presence of impurities such as Sulphur or Phosphorus, so that chemical
tests are advisable in cases of doubt. The best practical tests are forging
tests, hot and cold, to ascertain whether the metal will spread well without
cracking (Fig. 19), and welding tests.
Cold forging tests should be taken after heating and cooling rapidly
in water to prove that the Carbon content is sufficiently low to prevent any
hardening from taking place.
Steel.
There is no one substance that can be taken as representative of steel,
as the name includes a great variety of materials, having different
properties and uses. It is therefore necessary to classify steel into two
main divisions, namely. Carbon and Alloy Steels. All steels contain
Carbon, -affld the percentage of this element present has a deciding influence
on the properties of the material. Plain carbon steels also contain
Manganese, Silicon, Sulphur, and Phosphorus, the two latter constituents
being undesirable impurities to be kept as low as possible. Generally,
the addition of Carbon to pure Iron increases the tensile strength, and
decreases the ductility of the material. If heated to redness and rapidly
cooled, such material will harden to a degree dependent on the percentage
of Carbon present, so that the natural classification for such steels is low,
medium, and high Carbon. There is a good deal of confusion in com-
mercial and engineering work, as to the exact definition of mild steel.
Some suppliers, when asked for mild steel give carbon contents of anything
up to -5 per cent., but this is not a true mild steel. The latter
term should only be applied to steel that will not harden very appreciably
when heated to a high temperature and rapidly cooled. The commercial
limit for mild steels should be about -3 per cent. Carbon, and anything
above this and below -6 per cent. Carbon should be classed as medium
Carbon steel.
INSPECTION OF RAW MATERIALS 41
We therefore have the following classification for carbon steels : —
% Carbon.
■1 — -3 Mild Steels.
■3 — "6 Medium Carbon Steels.
'6 — 1'5 High Carbon Steels.
If other metals, such as Nickel, Chromium, Tungsten, etc., are intro-
duced into the steels, the properties of the material are considerably
affected, and the Carbon content ceases to be the decisive factor. Such
steels are therefore known collectively as Alloy Steels, and individually
as Nickel, Nickel Chrome, Tungsten, or Vanadium Steels, according
to the name of the special elements introduced.
The effect of various alloying elements on the properties of steel
may be summarised briefly as follows : —
Manganese.
This is always present in small quantities, but if present
between 1 and 5 per cent., decreases the strength and ductility of the
steel. A further increase to 14 per cent, improves these qualities, but the
tensile strength and elongation again decrease with more than 14 per cent,
of Manganese. The commercial form contains 1'2 to 1"5 per cent.
Carbon, and 12 to 14 per cent. Manganese, and gives sound castings with
a large contraction (I" per foot) and low tensile strength and ductility.
These are enormously increased by heating and quenching in water. This
material forges well but is unweldable. When cold, although soft to the
Brinell test, it is practically unmachinable, and is non-magnetic.
The rolled bar has a tensile strength of 40 tons per square inch and an
elongation of 5 per cent, on 8". These are increased to 60 tons and 40
per cent, after heat treatment.
Chromium.
This has a hardening tendency in steel and if increased to over 5 per
cent, the steel will become hard when cooled in air after heating. With
•2 per cent. Carbon and up to 10 per cent. Chromium the tensile strength
increases and ductility decreases, the maximum strength after heat treat-
ment being over 90 tons per square inch. "Stainless" Steel, used for
cutlery, valves, etc., contains 12 to 14 per cent. Chromium.
ENGINEERING INSPECTION
Tungsten.
This causes steel to be "self-hardening," and so is used for cutting
tools. Modern high-speed tools have up to 18 per cent. Tungsten and
some Chromium, but this does not imply that the steel is harder when cold
than a good high Carbon steel. It does indicate, however, that the steel
can be run with heavy cuts at a high speed, as tungsten steel does not lose
its hardness to any great extent when heated by this treatment, and this
gives it a great advantage over Carbon steel.
Molybdenum.
The effect of this element is somewhat similar to Tungsten, but it has
not been adopted to any great extent for cutting tools. Steel containing
"25 to "3 per cent. Carbon, '8 per cent. Chromium, 3 per cent. Nickel, and
■35 to 45 per cent. Molybdenum has been used for small crank shafts, giving
the following properties : —
Yield point, 58 tons per square inch.
Ultimate tensile strength, 65 tons per square inch.
Elongation, 20 per cent.
Impact (Izod), 67ft. lbs.
Nickel.
This is a toughening element in steel, and when used in conjunction
with Chromium, very high tensile strengths (over 100 tons per square
inch) can be obtained. Steel with 1 — 3 per cent. Nickel is very good for
forging, and is very easy to heat treat successfully. On account of their
high ductility Nickel steels are very useful whenever shock is to be met.
Vanadium.
This improves the tensile strength and ductility of steel, having its
best effect at about T per cent. Its action is generally supposed to be a
cleansing one, removing gaseous impurities from the steel, but its high
cost is a bar to its extended use. It is now used for automobile springs
and other pieces where high resistance to shock is required.
Aluminium.
This is used in small quantities to increase the fluidity of steel, but
if any appreciable quantity is present {e.g-, '5 to '75 per cent.) the steel
INSPECTION OF RAW MATERIALS 45
becomes difficult to cast, and pipes are produced in the ingots owing to
the high contraction. More than 2 per cent, of Aluminium produces
brittleness in the steel.
There is a still further complication as many articles such as gear
wheels, shafts, etc., are required to possess hard surfaces to resist wear,
and therefore have to be case-hardened. If a mild steed is to be case-
hardened, the Carbon content must not exceed '2 per cent., as otherwise
the steel is inclined to be crystalline and brittle after hardening. Alloy
steels also have to be case-hardened in some instances, and at other times
have to be hardened by heat treatment only. The classification of steels
may be expressed diagrammatically as follows : —
STEELS.
I I
Carbon .Alloy
I I
I I I ] \ I
Mild Med. Carbon High Carbon Case Oil Air
I Hardening Hardening Hardening
1 I
t; •OB
lai o a -a
Storage and Handling.
In view of the different varieties of steel that may be in stores or in
use at the same time, the storage and handling of steel is a matter of
supreme importance to the inspector concerned. It is his business to see
that the correct kind of steel is used on each job, and that no mistakes
are made.
The possibilities of error are numerous. The wrong steel may be
issued, the various issues may be mixed in the cutting off stores, the
issue may be taken to the wrong hammer at the forge, or to the wrong
machine in the shops, or one of the smiths may be short of material of
that particular size, and may take the wrong steel from the stock in the
smithy while nobody is looking. These sources of error are not imaginary,
but have actually been encountered in practice.
The utmost care should therefore be taken in drawing up that part
of the scheme which deals with the handling and inspection of steel bars,
billets, and plates. The steel should be "bonded" on arrival, and given
a definite bond or cast number. Where possible, it is advisable to get
44 ENGINEERING INSPECTION
the maker's cast number and employ it in the system, as, in case of
dispute, negotiations are greatly facilitated by this procedure. When the
steel is "bonded" in the stores, particulars of the consignment should
be sent to the Inspection Department, in duplicate, as a notification of
arrival and a request for examination, thus : —
BLANK MANUFACTURING CO. go^d No.
STEEL BOND. Maker's
Cast No.
To Materials Inspection Dept.
(or Laboratory). Date
Please inspect the following material received on the ij
from Messrs
Our Order No Requisition No
To be used for
Our Specification No British Std. Spec. No.
Class of Steel Maker's Brand ...
I As rolled
Maker's Marks Condition { Normalised
Heat Treated.
I Bright Bars
Black Bars
Billets
Plates \X'eight
_. , Black Bars Ouantity ...
f^°"" I Billets ~ T
Q LB
TO BE FILLED IN BY MATERIALS INSPECTION DEPT.
This Steel jg ^^j suitable for Specification No and must be ^'"•^^''t,^
Our Identification marks
Heat Treatment required
Signed
Inspsctor of Materials.
Date
Our Test No
The top half of the sheet is filled in by the Steel Bond storekeeper,
and the bottom half left blank for completion after the tests are taken.
In order to facilitate heat treatment and allocation of faulty steel, it is
INSPECTION OF RAW MATERIALS
45
advisable to keep a list of symbols (either letters or figures) allocated to
the various sources of supply, as follows : —
STEEL SUPPLIERS.
Name.
Symbol.
Name.
Symbol.
Name.
Symbol.
Aaron & Co.
Acton & Co.
Addison & Co....
AA
AC
AD
Marshall & Son...
Mistral & Co. ...
Moland & Nephew
M A
MS
ML
E. Smith & Co....
J. W. Smith & Co.
L. Smith & Jevons
SM
ST
SJ
Care should be taken in allocating symbols to avoid repetition and
also, if the identification mark includes figures after the letters, to avoid
the letters I and O, as these may be mistaken for figures (see second
column of symbols). Further, in case of the same name occurring several
times, the second letter or symbol must be changed, e.g., Smith may be
represented by Sm, St, Sh, or other combinations.
The steel in the stores should be divided into seven classes, and each
class kept strictly separate from the others, so that no mistake or mixing
may be likely to take place there. It is advisable also to keep the various
consignments or bonds of similar material separate, even after they are
passed for use, so that in case of mechanical or other defects being found
subsequently, the faulty consignment may be isolated. The various classes
of steel may be known by numbers instead of names, as follows : —
1. Mild forging or rolling.
2. Mild case hardening.
•3. Medium carbon.
4. High carbon.
5. Alloy case hardening.
6. Alloy oil hardening.
7. Alloy air hardening.
Subsequent figures may be used for plate or sheet work, thus : — -
8. Best CRCA (cold rolled close annealed).
9. Best pickled.
10. Deep stamping quality.
11. Copper soft CRCA.
12. Best polished stamping.
etc.
If the steel has to be heat treated, a further number may be allocated,
to indicate the kind of heat treatment required to give the desired result.
46 ENGINEERING INSPECTION
Thus the identification mark consists of one or two letters, giving the
name of supplier, a figure indicating the class of steel, and a figure
indicating the number of heat treatment required.
Example. — JC52 means Jones and Colman's steel, alloy case
hardening, heat treatment No. 2. This mark should be stamped on every
bar or billet, and transferred to every article made from that bar or billet.
When the articles in question reach the hardening shop for heat treat-
ment, the foreman hardener will look up card JC52 in his card index, and
obtain all particulars necessary for the heat treatment of the part in
question.
HEAT TREATMENT CARD.
Supplier. -
—Jones
and
Cohnan.
Mark
Braxd.
JC
52
— AHS
55.
Our Spec
.—No.
H3.
British
Std.
Spec-
-2 S 15.
HE.\T TRE.\T.MEN1
Carbonisi
.—900°
C.
Cool
slowly in
box.
Harden.—
-Reheat
rap
idlv
to 850° C,
soak 15 mins.,
quench in oil.
Temper. —
Re-heat
steadily
to 760° C
quench in wate
'.
Date
Thus if any materials are found to be faulty during the later machining
or hardening processes, all other parts made from the same brand of steel
may be separated out immediately, and the suppliers notified of the
trouble.
Surface Examination.
While steel is in bond a certain amount of time may be saved by
examining brands of known reliability for superficial defects. As these
defects are common to most brands of steel they are all included in this
examination.
Bars should be as straight as possible, particularly in the case of
bright bars to be used for machining. In the latter case also, particular
attention should be paid to the question of limits, as bright bar is fre-
quently ordered to a limiting size to save cost of machining. Suppliers,
however, do not always adhere to the limits imposed and therefore ring
gauges of the prescribed sizes should be provided. Gap gauges are
quicker in operation, but are not so useful, as the bars are sometimes oval
or irregular in section.
INSPECTION OF RAW MATERIALS 47
Standard limits for bright drawn bars are given in British Standard
.Specification No. 32.*
In the case of billets also, attention must be paid to the cross sectional
dimensions, as if they have to be cut up into standard lengths for making
forgings or drop stampings, billets of small section will not give sufficient
material to fill the dies, and the forgings will be scrapped.
Roaks or seams are common to bars and billets and consist of lines
along the surface following the direction of rolling. They are generally
caused by slag or scale which is rolled into the section and may frequently
be detected by observing the sheared ends of the billets where they often
open out into cracks. By filing or grinding a Vee notch across the billets
the depth of these defects may be observed, and if not too deep, the steel
may be allowed to go forward. If more than 1" deep the billets should
not be used for important drop forgings, especially when these have to
be heat treated, as the seams open out into cracks on hardening, unless
previously removed by machining. It should be remembered, however,
"that the depth of seam in the forging is generally greater than in the bar
from which it is made, and allowance should be made for this fact when
inspecting.
Bars that have to be machined may be inspected for seams, by
brightening up the surface at intervals of 3 or 4 feet with a grindstone
or emery wheel. If the seam does not pass right along the bar, faulty
«nds may be cut off and returned to the makers as rejections. These
defects are particularly objectionable in case-hardened work, as they
break the continuity of the case and tend to open out as cracks. In case-
hardening steel, they frequently appear below the surface, and are revealed
by machining. The appearance of the surface, however, is usually a fairly
good indication of freedom from these defects.
Piping is also met with occasionally in bars and billets. It consists
oi an irregular crack or flaw down the centre of the section, and is
■derived from the recess which forms during the cooling of the ingot from
which the billet is rolled. In most cases care is taken to see that the
top of the ingot is kept liquid until the lower portion has solidified, but
■even then some of the ingot has to be cut off before rolling. If this cut
is taken too high up, the cavity will be rolled into the centre of the billets,
but if taken below the " pipe," the billets will be sound. The only way of
detecting this is to examine the cut ends of the billets, particularly when
-these are being sawn up for forging purposes, as it is generally easier to see
* Engineering Standards Committee, 28, Victoria Street, London, S.W.I.
48
ENGINEERING INSPECTION
piping flaws on a freshly cut section than on one that is obscured with rust
or scale.
Hard centres are also occasionally met with, owing to carburised
material, which sometimes forms the top of the ingot, getting into the
body of the metal, whereas in the ordinary course it would be cut off.
/
S/a^Seam laps "Tongue's P//>in^ r/atv
f/g.20^ Defects in 5 fee/ Bars
Laps are somewhat similar in appearance to roaks, but while the
latter run from the surface towards the centre, the former, as their name
indicates, run in an oblique direction and are generally easier to machine
out as they do not penetrate so far into the material (Fig. 20).
For bars and billets, tensile and impact tests are advisable with an
occasional check by chemical analysis, particularly in the case of steels
O.O
/y i?/_ Bending Test on ml/d Sfee/ —
A.&B. are /fo//erj Ae/d in posihon
C /3 a^moyee/ fin pt/ffea up againsf
ffie £ar mfh &rce xjffieienf- io^endii"
Conyenienf- Dtmensfons ofTesf-Bar:
/}ia. jg' Le«gfh. 3'
that have to be heat treated. Bend tests are occasionally useful in the
case of mild steels, but are not so searching in character as impact tests
(Fig. 21). A good idea of the shock strength of a steel may be obtained
INSPECTION OF RAW MATERIALS
49
by making a notch or saw cut in the side of the bar and breaking it with
a hammer. The resistance to fracture and the appearance of the frac-
tured surface (whether fibrous or crystalHne) gives experienced observers
a good idea of the impact value likely to be obtained under test. A
Brinell hardness test is also a useful accessory, as this gives an idea of the
tensile strength, so that for many mild steels a fracture and Brinell test
will give all necessary information.
Alloy steels must be more carefully treated. The best procedure
for case-hardening or oil-tempering steels is to cut test pieces from the
bars or billets (say one test piece to 50 bars), forge down to about 1^"
diameter, and about 8 — 9" long (if necessary), and then submit these
pieces to the exact heat treatment found correct for that particular brand
in the past. If the steel is to be case-hardened the carbon must be turned
off the outside of the test piece before hardening, as otherwise it will have
to be ground off. After heat treatment the test pieces must be carefully
turned to finished size, taking care to leave the surface smooth and to
avoid sharp corners or small internal radii. Chemical analyses and
microscopic tests should always be taken for new brands and occasionally
as a check on regular supplies.
" -; * " I
RooGrH F^Ml&H
-J*i
(a) 7es^ P/ece /or CarSur/s/ng
FLa-ts 2 ^/iDE p*oR Bf^iMEuL. "Test
3^:
^E
No
1>^
l*"l
si"
f6j Te5f Piece reocfi/ /or Heaf- Treafmen/
fCarion ^urnet^ off Oiomefierj and Efisfsj
V
r
II ^
'i*m
4*'
1^
Th
HIS PAf^T MOST MOT OE l^noERCUT
3L
S
Notches
^ 1 I
Or
-^"
5i"
•r^
CI
a<-!
7ensi/e Tes/
fc) F/na/ fbrm oriesf P/ece
/moac/ Tes/
/y'g.BB^ Tensi/e & /mpoc/-Tesf- Pieces /n Caje/rarc/en/ng Siee/.
50 ENGINEERING INSPECTION
With sheets for press work, one of the chief questions is that of
surface. For many purposes, such as motor body panels, good surface is
essential, and in this case the material must be cold rolled, close annealed,
pickled to remove scale, and hydraulically flattened. In the last case
there will be an unusable strip of about 1 inch wide on each side of the
sheets where they are held during the flattening process. Open annealed
sheets with flaws and pitted surface, occasionally find their way into the
works as CRCA sheets, and must be rejected, as even if they will stand
the press work put upon them, the surface of the finished article, even
after painting, will show up the defects badly. The size and gauge of the
sheets must also be watched, to see that these fulfil the conditions of the
contract, otherwise scrap pressings may result. The pressing or drawing
qualities of the sheets may be estimated by flattening over one corner
(when they should bend over flat without cracking), and by taking an
an " Erichsen " or bulging test, already described.* In the writer's
experience, however, these tests have not proved entirely satisfactory in
indicating the ductility of material for deep pressings, so that even if
they are made, it is also advisable to try sample sheets in the dies to
ascertain whether they crack in pressing.
Boiler plates must be examined for buckling, pitting, scaling, and
lamination. The first three defects are self-explanatory, and the latter
(which also occurs in strip steel) is a crack in the plate which divides it
into two thicknesses and entirely unfits it for practical use.
They should always be tested chemically, as well as physically, as
the former test shows the quality of the material, and the latter its
condition.
Boiler plates should have approximately the following composition : —
Carbon ... T6— -18% Silicon ... -01— '02%
Manganese ... '25 — ■50% Sulphur and Phosphorus
not more than '04%
A strip about 2" wide cut from the plate should bend through an
angle of 180° and close down flat (after being heated and quenched in
warm water) without cracking. Drifting tests on pieces about 3" square
should enable a punched hole f " diameter to be expanded to 1|-" diameter.
Non-Ferrous Materi.\ls.
Non-ferrous materials are usually bought in the form of castings or
forgings, and these will be dealt with in Chapter V, but occasionally bars,
wire, and sheets of these materials are used in engineering practice.
* .See Chapter III.
INSPECTION OF RAW MATERIALS 51
Copper and aluminium bars and wire are extensively used in electrical
engineering as conductors, but their behaviour electrically is outside the
scope of this work, and so will not be considered. Mechanically, the
■only test that need be applied is the tensile test, and chemically the
percentage of impurities should be as low as possible to ensure the best
possible conductivity.
The non-ferrous materials generally used in this form are the various
.alloys of copper with zinc and tin, called respectively the brasses and
bronzes. Brasses containing high percentages of copper are malleable
and ductile, and as the percentage of zinc increases they become harder
and more fusible. Alloys containing about 15 to 20 per cent, zinc are the
most ductile, and up to od per cent, zinc can only be cold rolled or drawn.
Alloys with .io to 40 per cent, zinc can be either hot or cold worked.
Crenerally the melting point of brasses used in engineering is 900° to
1,000°C. Brass with 60 per cent, copper, and 40 per cent, zinc is
often made into bars of special section by "extrusion," i.e., by forcing
the heated ingot through a hardened die of the required shape by means
of hydraulic pressure. The ingots treated in this way must be free from
blowholes and oxide inclusions. The temperature of working must be
carefully regulated, as if worked too hot, the structure is too coarse, and
the bars are weak, and if too cold, the structure is distorted and strained
■by excessive cold work. Other non-ferrous alloys may be treated in this
way and the temperature of working varies with the alloy used.
The tensile strength is greatly increased by this process, and the
metal made more uniform than when rolled or drawn. The following list
gives the average strength of extruded bars : — *
Tensile strength
El
ongation
tons
per sq.
in.
on 2".
Aluminium ...
17
5
Aluminium brass (85 — 10-
~5)
47
18
Brass (60—40)
33
35
Copper
15
40
Delta Metal (No. 1)
44
20
Magnalium ...
32
10
Zinc ...
13
25
The effect of iron on brass is to harden the metal and increase its
tenacity, but it is difficult to dissolve in copper. This was overcome by a
* " Metal Industry " Handbook, 1921.
52 ENGINEERING INSPECTION
special process, and Mr. Dick introduced a special alloy containing 55 — 57
per cent, copper, 42 per cent, zinc, 1'2 per cent, iron, and traces of
phosphorus, which was the origin of the Delta metals, now covering a
range of alloys for different purposes.
Nos. 1, 2, 4 and 7 are used for bars. No. 'i for solid drawn tubes,
and No. 4 for sheet, wire, etc. Of these, the last is most widely used,
and is a malleable metal with high resistance to corrosion, and is also
suitable for castings and forgings. Its properties will be found in
Table 2.
Monel Metal is a nickel-copper alloy, containing 67 per cent, nickel,
29 per cent, copper, 3 per cent, iron, 1 per cent, manganese,* and is used
for valves, pump rods, etc. It usually contains also "10 — "15 per cent,
silicon, and carbon up to '3 per cent, increases its strength in a similar
manner (though in a less degree) to its effect on mild steel. Excess of
carbon causes trouble in forging, and if present in the graphitic state, the
metal is totally unforgeable.
Its range of forging temperatures is 900° to 1100°C.t and it cannot
be hardened by heat treatment and is not readily softened by annealing at
700°C. Although distinctly magnetic, it is not sufficiently so to enable
its swarf to be separated out from other materials in this way. It is
rather difficult to machine on account of its toughness, but is no harder
than mild steel when tested by the Brinell test. It has a high resistance
to corrosion, and melts at 1350°C.
Duralumin is a light alloy containing 93 — 95 per cent, aluminium,
"5 per cent, magnesium, 3 — 5 per cent, copper, and '5 — 4'5 per cent,
manganese. Its specific gravity is 2'75 to 2'84, and melting point about
650°C. It is generally used in the form of sheet and can be made into
tubes by pressing. It can be hardened by heating and quenching, but
reaches its maximum strength about 5 days after such treatment, without
decrease of ductility. If worked hot it must not be heated above 530°C.
and must be annealed in a salt bath.
Magnalium is an alloy of aluminium containing 3 — 10 per cent, of
magnesium, and has a specific gravity of 2'5. It can be cast, welded, and
forged, and can be obtained in the forms of sheet, wire, and tubing.
These are some of the alloys commonly met with in bar, wire, or
sheet form. Those mostly encountered as casting or forgings will be
dealt with in Chapter V.
* J. Arnott, " Metal Industry," XVI., 17.
t Above 1100° C Monel Metal tends to crumble under the hammer.
INSPECTION UF RAW MATERIALS -Vl
Non-Metallic Matf.rlvls.
The inspection and testing of non-metallic materials does not often
fall within the province of mechanical engineers, save in a few special
cases, and therefore this side of the subject will onlv he considered
Vjriefiy.
Stones and Earths.
Stones are usually tested in compression, being first dressed into the
form of a cube, and then compressed on two opposite faces lietween
flat platens in the compression tools of a tensile testing machine.
Alternatively a special hydraulic press with pressure gauge calibrated
to read in tons pressure on the ram, may be used for this purpose, but the
correctness of this method is bounded by the accuracy of the gauge —
always an uncertain quantity. In anv case, care must be taken to see
that the stone is properly bedded to the compression surface bv facing
with some packing substance to distribute the load uniformly over the
surface. Also the compression platens must be parallel to prevent the
load being applied to one corner or one side of the cube. For this reason
the platens should rest in spherical sockets so that thev niav automatically
adjust themselves when the load is applied.
The testing of Cement is completely described in FJrilish Standards
Specification No. 12 (Revised March, l!tlo)* and to this, interested readers
Fu.. -23.— (.'-■inriu 'IVvlini; .Marilin.'. ("'.■ipMcity l.LHIilb,.
(;;\' />fniii,s-ii,.ii .1/ ir. ami 'I'. .I;viv, I. Id.)
Briti^h Smndards Cuniinilii-c 28, \"ictoria Stn-ot, S.W. 1.
54
ENGINEERING INSPECTION
are referred. Briefly summarising this, the cement is tested for fineness
on sieves of 76 x 70 meshes, and 180 x 180 meshes to the inch, the residue
not to exceed 3 per cent, and 18 per cent, respectively. The specific
gravity is tested in a specific gravity bottle (not below 3'15 if fresh or 3'10
if over four weeks old) and the chemical composition checked by analysis.
Briquettes of special shape, each having a neck area of one square inch,
are made in standard moulds, and are tested after 7 and 28 days' soaking
in water. The briquette is placed in the gripping jaws of a special testing
machine (Fig. 2'i) where the load is applied to the specimen by means of
" lA'ca/-' Needle
Cement Briaueffe
soo
Ae ChaM/er " Gauge
/7^ 24— Cement Testing Apparatus
lead shot running into a can. This can is supported at the end of a lever
system which transmits the load to the briquette. As the lever ratio is
40 to 1, the full load of 1,200 lbs. is produced by 30 lbs. of shot. When
the specimen breaks, the supply of shot is automatically cut off by a trip
lever, and the can is hung at the other end of the main lever (or " steel-
yard "). The shot may then be weighed by a sliding weight on the
steelyard, which is graduated to read the actual load on the briquette, at
the time of fracture. In testing, care should be taken to set the shot
valve so that the load is uniformly applied at the rate of 500 lbs. per
minute, to see that the jaws are well greased where they touch the
INSPECTION OF RAW MATERIALS 55
specimen, and that the load is evenly applied at all four points of contact,
as otherwise premature fracture will take place. Time of setting is tested
by the " Vicat " needle (Fig. 24) which is 1 mm. square, and is
loaded with a weight of 300 grammes. When this fails to make an
impression on the surface of the cement, setting is presumed to be com-
plete.
Quick setting ... ... ... 10 to -lO minutes
Medium ,, 30 to 120
Slow ,, 120 to 300
Soundness is tested in the Le Chatelier mould which has pointers
1G5 mm. long. The cylindrical (split) mould is filled with cement and
boiled in water for six hours. The expansion (after cooling) is measured
at the pointers and should not exceed 10 mm. after 24 hours' aeration.
Timber.
Timber has recently become important on account of its application
to aircraft construction. As all timbers are far from uniform, tensile tests
are valueless. They are also difficult to carry out, owing to the tendency
of the timber to crush in the gripping tools. Crushing tests may be made
on cubes or short cylinders, care being taken to see that the ends are
properly bedded in some soft packing material so that the load may be
uniformly applied. Axial loading should be assisted by spherically seated
compression platens or ball seats as shown in Fig. 25. On account of its
lack of homogeneity, consignments should always be visually inspected,
and samples examined with a low power microscope.* The defects usually
found are shakes, dote, knots, dead wood and resin pockets, and in the
case of birch, "pith flecks" caused by insects.
The inclination of grain to length, as ascertained by a splitting test,
must not exceed 1 in 12 for walnut, 1 in 10 for ash, and 1 in 20 for spruce.
Moisture has a considerable effect on the strength of timber, but
should not be reduced to less than 10 per cent, as further drying causes
brittleness. The standard amount of moisture allowed for is 15 per cent,
on the weight of dried samples, but this varies about 2 per cent, with
the seasons.
The rate of growth has a considerable effect on density, and for silver
spruce and similar timbers, the number of annual rings should not be less
* For further particulars see ^^'. H. Barling on Aeroplane Timbers (Royal Aeronautical
Society, 1918).
56
ENGINEERING INSPECTION
than 8 to 10 per inch, but in ash, slow growing produces a preponderance
of weak spring wood, and in this case the number of rings should not
exceed 16 per inch.
Densities of timbers are expressed in lbs. per cubic foot at 15 per cent.
moisture. Walnut ;i5 lbs. cu. ft. ]
Ash -48 -
Silver Spruce ... 25 ,, ,, ,, j
In the case of fir, pine, elm, ash, maple, sycamore, aspen, and alder,
wood showing annual rings with only a slight curvature is strongest, other
things being equal.
/yjr. 25- Tesfs ^r T/'mber.
yy/'yy'y//y/yy/
'(M
I "Thick
V
W
2
w
2
IS"
3S
Bend/ng Tesf
S-reei-
C^i-
1
7777V7
Load
1
^ Bi_ow
J
< /
/ -n
^
p^-
1
N
H
-^
3*?
2 a
Bi-ow
Radius op- rSoxcwes-
/zc^ /mpac/- Tesf- P/ece^
G/ue Test
2"
-Gl
Tgj/ /3r 3P/t/ Wood
Bending tests on specimens 40" x 1" (wide) x 2" deep are now
generally substituted for tensile tests. The beam is loaded as shown in
Fig. 25, and the deflection measured across a measured length of 18 inches
as the loads are applied by means of rollers and steel saddles. The load
is applied at two points to produce a length " 1 " (18") of uniform bending
moment and no shearing stress, along which measurements can be taken.
INSPECTION OF RAW MATERIALS 57
The loads and corresponding deflections for any beam are plotted in
a curve, and the modulus of elasticity of the timber calculated as follows : —
W and d are the load and corresponding deflection for any point in the
straight part of the curve.
Z) = breadth of beam; /i = depth of beam.
E = modulus of elasticity of timber.
I — Moment of Inertia of Cross Section =x\7 6 /»^-
For thin ash a suitable test is to bend a lath \" thick round a semi-
circle 18" diameter. The lath should show no signs of fracture.
An impact test has also been adopted for aeroplane timbers. The
method of testing is similar to that in the Izod test (Page 27), but the
dimensions of machine and test piece are different.
Weight of pendulum, 20 lbs.
Radius of swing, 24".
Striking distance above centre of notch 2\"
Size of specimen, ^" square.
Dimensions of notch are shown in Fig. 25.
The impact strengths of various timbers are : —
Walnut, 9ft. lbs.
Ash, 10ft. lbs.
Spruce, 4 — 8ft. lbs.
Glued joints may be tested by gluing a block with sides 2" square
between two others, and applying a load to shear the glue until the joint
fails. A good joint should stand 3,000 to 7,000 lbs. when tested in this
way.
Three ply wood may be tested by cutting a section as in Fig. 25,
and pulling the section in a tensile testing machine, when the joints should
stand a load of 150 lbs. per square inch of area. The plies may also be
tested for separation by immersing a piece about 6" x 6" in water at 45°C.
for 3 to 6 hours, after which treatment the plies should not show signs of
separation at the edges.
For plywood of |" thickness or less, a specimen 12" x 2" may be
tested by bending round a circle of 18" diameter, when the wood should
not show signs of fracture, crack or parting.
The tests mentioned above are mostly for aeroplane timber, but may
be found useful in modified forms for other classes of work.
o8
EXGIXEERING INSPECTION
Fabrics.
Aircraft falirics are generally tested in tension, and as the load must
be uniformly applied at a given rate, the method adopted is similar to
that used in cement testing. The specimen is held in corrugated grips,
care being taken to ensure accuracy and straightness, and the load is
applied through a compound lever system, by running lead shot into a can
hung from the end of the steelyard. In this case, however, stretching
of the specimen must be allowed for, and a deflecting mechanism is pro-
vided to stop or divert the flow of shot, whWe the stretch is being taken
Fig. -20.— Fabric lasting Macliine. Capacity 1,200 and 240 lbs.,
to test fabrics up to (V wide.
( /m' l^cniiissioii of ir. and T. Avfiy, Lid.)
up by the straining screw. The machine also weighs the shot after the
specimen is broken, giving the actual breaking load in lbs. or kilogrammes.
The leverage is 50 to 1, so that 24 lbs. of shot are required to give the full
load of 1,200 lbs. (Fig. 2G).
Few aircraft fabrics require this load, and for these the machine may
be converted into a single lever machine with a maximum capacity of 240
lbs. .Special machines for aero fabrics are also made, with a capacity of
oOO lbs., and electrically operated cut-off gear.
INSPECTION OF RAW MATERIALS 59'
As sometinies there is a tendency for the load to Ije applied akmg
one edge of the specimen, it is advisaljle to put a small load on the
machine and to allow the specimen to slip slightly before tightening up
the grips. This ensures an even tension across the width.
II Mlill lUlWl
Fi(
'21. — < H-ip>
Trsiiii"- .Macliini;
The specimens are usually 7" long between the grips, and 2" wide
after trimming, or IS cm. and 5 cm. respectively. They are trimmed
to oljtain the same number of threads in each piece (Fig. 27).
60 ENGINEERING INSPECTION
The standard rate of loading for linen fabric is 150 lbs. per inch width
per minute. The specimen is usually soaked in water for 15 minutes or
more before testing, to avoid complications due to the percentage of
moisture in the atmosphere which varies from day to day. This increases
the strength from 20 to 40 per cent, above that of dry fabric. Specimens
2" and 3" wide give results about 4 per cent, and 13 per cent, less respec-
tively than would be expected from tests made on a specimen 1" wide.
Short specimens generally give slightly greater strength than long ones,
owing to the diminished chances of finding weak places in short lengths.
If the rate of loading is trebled, the increase of strength indicated
is about 6 per cent.
Good fabric gives tensile results varying about 7 per cent, to 10 per
cent., and poor qualities vary up to 20 per cent.
Fabrics are also judged by weight after examination for faults in
manufacture. Linen fabric for aeroplane work should not exceed 4 ozs.
per square yard, and should give minimum tensile tests of 92 lbs. per inch
width of warp, and 95 lbs. per inch width of weft.
Cotton fabrics vary from 3'8 to 1'3 ozs. per square yard, and give
strength of 70 to 28 lbs. per inch width respectively.
Balloon fabrics may also be tested by clamping specimens between
iron rings and applying pressure from an air pump until the fabric gives
way. Gas tightness may be tested in a similar way by observing the
quantity of gas which passes through a fabric disc under a given difference
of pressure.
For tyre and other industrial fabrics, tensile tests similar to those
applied to aircraft fabrics may be used, but larger specimens should be
employed.
Fabrics used as friction surfaces {e.g., clutch and brake fabrics) should
be tested for tensile strength, but are better tested on running surfaces,
the amount of power absorbed at different speeds and pressures, and the
co-efficient of friction being most important.
" Ferodo " fabric has a tensile strength of about 4,000 lbs. per square
inch, and a co-efficient of friction of about '30 under average running
conditions, but naturally this varies with the form of the surface and its
condition, whether dry or lubricated.
Rubber is tested tensionally in the " Schopper " testing machine.
The sheet is cut into rings which are mounted on rollers as shown in
Fig. 28. These are rotated, as the specimen stretches, by means of the
rack and pinion shown. The load is applied by water pressure from the
INSPECTION OF RAW MATERIALS
61
town mains, and is measured by raising a weighted pendulum, which
remains in position when the specimen breaks, indicating the breaking
■Suspenc/ecf /ree/y
from PenduJum fnd.
ndica^or
Specimen
paii-lystrefchgd
P/nion Gearing
ivith rdck and X',
rofafwgas ,
Specimen jfrefches
Groovedfloilei
/fack
Originaf Form
orSpecimen
fo hydrauiic P/sion
tvorired bi/ iVaier f^ain Pressure
Fir,. -iS.-
-Diagram of " Schopper
Rubber.
tensile test for
load. Other tests for rubber are outside the scope of the ordinary
engineer and are best dealt with by experts.
Oils and Fuels.
For dealing adequately with this portion of the subject, a separate
treatise would be required, but as it has already been described at length
in other works, a general summary only will be given here.
Oils are required either for their lubricating properties or for their
value as fuels, and the engineer is only concerned to know whether he is
getting good value for his money. To a certain extent this will be
demonstrated in practice, but to avoid trouble, delay, and expense, tests
are advisable when placing contracts and to see that the conditions of the
contracts are adhered to by the suppliers.
The object of lubrication is to keep two metallic surfaces separated
by a thin film of oil. The viscosity of the lubricant is tested by a Redwood
" Viscosimeter " by means of which a volume of the oil (raised to the
necessary temperature) is allowed to run through a hole at the bottom of
62
ENGINEERING INSPECTION
the vessel, 1'7 mm. diameter and 12 mm. long. This allows 50 cubic
centimetres of rape oil at 60°F. to pass through in 535 sees., and this
time is taken as a standard. The thinning of oil at high temperatures is
tested by this instrument, and is an important factor in most lubricating
oils. Mineral oils are better than vegetable oils in this respect.
Flashpoint is the temperature at which the oil gives off vapour which
■will burn in air. It is obviously undesirable for this to be low. Loss on
Seciiiirtal Viuw of "Railwii}' and HccordiriH Patterns."
F/^. 2d- Thurjfon's O// Tejf/ng Mac/j/ne
fjir W.H.Bai/e(/&C? Manc/iesf-erJ
heating should also be low for the sake of economy. Some oils have a
tendency to become " gummy " when exposed to the air, and this, together
with acidity (which promotes corrosion) should be avoided.
Chemical analysis is therefore advisable, and if possible, a distillation
should be made to determine the quantity and nature of residue left after
boiling. For running tests, special machines, such as the "Thurston"
oil tester (made by W. H. Bailey & Co.) may be used (Fig. 29).
INSPECTION OF RAW MATERIALS G■■^
Fuels are generally assessed on their heating value, as the object
IS to obtain the greatest possible number of heat units for a given
expenditure. Solid fuels should be carefully sampled from all parts of
ihe consignment, and after careful mixing, breaking up, and successive
" quarterings " of the sample, the final pile should be tested by heat (after
pulverising to a fine powder) to obtain the amount of moisture and ash
present.
It should then be tested in a bomb or other calorimeter to ascertain
its heating value, but if burnt in a Mahler bomb, the pressure of oxygen
■should exceed 20 atmospheres as otherwise results are likely to be low.*
Gaseous fuels are tested for quality by analysis, and for heating value by
the Junker or other similar calorimeter. For further particulars and
■description of the various calorimeters employed, the reader is referred
io "The Calorific Power of Gas," by J. H. Coste (Griffin & Co.) where
■the whole matter is exhaustively dealt with.
For liquid fuels, chemical analysis with specific gravity and flash-
point tests for light fuels, and distillation to examine the quantity and
quality of residue will give all necessary information. Calorimetric tests
should be made for all fuels, as it may be more economical to pay more for
a fuel because it has a higher heating value per pound, than the cheaper
variety. In the case of coal, too, the composition of the ash may be a
deciding factor, owing to its freedom or otherwise from clinkering troubles.
-* " E.Nperiments on a Bomb Calorimeter." Engineering, December 2nd, 1910.
CHAPTER V
INSPECTION OF PARTLY FINISHED MATERIAL
As previously explained, this chapter deals with material upon which
a certain amount of engineering work has been done, but which requires
further machining or other adaptation before it can form part of the finished
article.
Castings.
The inspection of castings may be considered under two heads, viz.,
material and dimensional inspection. In the case of important castings,
tensile tests should be taken from each consignment as it arrives at the
works, and the batch of castings should not be moved until the results of
this test are known.
In the case of mild steel or malleable iron castings, an impact test
is also advisable, the former generally giving 15 to 20ft. lbs. and the
latter about 5ft. lbs. when tested by the Izod test. When an impact
machine is not obtainable, a bend test may be substituted, but this is not
so searching as the impact test. In doubtful cases a chemical analysis
is advisable, and this may be supplemented by microscopic examination,
which is especially useful in the case of alloys. It should be noted that
chemical analysis is not an infallible guide in considering the suitability
of many materials, as the structure or disposition of the various con-
stituents has an important effect upon the physical properties of such
materials and upon their general suitabiHty for engineering purposes. In
this connection the microscope is a very useful accessory.
In making tensile and impact tests it is advisable, wherever possible,
to cut the test pieces out of the castings themselves. This involves the
scrapping of useful castings, but the cost may be minimised, in many
instances, by cutting up castings that are rejected for some physical
fault such as incorrect dimensions, flaws, etc. Many founders object to
this practice, as they can usually get better results from test pieces specially
cast for the purpose, and supplied with the castings. They prefer,
therefore, to supply test pieces, and this proposition is attractive to buyers
as it saves scrapping actual castings.
64
INSPECTION OF PARTLY FINISHED MATERIAL G5
There are several objections to this practice however. In the first
place, there is usually no proof that the test pieces have been cast from
the same metal and at the same time as the castings. Cases have been
known where a quantity of test pieces were made from a superior grade
of metal in the early stages of the contract, and one or two of these were
sent in with each consignment of castings. The practice of casting test
pieces on actual castings does not entirely remove this possibility, as
obviously the castings to which the test pieces are attached need not be
of the same grade as the castings which form the bulk of the supply.
It may be objected that no reputable foundry would do this sort of
thing, but the fact remains that it has been done in the past, and if the
reputation and character of supplying firms is to be taken as an infallible
safeguard, inspection of any kind is superfluous. Further, the thickness
and mass of the test piece are generally very different from those of the
actual castings, so that conditions of cooling in the two cases are by no
means similar. This factor has a great influence on the physical properties
of many castings, so that, even if the test piece is cast at the same time
as the actual castings, the physical properties of test piece and castings
may be very different indeed. It is sometimes argued on behalf of the
founders, that the practice of casting test pieces allows the various grades
of material to be compared under similar conditions. This is true and is
useful from their point of view, but it must be remembered that the user
is not particularly interested in the strength of material under the con-
ditions obtaining in the test piece, but in the actual strengh of the casting
itself. Castings are not homogeneous, and owing to variations in
thickness of section, internal radii, conditions of cooling, mixing of metal,
and other conditions, some parts are weaker than others. Test pieces
should therefore be cut, as far as possible, near to the place where the
maximum stress is expected, so that the factor of safety may be ascertained
with reasonable accuracy.
As castings contract in cooling, lack of uniformity in thickness is liable
to cause contraction cracks and flaws, particularly in the case of steel
castings. These are often covered by scale, and are invisible on arrival at
the works. It is advisable, therefore, to pickle highly stressed castings
(such as axles) in dilute Sulphuric Acid* to remove the scale. After this
treatment, any cracks or flaws are shown up distinctly, and the faulty
castings may be weeded out. After pickling, the acid must be thoroughly
* Mixture: 1 part acid !o about 30 of water. Keep Specific Gravity by fiydrometer at
about I'l.
66
ENGINEERING INSPECTION
washed out of the castings with hot water. Iron castings are often cracked
in transit, and when received, the broken castings may be sorted out by
a "ringing" test, made by tapping the castings with a hammer, while
suspended freely in air. Sound castings give a clear note, while cracked
ones sound "addled."
Spongy places and blow holes are not usually apparent on the outside
of castings, but are revealed by machining the surface. In the case of
castings for small gears, it is advisable to inspect these after rough turning
as otherwise a good deal of time may be thrown away in cutting teeth on
faulty castings, as teeth broken up by blowholes and spongy places are
generally useless. Such teeth not only scrap the gear itself by breaking
off in use, but tend to crumble up, and the small pieces carried away by the
lubricating oil and grease are liable to wear away other gears and bearings.
F/^.30^ Bench for Hof and Co/d Wafer Pressure Tesfina
rP?or-i Pump
^T^^f-^ doi*.
Many castings, such as cylinders, pumps, etc., are required to be
sound under water pressure. These are usually tested by pumping water
or oil into them under a pressure that is usually from two to four times the
working pressure (Fig. 30). For important aeroplane work where very
thin castings are used, a better plan is to immerse the casting in water
and pump air into it, testing for leaks in the same way as a bicycle
tyre is tested for punctures. Spongy places are revealed by bubbles of
air escaping from the faulty spots.
Monobloc and other cylinder castings for internal combustion work
INSPECTION OF PARTLY FINISHED MATERIAL GT
are better tested with warm water as this often reveals faulty places that
cold water testing does not detect.
If the casting is not very porous (in the case of iron and steel) it
may be filled with a solution of sal ammoniac or other rusting agency, and
left for several weeks, by which time the porous place is often " made-up,"
and the casting is then sound enough for most purposes. The same result
may frequently be obtained by leaving the castings in the open air to
"weather" for periods varying from one to six months. Some castings
are made water-tight by galvanising, but this is somewhat expensive as
they must be thoroughly cleaned before being coated.
Aluminium castings may be made tight by doping with water glass
(sodium silicate solution). After doping, all trace of this substance must
be removed by thoroughly washing with hot water to prevent further
■corrosion. The exact procedure is laid down in Air Board Specification
No. M8* Castings of complicated form, that have no great pressure to
withstand, may be tested by plugging up any holes, and filling with paraffin.
This will find out any bad places that are likely to be troublesome in
practice. Gun metal and other copper alloys may be improved by enamel
or other non-metallic coating, but care should be taken to see that the
coating is not liable to chip off, as otherwise the small pieces of enamel
may cause trouble. It is better to apply the coating to the outside of the
■casting if this can be made satisfactory, as then there is no possibility of
contaminating the working fluid.
A great trouble with steel castings is sand which is fused into the
surface owing to the high casting temperature. This can sometimes be
removed by pickling, but usually sand blasting is necessary. In very
obdurate cases, it may be necessary to dope or paint the surface of the
casting to stick the sand in position and prevent it dropping into lubricating
■oil or other working fluid. This is a policy of desperation however, and
should not be adopted, save as a last resort. It is the inspector's duty
-to see that the castings sent for machining are sound and free from, sand
or scale, as the latter may cause many expensive tools to be scrapped,
and earn for the castings an undeserved reputation for hardness. Hard
spots are a frequent trouble in iron, malleable, and steel castings, and
generally result from bad mixing or local chilling, but these can usually
"be detected only by machining. A Brinell test will often separate out
castings that are uniformly hard and require annealing, but it is not a
safe guide to the tensile strength of the castings themselves.
* Now issued bv the Britisli Standards Committee.
68 ENGINEERING INSPECTION
The extent to which dimensional inspection of castings is taken,
depends upon the scale of operations. If thousands of similar castings;
are required under a mass production scheme, machining allowances and
weights are reduced to a minimum, and the castings must fit accurately
in the jigs provided for machining. With smaller quantities, machining^
operations will not be so close, and inspection need not be so rigid.
For quantity production of castings, the allowable variation in
dimensions is very small indeed, as the cost of machining is reduced to a
minimum, and castings usually have locating spots or faces from which
the first operations commence. If these are displaced, the whole of
the subsequent operations are affected. An instance in point is a cylinder
casting where such displacement throws all the boring out of position, so
that one side of each cylinder is unduly thin. Distortion and wear of
patterns often causes the machining of castings to be thrown out to such
an extent that some surfaces will not clean up, and irregular thicknesses
of metal are common in such cases. This emphasises the necessity for
periodic examination of patterns, so that castings may be kept as close
as possible to standard sizes and shapes. Sometimes an arrangement is;
made whereby the founders themselves keep patterns in order, but where
large quantities of castings have to be made, this work is best performed
by the firm who have to machine the castings, as they know best the
difKculties met with in production. A man should therefore be kept for
the sole purpose of following up difficulties met with in machining castings,
and to see that all patterns are kept true and in good working order. He
thus becomes a sort of liason officer between the Inspection Department,
Machine Shop, Buying Office, and Foundry, but his duties are funda-
mentally inspection (Fig. 31, Report of Rough Viewing and Marking
Out).
The repairing of faulty castings is a matter that needs careful scrutiny.
Many castings showing flaws are repaired by patching or welding, and
when these arrive, a proof test of some kind is advisable to see if the
repair has been properly done. Many so-called welds fail to knit together
the sides of the flaw, and simply cover up all external indications of the
defect, without curing the trouble. This is like attempting to cure a
cancer by covering it with a plaster. In such cases the flaw should be
chipped out in the form of a Vee groove, and then welded up. The
operation of heating up the casting for welding is very liable to form
cracks elsewhere, and the casting should be viewed most carefully after
the welding has been done. When lugs are burnt on to castings
INSPECTION OF PARTLY FINISHED MATERIAL G9
(particularly in the case of aluminium castings) they should be examined
with a strong magnifying glass and tapped gently with a hammer to see
if the weld is sound. Generally aluminium welds are not very satisfactory.
Welds in steel and iron castings (in cases where a proof-load cannot be
REPORT OF ROUGH VIEWIXG AND MARKING OUT.
Date
Description
Part No Supplied by
(or Drawing No.)
Specification No Castings or Forgings
Quantity
General Condition and Finish.
Report on Marking Out.
! General Remarks
J'iewer.
Percentage of Rejections
(Approx.)
Fig. 31.
employed) should be rough machined before passing into stores to ascertain
whether the weld is sound, and whether the material used for welding is
too hard to machine. The latter is a frequent fault. Welded castings
should be annealed before use. Welding is also liable to distort machined
■surfaces, and therefore faulty castings should be thrown out as early as
possible, so that such distortion may be allowed for. This point should
be borne in mind by the inspector who sanctions repairs by welding.
70 ENGINEERING INSPECTION
Alloys.
The principal non-ferrous alloys used in castings are the copper and
aluminium alloys.
Brass varies in composition, with the size and uses of the castings,
but generally it contains about 67 — 70 per cent, copper, the remainder
being mostly zinc. As common materials are used, however, impurities
are bound to be introduced. From 1 per cefit. to 3 per cent, of lead
facilitates machining, but also reduces tensile strength. Up to 2 per
cent, the presence of tin increases hardness, but this constituent should
not exceed -3 per cent, as brittleness is also increased.
Muntz metal is usually composed of 60 per cent, copper and 40 per
cent, zinc, but the percentage of copper varies from 67 to 63 per cent.
It is forgeable at 750 — 800°C., and if quenched from these temperatures
becomes harder and stronger, but is less ductile. Its tensile strength is
about 22 tons per square inch, and elongation 40 per cent.
Manganese brass or bronze contains 58 — 60 per cent, copper, 41 — 39
per cent, zinc, and manganese up to 2 per cent. Some manganese
bronzes, however, contain very little manganese, and as they contain little
or no tin they are not bronzes. Aluminium and tin are generally present
up to 1 per cent., and sometimes a little iron is also present. Its tensile
strength is 41 tons per square inch, elongation 24 per cent, and melting
point 870OC.
The most important of the bronzes is Gun Metal, which covers a
range of alloys averaging about 89 — 90 per cent, copper. This has a
tensile strength of 15 tons per square inch and an elongation of 15 per
cent. Admiralty gun metal contains 88 per cent, copper, 10 per cent,
tin and 2 per cent, zinc, and has a tensile strength of 15 tons per square
inch with an elongation of 7^ per cent. Its melting point is 985°C.
Phosphor Bronze is generally used as a hard bearing metal, and for
this purpose should contain o to I'O per cent phosphorus. Many makers
supply less than this amount, but in this case the bearing is too soft for heavy
loading. Its tensile strength is 10 — 18 tons per square inch, and elonga-
tion 6 — 10 per cent. For excessive wear the percentage of tin should be
kept high. A typical analysis is : — Copper, 86 per cent. ; zinc, 1'5 per
cent. ; tin, 11'6 per cent. ; phosphorus, "9 per cent.
Aluminium Bronze usually contains about 10 per cent, aluminium
for casting purposes. In dry sand and chill castings respectively, it has
an elastic limit of 10 and 14 tons per square inch. Ultimate strength, 25
and 30 tons ; and elongation, 2 and 8 per cent.
No.
2.
No.
3.
No.
4.
No.
5.
No.
G.
No.
7.
No.
8,
INSPECTION OF PARTLY FINISHED MATERIAL 71
Delta Metal is made in several grades for different purposes :—
No. 1. High tensile forgings and castings. Tensile strength,
40 tons per square inch, and elongation 18 — 20 per cent.
Silver bronze for rods, forgings and castings.
Special for solid drawn tubes.
(Various grades) malleable bronze, tensile strength, 24
tons per square inch ; elongation, 21 per cent. For
castings, forgings, stampings, wire, etc.
Anti- friction bronze for bearings.
Improved gun metal for castings.
High temperature bronze for castings, stampings, etc.
9 and 9a. White anti-friction metals.
No. 4 alloy can be cast in sand and chill moulds, and hammering cold
considerably increases its strength, raising the tensile strength 60 — 70
per cent. On account of galvanic action it should not be placed in contact
with copper or gun metal, in sea water or other corrosive fluids. Its
forging temperature is 550°C., and forgings made at this temperature
have a tensile strength of 34 tons per square inch and an elongation of 25
per cent.
Aluminium Copper Zinc Alloys are greatly used for aeroplane and
motor engine castings. Sand castings with 12 — 14 per cent, zinc, and 2^
to 3 per cent, copper have been used extensively for crank cases and other
important castings, but this alloy is liable to porosity. Of impurities, lead
should not exceed 1 per cent., silicon 1 per cent., and iron 1 per cent.
This alloy has a tensile strength of 11 tons per square inch, and an elonga-
tion of 4 per cent. Its specific gravity is 3'0.
For light die castings, an alloy of 11 to 13 per cent, copper, with
impurities, zinc and lead not more than '1 per cent., silicon and iron not
more than 1"0 per cent., and the rest aluminium, has been extensively
used. It has a tensile strength of 9 tons per square inch, and is used for
pistons and other small parts. Its specific gravity is 2'83 — 2'94.*
White Metals.
These consist of hard crystals embedded in a comparatively soft
ground mass or "matrix," and are chiefly used for lining bearings. The
forces on bearings consist of a compression stress due to the loading
* For fui'thrr information see " Aluminium Alloys for Aeroplane Engines," by Prof.
F. C. Lea, .Aeronautical Journal, November, 1919.
72 ENGINEERING INSPECTION
and a tensile stress produced by friction. If the compression stress
exceeds the compression yield point of the "matrix," the latter will give
way, the hard crystals will be forced downwards, and the bearing will fail
by wear and spreading. If the tensile stress exceeds the yield point of
the " matrix " in tension, the hard crystals will be torn out and the bearing
fails by scoring.
In both compression and tension a tin base alloy is superior to a lead
base alloy, and tin base alloys are generally used for bearings. Tin base
alloys, however, rapidly deteriorate by burning if overheated for any length
of time, while lead base alloys are not greatly affected. Mechanical tests
are not of any great value for bearing metals. The scleroscope is some-
what more delicate than the Brinell test, but neither of them is of much use
in determining the suitability or otherwise of white metals for bearing
work. Chemical analysis and micro-structure are the best indications
available for this purpose.
The effect of various constituents on tin base alloys is as follows,: — *
Antimony — Increases hardness, wearing qualities and brittleness.
Bismuth — Should only be present as a trace to act as flux.
Copper — Prevents segregation and increases hardness. If more
than 5 per cent, present, promotes brittleness and
tendency to crack.
Iron — Undesirable, increases hardness and brittleness.
Lead — ^Increases fluidity and ductility. Not more than 1 per
cent.
Nickel - — Increases ductility and lessens liability to crack if not
more than "5 per cent.
Tin — Promotes rigidity and increases wearing qualities. Is
prime factor in cost.
Zinc — Creates tendency to seize, and increases hardness and
brittleness. Undesirable.
Alloys having a Brinell hardness of 34 have been found to have an
approximate compressive strength of 24,000 lbs. per square inch, and at
20, a strength of 42,000 lbs. per square inch before cracking, but this
is considerably affected by rate of cooling. The castings should be cooled
rapidly to prevent the separation of antimony tin cubes, which have a
tendency to rise to the top of the metal.
* Anti FricUon Bearing Metals," by I' W. Priestley, " Metal Industry," 1920.
INSPECTION OF PARTLY FINISHED MATERIAL T.'J
Forcings.
Forgings and drop stampings are sometimes ordered from outside
contractors, and in other instances are made "at home," but in the latter
case the general procedure for inspection is similar to the former, the
smithy being treated as an outside firm supplying forgings to the stores.
Here again, as in the case of castings, the inspection procedure is subject
to considerations of quantity. When large quantities of comparatively small
forgings are required, the machining must be cut down to the smallest
possible amount, and must be done in jigs. Forgings are not usually made
with sufficient accuracy to give the best results, and also highly skilled
men are needed to produce them, especially if they are at all complicated
in form. Drop stampings can be made to approximate very closely to
the finished form of the articles required, but as the cost of sinking the
dies necessary for making them, is comparatively great, and the risk of
dies breaking in use is considerable, it does not pay to produce drop
forgings in small quantities (usually not less than 1,000). The fundamental
difference between the two methods of forging is as follows : — Hand or
power hammer forgings are moulded by a large number of comparatively
light blows, but drop stampings are moulded by a few heavy blows. The
weight of drop stamps ranges from 10 cwt. to 6 tons, and the height of
fall is usually about 5 to feet. The force of the blow is regulated by,
varying the height of lift. As drop stampings are coming largely into
favour for light engineering and repetition work they will be considered
first.
Before inspection, all important drop stampings should be pickled
in dilute sulphuric acid, and thoroughly brushed with hand or revolving
wire brushes to remove the scale which adheres to the surface and hides
defects. The acid solution should have a specific gravity of about 11,
and the brushes (about 9" diameter) should revolve at a speed of about
800 revolutions per minute.
Seams and laps are formed in the stamping by incorrect formation
prior to placing the red hot metal in the dies. The steel billet from which
the stamping or forging is to be made, is heated in a coal, gas, or oil fired
furnace until it reaches a uniform temperature of about 1,100°C. It is
then withdrawn and (where necessary) roughly forged under the roughing
stamps to a shape approximating to the finished form of the stamping.
This is to ensure the dies being filled with metal, and to economise material
at the next operation. If this preliminary formation is not properly done
the dies may split or the metal may fail to fill the dies and faulty stampings
74 ENGINEERING INSPECTION
result. If a slight fold is produced during the forming operation, or
in the dies) this will be hammered into the body of the forging as shown
in Fig. 32, and will produce a hair line. These have a very similar
appearance to the seams in steel mentioned in Chapter 4, but the twO'
faults can be distinguished from one another in two ways. If a section'
is cut through one of the seams or laps, it will generally be found that
material faults are radial, and point toward the centre, while stamping-
laps are oblique, as shown in Fig. 20. Also as stampings are formed by
the flow of metal (in a plastic state), material faults which originate with
a flaw running along the steel billet, always follow the lines of flow of the
metal, while stamping faults usually occur near changes of section or deep
gaps in the stamping, and often do not follow the line of flow at all. It
is important to make this distinction, as when stampers are paid by the
piece, it is necessary to credit them, not with the number of stampings
produced, but with the number passed as good by the inspector. In this
OO oo
f/^.32— Fcmafion of Laps ih For^i'n^ a/oivn
/found Secf/onj
case the stamper will be credited with the work performed on faulty
material, but if the faults are due to the method of forging or stamping,
he is not paid for the number rejected. It is necessary, therefore, for the
inspector of stampings to be an experienced man who can say definitely
whether rejected work is due to the material or operator. Defects of this
kind are not so prevalent in forgings, as the formation of laps can better
be seen and prevented, and often the operator has more skill and
experience than the drop forger. Further, the amount of machining
allowance on rough forgings is generally greater than on drop forgings,
and this gives a better chance of machining out the defect at a later stage.
Overheated or burnt metal has been spoiled by heating the metal in
the furnace to 1,300°C. or more. It may be due to one end or corner of the
billet being exposed to flame in the furnace, or to the latter being kept at
too high a temperature. The best check on this is to observe the tempera-
ture of the furnaces periodically with a pyrometer (or Seger Cones) and to
select well-designed furnaces, where the flame cannot impinge on the
articles that are being heated. In important cases, however, a further
precaution is advisable, namely, to have a walking inspector in the stamp
INSPECTION OF PARTLY FINISHED MATERIAL
1 ■>
shop or forge, to watch for cases of overheating, on the spot. Forgings
deemed to be made at too high temperatures should lie stamped by him
immediately. This not only serves as a danger signal to the inspector
who examines the stampings when cold, but acts as a deterrent to the
operator, who has been known to scrap the stamping forthwith without
submitting it for inspection and payment.
Imc. SH. — " I'hi^li " <.f ovrrlir:i|((J ,l.ain|)i ny.
A reliable method of detecting cases of overheating is to examine
the "flash" which is sheared off the stampings after forging. This being
the thinnest part of the stamping, it will show by its crumbled and ragged
outline when forging has been done at too high temperature. (Fig. -Vo.)
Burnt stampings may be detected in the view room after pickling, by their
coarse surface, usually covered with a netv.'ork of fine cracks where the
material has crumbled under the hammer, and by their coarsely crystalline
fracture when broken.
76
ENGINEERING INSPECTION
The direction of flow of metal is an important point in all forgings.
Generally steel billets have a ' ' grain ' ' which runs in the direction of
rolling, and the forging should be so made that the main stresses coming
upon it in practice lie along and not across the grain. This is a point
worthy of notice by inspectors as, for their own convenience, smiths and
Good
a
F/0. 34- _ Oirechon of Fibre
rn r'orgin^ Gear t/Vheeh
dad
stampers may reduce considerably the strength of forgings by incorrect
formation in this respect. (Fig. •'!4.)
Wear in dies is responsible for much trouble, causing heavy forgings
and displacing location spots for machining. Many stampings are
scrapped through dies becoming " offset" so that the top die is out of
line with the bottom. If the amount of offset is greater than the machining
allowance, stampings fail to clean up in machining, or, when machined
inside, the walls become too thin locally.
VTT P^.
F/'g.JS^ " Offjefand EccenMc For^/rj^s due to diei
6ein^ oui- of Line.
Somewhat akin to this is the fault of eccentricity. When the top die
consists of a peg which makes a recess in the stamping (as shown in
Fig. 35) the inside becomes eccentric with the outside and fails to machine
up. The best way to examine for this fault is to mount the stampings on
a peg which can be rotated. A little experience will make viewers quite
INSPECTION OF PARTLY FINISHED MATERIAL 77
expert in judging from the "spinning" test, whether the stampings are
usable or not. (Fig. 36.)
For measuring the thickness of stampings and castings, the
indicating calipers and sliding vernier shown on page 117 are very
useful, but as far as possible, fixed "go" and "not go" gauges and
plate templets should be used. The first casting or stamping of a series
should always be marked out, and a report of the defects found should be
sent to the founder or stamper at the earliest possible moment. This should
be repeated when a definite number of castings or stampings have been
received, so that a check may be kept upon the condition of the patterns
or dies.
As the identification mark stamped on the steel billets or bars is
effaced during the operation of stamping, this should be written or stamped
on the Progress or other card which accompanies the batch of articles to
F/ffSS— Spinning Ta6/e -Gr Eccentric
forcings & Castings.
the view room. After the articles have been inspected, this mark should
be stamped on every one of the forgings, preferably in a place where
there is no machining, or if this is impossible, in a place from which it
will not be machined off during the first few operations. This transference
of the identification mark is most important, as it is obviously useless to
inaugurate an identification system if the marks are not permanently
retained. Castings should also be marked after inspection, with symbols
indicating the foundry from which they were obtained. Important castings
and forgings are also occasionally given serial numbers so that they can
be traced individually in case of trouble.
A useful procedure in complicated castings is to cut the earlier samples
into sections, so that the thickness of the various walls may be checked.
78 ENGINEERING INSPECTION
Die castings are now made for small pieces where material of lower
strength than iron is admissible, or where it is desired to get a good finish
to avoid machining. They can be made correct to x 1
flattening
(Fig. 37.)
Tube must
flatten and
double over
both when
lube must
flatten and
double over
when cold.
y J
cold and
red hot.
In addition to these mechanical tests, copper and brass boiler tubes must
-withstand a hydraulic pressure of 750 lbs. per square inch.
^y
^
^
J\
^
Bt//^/na
Drifh.
->§
F/ang/ng
f/affening
Doubling ovef
t'/g. 37^ Tests /S/- Copper & Brass Tubes
In the case of cold drawn steel tubes for aeroplanes, the process of
manufacture must be watched with the utmost care to ensure perfection in
•every way. The tubes must be smooth, true to section, free from scale,
-dirt, specks, longitudinal seams, lamination, grooving, and blistering, both
inside and outside. The limits for round tubes are + "004" for tubes of
"2" diameter or under, and -7^' for tubes over 2" diameter. The mean
oUU
inside diameter must not be less than the correct outside diameter minus
iwice the maximum permissible thickness, nor greater than same minus
80
ENGINEERING INSPECTION
twice the minimum permissible thickness. Oval and special section tubes
are scheduled in Air Board Specifications Til and 12*.
The mean thickness must not be less than the specified gauge, and
must not exceed it by more than '004" except tubes thicker than MJG" for
which the tolerance is to be 7^ per cent, of their thickness. The limit of
uniformity in thickness along the tube is —10 per cent, and +15 per cent.,
and the departure from straightness must not be more than 7^1- of the
length of the part that is bent.
Tension and compression tests are made as received, and if hard
drawn and blued, further tensile tests are made after softening, to prove
that the metal is not unduly soft after annealing at the ends for welding
or brazing. Full particulars of the standard to be attained for aircraft
tubes are to be found in Air Board Specifications Tl to 2(i,* but flattening,
Fig. 38. — Compression test on aeroplane tubes.
crushing and bending tests are also scheduled for tubes of different kinds
and materials. Hard drawn and blued tubes must stand flattening until
the sides are apart not more than three times the thickness of the metal
(not more than six blows to be used) and crushing until the outside
diameter is increased in one place by at least 25 per cent., or until one com-
plete fold is formed. (Fig. .'J8.) No crack must be observable after either
of these tests. Annealed tubes must stand flattening until the sides are close
together, and must stand crushing imtil one fold is formed or the diameter
is increased by 25 per cent, in one place. Soft tubes of less than -f"
diameter must stand being bent through 90° round a radius not greater
than 10 diameters without serious deformation in section or showing signs
of failure.
* Now issued by British Standards Committee, 28, Victoria Street, S.W.).
CHAPTER VI
INSPECTION OF FINISHED MATERIAL
The testing of material arriving at the works in the finished state is
necessarily a difficult proposition, as in many instances the various parts
are fitted together, and in most cases a good deal of machining work has
been done upon them before dispatch. If proof tests can be made to check
the quality of material without scrapping the parts themselves, the problem
is solved, but in most cases it is necessary to scrap some of the parts for
testing purposes, thereby losing, not only the value of the material, but
the work done upon it. An external inspector may be employed to check
the materials used at the suppliers' works, and to bring back unmachined
samples for inspection and testing, but unless this system is supplemented
by a bonding arrangement at the suppliers' works, the method is open to
many objections. In most cases, however, inspection has to be done upon
receipt of the goods.
After dissembling, castings and forgings may be cut up for tensile
and impact tests, for micro-examination, or analysis, according to the
conditions in each case and the importance of individual parts. Some
pieces are sufficiently unimportant to escape with dimensional inspection
only, or even with none at all.
In the case of springs, a proof test consisting of a tension or com-
pression load is put upon each spring to ascertain whether the rate of
extension or compression corresponds with that laid down on the drawing.
At the same time, the dimensions unloaded and at full load should be
checked to ensure interchangeability. (Fig. 39). All springs should be tested
either at the makers' works or upon receipt, by " scragging." The spring
is loaded, in the case of a laminated spring, until it is flat (or in the case of
a coil spring until it is solid), by pressing upon it with a plunger and
rapidly releasing the load, this operation being repeated a number of
times, depending upon the design of the spring*
This test reveals hidden defects in the material, and is also intended
to indicate the resilience of the spring, the dimensions of which should be
unchanged after the scragging test.
* For Railway Rolling Stock springs see British Standards Specification No. 24.
(British Standards Committee, 28, Victoria Street, S.W.I.)
81 G
8L'
ENGINEERING INSPECTION
In the case of laminated or leaf springs, leaves may be taken succes-
sively for tensile tests as follows : —
1st Consignment ... ... Top leaf of 1 spring.
2nd ,, 2nd ,, ,,
• !rd ,, ... ... -.trd ,, ,, ,,
And so on. In this way, if a y-leaf spring is used, new springs can be
made out of the remaining leaves, so that, in effect, only 1 complete spring
in 5 consignments is destroyed, though all the consignments are tested.
l'"!f,. o9. — M.-iL-liini.' fill- 'J'l.'sLing Coil and I.nniiiinli.d Springs
(ilict I'ii nll\' dri\"f'n], capacil\' 4 tiins.
(I'.v lirriiiissini, ,,( ]\\ ami T. .I;'i'lv, Ud.)
Chains also may be tested by proof loads, the usual procedure being
to take lengths of about !MI feet at a time (Fig. 40), and test these to a load
specified by the examining body (Lloyds, Board of Trade, etc.). Three
links are cut out and tested in tension until they break, and the minimum
breaking strength in these cases is also laid down by the various examining
bodies.* The two halves of the chain are connected by a special link put
in after the breaking test has been made and the proof load then applied,
as described above. Wire, hemp, and cotton ropes may be tested in a
* I'Vji Id(i)d's and ollu.i- RliIi-s spij " I-^noinfiji-'s Vuar Book," b\' il. R. Kniipr,
M. Inst. C.E., Section \'.
INSPECTION OF FINISHED MATERIAL
8:;
similar manner, but usually only short lengths are tested to destruction.
In this case, care must be taken not to cut the wire or rope in the grips of
the testing machine, as this will cause the test piece to break at a low load
and will not give a true indication of the strength of the material. Special
forms of grips are supplied by testing machine makers for the purpose.
(Fig. 41.)
F/0. 4-0 Gr/p5 ^r Continuous Cha/n Tesfiny.
The examination of hardened or heat-treated work is one of the most
important and most difficult tasks set before an inspector, as the outside
appearance of the material gives very little indication of its suitability or
otherwise for the requirements of the job.
GHp fir Lar^e %*?^
Gr/o for Jma// fiopes F/^. 4-1
In case hardened work it is not only necessary to see that the surface is
Tiard, but that the interior, or "core," is satisfactory, and also that the
internal condition of the case is good.
A case hardened part consists of two distinct portions : —
(1) An outside shell of hard, brittle material ("case") which
is a high carbon steel, and is designed to resist wear and to reduce
84 ENGINEERING INSPECTION
friction in running ; and (2) a soft core of tough material for the purpose
of withstanding shocl^s. (Fig. 42.) If there is a sudden transition between
case and core, there is a great tendency for the hard case to break away
from the "core" as a "shell," so that when the fractured surface is
examined, there should be a gradual transition from the hard white case
to the soft grey core. If this is too gradual, however, the toughness of
the core in small sections may be impaired, so that the case should not be
" diffused " or spread too far into the core. For a similar reason the case
must not be too thick, especially in hardened gears, as then there is insufifi-
Ki(^ 42. — Fi'actLir-' of case-hai-di'iicd part,
sli'.-w'Ing " ('asi' " and "' forf."
cient backing of soft material to give strength to the teeth, which are liable
to break off under shock. A thick case also has a tendency to " chip " —
especially on corners, or at the top of gear teeth. It is therefore advisable
to avoid corners wherever possible. (Fig. 4^i.) The opposite extreme of
thin case should also be avoided, as the thin layer of hard material has a
tendency to crush under heavy loads, and the hard particles not only cause
the failure of the gear in question, but get into the lubricating oil and
wreck other gears and bearings. The case should be fine grained, but not
crystalline, as in the latter condition it is liable to break or chip off.
The core also should be tough and comparatively fibrous, as if crystalline
or hard it is liable to break under shock or vibration. Extremely fibrous
cores, however, are not always good, as they merely indicate the presence
of excessive slag in the steel, and are not an evidence of quality.
INSPECTION OF FINISHED MATERIAL 85
The surface of case hardened work is best tested with a " second cut "
file, which should not bite into hard work. It is a mistake to rub this
backwards and forwards along the work many times, as once or twice is
sufficient, and further rubbing only wears away the file and considerably
reduces its life. Some work is locally tempered to give extra toughness
to the case, at the expense of hardness, and in such instances the file
should just bite. The amount of bite to be allowed is purely a matter of
experience, and varies in different classes of work. In some instances the
scleroscope is a useful indication of hardness, but the ball test should not
be applied, as it has a tendency to start cracks in the case, and is also an
incorrect indication of hardness, as the ball is partly flattened owing to the
fact that it is very little, if any, harder than the surface tested. The ball
F/a 4-3— fffedofc/i'fferenf- Thicknesses
of Case on strength of Gear Teeth
test, however, is a good indication of the condition of the core, and may
be used with advantage as an auxiliary to optical examination on fractured
surfaces.
The condition of the core may further be investigated by taking tensile
and impact tests after heat treatment as described on p. 49. If thought
advisable, a travelling inspector may put numbered and stamped test pieces
in the carburising* pots, the other articles in the pots being similarly
stamped. After hardening, the fracture of the test piece may be taken as
an indication of the condition of the other work carburised at the same time.
In doing this, care must be taken to see that the test piece is made
of the same steel as the other articles covered by the test,
and also that the section of the test piece is approximately equal
and similar in form to the other articles in the batch. This can be
* The process of case hardening is sometimes termed carbonising, but this term should be
restricted to the process of reducing a substance by the application of heat to a particular
form of carbon, as in the case of retorting coal. For the sake of clearness it is advsiable to
adopt a distinctive term for the penetration of carbon into steel or other materials, and the
word " carburising," being a convenient one for the purpose, is used in this book.
80 ENGINEERING INSPECTION
done in the case of gears, for instance, by sending to the hardening shop
all gears scrapped in machining. They can then be cut into a number of
sections radially, and one section used as a test piece for every 20 similar
gears carburised. One scrap gear will thus produce sufficient test pieces
to cover 80 or 100 similar gears.
If the test piece does not show satisfactory results, one of the actual
articles must be broken. If still unsatisfactory, this broken article may be
used as the test piece when the articles are again heat treated (if this is
possible). In all cases it is essential that the test piece shall be given
exactly the same treatment as the articles covered by the test, and also
that the physical condition of the test piece before heat treatment should
be similar to that of the other articles in question. If the latter are
normalised, then the test piece must also be normalised before proceeding
with the heat treatment, as similar results cannot be expected from
different starting points.
Any discontinuity of the case caused by a crack or seam is a source of
trouble, and where there is heavy pressure, as in cams, rockers, etc., such
defects are sufficient in many cases to justify rejection. Cracks are always
bad, and should be strictly avoided, but seams, if not open at the edges,
are not necessarily fatal.
There is often a difficulty in locating the source of surface cracks, and
they are usually attributed to faulty material or hardening, so that workmen
may be paid for work done upon the rejected articles. On investigation,
however, it often appears that surface cracks are caused by grinding
troubles, among which are the following : — Excessive pressure on wheel
{i.e., too deep cut), insufficient supply of cooling fluid, wrong grade of
wheel, or wrong grinding speed. When one or more of these factors is
properly adjusted, the trouble frequently disappears.
In some instances it is necessary to leave soft, certain parts of case
hardened articles, so that further work may be done upon them after the
hardening process. This is generally done by machining off the case after
carhurising (and before hardening), by copper plating the part to be kept
soft, or by enclosing the latter part in sand, clay, or other protective media.
There are also special paints on the market for preventing carbon penetra-
tion. If, for some reason, the preventive is not satisfactory, and the
protected part becomes fairly hard, taps or drills may be broken in the
work. A common procedure is then to heat up the work locally to assist
in removing the broken tools. After this is done, the articles should be
returned to the view room for hardening inspection, as frequently the
INSPECTION OF FINISHED MATERIAL ST
hardened part is made soft by this process, with disastrous consequences to
the finished article. The writers have in mind a number of cam-shafts
which gave continual trouble by seizing up in engines. Great care was
taken on these shafts to see that the running surfaces were hard, but with-
out avail. On investigation, it was found that a small hole had to be
drilled near the end bearing in the erecting shop, and if the drill broke in
this hole, the shaft was heated up locally to remove the broken piece.
Consequently all the care taken in hardening was wasted, as the end
bearing was softened in the final assembly. All rectifications of this
nature should therefore be notified to the inspector on the job, and the
articles themselves should be sent back to the view room for examination
after the rectification process has been performed.
Local softness on the case may be due to two causes. If the quenching
medium (water or oil) is dirty, or unsuitable, cooling of the surface will
proceed more slowly in some parts than in others, and consequently soft
spots or patches will be obtained. Soft patches may also result from
grinding operations, as if the surface becomes heated (owing to insufficient
coolant or excessive pressure), the case may crack or soft spots may be
obtained. A file test should therefore be imposed on all hardened surfaces,
after grinding. Distortion is a very prevalent trouble with hardened gears
and gauges, and it is often very difficult to tell in which direction distortion
will take place. Generally articles distort more when hardened in water
than in oil, but much trouble may frequently be avoided by careful
quenching. Long, thin articles, such as cam-shafts, should always be
dipped "end on" and not sideways, and in other cases experiment will
show the best way of quenching an object to avoid distortion.
Hardening troubles may also be traced to the quality of
carburising compound used, and it is advisable to check this by analysis
at frequent intervals. The following are typical analyses of carburising
compounds in common use : —
Water
. 8-76
5-89
305
1-42
Oil
. 0-30
3-35
19-86
14-00
Carbonaceous Matter ..
. 8;319
79-30
.53-93
2519
Phosphoric Acid
—
—
6-57
19-17
Lime
107
247
10-80
31-93
Sihca
0-71
442
0-66
—
Sulphuric Anhydride
. 0.57
—
—
0-38
Ferric Oxide
. 0-99
211
0-43
o-(;2
Alumina
—
—
1-27
8-38
88 ENGINEERING INSPECTION
The compound should be free from Sulphur, as this has a very
deleterious effect on the case.
Also, some compounds shrink considerably during the heating period,
and it will be found that with such compounds the articles at the top of
each box become uncovered during the carburising period, unless due
allowance is made for this shrinkage.
The testing of heat-treated work which is not case hardened, is usually
performed by cutting up one article in 50 or 100 into tensile and impact
test pieces, and taking the results obtained as representative of the whole
consignment. It is necessary, however, to make sure that the heat treat-
ment has been uniformly applied, and that the various articles are all made
of similar steel. For this purpose the Brinell test is a very useful guide,
and it is customary to specify Brinell limits on material specifications, so
that if any of the articles are outside these limits when tested, they are sent
back for re-heat treatment, or rejected altogether. If heat-treated work
fails to meet the specification after the third heat treatment, it is advisable
to scrap it. Generally large forgings, such as crank shafts or axles, should
not differ in Brinell diameter by more than ■2mm. at any two points, and
medium carbon steel forgings should have Brinell impressions between 4'0
and 4'6mm. diameter after heat treatment. If the impressions are smaller
than 4'Omm. the work is likely to be brittle, and if greater than
4'6mm. it is soft and poor in tensile strength. It should be emphasised
that the Brinell test is only an approximate guide to the tensile strength of
the material, but is of great value in deciding uniformity of heat treatment
and in detecting mixing in the steel or other material supplied.
Forged test pieces are occasionally sent with the work, but it is
preferable (though more costly) to cut up an actual forging after heat treat-
ment, on account of the differences in sectional area, form, and size. In
this connection, scrap forgings are very useful. These should be heat
treated with the consignment, and then cut up for test. If test pieces are
sent, they should always be attached to the forgings themselves, or, in the
case of drop forgings, to the " flash " from which the forgings have been
sheared. (See Fig. 33.)
For articles having surfaces of deposited metal {e.g., silver or nickel-
plated work), the first point to be noted is the thickness and uniformity of
the deposit. This may be ascertained in some cases by weighing the
articles before and after the deposition, but to enable this to be done, each
article must bear an individual number, as absolute uniformity in weight is
very unlikely. This is rather costly, involving, as it does, a duplicate
INSPECTION OF FINISHED MATERIAL 89
system of records (by the supplying and receiving firms) and the probability
of disputes if any mistake is made by either firm. This was minimised, in
the case of aeroplane cylinders which had to be copper-plated, by stamping
the weight of each machined cylinder on its flange before sending out for
plating. The cylinders were re-weighed on their return, the difference
giving the amount of copper deposited on each one. There was a
tendency, however, for the copper to be deposited thickly on the outside
edges of the fins and on the top flange, and for the recesses to have very
thin deposits. This can be ascertained by measurement, where possible,
or alternatively by cutting sections through some of the articles and
observing the thickness at different points with a lens or microscope. This,
however, is more costly, as it involves scrapping some of the articles. The
character of the deposit, whether spongy or compact, may be observed
with a lens, and its adhesiveness should be tested with a small chisel or
other tool, as if the surface has not been properly cleaned before plating,
the deposit will peel off. A further point to be noted when finished articles
have been sent out for plating, is the danger of corrosion. In many
instances some of the finished surfaces are not required to be plated, and
must therefore be protected while in the plating bath. If the protecting
medium does not cover the surface, or the joints are bad, permitting
leakage, the finished surface will be badly eaten away, and this must be
noted immediately the articles arrive from the platers, so that a complaint
may be lodged at once. This is a serious matter, as, not only is the value
of the plating lost, but also the whole of the material, expensive
machining, and other work done upon the articles. Copper plating is often
•used to protect surfaces that are not required to be carburised during case
hardening. For this purpose the thickness of the deposit is not so
important as its density and adhesiveness. It has been found* that a
thickness of "0004" is sufficient to prevent carbon penetration, but if the
deposit is spongy or not adhesive, the carbon will penetrate, whatever the
thickness, and therefore particular attention should be paid to this point.
Galvanising is also dependent for its effectiveness upon the cleanliness
of the sheet or casting before dipping, as otherwise a uniform deposit will
not be obtained, and in bad cases bare places will be left.
The strength of fastening appliances, such as nuts, bolts, rivets, etc., is
obviously important, as the stability or safety of most engineering machines
and structures is entirely dependent upon the media by which they are
* " The effect of copper plating on carburisation," F. Zimmerli, " Metal Industry,"
May 13th, 1921.
90
ENGINEERING INSPECTION
connected. The large quantities used in most works renders any individual
examination impossible, and therefore the only systematic way of dealing'
with such parts is to take periodic tests of the raw material at the makers'
works, and to check these by tests made on a small percentage of the
finished product arriving at the consumers' works. Rivets are usually too
small to allow tensile tests to be taken, and must therefore be submitted ta
such workshop tests as may be advisable (Fig. 44). Such tests are
Co/d Bend Te^
ffof Bend TesJ-
Z-S D
//a/ F/aff^ening Tes^
f/^. 44^ l^orhsfiop Tesh for Rivets (3i-ee/J
specified by the Admiralty, Lloyds, and Bureau Veritas. An occasional
check test for fracture and chemical analysis should be made, and a few
bags examined for cracks in the rivet heads and other physical faults. Nuts
and bolts must also be watched for cracks and physical flaws, and a small
percentage (say, 1 in 500) fractured in each consignment to ensure a
reasonable standard of toughness in the material. In the case of alloy steel
or heat-treated bolts, tensile tests should be taken whenever possible, and
supplemented by Brinell tests on the bolts used for the fracture tests.
CHAPTER VII
GAUGES AND MEASURING INSTRUMENTS
The machining of material to sizes indicated by drawings, necessarily
involves some method or methods of measurement, which must indicate
(1) that the dimensions given on the drawings have been worked to with
reasonable accuracy ; (2) that the amounts by which the machined part
exceeds or fails to reach the nominal dimensions, fall within the limits
laid down on the drawing.
The old methods of measurement by rule and caliper, although,
sufficiently accurate for many purposes, have largely fallen into disuse
for three reasons : — (1) because the time taken to set the calipers or
similar measuring instruments is too long ; (2) because the readings
obtained are not accurate enough to enable filing, scraping, and other
expensive fitting operations to be dispensed with ; (3) because the calipers
are not sufficiently rigid and often move while in use. The supply of
spare parts for repairs or replacement is also facilitated by accuracy of
measurement, as it is both inconvenient and costly to modify such parts
on arrival at a distant place where few, if any, engineering appliances are
available, and where skilled labour cannot always be obtained.
For these and other reasons, standardisation of dimensions is now
generally resorted to, and as this entails close and accurate measurement,
the measuring appliances or gauges must be so designed and made, that,
once "set" or fixed they are not liable by distortion or other agencies,
to alter their size or shape until by reason of wear, they are either discarded
or converted into smaller or larger sizes.
The terms "tolerance," "allowance," and "limit," are so often
used in measurement that it is advisable at this stage to explain their
meanings.
Tolerance is the variation from nominal or standard size allowed in a
gauge or piece of work to cover small discrepancies or errors in workman-
ship.
Allowance is the difference between the dimensions of two parts that
are required to fit together, and this difference varies with the size of the
parts or the fit demanded. Thus the allowance for a push fit is
91
92
ENGINEERING INSPECTION
less than for a running fit, and such allowances will be greater for a
2" bar than for a 1" bar.
Limit is the term used to indicate the maximum and minimum dimen-
sions that may be allowed in machining any part. No engineering work
is absolutely accurate, and the limits placed on a job indicate the degree of
accuracy called for.
The existence of limits implies some standard which may be used as a
basis from which the limits can start.
As cylindrical work is the simplest and most common instance, the
case of a shaft working in a hole is taken as an example. If the nominal
Touew
/^ULM'^/^riCE
Hoi-E
//////////////
Shaf-t _>|^|<_ Houe.
F/g. 4-5— D/agram il/ujfrahny Uni/af^ra/ 5i/sfem of^Limif Gacyg/n^.
diameter of shaft and hole is 2 inches, there are four possible dimensions
to consider (Fig. 45) : — ■
(A) The minimum diameter of the hole.
(B) The maximum diameter of the hole.
(C) The maximum diameter of the shaft.
(D) The minimum diameter of the shaft.
If all the .shafts made are to enter any of the holes, it follows that (A)
must be greater than (C), but under these circumstances a shaft made
to diameter (D) might be assembled with a hole made to diameter (B), so
that the clearance between (B) and (D) must be suitable for the kind of fit
required.
GAUGES AND MEASURING INSTRUMENTS 93
Such fits may be divided broadly into three categories : — *
(1) Clearance fits when there is a positive allowance between
(A) and (C), i.e., when (A) is greater than (C).
(2) Interference fits when (A) is less than (C).
(3) Transition fits which come midway between (1) and (2) and
which cover cases where (A) is equal to (C).
The various terms used in practice, starting with the slackest fit,
are : —
(a) Running fit )
(b) Push fit j Clearance fits.
(c) Key fit ]
(d) Light drive fit]'^^^"^'*'°" ^^'-
(e) Drive fit ]
(f) Force fit jTnterference fits.
(g) Shrink fit j
The tolerance allowed on shaft or hole may be arranged on the uni-
lateral or bi-lateral systems. In the uni-lateral system, the minimum hole
(A) is the nominal size, and all tolerances of holes are above this.
The maximum size of shaft (C) in this case is (A) minus allowance,
and (D) is (C) minus tolerance on shaft.
As an example, for a 2" diameter hole with running fit : —
(A) is 2-000", (B) is 2001", (C) is 1-997", (D) is 1-995".
Thus the maximum clearance between shaft and hole is -006", and the
minimum clearance is -003".
The bi-lateral system has the nominal size midway between the
maximum and minimum holes, i.e., between (A) and (C). In other words
the nommal diameter is -= — —iy^ — -
The former has been most widely adopted in this country and also in
Germany, Switzerland, and America, but where the bi-lateral system
already exists there is considerable difficulty in changing over, and there-
fore the British Engineering Standards Committee considered this matter,
and came to the conclusion that it would be possible to standardise a series
of shafts that could be employed satisfactorily either in a uni-lateral hole
or in a bi-lateral hole.
In the above, it is assumed throughout that the size of the hole is
taken as the standard or nominal dimension, but instead of this the size
* " The Principles of Limit Gauging," by A. A. Remington, M.I.Mech.E.,
" Engineering," April 15th, 1921.
94 ENGINEERING INSPECTION
of the shaft may be adopted. The former is the more convenient method,
as otherwise, drills, taps, reamers, etc., would have to be made specially
to suit different systems and allowances, but in some cases the latter has
advantages. With long shafting, for instance, it is convenient to take
the shaft as a basis and bore out bearings to suit.
The standard hole is, however, the better system and is in general
use.*
Gauges and measuring instruments may be divided roughly into three
classes : —
(1) Fixed gauges whose size is not controllable by operator.
(2) Self-recording instruments, the readings of which are taken
by the operator. This class includes dial gauges.
(3) Instruments of variable size where the adjustment is made
and readings taken by the user. This class includes calipers,
micrometers, verniers, and measuring machines.
Instruments of the third type need more care and skill than those of
the first and second, because they have to be adjusted to the required
size and then applied to the work. The sense of touch or " feel " involved
in making such measurements is an important factor, and a certain amount
of practice and experience is necessary to acquire the delicacy required
in making fine measurements by these means. Self-recording instruments
of the dial indicator types are used for detecting irregularities of form
rather than in making definite measurements. The only care required is
that of setting up the instrument and in preventing damage to the gauge.
Fixed gauges are most commonly used, and, provided care is taken not to
■damage or distort the gauges, they are by far the most satisfactory. The
size of each gauge is stamped upon it, and the viewer has only to see that
the size issued to him corresponds with the drawing to which the operator
is working and that it is kept free from damage or distortion.
Some snap gauges are purposely made of brittle material, such as cast
iron, so that in the event of their receiving a blow they will break rather
than bend or distort.
Gauges for measuring diameters, projections, or gaps in work are
usually made of the " Go " and " Not Go " types.
The working, or " go," side must enter the hole or gap, or pass over
* For further information re limits and limit gauging the reader is referred to The Thomas
Hawkesley Lecture on " Limit Gauging," by Sir Richard Glazebrook, Proc. Inst., Mech.
E., November, 1920, and April, 1921, and " The Principles of Limit Gauging," by A. A.
Remington, M.L, Mech. E. Engineering, April 15th, 1921.
GAUGES AND MEASURING INSTRUMENTS
Do
ihe diameter or projection, and the "not go" side must not. In some
instances the two gauges are made separate, as there is more wear on the
"go" side, but the convenience and greater speed of operation, which
results from having the two gauges in one piece, makes the latter form
very popular where large quantities of work have to be produced. In either
case it is advisable for the " not go " side to be painted red or some other
convenient colour, so that it can readily be distinguished, thus diminishing
the liability of mistakes being made. The " not go " side is usually made
much shorter than the " go " side, because the wear on the former is much
less than that on the latter.
The most common types of gauges are : —
(1) Plug gauges for holes.
(2) Ring or snap gauges for diameters.
(3) Taper gauges for conical measurements (plug, ring, and plate
types).
(4) Height gauges.
(5) Thickness gauges or feelers.
(6) Profile gauges for surfaces.
(7) Screwed plug gauges for internal threads.
(8) Screwed ring gauges for external threads.
(9) Reference and special gauges.
Plug Gauges. (Fig. 46.)
There are many types of plug gauges, but the following examples
illustrate the principles of those in common use. The ordinary gauge
do
Mo-r
do
F/g. 4£— P/ug Gouges
consists of a piece of hardened steel, ground truly cylindrical, with a
roughened or knurled handle to prevent slipping in the fingers. (See
96
ENGINEERING INSPECTION
Fig. 46.) There is a clear space ground on the side of the handle,
upon which is stamped the number of the gauge and its size,
together with any other particulars that may be necessary or
useful, such as the number or name of the operation after which
it has to be used. It is advisable to make the handle smaller in
diameter than the gauge surface, so that long holes may be gauged without
having a gauge surface of excessive length. Sometimes a hole is drilled
up the centre of the handle to facilitate the escape of air from blind holes,
but more often the sides of the gauge are cut away, as shown in Fig. 46 (b).
The latter method is also convenient for testing whether a hole is truly
circular or not.
A cylindrical gauge only tests the minimum distance across the hole
(Fig. 47), but the "cut away" gauge can be turned round to test any
diameter. As the wearing surface is much smaller in the latter type, it
has a shorter life than the cylindrical gauge.
■«UOE' ^l.i'KlK
/^^.■47__ C/se of P/ujr Gauges /n £///pHca/ No/es.
Limit gauges may be of the double or single ended type
(Fig. 46, c and d). The latter has the advantage that only a single move-
ment is necessary in gauging, but unless the " go " end is made very long,
it is impossible to gauge the centres of long holes. Further, the hole must
be of greater depth than the " go " end to enable the " not go " part of
the gauge to be used at all.
It has, however, the advantage that no mistake can be made between
the " go " and " not go " ends. Sometimes a gauge has its middle portion
ground to the " mean " diameter of the hole, as this is the size generally
aimed at. In this case it is advisable to make the mean diameter nearer
that of the " go " end than that of the " not go " end.
In grinding, it is difficult for the operator to see how near he is
approaching to the drawing size, and for this reason operators' gauges are
sometimes tapered "005" to "010" below the "go" diameter for a short
distance to enable work to be finished more rapidly, but this is not neces-
sary or advisable in the case of inspectors' gauges.
GAUGES AND MEASURING INSTRUMENTS
97
Large gauges sometimes have the ends made separate from the
handle for the sake of economy and weight, but in this case care is
necessary to ensure that the ends are not liable to distortion after hardening
and grinding, or in use. The form shown in Fig. 48 (a) is liable to give
trouble, as unequal stresses are set up in hardening, and such gauges may
be seriously affected by temperature changes. The modification shown in
f}g. 4S^ Large P/u^ Gaug-e^.
Fig. 48 (b) is better, as the stresses are equalised. Lightening large gauges
by drilling holes in the web necessitates careful hardening to avoid cracking
or internal stresses, and when the ends are forced on to a mild steel handle
care must be taken to avoid distortion.
Plug gauges are generally made of high carbon steel, hardened
throughout and tempered to remove hardening stresses, seasoned for a
period of about three months to ensure permanence, and ground to size.
r/£. 49 _ Pro/ec fed Centre.
All sharp edges should be taken off plug gauges to avoid "burring," and
protected centres (Fig. 49) are advisable to allow of work being " spun "
when a mandrel or test bar is not available.
H
98
ENGINEERING INSPECTION
Ring Gauges.
These are simple in form, as shown in Fig. 50, and are ground on their
inside diameters to the required size, after hardening. They are roughened
or knurled on the outside diameter to enable them to be securely gripped
with the fingers, and the size, number, and other particulars are marked
on one of the flat faces. Ring gauges are subject to a similar objection to
that obtaining in the case of plug gauges. They only measure the maximum
f/g. 30- ft/n^ Gau^e
diameter of the work, and do not reveal any irregularity or eccentricity in
the section. They are convenient, however, for gauging long work, such
as steel bars, etc., when any taper or local increases in diameter are
immediately detected by them. The faces should be ground at right angles
to the inside diameter, so that shoulders turned on shafts may be checked
for truth and flatness, and also the sharp corners should be radiused off to
avoid damage to work or gauge.
Snap Gauges.
Snap gauges are of three types — solid, adjustable, and built up.
Simple snap gauges (Fig. 51, a) are generally high carbon steel forgings,
which are hardened, seasoned, ground, and lapped to size on the measuring
surfaces. They are sometimes cut out of simple plates |^-inch to J-inch
thick, but, wherever possible, should have wide gauging surfaces to
decrease wear. As in the case of plug gauges, snap gauges can be made
either of the single or double ended types, and the gauging surfaces may
either be forged with the body, or else a cast-iron body may be used with
gauge points of hardened steel inserted (Fig. 51, b).
Adjustable gauges have the plugs screwed in position, and these are
secured from movement by locking screws through the body. Such
adjustable gauges have the advantage that they may be set to suit different
jobs, but such modifications must be carefully watched to avoid mistakes.
The gauging points are usually bevelled off at the edge to avoid scratching
GAUGES AND MEASURING INSTRUMENTS
99
.or marking the work. In some instances (as in the Johannsson Limit Snap
•Gauge) all four points are made adjustable and for rapid work, the lower
jaw is made of a flat plate (Fig 51, c), the two upper gauge points being
.adjustable.
J
1 1
L.
1 ! !
.1 • 1 1
(
:•)
11 i 1
Not
Go.
F/g.S/^ 3 nap Gaujres.
With double-ended gauges the constant tapping on the body, of work
that passes the " go " gauge sometimes causes the body to open slightly
and makes the gauge inaccurate. For this reason rubber or spring buffers
are often placed at the boftom of the jaw to absorb the shock.
Care should always be taken to ensure the gauge plugs being true with
the axis, otherwise incorrect results will be obtained. If the surfaces,
although parallel, are not "square," the readings obtained will be larger
than the correct size of the work.
Built-up snap gauges consist of two plates separated by a distance
piece, as shown in Fig 51, d. The difference between the "go" and
" not go " ends is provided by grinding a step in one of the side plates.
"By dissembling the gauge and using different sizes of distance piece this
type of gauge can be used for a variety of jobs. Being cheap to manufac-
ture, it is suitable for work of a temporary nature, as it saves the cost of
.making special gauges for small orders.
100
ENGINEERING INSPECTION
Taper Gauges.
The gauging of taper holes or surfaces depends upon the translation of
diameter tolerances into lengths. If a taper piece A B C D (Fig. 52) is tO'
be measured, the taper is expressed on the drawing in inches per foot. As.
A
H
/yjr. S2 Diagram ///usfrafinf Princ/'p/e of
Taper (PaujrJn^,
an example, suppose the taper given is ^-inch per foot. This means that
for every 12 inches of F B (or length of centre line G H), the difference
between A D and B C is i-inch or "5 inch). If F B or G H are less than
12 inches long, the same ratio still holds good.
Therefore in this case
FB _ 12 ^ 24
AD— FE ■" -5
Now, in considering
tolerances, the same relationship is true. If we assume that the accuracy
required on the large diameter A D is "001", then this corresponds to a
difference in length of -001" x 24 = -024".
For other tapers similar calculations may be made, and the difference
of diameter, or limits, expressed in terms of the length. Thus, a taper
plug gauge has its limits indicated by a piece ground off the large end of
the taper, so that it must enter the hole for such a distance that the " go "
edge of the gauge is inside, and the " not go " edge outside the hole. The
same purpose may be served by inscribing two lines on the plug or ring'
gauge (Fig. 53).
The truth of the taper itself may be tested by thinly smearing the plug^
or ring with Prussian blue or other marking medium. Another method is
to mill off the sides of the gauge, as in the case of simple plug gauges
(Fig. 46, b), or to cut away the middle portion of the gauge (Fig. 53, b), in
GAUGES AND MEASURING INSTRUMENTS
101
pq
.^
I
I
u
(I
L
D
1
5
(Q
I'
y
o:
D
r
^
^
102
ENGINEERING INSPECTION
which case want of accuracy is detected by the " play " of the gauge in the-
hole.
Adjustable or plate gauges may be made by setting two plates at the-
required angle on a stand, and in this case accuracy may be achieved by
setting the plates to two plug gauges, as shown in Fig. 53.
Some gauges are made with grooves along the diameter to catch dust
or dirt, and to enable the air to escape more freely from blind holes.
Height Gauges.
These are of various kinds, and are used for measuring the height of
projections above a plane surface, or the location of bushes on jigs and
machine parts. A simple height gauge may be used for testing small work
on a surface plate. More elaborate gauges for testing larger pieces are-
made adjustable on the Vernier principle (to be described later). (Fig 54. )i
TKi.j^niinTXA,
-i-i 10 - ;9 Ms
JiiiiiiiiiiiiiliiilJiiiiiiiiiiUiW^^
[myiiilMln!
Fig. -54. — \'ernicr Height Gauge.
(i>v permission of Messrs. L. S. Starrctt Co., Ltd.)
These take reading accurate to -(Ifll", and are graduated on one side for
internal measurements, and on the other for external measurements.
Special arrangements are made for getting over a bar or projection and for
using close to a projection. It is advantageous for the under side of the
projecting arm to be rounded to a small radius, so that the position of holes-
may be correctly gauged.
Thickness G.'Vuges or Feelers.
Feelers are made up in sets varying in thickness from '0015" to ■025",.
and are used for checking clearances and "play" in assembled work.
The tapered end types have the advantage over the plain rounded ends
that it is easier to get them in between a shaft and bearing, or between a
GAUGES AND MEASURING INSTRUMENTS
103
plug gauge and hole. Feelers are also used on milling and planing machine
tables to ensure that the work is flat or " down " when secured to the tables
for machining.
Profile Gauges.
The measuring surface is formed to the profile required (as in a
templet), and where limits are required, two such gauges are provided, one
being made to the maximum sizes and one to the minimum sizes allowed.
In some instances such gauges are made to suit a series of operations, on
to test the collective result of a number of operations at one time, but this
is only done where great accuracy is not required. An example is that of
a number of steps or collars in a turned shaft. The accuracy of positioning
and the form of any irregular part may be tested by a plate gauge cut to
suit.
//gSS Pro/P/e or Form Gau^&s.
Profile gauges may be cut from a solid sheet, or built up of a number of
strips screwed and dowelled on to a base plate of mild steel. These strips
may be hardened, and thus will stand a considerable amount of use when
large quantities of work are being produced and inspected.
Examples of the use of profile (or " form ") gauges will be found in
Fig. 55.
Screw Gauges.
A screw thread consists of a number of ridges of triangular, square,
or rounded form imposed on a cylindrical "core." The pitch "p" is
the distance in inches or millimetres between the centres or sides of two
consecutive threads as measured on a diameter midway between the top
104
ENGINEERING INSPECTION
and bottom of the threads. This is called the "effective" or "pitch"
diameter. The top and bottom of triangular threads are usually rounded
or cut off square for strength and convenience, and the angle of the threads
is chosen to give greatest strength. (Fig. 56.)
f^OO-r aiK. (^o-r-roi>-i
S(_
f/^.S5^ 3 crew Threads
Some of the various forms and angles in common use are as follows : —
Bounded or
Eadiusof Proportion of
Angle.
flat.
top and bottom, thread cut off.
55°
Rounded
•1373 p. -IGp.
60°
Flat
— -108 p.
47io
Rounded
t\P- -2 p.
/Depth of
29°
Flat
thread = "Sp. + -01"
— ) Width of flat
I on top = -3707 p.
Whitworth threads
Sellers (U.S.A.)
British Association (B.A.)
Acme
The errors which may be expected in screw threads are as follows : —
(1) Outside and core diameters too large or too small.
(2) Effective diameter too large or too small.
(3) Pitch wrong, either progressive or periodic.
(4) Radius or flat at top or bottom wrong.
(5) Angle wrong.
(6) Core and outside diameters not concentric.
Screw gauges of similar forms to those shown in Figs. 5TA and 57c
may be used for general checking, but the information given by screwed
GAUGES AND MEASURING INSTRUMENTS
105
plugs or rings is far from complete, as the threads may only be touching
at certain points.
In report No. 38 of the Standards Committee, Mr. Taylor recommends
the following screwed gauges for general use (Fig. 57) : —
rioT Go
Coni^ CDi^
Go. /\ fJoT Cro
FLu& Gmo«seis>
Wo««
E> Not Cra
Cr^utjEi
do. Crf=\occEi
rot "TT-iR EMo
m
wA
WA
RiiNCr G/»>uo.ea
rl OT Cro Ct/^uge
/^ia.57 3 ere IV Thread Cau^ei
Plug Gauges.
(1) " Go " screwed gauge to enter the hole.
(2) " Not go" plain gauge to test core diameter.
(3) "Not go" screwed gauge to test effective diameter.
(4) "Not go " gauge for outside diameter.
Ring and Gap Gauges.
(1) Complete " go " ring screwed gauge to test entire length of
thread.
(2) " Not go " plain gauge for testing outside diameter.
(3) "Not go" 3-point gauge for testing effective diameter and
pitch.
(4) In cases where the tensile strength of the core is important, a
" not go " gauge for core diameter.
106
ENGINEERING INSPECTION
Errors in pitch, form of thread, angle, or radius may be tested by
mounting the gauge or work under a microscope with a screw attachment
for moving the screwed object across the field of view. A cross hair in the
eyepiece is set across the centre of one thread and the screw moved until
the hair line is directly over the next thread. The difference between the two-
readings of the microscope screw gives the pitch of the thread. The depth
of the thread may be measured in a similar manner, and any irregularities-
in form noted. By this means, irregularities in pitch can be measured
within "00004", and the angle may be ascertained within 5' by rotating the
eyepiece. A better method, however, is to use a projection apparatus.*
The gauge or screw is mounted in a suitable manner in the light
thrown by a lamp and condenser. Magnifying lenses are then arranged to^
throw an image of the thread, about 50 times full size, on a screen, where
it can be compared with a correct outline drawing of the thread to the
same scale. The lenses should be chosen to give a uniform magnification
and to avoid distortion. These methods, however, are applicable to male
or plug threads only. The threads of ring gauges may be examined by
taking plaster casts of the inside of the gauges and measuring these as in
the case of plug gauges.
L.E H GrTH
F/g.SS -- C urines of P/fch Error.
Pitch errors may be progressive or periodic — that is to say, the error
may increase in magnitude as longer lengths of thread are taken for
measurement, or it may rise to a maximum and then diminish again
periodically (Fig. 58).
* For various methods of applying tliese see National Physical Laboratory Report, 1919.
GAUGES AND MEASURING INSTRUMENTS
lor
Snap gauges with conical points ground to the exact angle of the thread,
are used for testing pitch, or effective diameters. These may have two or
three points. In the former case the two points are offset a distance equal to-
half the pitch, and maybe made to suit the upper and lower hmits of the job.
Snap gauges with two lower and one upper point may be used for
testing the effective diameter and accuracy of pitch at the same time. In
this case, the two lower points are set a definite number of pitches apart,
so that if the actual pitch of the work is too wide or too narrow, the points-
will not enter.
As points are easily worn away, an alternative method is to fill up the-
hollows of the thread with wires of such a size that they will touch the
r
AA/WV
w\
vySAAA/
T
v3 va'if?!
I WiRC
[Methods of=" M^.'vsLjffirHcs
/yjr.SP— The Measuremenf of5crew Threads
threads on the effective diameter, and to measure the distance between the
outside faces of the wires (Fig. 59). Suitable sizes of wire are as follows :.
Whitworth threads diameter of wire = -5637 p.
Sellers (U.S.) threads ,, ,, ,, = '5774?.
British Association (B. A.) threads ,, ,, ,, =:o463p.
108 ENGINEERING INSPECTION
The outside diameters of threads should be checked at different points
along the screw to detect any tapering, and on different diameters to see
that the thread is truly round.
Ring and plug gauges frequently have the bottom of the thread made
sharper than the standard size, so that any dirt or chips may collect there.
As screw gauges have small wearing surfaces and considerable friction,
it is advisable that they should be hardened, and this operation, by
distorting the threads, often causes considerable trouble. Case-hardened
gauges of mild or nickel steel* are now in general use, and for further
information on methods of hardening with minimum distortion, the reader is
referred to a paper read by Mr. W. J. Lineham, B.Sc, before the Institution
of Mechanical Engineers, entitled " The Hardening of Screw Gauges with
the Least Distortion in Pitch " (April, 1920).
Reference gauges which have very little work to do are often left
tanhardened to avoid these difficulties and to secure greater accuracy.
As accurate work cannot be produced with defective tools, the taps
and dies used in screwed work must be carefully inspected before being put
into use, to ensure that none of the errors described above are present to a
sufficient extent to affect materially the accuracy of the work to be done
by them.
" Not go " screw gauges will only test one element of a thread, and
as it is impossible to provide ' ' not go ' ' gauges for each element, the
systematic checking of threading tools forms the best safeguard in practice.
If the tools are right, the work of inspecting the product is considerably
simplified, and manufacture may proceed with greater confidence and
security.
Combination Angle Gauges.
The combination angle gauge gives a very convenient and quick
method of checking angles accurately. Previous to the introduction of the
fixed angle gauge the best-known method of checking angles was by means
of the sine bar or bevel protractor vernier. The vernier protractor is a
good instrument for measuring different angles, but it only registers
angles in one-twelfth degree and therefore is only suitable for lovsfer
standards of accuracy. This is because a good deal is dependent on sen-
sitiveness of touch and setting, also vision (often assisted by lenses).
Therefore it is not sufficiently reliable for gauge or jig work.
* 3 per cent, nickel steel is very suitable for this purpose.
GAUGES AND MEASURING INSTRUMENTS
109
" Johansson " combination angle gauges (Fig. 60) can be obtained in
a series of 15 blocks with an angle at the four corners of each, the angles
increasing in minutes from 10° to 11°. A second series consists of 40
gauges embracing angles 0° to 90° in increments of 1°. The first block has
rectangular sides, and the succeeding six have an angle at each of their
four corners, whereas the remaining gauges of the set have two angles only.
IO"28-
Ho. 5" _ -*5ViMD •4-+''
C
Mo. e _ 90° 2o' 9°
«f>
*s
^
F/^j60^ Cofn6//7afion An^/e Gauges.
The third series of 30 gauges includes the angle range of 89° to 90° in
minutes. All the angles are marked on each gauge and each gauge is
numbered.
A holder is supplied for clamping two angle pieces together, and both
male and female angles can be fixed up.
110
ENGINEERING INSPECTION
Fig. 60 (a) shows the holder with two angle gauges clamped in
position. Gauge No. 1 represents male 55°, and Nos. 2 and 3 female
gauges set to 55°.
Fig. GO (b, c, and d) illustrates angle gauges marked 4, 5, and 6
respectively. No. 4 is 10° 28', No. 5 is 45° and 44°, and No. 6 is 90° 20'.
■"Johansson" Gauges.
These reference gauges are the most accurate made, being a series of
Tectangular blocks of " Invar " steel, carefully machined, seasoned, ground
.and lapped on both sides. The parallel sides of each block are correct to
"the nominal dimension within '00001", and they are supplied in various
■sets for different purposes. Set No. 1, comprising 81 blocks, is divided
into four series. The first series contains nine blocks from '1001" to '1009"
lay increments of 0001". The second has 49 blocks from "101" to "149",
-the third has 19 blocks from 050" to -950", and the fourth 4 blocks of
Roi-ueil
ROULEI?
f^inCr
f/^. 6/ - Te^f/n^ /nfernal Dia. offi/n^
iv/th Johansson Gauges .
1", 2", 3", and 4" thickness respectively. With this set, measurements
•can be made from "0500" to 10". When supplied with standard plugs and
holders, over 100,000 different gauges can be built up. Other series give
readings up to 20", and readings in quarter-thousandths may be obtained
"by adding blocks of "10025" and "10075" thickness. In use, the gauges
are slid together with slight pressure, and the air is thus squeezed out,
-enabling the blocks to stick together and form a single unit.
With these, ring gauges may be checked by inserting rollers of known
«ize at each side of the measuring blocks, as shown in Fig. 61. For gauge
^nd tool room purposes, these blocks are indispensable, but they are too
costly and delicate for production or shop use. By means of such blocks,
■duphcate reference gauges are often rendered unnecessary, as the desired
standard can be set up with great accuracy in a very short time.
TVlASTER AND REFERENCE GAUGES.
When work is being produced to limits it is advisable that the gauges
used by the operator should be within the limits of those used by the
inspector in the shop, to ensure that, even if slight wear takes place in
GAUGES AND MEASURING INSTRUMENTS 111
the operator's gauge, the work which passes the latter will also pass the
viewer's gauges.
As the viewer's gauges are checked periodically, their tolerance again
will be slightly less than the reference gauges used for checking purposes
-or by the purchaser's inspector. Where Johansson gauges are not used
or are inadmissible, duplicate reference or master gauges, consisting of a
male and female part, must be provided for checking periodically those in
-use. In some instances a certified set of standards is kept in the gauge
room, and with these, the working master gauges are compared from time
io time. In no cases should the master gauges be used in the shops.
They must be kept strictly for checking purposes, each working master
plug gauge being kept with a corresponding ring gauge, and vice versa.
If a workman or viewer accidentally drops or damages a gauge it should
always be returned to the gauge room for checking before being used.
Gauges must never be used while work is in motion, and must not be forced
•on to the work, otherwise they will very soon become useless.
TMlCROMETERS.
The ordinary micrometer for measuring outside diameters (Fig. G2a)
■consists of a frame of drop forged or cast steel (or aluminium in large
sizes), having one fixed point of hardened steel and one movable point.
The latter is also of hardened steel, and is advanced to, or withdrawn from
the fixed point by means of an accurately-made screw having usually 40
threads to the inch.
Every turn of the screw, therefore, moves the spindle through J^" or
^025", and lines are engraved on the sleeve to indicate each "025", with every
fourth line longer than the others. Each of the larger divisions, therefore,
•corresponds to "01", and as there are usually 10 of these, the total range
of measurement is 1". The outer sleeve or "thimble" has a bevelled
■edge, divided into 25 divisions, so that each of these divisions corresponds
to 'OOl". Finer readings to "0001" may be taken on a vernier supplied
Tvith some instruments.
Although the total range is 1" (or in some cases 2"), or 25mm.
(reading to 'Olmm.), sets are provided reading from 1" to 2", 2" to 3",
3" to 4", etc., up to 12" or 20". Corresponding metric sizes are also made.
Special micrometers with deep gaps are made for measuring the thickness
of plates, and with measuring points of large size for soft materials, such
as paper or fabric. Micrometers of special form with rounded points are
made for measuring the thickness of tubes, or for places where the
■ordinary micrometer is too large to penetrate.
112
ENGINEERING INSPECTION
Some micrometers are provided with a milled nut for locking the
movable points in position after setting to a definite size, and quick-
adjusting micrometers, where the nut is disengaged from the screwed
One-inch Micrometer.
V .-^
\
--_
\ ^
r-.. - --- .-
1-e .125
y -
i-4.2S0
^^.^^
3-8.375
1-2. SOO
6-B.e25
3-4.750 ,
■7-3. 67 5- \
lettiB. \
/
1 .0625 \.
3- .1375
V 5 .312
^•»
S
21-
No 2 27 .6437
29.9062
\ 7 .4375
\ 11.6875 "■'
\t 13.8125
i,^^^ 15.q375
31.9637 ^
Two-inch Micrometer, with Extension Pie
Tube Micrometer (Ratchet Stop).
Fig. Ci?A. — Micrometers.
(By permission of Alessrs. L. S. Starrelt Co., Ltd.)
spindle by pressing the end of the thimble, save a good deal of time and
wear on the thread when adjusting the instrument.
Micrometers are liable to open out and become inaccurate when.
GAUGES AND MEASURING INSTRUMENTS
113
Micrometer for Fabric or Paper.
Deep Frame Micrometer for Plates or Sheets.
Six-Inch Adjustable Micrometer.
Fig. Cylv.. — Micrometers.
(By l^erniissicDi of Mcss>s. L. S. Starrclt Co., Ltd.
114
ENGINEERING INSPECTION
handled (owing to expansion with temperature), and therefore special
wooden handles or rubber grips are provided on the frames of large sizes.
If not, the same result may be achieved by covering the frame with a
wrapping of asbestos string.
As accuracy in making micrometric measurements is bounded to a
certain extent by the pressure between the micrometer and the work,
ratchet stops are provided on some instruments, so that the ratchet pawl
slips when more than a certain pressure is applied. This device is also
useful when measurements have to be taken by different persons, as it
eliminates the personal factor, being automatic in its action. The fixed
points or anvils are sometimes made adjustable or interchangeable, to
increase the range of the instrument, and a micrometer is also made that
will measure round work of any size up to 4j" diameter, and flat work up
to (i", by sliding the micrometer head along a bar and locating it in the
desired position by plugs passed through hardened steel bushes in the
slide (Fig. 02b). Bench micrometers are mounted on heavy cast-iron bases
and are both rigid and accurate.
The use of micrometers for measuring threads necessitates the
replacement of the ordinary flat measuring surfaces, by pieces ground to
the form of the thread. If two simple points are used, they must be
ma
(/.'v prnilis^i.nl of Missrs. L. S. Slnnctl C'<
"offset" a distance equal to half the pitch of the thread, but a better
method is to make the movable point in the form of a Y, and the fixed
point to fit over the top of the opposite thread, as shown in Fig. (Vi.
When point and anvil are in contact, the line A B corresponds to the
( ) position of the micrometer scale.
The effective diameter of the thread is the outside diameter, less the
depth of one thread.
GAUGES AN'D MEASURING INSTRUMENTS
11.')
For Whitworth threads —
Pitch diameter or effective diameter = I) -
■fitO
1^'
Where D = outside diameter in inches.
N = numljer of threads per inch.
For U.S. and A.S.M.E. standard threads-
Effective diameter = D - — -- .
N
Points for measuring the pitch diameters must be cut clear of the
threads at the top and liottom, so that fiearing is onlv olitained imt the sides
of the threads.
With micrometers for measuring the core diameters, the "offset"
varies ^^•ith the pitch of the thread, and for this reason the anvil is some-
times mounted on an adjustable cross slide, where its position can l)e
altered to suit the thread in question.
Ball points are used for comparativelv coarse threads, but these are
uselss for threads as fine as 1(1 pitch, and where used, should be solid and
not made to slip over ordinary micrometer points, as in the latter case
there is great liability for errors to occur. If thread measurement is only
occasional, the three-wire system described on page 11)7 mav he used in
conjunction with a micrometer instead of a gauge.
Inside micrometers are used for measuring the diameters of holes,
and may be made of the two-point or three-point patterns.
The two-point micrometer is illustrated in Fig. (i4, and is operated
in an exactly similar manner to the outside micrometer. The lengthening
a
Fir. Ci-l. — r\vci-|)iilni m^ldc .\1 irrunn-li r, \xiih cxI.n^i.Hi 1
illy l:crwissi,ni ,./ Mnsr^. I.. S. Shinrll (',.., /,/,/.)
bars are used to increase the range of the instrument, and can be obtained
suitable for measurements up to 107". The three-point micrometer
(Fig. (;.)) is extensively userl for accurate measurements. The three leo-s
are at an angle of 12(1° to each other, and measure the distance between
IIG
ENGINEERING INSPECTION
three equi-distant points on the circumference. This type of gauge does
not need such skilled handling as the two-point form, and so is very
popular, especially for large diameters. Fixed gauges of this type are also'
I-'ic. ()5. — Three-point iiuernal Microni(l»;r.
(By f'rnitission of Messrs. L. S. Slurrctt Cik, Ltd.)
made, the only difference being the absence of the micrometer head. The
two-point gauge or micrometer, however, is simpler for checking purposes.
Depth and height gauges (Fig. 54) are also made on the micrometer
principle for checking slots, shoulders, etc., and their use is similar to that
of fixed gauges made for the same purpose, save that in this case definite
measurements can be made.
Space does not permit of the description of other applications of the
micrometer principle, but many special forms are made for different
purposes, the principle of measurement, however, being the same in all
cases.
Vernier Calipers.
Vernier calipers are graduated in inches, tenths, and fortieths of an'
inch (025"), but the vernier scale which slides along it has 25 divisions.
I 2
4- 5" S 7 S 3
lilllllllllllllllli
6 S lo IS za zs
f/£r. S6_ Mefhod of reac/ing Vernier 5ca/e .
(Fig. GG), which occupy the same length as 24 divisions on the caliper
body. Thus the difference between a vernier division and a body division.
GAUGES AND MEASURING LNSTRUMENTS
li:
is one-twenty-fifth of one-fortieth or j J^_". To read the instrument, note
how many inches, tenths, and fortieths of an inch the O mark on the
vernier is from the O mark on the body. Then note the number of
divisions on the vernier from to a line which exactly coincides with a
line on the body. In the illustration (Fig. GG) the vernier has moved
1" + four-tenths + one-fortieth (■025), and the 11th line on the vernier
coincides with a line on the body. Therefore the reading is
1 + '■! + '025 + 'Oil = l'4-!(i". Similar readings may be taken on a
metric scale, in which 10 divisions on the vernier coincide with 9 divisions
on the body. Vernier calipers (Fig. 07) are made with a slide that can be
•clamped in position, and this is attached by a screw to a measuring jaw
liliiUnldiiiliiiliiliiiliiilniliiiliiiliiiliiiliiiliiiliiiliiiliiiliiiliiiiiiiliiiliii
Fl-dllt .Siclr.
Back .Sid.;-.
Fig. G7. — \'eriiiei' Caliprrs.
(By pcnnission of Ulcssys. L. S. Slarrdt C:>., Ltd.)
which carries the vernier. The screw is used for making fine adjustments.
■Outside diameters are measured by placing the jaws of the caliper over
the work, and inside diameters by using the ground projections on the
measuring jaws. As these are only about j" long, however, the ends of
the holes only can be gauged in this way. For inside measurements the
distance across these projections must be added to the readings to obtain
the correct result. This can be obtained from the makers, or measured by
means of an outside micrometer or vernier caliper when the jaws are in
the closed position.
lis
ENGINEERING INSPECTION
Verniers are made up to 24" long, and if made in metric sizes are
accurate to J^j mm.
A special and important use of the vernier principle is its application
to the measurement of gear teeth. A gear-tooth caliper is shown in
Fig. liS, and consists of an ordinary vernier with a tongue that can be
. -3"'-"
lIlKllilIiillilillllllliL-,
Fii'.. n,^. — Crar I'ooih \'ornier.
(/.'y prniiiisi, II, of Mi'Sii's. L. S. Starrcit Co., Ltd.)
moved at right angles to the jaws and marked in a similar manner to the
ordinary vernier. Both the sliding jaw and the tongue have adjusting
screws, and compensation can be made for any variation that may occur
in either a gear blank or rack.
Depth gauges are also made on similar lines to the micrometer depth
gauge, save that in this case the readings are taken on the vernier principle,
and the sliding tongue is flat instead of being round, as in the case of
micrometer depth gauges.
Measuring Machines.
Standard measuring machines consist of a combination of the
micrometer and vernier principles of measurement, together with some
device for indicating the pressure between the measuring surfaces. As the
GAUGES AND MEASURING INSTRUMENTS
11!)
readings are taken to an accuracy of j-^y,T,T, to jv,vhnn oi an inch, it is
necessary to mount sucli machines on a rigid bed and to ensure an even
temperature in the room where they are placed.
Elal")orate precautions are taken to ehminate personal errors, and as
the accuracy of such machines is far greater than is necessary in the shops,
they are only used for checking purposes. Descriptions of the best-known
measuring machines may be found in " Machinery," Nov. 2nd, 11)10, under
the heading, " Gauging and Inspection Methods."
Indicating G.m'ges.
Gauges of the self-indicating type are mostly used for the purpose of
comparison, for detecting eccentricity, irregularities of surface, distortion
after hardening, and other similar measurements.
The range of indication is usually small, lieing in the neighbourhood
of j;\; " for dial indicators, and therefore such instruments cannot be used
for making dehnite measurements of sizes, unless supplemented by some
other i/aus-re or indicator.
The simplest gauge of this type is the multiplying lever form, shown
in Fig. li!t. The short arm of the lever rests against the work, which is
mounted on a centring device of some kind and turned round by hand.
The needle, being set at zero for one point in the circumference, will
(By pcrnmsuni tif Messrs. L. S. SturrrtI Cn., Ltd.]
indicate on the scale to a greatly magnified extent any irregularity in the
surface, eccentricity, or departure from a truly circular form. The indicator
shown gives readings to (JOl", and by mounting on a suitable support, such
as a scribing block body, may be moved over a flat surface to indicate any
defects of form or setting.
120
ENGINEERING INSPECTION
A very accurate form of lever gauge, which can be adapted to almost
any class of work by arranging suitable holding devices, is the Hirth
minimeter (Fig. TO). In this instrument the short arm of the lever is made
■very small and accurate, being the distance between two knife edges
A and B. This distance is made slightly variable to allow adjustment for
the instrument. A plunger in contact with the work presses on the knife
pa 70 _ Diagram shown^
Prmaple oTHirth Miniwefer.
edge A, and any small movement of A causes the long arm C to move
across the scale, which may be graduated to read in divisions of '001" or
'0001", according to the ratio between the two arms of the lever. A
■spring D keeps the seating block E in contact with the knife edge, and
returns the plunger F to its lowest position after measuring, thus bringing
back the pointer C to zero. The entire mechanism is enclosed in a tube
with an opening at the top to enable the scale and pointer to be seen when
measuring.
Dial indicators are also operated by a plunger in contact with the
work, and in this case the movement is either multiplied by a lever and
segment or by a rack and train of gears. In the former case (Fig. 71b) the
plunger is pressed to its lowest position by means of a spring A. When
it is moved over the work, any irregularity causes the plunger to rise, and
the motion is transmitted and magnified by the lever B, which carries a
toothed segment C. In moving from left to right about the centre D, the
small gear wheel G is caused to rotate, carrying with it a needle which
GAUGES AND MEASURING INSTRUMENTS
121
works on a graduated scale. Thus a very high degree of magnification
can be obtained, and readings to "001" or "0001" may be made with
different instruments of this type.
The principle of a rack-operated indicator is shown in Fig. 7lA. Here
the spindle plunger A works in lapped and hardened bushings, and has a
rack cut in it which turns a pinion B. This rotates a pinion C mounted
on the same spindle, and C, in turn, drives pinion D, upon the spindle of
/
/
Ci'
\
^
\
\
\
/*
/
/
A
'^////////
\r^OR K
////////////
Work
/v^. 7/_ Diagram ofRach&P/n/on f;g, 7/_ £)/a^ram of lei/er & Segment
£)/a/ /nd/cafor. ^, ^/ /nc//cator
(A) (B)
which the indicating needle is mounted. Thus the degree of magnification
depends upon the ratio of the various gears, and may be made greater or
less as required. A fourth pinion is generally added to prevent back-
lash and thus to give greater accuracy.
Other magnifying devices are used in different dial indicators, but
these, together with the various uses of dial gauges, cannot be discussed
here. A leference to articles and papers describing various types and
applications in detail is given at the end of this chapter. Dial indicators
need to be checked periodically by means of a setting block or other
standard to ensure correctness and accuracy of working.
The Gauge Room.
This is a necessity with any inspection system, and is the place where
gauges may be kept for replacement, master gauges and measuring
machines stored, and appliances for checking and standardising gauges
installed. It is therefore the "heart" of the gauge system.
122 ENGINEERING INSPECTION
The following tools and measuring instruments form the nucleus of
such an equipment, but special tools and appliances must be added for
different classes of work : —
(1) Inspection table and surface plates.
(2) Heads with testing centres.
(3) Newall or other measuring machine.
(4) Johansson gauges.
(5) External and internal micrometers.
(6) Height and depth gauges.
(7) Set of vernier calipers (li", 6", 12", and 24").
(8) Straight-edges and test bars of various sizes.
(9) Universal bevel protractors and combination angle gauges,
(10) Radius gauges, feelers, and thickness gauges.
(11) Dial indicators to -001" and -0001".
(12) Speed indicator.
(13) Gear tooth vernier.
(14) Screw pitch gauges and wires for checking screw gauges.
(15) Reference or master gauges.
(16) Try square, spirit level, and thermometer.
(17) Hoffmann standard rollers and balls.
(18) Screen and projection apparatus for threads.
The gauge room should be kept at a mean temperature of about
62° F. or 16.7° C. This is not vitally necessary, but the effect of tempera-
ture should be borne in mind when checking gauges made by outside
contractors. Gauges made by the National Physical Laboratory are usually
tested at that temperature.
The ordinary measuring instruments, such as rules, squares,
protractors, calipers, scribers, wire gauges, etc., are not described in the
foregoing pages, as it is assumed that the reader is sufficiently familiar
with these, and, if not, their principles can readily be grasped by reference
to any toolmaker's catalogues. Also, where detailed descriptions have
been given, the use of such tools is described from an inspector's standpoint
only, so that many useful and well-known applications have been omitted.
GAUGES AND MEASURING INSTRUMENTS 12S
Apart from the various catalogues issued by tool suppliers and makers,
the following papers and articles on gauges and gauging may be consulted
with advantage : —
Thomas Hawkesley Lecture on " Limit Gauging," by Sir Richard
Glazebrook, K.C.B. (Proc. Inst. Mech. Eng., Nov 1920).
" The Hardening of Screw Gauges with the Least Distortion in Pitch,"
by W. J. Lineham, B.Sc. (Proc. Inst. Mech. Eng., April, 1920).
"The Manufacture of British Association Screw Gauges," by T. F.
Davey (Proc. Inst. Mech. Eng., Feb., 1921).
"The Principles of Limit Gauging," by A. A. Remington, M.I. Mech. E.
("Engineering," April 15th, 1921).
"Capstan Dial Gauge" ("Engineering," March 18th, 1921).
"The Sykes Gear Tooth Comparator" ("Engineering," July 15th,
1921).
"Gauging and Inspection Methods" ("Machinery," Oct. 26th,
Nov. 2nd, and Nov. 9th, 1916).
"Profile and Indicating Gauges" ("Machinery," Dec. 14th, 1916).
" Making Limit Gauges " (" Machinery," Dec. 21st, 1916).
"Notes on Screw Gauges," by Col. R. E. B. Compton (Inst.
Automobile Engineers, 1917).
"Article on Threads and Screw Gauges" ("Machinery," March
15th, 1917).
" Master Whitworth Thread Gauges" ("Machinery," Oct. 25th, 1917).
"Gauging and Inspecting Threads" ("Machinery," June, 1917).
"Common and Special Micrometer Calipers" ("Machinery," Feb.
14th, 1918).
CHAPTER VIII
MACHINE SHOP INSPECTION
The inspection of machined, or partly machined details is largely
bound up with the question of jigs and gauges, but there are certain
guiding principles that must be observed whatever may be the nature of
the job and the degree of accuracy required.
The system of inspection used, depends largely upon the lay-out
•of machinery and sequence of operations, and in cases where a works or
machine shop is being planned " de novo" it is a simple matter to make
arrangements for inspection after each operation, or upon completion of
each detail as may be required. In many instances, however, the
inspection system has to be grafted on to the existing organisation, and
thus its arrangement is bounded by limitations of space and other
•considerations which often make it impossible to adopt the best procedure.
The weight of parts produced exercises a good deal of influence on
the arrangement of machine tools. Where heavy parts are being made,
it is sometimes more convenient and cheaper to take the machine to the
work, than to take the work to the machine. In such instances the basis
for the system is the Job or Order Number, one or more of these being
placed in charge of a viewer or inspector to see that all operations are
correctly performed, that nothing is omitted, and that the job is sent out
of the shop in a complete state to the next shop, or series of operations.
The number of viewers required, and their arrangement, will therefore
ij Ijoi'J
3.
Poojl, >y5. oPc^cip
u
Roujt) rjjfll ftodg.o^ l)\Hl.EW. Bono Oftuow>r>gTgp. ■
solid fuels is the "Bomb" type shewn in Fig. 76. In this instrument
the steel bomb is filled with oxygen, at a pressure of 20 to 25 atmospheres
(300 to 370 lbs. per sq. inch) and the fuel is burnt on a platinum tray
which holds one gramme of the powdered fuel. The sample is fired
electrically by a thin fuse wire, and the heat thus liberated is absorbed by
the water in which the bomb is immersed. A thermometer graduated to
^^o°C. is provided to register the rise in temperature thus produced, and
150 ENGINEERING INSPECTION
a stirrer keeps the water in constant motion, so that the temperature
throughout the water is as uniform as possible. The calorimeter is well
lagged outside to prevent the escape of heat to the atmosphere, and the
readings are worked out as follows : —
Weight of coal burnt in platinum tray = l gramme
Rise in temperature of water =2'8°C.
Water equivalent of calorimeter = 510 grammes
Weight of water in calorimeter =2200 grammes
Therefore total equivalent weight of water = 2710 grammes
Total calories given to water by burning coal = 2710 x 2'8
= 7580
Therefore heating value of coal = 7580 calories per gramme
But 1 calorie per gramme = 1-8 British Thermal Units per lb.
Thus calorific or heating value of coal = 7580 x 1-8
= 13600 B. Th. U. per lb.
It is advisable that the combustion in the calorimeter should not take
place at a pressure of less than 2D atmospheres, as below this pressure
complete combustion is not always obtained.*
For complete tests it is advisable, in the case of solid fuel, to obtain
the amount of moisture and ash in the coal. For this purpose the coal
is sampled by " quartering "f until about 1 lb. is left, and some of this is
ground up until it passes a sieve of 80 meshes to the inch. A portion of
this is used for the calorific test, and a further 1 gramme is weighed out
for the moisture test. It is then heated for some time in an oven kept
at a temperature of 110°C., so that all moisture is driven off, and the
contents are then re-weighed. If the net weight of fuel is then '95 gramme
the moisture content is 100-95 or 5 per cent. A similar procedure is
adopted for getting the percentage of ash, save that in this case the
procelain crucible containing the fuel is put into a muffle kept at a bright
red heat. The fuel should be continually stirred with a platinum wire
until all traces of black have disappeared. The ash is then weighed and
* For further particulars see " Experiments on a Bomb Calorimeter " (" Engineering,"
December 2, 1910), by E. A. Allcut, M.Sc.
+ To sample solid fuel by " quartering," the large sample (obtained by putting out a
shovelful of fuel at regular intervals when firing) is broken up into lumps of equal size, and
after mixing up is divided into four equal parts. One of these parts is taken and broken up
evenly into smaller pieces and again divided into four equal parts, the process being repeated
until about lib. of fuel is left. This is transferred to a corked bottle and kept airtight until
the analysis, moisture, and calorific tests are taken.
FINAL TESTS 151
its percentage calculated. Thus all the conditions of the fuel fed into the
furnace, boiler or engine are known.
The heating value of gaseous fuels is calculated in a different way.
The Junkers calorimeter (Fig. 77) consists of a copper vessel in the middle
of which the gas burns at a constant rate which is observed by means
of a small gas meter. This combustion chamber is surrounded by a water
jacket through which pass tubes which take the hot products of combustion
through the water. Thus they are cooled down and pass out into the air
practically at atmospheric temperature. The water formed by the com-
bustion of any hydrogen or hydrocarbons in the gas, is condensed by this
cooling action, and flows away from the instrument into a measuring glass.
The cooling water is regulated to pass through the jacket at a constant
rate, and thermometers reading to j\^ degree Centigrade are placed at
the inlet (bottom) and outlet (top) of the calorimeter, and the quantity
used during the test is also measured by running the outlet water into a
measuring tank or glass.
When the conditions are constant, readings of the volume of gas used,
weight of cooling water, weight of condensed water, and inlet and outlet
temperatures of gas are taken over a definite period, say 10 or 20 minutes
and the heating value of the gas calculated as follows : —
W = lbs. of cooling water used in time T.
ii and ^2 = inlet and outlet temperatures of cooling water in °F.
V = cubic feet of gas burned in time T.
w = \hs. of water condensed from burning gas at 0°C. and 760 milli-
metres pressure in time T.
Then Higher Calorific Value= — -}, — ^ B. Th. U. per cu. ft.
Lower Calorific Value= —~ — ^ '- — B. Th. U. per cu. ft.
Wet and dry bulb thermometers are used to indicate the amount of
water vapour present in the atmosphere, and a barometer to indicate the
atmospheric pressure, so that the results may be reduced to standard
conditions for the purpose of comparison.
To correct volume of gas used, to 0°C. and standard atmospheric
pressure (760 millimetres of mercury)
Let i°C. be the temperature at which test was taken.
f = pressure of gas in inches of water.
Z = tension of water vapour at t°C. in millimetres of mercury.
(Note. — 1 inch water pressure = l"87 millimetres of mercury).
B= height of barometer in millimetres of mercury.
152
ENGINEERING INSPECTION
5^ra//)er
Temperafure of IVa/er
Temperafure
of/n/ef-
Cock
Dra/n^^
Wa/erJacAef
Gas
Outlet
Cor7iJer7set/
lyafer OuHef
(^as coo//na
Tu6es
IVafer
/4/rJac/(ef-
/% /Z- D/a^ram orjun/fers' Gas Ca/or/mgfer.
FINAL TESTS 15:3
Then actual pressure of gas P = (B + l-87f -Z) millimetres of mercury at
t°C., and Volume of gas at 0°C. and 760 m.m. mercury
760^+273
Example.
Time of test = 20 minutes.
Average inlet temperature of cooling water = 10"7°C.
,, outlet ,, ,, =20-50C.
Temperature of air = 18"83°C.
Temperature of wet bulb tliermometer = 17"l°C.
Temperature of gas leaving calorimeter = 12'03°C.
Volume of gas used during test = 2'29 cu. feet.
Pressure of gas in inlet pipe = 4'8 inches water.
Volume of water condensed from gas = 13 cubic centimetres.
Weight of cooling water used = 24 lbs.
Rise in temperature of cooling water = 20"5 — 10"7 = 9'8°C.
= 9-8xl-8°F. (1-80F.=10C.)
= 17-60F.
.-. Higher* Calorific Value of gas = ?^^^^ = 184-5 B.Th.U. per cu. ft.
Wt. of water condensed per c. ft. of gas = rr— r?; — ='0125 lbs.
^ 6 2-29
(1 cu. cm. water weighs '0022 lbs.)
.•. Latent heat of water condensed
= •0125x966 = 12 B. Th. U. per cu. ft. of gas.
.'. Lower Calorific Value of gas = 184'5 - 12
= 172-5 B. Th. U. per cu. ft.
Corrected Figures of Calorific Values for Standard Pressure and
Temperature.
Barometric pressure of atmosphere = 744 m.m. mercury.
Vapour tension of water at 17"1°C. = 145 m.m. mercury.
.-. Actual pressure of gas = 744 + l"87 x4-8- 14-5
= 738'5 millimetres mercury.
Therefore corrected volume of gas at 0°C. and 760 m.m. mercury
„ „_ 738-5 278
= 2 29 X — — — X;
760 273+17
= 2-09 cubic ft.
* The Higher Calorific value is the heat given out by the burning gas, if all the products
of combustion are cooled down to atmospheric temperature, and thus includes the latent
heat of the water formed during combustion. In gas engines, boilers and other engineering
apparatus, the exhaust gases are generally at a much higher temperature than the boiling
point of water (100° C), and therefore this latent heat is usually deducted and the Lower
Calorific value used for calculations in practice.
154
ENGINEERING INSPECTION
.". Corrected Higher Calorific Value
= 184-5 X 1^ = 202 B. Th. U. per cu. ft.
and Corrected Lower Calorific Value
= 172-5 X f^ = 189 B. Th. U. per cu. ft.*
Chemical analysis is also a valuable guide in the testing of fuels and
prime movers. The calorific value of coal may be calculated from its
analysis as follows : — C, H, and S are the percentages of Carbon,
Hydrogen, Oxygen and Sulphur respectively present in the coal used for
test.
Then Calorific Value= 145 [0 + 4 28 (h- ^j + -28 s] B.Th. U. per lb.
and this may be used as a basis where no calorimeter is available.
The analysis of gases also enables the calorific value to be calculated
as follows : —
Higher Calorific Value.
B Th. U per cu. ft.
Lower Calorific Value.
B Th. U per cu. ft.
Hydrogen
Marsh Gas (CH4)
Carbon Mon Oxide (CO)
Ethylene (C2 H2)
Acetylene
Alcohol (Absolute)
Benzine
347
1072
342
1660
1588
1597
3920
292
963
342
1550
1539
1451
3780
The above calorific values are given at 0°C. and 760 m.m. mercury,
and corrections must be applied to obtain the actual calorific value under
test conditions.
If the actual temperature be t°C. and the pressure of gas p m.m.
mercury, the corrected calorific value will be : —
273
Calculated Calorific Value x
P
273 + r 700
The calorific values of oil fuels vary considerably with their com-
position, but usually range in the neighbourhood of 18,000 to 20,000
B. Th. U. per lb. Petrol has usually about 18,500 B. Th. U. per lb.
In engine testing the fuel or steam consumption is most important,
and the latter is taken by measuring or weighing the amount of feed water
put into the boiler while the engine is on test (always provided that the
* Further details of construction and descriptions of other calorimeters may be found in'
'• The Calorific Value of Coal Gas," by J. H. Coste, F.I.C., and " Producer Gas," by
Dowson and Larter.
FINAL TESTS
155
running conditions are steady, and the boiler is supplying only the engine
under test). The amount of moisture in the steam may be obtained by
means of a steam calorimeter*, so that the weight of dry steam supplied
to the engine under test conditions can be ascertained. The result is
usually plotted on a consumption curve, which gives the weight of steam
or fuel used per hour for every 1-horse power (Indicated or Brake)
developed by the engine, when running at various loads. In this way the
cost of running can be calculated.
"To F^E*aui«
Watc^
^'£- 73— Gau^e /or Jma// Pressures
Pressure readings may be taken with an ordinary water or mercury
gauge when the pressure is small, and slight differences in pressure may be
read by the magnifying pressure gauge shown in Fig. 78, where the reduc-
tion of area in the tube, together with the difference in density between
water and paraffin, are utilised to obtain readings of greater magnitude
than can be obtained with an ordinary water gauge. High pressures are
indicated by means of pressure gauges, which should be checked at
frequent intervals against a standard gauge or loaded plunger (Fig. 79).
The pressure gauge shewn in Fig. 78 may be used for measuring
quantities of air or clean gas passing along a pipe (Fig. 80). When a large
* For description see " Steam and Steam Engines," by Prof. Jamieson, and other
standard works on Steam Engines. It should be noted, however, that the correct sampling
of steam to obtain the dryness fraction is a very difficult matter, and, for this reason, the
results given by throttling calorimeters are liable to be misleading. Such calorimeters
correctly indicate the wetness of the steam passing through them, but there is no certainty
that this steam is similar to the average quality of steam entering the engine.
156
ENGINEERING INSPECTION
gas meter or holder is not available for measuring volumes, the drop in
pressure dovs^n a long straight pipe may be used to indicate the. velocity of
the gas. This drop is very small, and the pressure gauge shown in Fig. 78
gives a reading which indicates the volume of air or gas passing through
the pipe per minute or per hour.
To P&M
/r/cf/ona/ flesisfance 6e/ore fa/f/n^ /feadings.
F/'g. 7S^ Tesf^/hg Pressure Gouge.
For this purpose the pipe and gauge must be calibrated against a gas
meter or holder, so that the velocity or volume which corresponds to a
given reading of the gauge may be known. A calibration curve of this
kind is shown in Fig 81.
Deposits of tar, moisture or dust on the inner surface of the pipe
affect its accuracy, so that this method can only be used with clean gases.
The pipe should be straight, uniform in section, and smooth inside.
Gffls
Pre WITM Smooth, UMiF-oi^r-^ &on.e.
Grns,
/
@=@
0»o£eB
\
l^uaocf^ TuOirtCa
/yg. 80— /Irran^emenf o'TD/fiirenfia/ Gou£re ^r
measuring i^e/oc/f/es or QuanW/es oPGaj orAff'-
Standard orifices and Venturi meters are more frequently used for this
purpose, particularly in testing the output of air compressors.
The power developed in an engine cylinder is obtained by taking
indicator diagrams which give a record of the variations of pressure during
each stroke made by the engine. Engines of medium speed may be
FINAL TESTS
157
indicated by a spring-controlled piston (Fig. 82), which automatically
draws a diagram similar to those shown in Fig. 83, but the inertia of the
F/d: 81 — Ca//6ration curi/e far cf/fferenf/a/ ^au^e rcess/^e //7er//ar or
iVea^^pr/n^ or? /ricZ/cai'or
L ea/rt/3//e/e Hy/ve
/^oo /a A;
/nffy'cf/ Con<^enso'ffon
c>nc^ /fe-e^aporat/on .
O^er ■ running port /iice
f'xhausf oper?€€/
Aoo eejr/^
//7£//C€f/i?r stops c'uf
ofFpart of£//'cMjfrarr}
This is the best and most convenient form of brake, but the rope brake
IS cheaper, and can readily be fitted up from materials usually kept in
:stock in engineering works.
The foregoing is a brief description of the principles involved in
measuring temperatures, pressures, calorific values, speeds, volumes, and
power in connection with performance or consumption tests. Space does
160
ENGINEERING INSPECTION
not permit of this part of the subject being dealt with in great detail, but
for other apparatus and engine testing devices, reference should be made
to standard works on Heat Engines or Experimental Engineering.
(3) Acceptance Tests.
These tests may include any of the tests mentioned above, but the
title is here applied to the final tests of the completed engine, machine,
vehicle, or ship, upon the results of which the work is accepted or refused
by the customer. The tests coming under (2) are often made on various
separate components of the completed job, but it is necessary to show that
f/^. 84- fiope Brake ^r £n^/ne TesHng.
when these components or parts are assembled together they will run in
harmony with one another, and produce the results required by the
customer or guaranteed by the makers. For instance, automobiles are
given a test on the road, on hills, and under abnormal conditions to
demonstrate the power, flexibility, silence, and accelerating properties of
the cars, the power of the brakes, ease of gear changing, and general
performance of the complete vehicle. Bridges and other structures are
tested with fixed and moving loads of definite intensities to demonstrate
the load-bearing capacity of the structure, and the absence of undue strain
as shown by the deflection in each span. Speed trials for ships and flight
trials for aeroplanes show their capability of resisting the forces of nature,
and of producing the speed and reliability that was anticipated by the
designers. These qualities can only be tried by a test made under working
FINAL TESTS
161
conditions. Although some of those conditions may be approached by
suitable testing arrangements in the works, the final test must always be
the performance of the machine in practice. A farmer is not particularly
interested to know the amount of horse power developed by his tractor
engine when run under ideal conditions on the testing bench — he wants to
know whether it will plough his land, and nothing short of this will satisfy
C-^sinc
RoTor^
CASlMCi
Pi_/^r>,
/y^ &?_ froude y/afer Brake
him. Therefore, although the performance test gives useful information
both to manufacturer and customer, in such cases the ploughing test should
be applied as a final proof. This need not be applied in every case, as,
after a number of similar machines have been made, their potentialities in
this direction became sufficiently well known to enable their performances
to be predicted within fairly narrow limits.
After one or more of these tests have been made, it is customary to
strip down the machine and examine details for undue wear, pitting,
M
162 ENGINEERING INSPECTION
burning, or over-heating, any faulty parts being weeded out and replaced
by new ones before the final test is made. Any such replacements should
only be made by the stores on receipt of a credit slip signed by the foreman
inspector, as, if unsuitable parts arrive at the finished machine, one (or
more) of the viewers is probably at fault, and this procedure enables the
foreman to check the work done by the inspectors and viewers, and to fill
up gaps in his own organisation through which unsatisfactory work might
pass on future occasions.
CHAPTER XI
REPAIRS, RECTIFICATIONS, AND OBSOLETE PARTS
In a mass production scheme, articles produced in the ordinary run of
manufacture follow certain well-known and recognised channels, and a
special organisation is laid down to see that the proper sequence of
operations is observed. Rejected articles, put aside for rectification
processes, are in a sort of backwater, and before they can be returned to
the main stream of production, various special machining or other opera-
tions must be done upon them.
As it does not pay, and in many cases is impossible, to set up
machines which are already occupied on production work, to deal with
these articles, it is usual to set aside a special section of the machine shop
for any machining work that comes outside the normal production
programme, and there, repairs, rectifications, and obsolete parts are dealt
with.
The work done on this section is of a varied character, and requires
not only a large number of drawings and gauges, but in many cases
acquaintance with the machines turned out by the manufacturing firm over
a period of many years. Some firms were systematised at an early date,
and good drawings and records were kept, of early designs and types of
machines. In many cases, however, sufficient information is not available
to enable parts of machines made many years ago to be manufactured with
that certainty and precision that is possible with present-day drawings,
jigs, and gauges. It may be that, when the particular machine was made,
■for which a replace part is required, the working drawings contained few,
if any limits, the machining was not done in jigs, and gauges were of a
-very rudimentary description. In such cases familiarity with old types of
machine, and with methods and difficulties of manufacture at that time,
considerably assist in producing a satisfactory job when the complete
machine is not available. The viewer, therefore, must not only be an
•experienced workman, but, if possible, should be an old employee of the
firm, so that his memory can supplement the information given on the
■drawings.
163
164 ENGINEERING INSPECTION
These points do not arise, however, in the case of work to be rectified.
This is usually current production work, which, by reason of faulty material
or workmanship, cannot be taken through the ordinary production pro-
cesses, but must be specially and carefully operated upon, to enable the
remaining production processes to be performed at the earliest possible
stage.
For this reason the ordinary production gauges cannot be used during
the rectifying processes, and considerable caution must be exercised in
allowing departures from standard dimensions, as, for interchangeability,
the final product must conform to the standard gauges, and for reasons of
economy special processes must be reduced to a minimum. It is desirable,
therefore, that the rectifying processes should be so arranged as to bring
the dimensions of the articles to correspond to those in one of the ordinary
production stages, as soon as possible, so that they may then be stamped,
up by the ordinary production viewer, and returned to the normal channel.
In the meantime, however, such parts must be carefully watched and.
specially marked to ensure that they are not taken away and put into-
production operations before the rectification is completed and passed.
Care should also be taken to ensure that the method of rectification chosen
does not spoil any work previously done. For instance, it is quite common
for broken drills or taps to be extracted from work by heating, but if that,
work has previously been heat treated, the advantage of such treatment
may be completely lost, and the article made unfit for use. In such
instances the article should be returned to the hardening shop for re-heat
treatment before any further machining is done. This, however, may
distort or scale the work to such an extent that it becomes useless, and
therefore it is often cheaper to scrap the faulty work in the first case rather
than to attempt rectification. Also, if the rectifying process costs more-
than the partly-finished articles are worth, it is obviously uneconomical,
and should not be proceeded with, unless the parts are urgently required
for production purposes, when their value is artificially and temporarily
raised. If, and when, any departure from standard dimensions is made to
avoid scrapping a very important or expensive part, it is necessary for the-
inspector to keep a record of such departure in a suitable reference book.
This book should be designed and indexed in such a way that variations
on particular machines can be readily and quickly referred to when:
repairing or overhauling the machine, or in making replacements.
A further point is the scrapping of important castings due to blow-
holes or other imperfections which are revealed in the course of machining.
REPAIRS, RECTIFICATIONS, AND OBSOLETE PARTS 1G5
Such castings may often be saved (in the case of a bored hole, for instance)
by taking a little more off the machined surface and inserting a plate or
bush to restore the part to its correct dimensions. Unfortunately, many
•designers do not allow for this, but it should always be borne in mind that
such operations may save materiaL and workmanship amounting to
hundreds or even thousands of pounds in the course of a year. It is there-
fore advisable, in places where blow-holes or other troubles are anticipated,
to allow sufficient metal for machining out such imperfections, so that
repairs of the nature outlined above may be made.
Broken or defective machine parts returned from customers are often
unaccompanied by a clear statement as to the manner of breakage or
failure, the conditions obtaining at the time, or in some instances by any
information at all. If the machine is supplied under a guarantee it is
necessary to find out definitely the cause of the failure, so that the
examining inspector may advise free replacement or not. In many cases,
fractures or cracks can be traced to flaws which were invisible when the
parts were made. Breakage through blow-holes or other internal cavities
are also quite frequent and easily assessed. Undue wear, however, or
fractures due to fouling are less easily dealt with. Wear may result from
soft material, faulty lubrication, grit or dirt between wearing surfaces, or
improper fitting. Some of these are faults of manufacture, and others are
due to bad usage, and often a very careful scrutiny is necessary to decide
between the two. Sometimes repairs are possible to make the faulty part
again usable, and in that event the examining inspector must state on his
certificate what steps are necessary for this purpose, and a copy of this
certificate should be sent to the inspector in the department where the
work is to be done, so that he will know what steps to take when the faulty
part arrives on his section.
CHAPTER XII
THE HUMAN ELEMENT
The personnel of the Inspection Department is one of the crucial
factors that determine the success, or otherwise, of the system in any
works. During the Great War, dilution of labour, due to the demands of
the military, made it necessary to employ as inspectors many men and
women who had no previous acquaintance with, or knowledge of, engineer-
ing processes. Some of these rose to the occasion, but the ignorance and
arbitrariness of many others brought inspection into disrepute, and made
permanent enemies of many engineers and mechanics. The bad reputation
gained for inspection by this class of labour has not yet been lived down,
but the better class of mechanic now available for inspection purposes
should considerably assist in making the department more efficient and
helpful in quantity production work.
In the first place, it should be clearly understood that there are two
distinct classes of inspectors, namely, the " viewers," who merely examine
the work for size, faults, and general suitability, and the higher class of
inspectors, who have a considerable amount of discretionary power and
responsibility, and it is upon the latter that the success or failure of the
inspection system depends to a very great degree.
The viewer is often merely a gauge operator — he has no discretionary
power and simply examines the work produced, to see if it will or will not
pass the gauges supplied for measuring the operations viewed by him. In
many instances, therefore, the viewer need not be a skilled worker, but
this depends to a great extent upon the nature of the operation viewed.
In viewing gear wheels and other articles for hardness, drop stampings for
flaws and other faults, important forgings and castings for machinability,
etc., a considerable amount of skill and experience is necessary, and the
viewer concerned must be a first-class man. Viewers of this class rise
eventually to the grade of foreman, where the same characteristics are
required in a more marked degree, as the amount of responsibility
increases.
The viewer must be absolutely reliable, as any scamped or missed work
on his part may cause considerable trouble and expense at a later stage.
He must naturally have good eyesight to make a proper use of the
166
THE HUMAN ELEMENT 167
gauges and measuring instruments supplied to him, and to detect minute
flaws and defects which might otherwise escape observation. He must
also have a good memory to retain the numerous instructions and warnings
that are issued from time to time in connection with his work. In this
connection, it should be noted that an instruction may be issued several
months before the work in question actually reaches the viewer, and for
this reason all instructions should be written and not verbal, so that no
excuse can be put forward by the viewer in case of mistakes being made.
It is not always possible, however, for the viewer to look back through all
his instructions as each job comes along, and therefore it is advisable that
he should carry as many of them as possible in his head.
He should also be of steady temperament and habits, as an excitable
or unstable nature is unsuitable for this class of work, which generally
demands a clear head and steady hand. A certain amount of tact is alsq
advisable, so that he can induce the operator to do work correctly, or
rectify faulty work, without the continual necessity of approaching the
foreman. Nothing is so troublesome or causes so much friction as a
cantankerous or quarrelsome viewer who is always in conflict with the
operators whose work he has to inspect, and the irritation caused by him
often reflects on the department as a whole. In addition, the time of the Chief
Inspector is often wasted in dealing with situations created by his actions.
Above all (and this remark applies to inspectors of all grades) the
viewer must be honest and conscientious in his work, and must not be in
league with any of the men whose work he has to inspect. When the
operators in question are paid upon the work passed by the inspector, there
is a direct inducement for them to ingratiate themselves with him, and to
obtain by any possible means his connivance in passing bad or doubtful work.
In too many works the inspector becomes a sort of industrial Ishmael
— " his hand against every man, and every man's hand against him." He
is frequently regarded as the official scrapper whose sole duty is to reject
as much work as possible. This is absurd, as no inspector is likely to
scrap work for the fun of the thing, but some men, by their over-bearing
attitude, get the reputation of doing so. These are exceptional cases,
however.
The Chief Inspector must be something of a diplomat, and must
possess abundance of tact, as he has to deal with many tangled situations,
and also with men who have their own ends to serve. It is frequently found
that the superintendent of a machine or fitting shop is more interested in
output than in quality and interchangeability. The Chief Inspector must
168 ENGINEERING INSPECTION
then see that the necessary standard is adhered to, and when this entails a
considerable amount of scrapping, he must be prepared to discuss the
matter with the superintendent concerned, and must be strong enough to
carry his point, otherwise future decisions will not be respected. He must
therefore be firm, but not obstinate. If fresh evidence is brought forward
which tends to reverse his decision, he must be prepared to examine the
situation again, and to weigh up the value of the new evidence in relation
to that upon which his previous decision rested. Having ascertained all
the known facts, he must be a man of quick decision, as it is very confusing
and irritating to have a number of batches of work lying about, waiting for
the inspector's decision. An incompetent inspector generally delays his
decision as long as possible, in the hope that something will turn up to
take the responsibility off his shoulders. There is no room for Mr.
Micawber in modern engineering ; he is too costly a luxury.
With quickness of decision, abundant common sense and caution must
be blended, as much money may be wasted by indiscriminate scrapping,
and no firm can survive that treatment for long. All the available facts
must first be obtained and sifted by a logical mind, so that the right con-
clusion may be arrived at in the shortest possible time. In many cases
that he is called upon to decide there is a mass of evidence, relevant and
irrelevant, the latter being sometimes brought in for the express purpose
of clouding the issue. The inspector must therefore have the ability to
distinguish essentials from non-essentials, and to classify the facts in order
of importance.
It is very advantageous for the Chief Inspector to have a knowledge
of design and stresses, in addition to machining and fitting processes, as
then he is in a better position to decide on the relative importance of many
factors in the case, and this knowledge may enable him to make recom-
mendations to the designers and producers which will have important
results in the quality or cheapness of the product. This also minimises the
risks taken, by reducing the number of unknown factors in doubtful cases.
It is obvious that an inspector must possess a vigilant eye to detect
faults and irregularities, and that he himself must be regular and punctual,
as otherwise his department is likely to get slack, which is the worst thing
that could possibly happen to any inspection system.
As the inspection department is not usually a compact unit, but is
scattered all over the works, the Chief Inspector must be a good and
systematic organiser, in order to see that the whole works is adequately
covered by his organisation, and that each man in the system has plenty of
THE HUMAN ELEMENT 169
employment. There is a great tendency in most inspection departments
towards over-staffing and consequent under-employment or overlapping
vyhen times are good, and to fly to the opposite extreme of under-staffing,
overwork, and big gaps in organisation when times are poor, so that a good
deal of work then passes through without examination.
As the Chief Inspector has many dealings with people outside the
works, such as contractors, representatives, etc., it is necessary that he
should be very discreet, as by the nature of his position he gets to know
all the troubles and weaknesses of the works and its products, and gathers
much information of a confidential nature about his various suppliers. The
reputations of his own and other firms are largely in his hands, and accord-
ingly he must be absolutely honest and trustworthy.
He will also have many letters and reports to write, and these must be
lucid, brief, descriptive, and to the point. They must give all necessary
information in an easily accessible form, and must be strictly accurate. It
should always be borne in mind that these letters and reports may be
brought as evidence in case of legal action, and therefore they must be so
worded that they cannot be read in a sense detrimental to his firm by
prejudiced parties.
These are the qualities that it is desirable for an inspector to possess.
Obviously no one man can have all these characteristics, but the successful
man selects his staff in such a way that he has as many as possible of the
above qualities at his command.
It is a common saying that "to err is human," but there is a distinct
difference between errors and mistakes. An error is a matter of judgment,
and gives an approximation to the desired result which might have been
much closer. Some errors are permissible, as that in working a slide rule
or micrometer, where the degree of accuracy is bounded by the mechanical
perfection of the instrument. Others, such as those due to defective eye-
sight or sense of touch, vary with the individual concerned, and have a
purely human basis. As all inspectors are human, it is only natural to
assume that errors will take place, and that the extent of these will
diminish as the inspectors become skilled in their work.
Mistakes are largely temperamental in their origin, and result from
inaccurate observation, incorrect deductions, and want of memory. While
errors are to a certain extent unavoidable, mistakes are distinctly avoid-
able, and are generally the result of carelessness. An inspector who, after
warning, makes repeated mistakes, can usually only be cured by his removal
to another sphere of activity.
APPENDIX 1
APPENDIX I
Physical Test and Acceptance Sheet
Description of Material.
Makers — British Crankshaft Co.
Kind of Material — Steel Stampings.
Size and Shape —
Condition of Material — Normalised.
Heat Treated by — Hardening Shop.
Quantity— 20.
No. 3421.
Date— 15/12/20.
Order No.— X 324.
Bond No.— 2890.
Material Mark — BY 51.
Previous Test No. —
F"or Part No.— S 234.
Shop Order No.— M 316.
Test Pieces.
Tests Required
No. of Tests
Dia. of Test Pieces — Ins.
Area of Test Pieces--Sq. in.
Tension Compr'n Impact Brinell Twisi
2
•564
25
Bend Flatten
Drift Fracture
Form of part from which Test Piece is cut — End of stamping.
Condition of Test Piece (1) As cut — Normalised. (2) When tested — Heat treated.
Date Machined— 16/12/20.
Test Results.
Specification No. CS 5.
Actu
al Fig-L
res
13-6
14-3
15-8
16-7
■40
•32
•425
45
3'7
•445
32
3^55
P.
F.
Tension or ) -.. , , „ , .
Compression | ^"='<^^ fon^/sq^ '"•
f Ultimate
\ Strength „
I Elongation
" \ Reduction. Per cent.
j Reduction of Area
" ( Increase of Area ,,
Impact (Izod) Ft. Lbs
Brinell Impression — Mm. Dia.
,, Number
Twist (No. of Turns or Angle)
Bend (Angle)
,, (Radius)
Flatten
Drift
Fracture
Calculated Figures
Specifi'n
Figures
54^4
57^2
50
63^2
66^8
60
20
16
17
43
45
3-7
269
38
32
3-55
293
40
35
3-4/3^7
321/269
P.
P.
Remarks
General Remarks-
All fractures satisfactory.
Test made by— C. Jones. Date— 17/12/20.
Witnessed by — M. Withers.
Passed by —
Material accepted —
(Signed) L. \'ANNAN, Inspector.
Date— 17/12/20.
APPENDIX I
173
TABLE I
Reduction of area in tensile test pieces expressed as a percentage of
the original area.
Reduced
Area
Reduction of area
Reduced
Ari;a
Reduction of area
diameter
Original
Original
diameter
Original
Original
ins.
sq. ins.
diameter
diameter
ins.
sq. in.
diameter
diameter
■564 ins.
■399 ins.
■564 ins.
■399 ins.
•564
•250
•405
•129
48^4
•560
■247
1^2
■400
•126
49^6
•555
•242
3-2
•395
•123
50-8
1-6
•550
•237
5-2
■390
•120
52^0
4-0
•545
•234
6^4
■385
•117
53-2
6^4
■540
■229
84
■380
•114
54-5
88
•535
•225
10^0
■375
•111
55^6
11-2
•530
•221
11^6
■370
•108
56^8
13^6
•525
•216
13-6
■365
•104
58^5
16^8
•520
•213
14^8
•360
•102
59^3
18^4
•515
•208
16-8
•355
•099
60-4
20^8
•510
•204
18-4
•350
•096
6V&
23-2
•505
•200
20^0
•345
•093
62-S
25-6
•500
•196
21-6
•340
•091
63-6
272
•495
•192
23-1
•335
•089
64^0
29^6
•490
■188
24^8
•330
•086
65^8
32^0
•485
•185
26-0
■325
•083
66 8
33^6
•480
•181
27-6
•320
•080
68^0
36^0
•475
•177
29-2
■315
•078
68-9
37^6
•470
•173
30-8
■310
•075
69-8
40-0
•465
•170
320
■305
•073
70^7
4r6
•460
•166
33-6
■300
■071
71^8
43^1
•455
•163
34-8
■295
■068
72^6
45^2
•450
•159
36-4
•290
•066
73-6
47^2
•445
•155
38-4
■285
■064
74-4
488
•440
■152
39^6
■280
■0615
75-5
50^4
•435
•149
40-4
■275
•0593
76^3
52-8
•430
•145
42^0
•270
•0572
77^1
54^5
•425
■142
43^2
•265
■055
78'C
56^0
•420
•138
44^8
■260
•053
78^8
57^6
•415
■136
45-6
•255
■051
79-7
59^2
•410
■132
472
■250
■0492
80^5
60^8
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