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Bureau of Mines Information Circular/1987 
 
 Water-Jet-Assisted Drag Bit Cutting 
 in Medium-Strength Rock 
 
 By J. E. Geier, M. Hood, and E. D. Thimons 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
Information Circular 9164 
 
 Water-Jet-Assisted Drag Bit Cutting 
 in Medium-Strength Rock 
 
 By J. E. Geier, M. Hood, and E. D. Thimons 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 Donald Paul Hodel, Secretary 
 
 BUREAU OF MINES 
 
 David S. Brown, Acting Director 
 

 yf»' 
 
 Library of Congress Cataloging in Publication Data: 
 
 Geier, J. E. (Joe E.) 
 
 Water-jet-assisted drag bit cutting in medium-strength rock. 
 
 (Information circular; 9164) 
 
 Bibliography: p. 9-10 
 
 Supt. of Docs, no.: I 28.27: 9164. 
 
 1. Drag bits (Drilling and boring) 2. Jet cutting. 3. Water-jet. I. Hood, M. (Mike) II. 
 Thimons, Edward D. III. Title. IV. Series: Information circular (United States. Bureau of 
 Mines); 9164. 
 
 TN295.U4 [TN281] 
 
 623 s 
 
 [622'.23] 
 
 87-600274 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Theories of water-jet assistance 2 
 
 Lubrication 3 
 
 Stress corrosion cracking 4 
 
 Suppression of secondary chipping 4 
 
 Hydraulic wedging 6 
 
 Erosion of crushed rock 7 
 
 Conclusions 9 
 
 References 9 
 
 ILLUSTRATIONS 
 
 1. Typical chip formation cycle 4 
 
 2. Results of size analyses of chips formed by cutting in Indiana limestone 
 
 with and without water-jet assistance 5 
 
 3. Samples of broken rock 5 
 
 4. Plots of F c versus distance traveled by bit for typical chip formation 
 
 cycles 7 
 
 5. Force reductions achieved in Indiana limestone for 15-mm-deep cuts 8 
 
 6. Force reductions achieved in Grindleford sandstone for 10-mm-deep cuts 8 
 

 UNIT OF MEASURE ABBREVIATIONS 
 
 USED IN 
 
 THIS REPORT 
 
 cm 2 
 
 square centimeter 
 
 kJ/m 
 
 kilojoule per meter 
 
 cm/s 
 
 centimeter per second 
 
 kN 
 
 kilonewton 
 
 deg 
 
 degree 
 
 m 
 
 meter 
 
 J 
 
 joule 
 
 mm 
 
 millimeter 
 
 J/m 
 
 joule per meter 
 
 MPa 
 
 megapascal 
 
 J/m 2 
 
 joule per square meter 
 
 m/s 
 
 meter per second 
 
 J /ram 
 
 joule per millimeter 
 
 
 
WATER- JET-ASSISTED DRAG BIT CUTTING IN 
 MEDIUM-STRENGTH ROCK 
 
 By J. E. Geier, 1 M. Hood, 2 and E. D. Thimons 3 
 
 ABSTRACT 
 
 This Bureau of Mines report reviews hypotheses for the mechanism by 
 which water jets reduce the specific energy of cutting for drag bits. 
 Several of these hypotheses are shown to be inconsistent with published 
 evidence and new observations. The notion of a limiting cutting speed, 
 above which water jets would be incapable of rendering assistance, is 
 shown to be improbable. The hypothesis that seems most plausible is 
 that specific energy reductions are mainly due to the erosion of crushed 
 materials from in front of the bit. This hypothesis leads to a predic- 
 tion that, for a given rock type and jet-bit configuration at a constant 
 depth of cut, the reduction in bit forces should be a function of dW/dx, 
 the jet energy spent per unit length of cut. This dependence upon dW/dx 
 is shown to be consistent with other workers' results. 
 
 Graduate student, University of California, Berkeley, CA. 
 ^Associate professor, University of California, Berkeley, CA. 
 
 ^Supervisory physical scientist, Pittsburgh Research Center, Bureau of Mines, 
 Pittsburgh, PA. 
 
INTRODUCTION 
 
 The use of water jets to augment mec- 
 hanical rock cutting has advanced greatly 
 since the discovery a decade ago (1_)„ 
 that water jets diminish the forces act- 
 ing upon drag bits cutting in strong 
 rock. Other research workers cutting in 
 a wide variety of rock types and with 
 various geometries of bits unanimously 
 confirmed that substantial benefits to 
 the cutting process, including signifi- 
 cant force reductions, are observed under 
 a broad range of cutting conditions (2- 
 4_) . Following these early, mainly lab- 
 oratory, studies, prototypical systems 
 using this technology were tested, and 
 many of the benefits predicted from the 
 laboratory investigations were realized 
 in the field. In particular, the use of 
 water jets has been demonstrated to en- 
 hance ability to cut harder rocks, reduce 
 bit wear, diminish machine vibrations (_5, 
 6_) , and dramatically reduce the dust pro- 
 duced at the face (_Z_~~_9) . However, ob- 
 servations on the effects of water jets 
 on cutting efficiency have been seemingly 
 contradictory. For example, Fairhurst 
 (10) and Kovscek (_7) claim that, at the 
 high bit velocities (1 to 3 m/s ) typical 
 of rotating-drum machines such as shear- 
 ers, the benefit of reduced bit forces 
 decreases, in some cases to zero. These 
 findings appear to conflict with results 
 from the field trials with roadheaders, 
 where the forces acting were decreased 
 substantially. 
 
 Commercial development of this tech- 
 nology is greatly hindered by this incom- 
 prehension of the water-jet assistance 
 mechanism, both because design parameters 
 such as jet position and pressure cannot 
 be optimized via a quantitative theory 
 and because there is no way of knowing 
 
 which parameters to consider in empirical 
 optimization studies. Hence if the full 
 benefits of water-jet assistance are to 
 be realized, a better physical under- 
 standing of the assistance mechanism is 
 essential. 
 
 Since the mechanism of water-jet assis- 
 tance is still poorly understood, the 
 only guide for the commercial development 
 of this technology is a body of labora- 
 tory data that is deficient in at least 
 three respects: 
 
 1. The data have been gathered from 
 cutting tests in an assortment of rock 
 types, using bits of various geometries 
 and employing a variety of jet-bit con- 
 figurations. These data are not neces- 
 sarily useful for predicting water-jet 
 effectiveness in other rocks, using bits 
 and jet-bit configurations other than the 
 ones specifically tested. 
 
 2. Although laboratory studies have 
 measured the effects of individual pa- 
 rameters such as jet pressure and cutting 
 speed, in general these studies have 
 considered only one or two of these 
 parameters. 
 
 3. For many of these studies, the 
 statistical significance of the results 
 has not been assessed. These deficien- 
 cies arise largely because, in the ab- 
 sence of an established mechanistic 
 theory, so many parameters must be con- 
 sidered that a comprehensive, empirical 
 study is not practical. An improved 
 understanding of the mechanism of water- 
 jet assistance would reduce the number of 
 parameters to be considered and thus 
 facilitate compilation of a more complete 
 and reliable set of data which could be 
 used for design purposes. 
 
 THEORIES OF WATER- JET ASSISTANCE 
 
 In the present paper, the term "water- 
 jet assistance" refers to the degree to 
 which the mechanical portion of cutting 
 
 Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this report. 
 
 energy is reduced when high-pressure (10- 
 to 70-MPa) water jets are used in con- 
 junction with drag bits. Since the mec- 
 hanical portion of the cutting specific 
 energy E s is 
 
 Es = df I F * V dt > 
 
where F is the force applied to the rock 
 by the bit, v is the bit velocity or 
 cutting speed, t is the time, and V is 
 the volume of rock excavated by the bit, 
 this reduction in specific energy may be 
 manifested as reduced components of F, if 
 the volume excavated per unit length of 
 cut, dV/dx, is held constant, or as in- 
 creased cutting rates, if F is held con- 
 stant. In general, F is more directly 
 measurable in the laboratory than is E s , 
 and so it is convenient to speak of jet 
 assistance in terms of the force re- 
 ductions observed when dV/dx is held 
 approximately constant. 
 
 Most researchers on the subject have 
 measured water-jet effectiveness in terms 
 of the difference between bit forces 
 measured for identical bits cutting with 
 and without the aid of water jets. Mor- 
 ris (11 ) suggested that a more realistic 
 measure of water-jet effectiveness would 
 be the rate at which forces increase due 
 to bit wear over the life of the bit. 
 This is true, if the objective of a study 
 is simply to optimize a particular water- 
 jet-assisted cutting system, or to demon- 
 strate the benefits that can be achieved 
 through the use of water jets. However, 
 if the objective is to determine the 
 precise mechanism of water-jet assis- 
 tance, this approach is inappropriate, 
 since it would confound the long-term ef- 
 fects of bit wear with the instantaneous 
 effect of a water jet in reducing E s . 
 
 While the observation that water jets 
 reduce bit wear may explain the greater 
 portion of the force reductions seen over 
 the life of a bit, it does not explain 
 why force reductions are observed when 
 water jets are used to assist bits in any 
 state of wear. Furthermore, as argued by 
 Cook (12) , wear occurs largely due to the 
 heat generated on the bit wearflat by the 
 frictional force, which is proportional 
 to the normal force. Thus if the water 
 jets reduce bit forces for a given depth 
 of cut, wear reductions may occur due to 
 the force reductions as well as the cool- 
 ing mechanism. For these reasons, the 
 present paper examines water-jet assis- 
 tance in terms of the instantaneous ef- 
 fect of a water jet in reducing E s . In 
 the following sections, five hypotheses 
 
 on the phenomenon of water-jet assis- 
 tance are reviewed. 
 
 LUBRICATION 
 
 A notion that persists in the litera- 
 ture is that the force acting upon a bit 
 is reduced by water jets because the 
 water behaves as a lubricant, allowing 
 the bit to slide more easily over the 
 rock surface. This is equivalent to sup- 
 posing that the coefficient of friction 
 between bit and rock is reduced by the 
 water jets. 
 
 This hypothesis is clearly inconsistent 
 with the work of Hood ( 13 ) , who measured 
 the forces acting on a drag bit of nega- 
 tive rake angle, cutting with and without 
 water jets. High-speed photography 
 showed that only the bottom edge (i.e., 
 the wearflat) of the bit was consistently 
 in contact with the rock. Hence the co- 
 efficient of friction y e between rock and 
 bit was the ratio of the cutting force F c 
 (the component of F in the direction of 
 bit travel) to the normal force F n (the 
 component of F normal to the rock sur- 
 face). It was found that the use of 
 water jets reduced F n by a greater 
 percentage than F c , and thus the water 
 jets effectively increased y e in these 
 experiments. 
 
 For a bit of positive rake angle, it is 
 more difficult to isolate the effect of 
 water jets on y e » since for such a bit 
 the front face is generally in contact 
 with the rock, as in figure 1, and hence 
 F c includes both a frictional component 
 and a "plowing" component. However, 
 experiments with such bits (2^ J^4) have 
 consistently shown that F n is reduced by 
 a greater percentage than is F c . Thus 
 there is no reason to suspect that the 
 water jets reduce the coefficient of 
 friction seen by these bits. 
 
 An explanation of the observed increase 
 in y e is that the water jet washes away 
 the crushed rock that forms ahead of the 
 bit, and therefore the bit slides on the 
 rock surface rather than on a thin layer 
 of crushed rock between the rock and the 
 bit. This layer would presumably behave 
 as a lubricating soil beneath the bit, 
 increasing F n but decreasing \i e > by 
 
VAWAV/A 
 
 Intact rock 
 
 B 
 
 WXWA 
 
 Crushing ahead 
 of bit 
 
 Increased 
 crushing 
 
 '^$$£$ffiizi0i3> 
 
 vrnmj^ 
 
 FIGURE 1.— Typical chip formation cycle. A, crushed debris ahead of bit after formation of a major chip; S, crushing of intact 
 rock just ahead of bit as the bit advances; C, formation of minor chips and initiation of fractures; D, propagation of one of these 
 fractures to form the next major chip. 
 
 washing away 
 would remove 
 crease the 
 friction. 
 
 this layer, the water jet 
 the lubricant and thus in- 
 effective coefficient of 
 
 STRESS CORROSION CRACKING 
 
 The possibility that water introduced 
 by the water jets might lower the frac- 
 ture strength of the rock, by chemical at- 
 tack was investigated by Tutluoglu (15). 
 He determined that the maximum crack 
 propagation speed at which this mechanism 
 could influence the energy needed to 
 drive the cracks was about two orders of 
 magnitude less than the crack propagation 
 speeds that occur during drag bit cut- 
 ting. Hence this mechanism, known as 
 stress corrosion cracking, could not play 
 a significant role in water-jet-assisted 
 cutting. 
 
 SUPPRESSION OF SECONDARY CHIPPING 
 
 The process by which a drag bit cuts 
 through rock is known to be cyclic in 
 nature. When water jets are not used, 
 the typical chip formation cycle occurs 
 as shown in figure 1. After a large chip 
 forms, a certain amount of crushed rock 
 remains in the path of the bit. As the 
 bit advances into the intact rock beneath 
 
 this debris, more debris is created by 
 crushing and by the formation of smaller 
 chips, until eventually a second large 
 chip forms. 
 
 Recently the suggestion was made (16) 
 that water jets might improve the ef- 
 ficiency of the cutting process by sup- 
 pressing formation of the smaller rock 
 chips prior to formation of the large 
 primary chips. However, laboratory evi- 
 dence indicates that water jets do not 
 significantly suppress this secondary 
 chip formation. Tutluoglu ( 15 ) performed 
 size analyses on the broken rock col- 
 lected from cuts made with and without 
 water jet assistance, using chisel-type 
 bits in Indiana limestone. The results 
 of these size analyses (fig. 2) indicated 
 that there is no significant difference 
 between the size distributions of broken 
 rock formed by the two processes. This 
 finding is consistent with the authors' 
 qualitative observation that, over a 
 range of jet pressures, flow rates, and 
 cutting speeds, there is no obvious vari- 
 ation in the size distribution of the 
 chips that are formed. Figure 3 shows 
 typical samples of the broken rock that 
 were collected after making 15-mm-deep 
 cuts in Indiana limestone, using the same 
 type of bit as was used by Tutluoglu 
 (15). 
 
S2 | 
 
 LU w- 
 > 2 
 
 < «_ 
 _J o 
 
 O 
 
 00.0 
 
 10.0 - 
 
 .0 
 
 0, 
 
 KEY 
 
 • With jets 
 o Without jets 
 
 0.001 
 
 0.010 
 
 0.100 
 
 .000 
 
 10.00 100.0 
 
 SIZE, mm 
 
 FIGURE 2.— Results of size analyses of chips formed by cutting in Indiana limestone with and without water-jet assistance. 
 
 FIGURE 3.— Samples of broken rock from cuts made A with no water-jet assistance, B with dW/dx = 12 J/mm, and C with 
 dW/dx = 47 J/mm. 
 
 The force exerted by the water jet on 
 the rock surface has also been proposed 
 as a mechanism whereby water jets dimin- 
 ish the cutting efficiency when the cut- 
 ting speed v is sufficiently high that 
 the water jet cannot penetrate the rock. 
 That a pressure on the upper surface of 
 rock increases the energy needed to form 
 chips is a well-known phenomenon in deep 
 drilling, where it is referred to as chip 
 
 holddown. When a water jet strikes the 
 upper surface of a forming rock chip, but 
 does not penetrate the chip, the water 
 jet exerts on the chip a downward force 
 F, 
 
 WJ 
 
 F W j ~ 
 
 a 2 p. 
 
 where p is the jet pressure and a 
 jet diameter. 
 
 is the 
 
Fairhurst (10) suggested that chip 
 holddown due to F w j may be important in 
 water-jet-assisted cutting, at v suffi- 
 ciently high that the water jet cannot 
 penetrate the rock. This mechanism is 
 invoked to explain the observations that, 
 at v > 1.4 m/s or so, water jets appeared 
 to produce an increase in bit forces. 
 However, this mechanism requires that, 
 with increasing v, the peak bit forces 
 should increase by a greater percentage 
 than the mean forces, since this mechan- 
 ism would act during the formation of the 
 large chips, if at all. This is not seen 
 in the published data. Moreover, in 
 those experiments little force reduction 
 was observed at any cutting speed, and it 
 is difficult to say whether the trends 
 cited are significant relative to the 
 unexplained variance apparent in the 
 published graphs of the average cutting 
 and normal forces. 
 
 The present authors speculate that the 
 force reductions seen by Fairhurst (10) 
 may have been low due to the 20° angle 
 between the water jet and the bit face, 
 which may have caused much of the jet 
 energy to miss or be deflected away from 
 the narrow crushed zone ahead of the bit, 
 and to the low levels of dW/dx (4 to 16 
 J/mm). The critical importance of water- 
 jet position with respect to the bit has 
 been established by Tutluoglu (14). 
 
 HYDRAULIC WEDGING 
 
 A fourth hypothesis to explain the re- 
 duction in mechanical specific energy E s 
 when water jets are used is that water 
 pressure aids in driving the cracks that 
 form the major rock chips. This mecha- 
 nism was proposed by Hood ( 13) , who per- 
 formed experiments using a sliding 
 indenter as the cutting tool. A tensile 
 crack chip can be initiated at relatively 
 low indenter loads, but propagation of 
 this crack takes place only after much 
 mechanical energy is spent in crushing 
 the rock beneath the indenter (17). Hood 
 (13) proposed that the reduction in E s 
 that is seen in cutting experiments with 
 water jets occurs because the water jets 
 penetrate the cracks that form at low 
 
 levels of bit force, and then drive these 
 cracks without the need for further 
 crushing and consequent wasting of energy 
 beneath the bit. Quasi-static indenta- 
 tion tests confirmed that chips do form 
 at substantially reduced indenter force 
 levels when water jets are employed. 
 
 This hydraulic wedging mechanism of 
 water-jet assistance, while it may be 
 important for indentation tools such as 
 blunt drag bits and disc cutters, is 
 probably not significant for sharp drag 
 bits. (A sharp drag bit is considered to 
 be one for which F c >>F n , and a blunt drag 
 bit is one for which F c <F n .) With a 
 blunt bit, crushing takes place predomi- 
 nantly beneath the wearflat; indeed, as 
 noted earlier, the leading face of a 
 blunt bit is often not in contact with 
 the rock, in which case crushing can oc- 
 cur only beneath the wearflat. With 
 sharp bits, however, crushing takes place 
 mainly ahead of the leading face of the 
 bit. Observations of the cutting process 
 (fig. 1) show that after a large chip is 
 created, the area of rock confronting the 
 bit is not sufficient to transmit the 
 high force needed to drive a crack and 
 produce a second large chip. Hence as 
 the bit moves forward, it crushes the 
 rock until the area confronting the bit 
 is sufficient for a second large chip to 
 be produced. Much energy must be spent 
 on compressing, shearing, and recrushing 
 the debris thus formed as the bit ad- 
 vances, especially if the debris is con- 
 fined in front of the bit. 
 
 Examination of force versus distance 
 plots for typical chip formation cycles 
 with and without water-jet assistance 
 (fig. 4) shows that the dominant influ- 
 ence of the water jets is to reduce the 
 energy spent on this crushing stage. As 
 evidenced by the shaded areas (which 
 represent the mechanical energy), the 
 energy needed to drive a crack after it 
 begins to propagate is small relative to 
 the energy spent on crushing. Indeed, a 
 simple calculation shows that the energy 
 needed to drive the crack to form a large 
 chip is trivial; therefore, if this were 
 the major influence of the water jet, the 
 reduction in E s would be minuscule. For 
 
KEY 
 
 Energy required to form chip 
 Energy spent in crushing 
 
 role of the hydraulic wedging mechanism 
 must be minor. 
 
 EROSION OF CRUSHED ROCK 
 
 Ld 
 O 
 
 o 
 
 h- 
 f- 
 
 3 
 O 
 
 12 
 10 
 
 8 
 6 
 
 Without jets 
 
 Formation of 
 major chip 
 
 Wmm 
 
 aJHI 
 
 5 6 
 
 DISTANCE, cm 
 
 7 
 
 FIGURE 4.— Plots of Fc versus distance traveled by bit for 
 typical chip formation cycles observed with jets (dW/dx = 19 
 J/mm) and without water-jet assistance. 
 
 example, wet Indiana (Salem) limestone 
 has a fracture energy R = 40 J/m 2 (18). A 
 15-mm-deep cut in this rock produces 
 chips that generally have a surface area 
 measuring less than 50 cm , so the for- 
 mation of a large chip requires an energy 
 expenditure of about 0.2 J. Over a dis- 
 tance of 1 m, about 25 chips of this 
 order of size are formed, so the energy 
 spent in producing these chips is about 5 
 J/m. As reported by Tutluoglu (15), the 
 total energy spent in making such a cut 
 is about 4.5 kJ/m. Thus it may be con- 
 cluded that, for sharp drag bits, the 
 
 The most promising explanation of 
 water-jet assistance is the hypothesis 
 that the water jet removes crushed rock 
 that would otherwise interfere with the 
 advance of the bit into the intact rock. 
 As discussed in the previous section, for 
 a sharp bit the energy consumed in ad- 
 vancing through the crushed rock ahead of 
 the bit constitutes the major portion of 
 the energy spent. Also, from observa- 
 tions of the groove left by the bit, it 
 is clear that some debris is forced 
 underneath the bit, where it produces 
 high normal forces and, in the case of 
 bits of negative rake angle which act 
 mainly as indenters ( 13 ) , acts as a 
 cushion that reduces the stress concen- 
 trations in the rock adjacent to the bit 
 and thus necessitates higher indentation 
 forces. Hence removal of this debris by 
 water jets should improve the efficiency 
 of the cutting process. 
 
 If erosion of crushed rock from ahead 
 of the bit is the main mechanism of 
 water-jet assistance, the improvement in 
 cutting efficiency due to water jets 
 should be controlled by the same param- 
 eters that control how much debris can be 
 eroded from in front of the bit. It can 
 be argued that the rate of debris erosion 
 is a function of the jet power dW/dt that 
 can be delivered to the crushed zone 
 ahead of the bit. Therefore at a given 
 point along the cut, the amount of debris 
 eroded should be a function of the water- 
 jet energy delivered to that point, i.e., 
 the amount of debris eroded per length of 
 cut should depend upon the water-jet 
 energy delivered per length of cut, 
 
 dW dW 1 . pQ 
 
 dx dt v v 
 
 where Q is the jet flow rate. Thus if 
 the main mechanism of water-jet assist- 
 ance is indeed the erosion of crushed 
 rock from in front of the bit, then the 
 
improvement in cutting efficiency due to 
 water jets should depend upon dW/dx. 
 
 Evidence of a dependence on dW/dx was 
 found by Hood (13) , who measured the 
 force reductions achieved with water-jet 
 assistance for chisel-type bits cutting 
 15 mm deep in Indiana limestone, for the 
 matrix of parameter levels: 
 
 Jet pressure p = 10, 20, 35, 50, and 
 
 7 MPa, 
 
 Jet diameter a = 0.65, 0.83, and 1.05 
 mm, 
 
 Cutting speed v = 0.16 and 0.42 m/s. 
 
 Figure 5A shows the percentage mean 
 cutting force reduction R c plotted versus 
 dW/dx, where 
 
 R c = = x 100 pet, 
 Fco 
 
 with F c being the mean cutting force and 
 F co being the mean cutting force for a 
 cut made without water-jet assistance 
 (3.4 kN for these tests). The substan- 
 tial overlap of the 90-pct-conf idence 
 bands for the data indicates that R c is a 
 function only of dW/dx when p and v are 
 varied independently for the case a 
 = 0.65 mm. Comparison with data for the 
 other levels of a indicates that R c is a 
 function only of dW/dx when all three 
 parameters are varied independently. A 
 similar analysis of the percentage mean 
 normal force reduction R n is also a 
 function of dW/dx, as evidenced by figure 
 55. 
 
 Further evidence that jet assistance 
 depends upon dW/dx is provided by data 
 for 10-mm-deep cuts taken at a cutting 
 speed v = 1.10 m/s in Grindleford sand- 
 stone, which are replotted in figure 6 
 from the paper of Fowell (16). In that 
 paper the confidence limits for the data 
 are not given, but if variances similar 
 to those seen by other researchers (14) 
 are assumed, then at least up to the 
 level dW/dx = 10 J/mm there is evidence 
 that R c and R n depend upon dW/dx when p 
 and a are varied independently. 
 
 Further evidence in favor of the 
 erosion hypothesis is provided by 
 
 observations of the amount of finely 
 crushed material remaining in the groove 
 left after passage of the bit. For 15- 
 mm-deep cuts with chisel-type bits in 
 
 o 
 
 I- 
 
 o 
 
 Q 
 Ld 
 Cd 
 
 Ld 
 
 o 
 cc 
 o 
 
 100 
 80 
 60 
 40 
 20 
 
 
 1 1 — i — i — i — i i 1 1 
 
 A, Cutting force reduction 
 
 KEY 
 
 V77A 16 cm/s 
 42 cm/s 
 
 _i i i i i '''i 
 
 10 
 dW/dx, J/mm 
 
 100 
 
 FIGURE 5.— Force reductions achieved in Indiana limestone 
 for 15-mm deep cuts. Shaded regions are the 90-pct- 
 confidence bands for the data. 
 
 I00 
 
 u 
 
 Q. 
 
 o 
 
 r- 
 U 
 Z) 
 Q 
 L±J 
 
 or 
 
 LU 
 
 o 
 q: 
 o 
 
 80 1- 
 
 60 
 40 
 20 
 
 1 — i — i — i — i 1 1 1 1 1 — 
 
 Cutting force reduction 
 
 KEY 
 
 - a 0.6 mm 
 • 0.9 mm 
 ■ 1. 2 mm 
 o |. 5 mm 
 
 dW/dx, J/mm 
 
 FIGURE 6.— Force reductions achieved in Grindleford sand- 
 stone for 10-mm-deep cuts. 
 
Indiana limestone, the authors observed 
 that the amount of this finely crushed 
 material diminished with increasing pres- 
 sure and flow rate and with decreasing 
 cutting speed. For dW/dx < 2 J/mm, the 
 amount of crushed material remaining in 
 the groove was not visibly less than that 
 seen for dry cuts, but as dW/dx = 30 J/mm 
 or so, there was almost no crushed ma- 
 terial remaining in the groove. Begin- 
 ning at roughly 20 J/mm, discontinuous 
 pitting of the bottom of the groove was 
 observed which progressed to continuous 
 slotting for dW/dx > 60 J/mm or so. This 
 is interesting because, as figure 5A 
 
 shows, 20 to 30 J/mm is the range over 
 which the greatest cutting force reduc- 
 tions were obtained. Thus the greatest 
 reductions in cutting force occur when 
 most of the crushed material is flushed 
 away before it can be trapped under the 
 bit, but before the energy density is 
 sufficient to continuously slot the rock. 
 This dependence upon dW/dx might ex- 
 plain why marked improvements in cutting 
 rate have been observed with a roadheader 
 (6_), for which dW/dx was roughly 3 J/mm, 
 while no significant increase in cutting 
 efficiency was observed for a shearer for 
 which dW/dx was about 1 J/mm (7). 
 
 CONCLUSIONS 
 
 A review has been given of file pro- 
 posed mechanisms of water-jet assistance. 
 Of these, lubrication and suppression of 
 secondary chipping are seen to contra- 
 dict previously published evidence, while 
 stress corrosion cracking cannot play a 
 significant role at the crack propagation 
 speeds prevailing in cutting operations. 
 A fourth mechanism, hydraulic wedging, 
 may occur, but its role is probably minor 
 relative to that of a fifth, the erosion 
 of crushed rock from in front of the 
 bit. 
 
 The assumption that erosion of crushed 
 rock is the principal mechanism leads to 
 a prediction that the force reductions R c 
 
 and R n should depend only upon dW/dx, 
 when jet pressure, nozzle orifice diam- 
 eter, and cutting speed are varied in- 
 dependently. This prediction is con- 
 firmed by the available data. 
 
 The observed dependence of R c and R n on 
 dW/dx implies that, at least within the 
 typical range of cutting speeds, there is 
 no limiting speed above which water-jet 
 assistance is fundamentally impossible. 
 No substantial evidence has emerged to 
 indicate otherwise. The hypothesis that 
 the confining force exerted by the water 
 jet significantly hampers cutting at high 
 speeds is not consistent with the avail- 
 able evidence. 
 
 REFERENCES 
 
 1. Hood, M. Cutting Strong Rock With 
 a Drag Bit Assisted by High Pressure 
 Water Jets. J. S. Afr. Inst. Min. and 
 Metall. , v. 77, No. 1, 1977, pp. 79-90. 
 
 2. Ropchan, D. , F. D. Wang, and 
 J. Wolgamott. Application of Water Jet 
 Assisted Drag Bit and Pick Cutter for the 
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 contract ET-77-A-01-9082, CO Sch. Mines). 
 NTIS DOE/ET12463-1, 1980, 230 pp. 
 
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 of Water-Jet-Assisted Drag Bit Cutting of 
 Rocks. Paper in Proceedings, 1st U.S. 
 Water Jet Symposium (Golden, CO, June 
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 1981, pp. II.4.1-II.4.11. 
 
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 10,000 psi Prototype System. Paper in 
 
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 Pittsburgh, PA, May 26-28, 1982. U.S. 
 Dep. Energy, 1982, pp. Cl-Cll. 
 
 5. Morris, A. H. , and M. G. Tomlin. 
 Experience With Boom-Type Roadheaders 
 Equipped With High-Pressure Water-Jet 
 Systems for Roadway Drivage in British 
 Coal Mines. Paper in Water-Jet-Assisted 
 Cutting. Proceedings: Bureau of Mines 
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 June 21, 1984. BuMines IC 9045, 1985, 
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 6. Barham, D. K. , and M. G. Tomlin. 
 High Pressure Water Assisted Rock and 
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 Effect of Varying Water Pressure and 
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