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Effects of diamond grain characteristics on sawblade wear
Int. J. Mach. Tools Manufact. Vol. 33, No. 2, pp. 257-266, 1993. Printed in Great Britain
EFFECTS
0890-6955/9356.00 + .00 © 1993 PergamonPress Ltd
OF DIAMOND GRAIN CHARACTERISTICS SAWBLADE WEAR
ON
S. Y. Luot and Y. S. LIAO~ (Received 17 September 1991; in final form 13 January 1992) Abstract--One-segmented diamond sawblades containing diamond with varying degrees of etch pits were manufactured to study the wear behaviour of diamond grains during the sawing of granite. The worn surfaces of the diamond segments were analysed by a scanning electron microscope (SEM), and the forces acting on the tool and the wear performance were measured. The results showed that a sawblade containing diamond with a small number of pits during sawing displays predominantly microfractured worn particles on the working segment surface, a lower sawing force, and a better wear performance. When a tool containing a greater number of extensive pits on the diamond grains is used, a higher proportion of macrofracture occurred and wear fiats appeared on the worn diamonds. In addition, the sawing forces are relatively higher and the blade performance is poorer.
1. I N T R O D U C T I O N
A CIRCULARsawblade used in the stoneworking industry consists of a steel core with radial slots, and between each slot, diamond impregnated segments are brazed on the periphery. The segments are composed of a large number of diamond particles randomly distributed in a metal matrix. The sawing of stone with a metal bonded diamond tool resembles the cutting operation on a grinding machine and the chips in sawing stone are formed through a brittle fracture process by compressive spalling. In general, sawblade wear can be broadly classified into attrition, diamond fracture and pull-out of diamond particles. Each classification will be influenced by the specification of the diamond segments, the cutting conditions, and the properties of the workpiece material. The performance characteristics of the diamond segments are determined by the diamond type, the size of diamond, the concentration of diamond and the properties of the metal bond. Performance is also dependent on the effect of manufacturing method including cold pressing and sintering, hot pressing, etc. In previous studies, Mamalis et al. [1] have studied sawblade wear on the slotting of granite using a one-segment circular sawblade. The results of their investigation show that the normal and tangential cutting forces are, in general, closely associated with flattened diamond particles or fractured diamond particles on the worn segment surface. Wright and Wapler [2] investigated the progressive wear of individual diamond particles in the sawing process and found that during the sawing process, the particle may be lost due to poor retention, or, if the matrix is eroded to a point where the diamond reaches the working height once again for a short period of time, then, as erosion progresses, the particle is lost. Work related to the cutting of concrete and other hard, brittle materials by diamond sawblade has been reported by several investigators [3-6]. These studies have been mostly devoted to the influence of cutting conditions on the life of the diamond sawblade and blade wear, but the effects of surface characteristics of the diamond grains on sawblade wear during sawing stone has not been studied. In the present investigation, the role of surface topography of the diamond crystals on sawblade wear in the sawing of granite using an experimental one-segmented
t D e p a r t m e n t of Mechanical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C. :~Author to whom correspondence should be addressed. 257
258
S . Y . Luo and Y. S. LIAO
diamond sawblade is discussed in detail. The worn diamond particles, the sawing force and the wear performance are also discussed. 2. EXPERIMENTAL PROCEDURES
2.1. Sawing tests Experiments were carried out on an experimental sawing machine equipped with a variable speed spindle and a variable speed hydraulic drive table. The circular sawblade used in the tests had a diameter of 205 mm and a core thickness of 5 mm. A diamond impregnated segment (size of 40 × 7 x 10.5 mm) was brazed to the periphery of a circular steel core. All tests were conducted with a reciprocating movement. The sawing conditions were: blade surface speed Vs = 30 m s - l ; traverse rate Vw = 1 m min ~; and depth of cut a = 0.2 mm. The cutting fluid was water. Rectangular blocks of Indian imperial red granite with approximate dimension of 150 x 100 x 80 mm were used as the workpiece material. Diamond grain wear in the working segment surface was measured by counting the worn diamond particles using a toolmaker's microscope and an SEM after every 90 cm 2 of area of material sawn. The radial wear was measured at three points per segment by means of a toolmaker's microscope equipped with a vertical illuminator. Performance of the sawblade during sawing granite was evaluated in terms of wear performance (defined as the ratio of area of material sawn to the radial wear of the sawblade, while the area of material sawn is the product of the length and depth of the workpiece being sawn). The vertical and horizontal sawing force components, Fv and Fh, were measured with a quartz piezoelectric type dynamometer ( K I S T L E R 9257 A) connected to an A / D converter and a personal computer. The force signals for every single revolution of the sawblade were recorded to provide detailed information. 2.2. Diamond segments The microstructure of the typical diamond segment is shown in Fig. 1. The specification of the diamond segments for three types of sawblades is given in Table 1. The abrasive used was MBS-70t, a light yellow-green, tough and blocky cubo-octahedral crystal with predominantly smooth faces of M a n - M a d e t diamond widely used for sawing stone. The 40/50 diamond size corresponds to a diameter of 300-425 ~m. The porosity of the metal bond in all segments was controlled to the amount of 4 - 6 % . Figure 2 shows the micrographs of the diamond grains before and after being
@
FI6. 1. Photomicrograph of a diamond segment.
t Trademark of General Electric Company, U.S.A.
Diamond Grain Characteristics and Sawblade Wear
259
TABLE 1. DESCRIPTION OF THE DIAMONDSEGMENTS IN THE SAWBLADES
Sawblade no.
Type of diamond
Mesh size of diamond
Diamond concentration
Bond
Bond hardness, HRB:~
1 2 3
MBS-70t MBS-70 MBS-70
40/50 40/50 40/50
20 20 20
Metal Metal Metal
95 98 106
tTrademark of General Electric Company, U.S.A. ~:Average value.
FIG. 2. Micrographs of (a) virgin diamond, and (b) recovered diamond.
fabricated into the diamond segments. Macro- and micro-observations of the virgin diamond, and the diamond taken from these segments are shown in Figs 3 and 4, respectively. It can be seen that the basic morphology of the diamond in these segments was not affected significantly by the manufacturing environment; the sharp cutting points and edges of these grits were also maintained. The surface characteristics of the diamond grains taken from sawblade 1 (refer to Fig. 4(b)) show the greatest number of extensive etch pits among all sawblades, and looking black in appearance. A greyblack tinged with a little light yellow colour was observed for the diamond recovered from sawblade 2, and the diamond surface showed a relatively smaller number of pits (Fig. 4(c)). Tile diamond taken from sawblade 3 appeared light yellow and its surface contained the smallest number of pits (see Fig. 4(d)) of all the sawblades, but still, the pits can be seen very clearly. 3. RESULTS AND DISCUSSION
3.1. Observations of worn diamond particles A typical worn segment surface obtained after sawing with sawblade 1 is shown in Fig. 5 from which the wear behaviour and distribution of diamond particles in the working surface can be roughly determined. After examining several worn surfaces, the wear of the; diamond particles could be classified as good (without significant wear), polished (flattened), microfractured, macrofractured, and pulled-out. Figure 6 shows the appearanc,z of these classifications. The worn diamond particles of various classifications were counted, and the percentage variation of each classification plotted against the material sawn area for three types of sawblades under identical sawing conditions are shown in Fig. 7. The percentage of particles with no significant wear drops almost exponentially with increasing material sawn area for sawblade 1 (Fig. 7(a)); the curve obtained in the case of sawblade 2 shows less variation. It can be seen that the proportion of good diamond generated when using sawblade 3 is higher than those of sawblades 1 and 2. The difference in the
260
s.Y. Luo and Y. S. LIAO
FIG. 3. Macro-observations of etched features for (a) virgin diamond, (b) diamond in sawblade 1, (c) diamond in sawblade 2, and (d) diamond in sawblade 3. proportion of good diamond during sawing was attributed to diamond integrity. Because the diamonds in sawblade 3 contain a smaller incidence of etching than those of sawblades 1 and 2 (see Fig. 4), the diamond grains in sawblade 3 display better diamond integrity, and are less likely to be broken or fractured during sawing. Hence, sawblade 3 results in a higher proportion of good diamond. The amount of polished or flattened diamond for sawblade 1 increased with increasing sawn area to reach a quasi-constant value, and thereafter decreased until the end of sawing (Fig. 7(b)), while it increased parabolically for sawblade 2. Numerically, the amounts of polished diamond for sawblade 1 and 2 were higher than that in sawblade 3. When there were a sufficiently large number of polished or flattened diamond particles at the sawblade's working segment surface, the sawblade often had a glazed appearance. Figure 7(c) shows the variation of the microfractured diamond with the material sawn area. There was a rather large difference in the percentage of microfractured particles produced for sawblades 1 and 2 during the initial period of sawing. This may be attributed to dissimilar conditions of working surface after dressing, but, in the final period, both blades exhibited similar behaviour. Comparatively, sawblade 3 displayed the highest proportion of microfractured diamond in the final period of sawing. The reason why this occurred is that the diamond particles in sawblade 3 had a better crystal integrity, so that the points of the cutting edges were maintained for a longer period of time during sawing, which in turn caused sawblade 3 to have the highest proportion of microfractured grits. However, the presence of a greater number of surface etch pits on the diamond particles in sawblades 1 and 2 (see Fig. 4(b) and (c)) may have weakened grain integrity; hence, the cutting edges of microfractured grits in sawblades 1 and 2 during sawing lasted for a shorter period of time, therefore, leading to macrofracture of the cutting edges. For this reason, a lower proportion of microfractured particles was observed when using sawblades 1 and 2 than with sawblade 3. It should
Diamond Grain Characteristics and Sawblade Wear {10o} face
261
{ill}face
F16.4. Micro-observationsof etched features of the {100} face and {111} face for (a) virgin diamond, (b) diamond in sawblade 1, (c) diamond in sawblade 2, and (d) diamond in sawblade 3. be noted that microfractured grit is considered to be beneficial as new cutting edges are created so improving cutting ability. The percentage of the macrofractured diamond for sawblade 1 decreased in a nonlinear manner with the sawn area, finally reducing to a low value. Similar variation was observed for sawblade 2 (see Fig. 7(d)). The proportion of macrofractured diamond obtained during sawing was the highest for sawblade 1 and lowest for sawblade 3. The macrofractured diamond formed during sawing only protruded a little above the working surface (see F:ig. 6(d)), which significantly reduced the cutting ability. The percentage of the diamond particles pulled out from the bonding when using
262
S.Y. Luo and Y. S. LIAO
Fie. 5. Micrographof a worn segment surface after sawing. sawblade 3 tended to increase with an increase in sawn area (Fig. 7(e)). The proportion of pulled-out particles was slightly greater for sawblade 3 than for sawblades 1 and 2. Comparing Fig. 4 with Fig. 7(e), it is noted that roughening of the diamond surface texture seemed to improve the bonding strength between the diamond and the matrix bond and, hence, prevented excessive pull-out of diamond particles from the matrix bond. However, the degree of diamond particles pulled out during sawing depends mainly on the interracial bonding at the diamond-bond interface, the wear resistance of the bond and the state of stress induced. Figure 8 shows micrographs of the diamond-matrix interface of the worn surface for three types of sawblades. The worn surfaces at the diamond-matrix interface of sawblades 1 and 2 (Fig. 8(a) and (b)) show good interfacial bonding without cracks. Figure 8(c) shows the occurrence of cracks at the diamond-matrix interface of sawblade 3. In this case, the interfacial bonding is poorer and the diamond particles were more easily pulled out from the matrix bond. Generally speaking, it is desirable to control pull-out of the worn diamond particles since the bond will wear faster and new diamond grits will then be exposed to facilitate constant, efficient cutting; otherwise, the sawblade develops a glazed appearance. From the discussions above, two modes are proposed to describe the wear of diamond in the working segment surface during sawing. First, freshly exposed diamond particles containing a relatively greater area of rough surface become blunt or break down after initial engagement with the workpiece. These grits develop a higher proportion of macrofracture and wear fiats and are pulled out until the bond can no longer support the diamond. The other mode shows that the points of freshly exposed diamond, that contain a relatively smaller number of etch pits, break off as sawing progresses resulting in microfracture of the grits. After repeated microfracture, the particle is finally plucked from its bond. 3.2. Sawing forces and wear performance The sawing force is an important measurement for evaluating the sawability of material. The variation in sawing force and force ratio with sawn area for the up- and down-cutting mode are presented in Fig. 9. The results are related to those in Fig. 7. It can be seen that sawblade 3 seemed to have a lower sawing force than the other sawblades, which can also be implied from Fig. 7. This is primarily due to the existing polished and macrofractured diamond particles in the worn working segment surface. For sawblade 3, only small numbers of polished particles and macrofractured grits were generated during sawing, while a larger proportion of good particles and microfractured diamonds were present as sawing progressed. Hence, sawblade 3 exhibited freer cutting behaviour during sawing. Conversely, sawblades 1 and 2, which had a higher proportion of polished and macrofractured particles (see Fig. 7(b) and (d)) during sawing, resulted in higher sawing forces.
Diamond Grain Characteristics and Sawblade Wear
263
FIG. 6. Micrographs showing the various classificationsof worn diamond particles: (a) good, (b) polished, (c) microfractured, (d) macrofractured, (e) pulled-out. The sawing force and force ratio obtained when cutting in the up direction were higher than those in down-cutting. In up-cutting, the vertical sawing force was about 3-5 times greater than the horizontal force, while the vertical force was about 6-8 times greater than the horizontal force in down-cutting. In grinding 117], the force curves can be divided into three regions as grinding progresses. The normal force increases rapidly at the end of region II and grinding becomes progressively less efficient, hence, the wheel must be dressed at this point to restore its cutting ability. However, the sawblade is, in general, not dressed during sawing, and relies on the self-dressing action to maintain its free cutting ability. Hence, if the sawing force increases rapidly, it causes the sawblade to deviate from its path (especially for a larger diameter blade) and may, in extreme cases, lead to sawblade failure. The wear performance (defined previously in section 2.1) computed for three types of sawbtades is shown in Fig. 10. It indicates that the wear performance is the highest
264
S . Y . L u o and Y. S. LIAO
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FIG. 7. Variation of the various classifications of worn diamond particles with material sawn area. Sawing conditions: Vs = 30 m s -1, Vw = 1 m min -1, a = 0.2 m m . Material: Indian imperial red granite. (a) Good, (b) polished, (c) microfractured, (d) macrofractured, (e) pulled-out.
Diamond Grain Characteristics and Sawblade Wear
265
FIG. 8. SEM observations of the diamond-matrix interface for (a) sawblade 1, (b) sawblade 2, and (c) sawblade 3.
for sawblade 3, while it is the lowest for sawblade 1. Comparing Fig. 10 with Table 1, Fig. 10 with Fig. 3 and Fig. 10 with Fig. 9, it can be shown that the wear performance increases with higher bond hardness. This also indicates that the sawblade that contained diamond particles with fewer etch pits gave a higher wear performance and a lower sawing force. 4.
CONCLUSIONS
Based on the results of this investigation, it can be concluded that the segments that contain diamond particles whose surfaces are completely covered with small welldefined etch pits, lead to diamond particles that are dominantly microfractured with only a small number of macrofractured and polished particles on the worn working surface. This produces a relatively lower sawing force, and the sawblade displays a higher wear pertormance. Conversely, when using the segments containing diamonds whose surfaces contain a HTH 33:2-K
266
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Moteriol sown oreo (era2) FIG. 9. V a r i a t i o n o f sawing forces and force ratios with material sawn area in the (a) up-cutting and ( b )
down-cutting mode (the sawing conditions and material are the same as in Fig. 7).
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FIG. 10. Wear performance for three types of sawblades (the sawing conditions and material are the same as in Fig. 7).
large number of extensive pits, the resulting worn particles occur mainly as microfractured and pulled-out grits while the number of polished and macrofractured grits would be very high as well. The sawing force is characteristically higher and the wear performance o f the sawblade is worse. Acknowledgements--This work was sponsored by the Taiwan Diamond Industrial Company Ltd and General Electric Company. The authors are especially grateful to Dr C. C. Chou, General Manager, and Mr C. L. Chen, Manager, for their help and advice and for permission to publish the work. REFERENCES [1] [2] [3] [4] [5] [6] [7]
A. G. MAMALIS,R. SCHULZE and H. K. TGNSHOFF, Proc. 20th M T D R Conf. (1979). D. N. WRIGHT and H. WAPLER, Ann. C1RP 35, 239 (1986). M. W. BAILEY and G. I. BULLEN, Ind. Diamond Rev. 39, 56 (1979). W. ER1aNGSHAUSEN, Ind. Diamond Rev. 45, 254 (1985). A. B0rrNER, Ind. Diamond Rev. 34, 89 (1974). D. N. WRIGHT and V. B. CASSAm, Ind. Diamond Rev. 45, 84 (1985). S. J. PANDE and G. K. LAL, Int. J. Mach. Tool Des. Res. 16, 179 (1976).
EFFECTS
0890-6955/9356.00 + .00 © 1993 PergamonPress Ltd
OF DIAMOND GRAIN CHARACTERISTICS SAWBLADE WEAR
ON
S. Y. Luot and Y. S. LIAO~ (Received 17 September 1991; in final form 13 January 1992) Abstract--One-segmented diamond sawblades containing diamond with varying degrees of etch pits were manufactured to study the wear behaviour of diamond grains during the sawing of granite. The worn surfaces of the diamond segments were analysed by a scanning electron microscope (SEM), and the forces acting on the tool and the wear performance were measured. The results showed that a sawblade containing diamond with a small number of pits during sawing displays predominantly microfractured worn particles on the working segment surface, a lower sawing force, and a better wear performance. When a tool containing a greater number of extensive pits on the diamond grains is used, a higher proportion of macrofracture occurred and wear fiats appeared on the worn diamonds. In addition, the sawing forces are relatively higher and the blade performance is poorer.
1. I N T R O D U C T I O N
A CIRCULARsawblade used in the stoneworking industry consists of a steel core with radial slots, and between each slot, diamond impregnated segments are brazed on the periphery. The segments are composed of a large number of diamond particles randomly distributed in a metal matrix. The sawing of stone with a metal bonded diamond tool resembles the cutting operation on a grinding machine and the chips in sawing stone are formed through a brittle fracture process by compressive spalling. In general, sawblade wear can be broadly classified into attrition, diamond fracture and pull-out of diamond particles. Each classification will be influenced by the specification of the diamond segments, the cutting conditions, and the properties of the workpiece material. The performance characteristics of the diamond segments are determined by the diamond type, the size of diamond, the concentration of diamond and the properties of the metal bond. Performance is also dependent on the effect of manufacturing method including cold pressing and sintering, hot pressing, etc. In previous studies, Mamalis et al. [1] have studied sawblade wear on the slotting of granite using a one-segment circular sawblade. The results of their investigation show that the normal and tangential cutting forces are, in general, closely associated with flattened diamond particles or fractured diamond particles on the worn segment surface. Wright and Wapler [2] investigated the progressive wear of individual diamond particles in the sawing process and found that during the sawing process, the particle may be lost due to poor retention, or, if the matrix is eroded to a point where the diamond reaches the working height once again for a short period of time, then, as erosion progresses, the particle is lost. Work related to the cutting of concrete and other hard, brittle materials by diamond sawblade has been reported by several investigators [3-6]. These studies have been mostly devoted to the influence of cutting conditions on the life of the diamond sawblade and blade wear, but the effects of surface characteristics of the diamond grains on sawblade wear during sawing stone has not been studied. In the present investigation, the role of surface topography of the diamond crystals on sawblade wear in the sawing of granite using an experimental one-segmented
t D e p a r t m e n t of Mechanical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C. :~Author to whom correspondence should be addressed. 257
258
S . Y . Luo and Y. S. LIAO
diamond sawblade is discussed in detail. The worn diamond particles, the sawing force and the wear performance are also discussed. 2. EXPERIMENTAL PROCEDURES
2.1. Sawing tests Experiments were carried out on an experimental sawing machine equipped with a variable speed spindle and a variable speed hydraulic drive table. The circular sawblade used in the tests had a diameter of 205 mm and a core thickness of 5 mm. A diamond impregnated segment (size of 40 × 7 x 10.5 mm) was brazed to the periphery of a circular steel core. All tests were conducted with a reciprocating movement. The sawing conditions were: blade surface speed Vs = 30 m s - l ; traverse rate Vw = 1 m min ~; and depth of cut a = 0.2 mm. The cutting fluid was water. Rectangular blocks of Indian imperial red granite with approximate dimension of 150 x 100 x 80 mm were used as the workpiece material. Diamond grain wear in the working segment surface was measured by counting the worn diamond particles using a toolmaker's microscope and an SEM after every 90 cm 2 of area of material sawn. The radial wear was measured at three points per segment by means of a toolmaker's microscope equipped with a vertical illuminator. Performance of the sawblade during sawing granite was evaluated in terms of wear performance (defined as the ratio of area of material sawn to the radial wear of the sawblade, while the area of material sawn is the product of the length and depth of the workpiece being sawn). The vertical and horizontal sawing force components, Fv and Fh, were measured with a quartz piezoelectric type dynamometer ( K I S T L E R 9257 A) connected to an A / D converter and a personal computer. The force signals for every single revolution of the sawblade were recorded to provide detailed information. 2.2. Diamond segments The microstructure of the typical diamond segment is shown in Fig. 1. The specification of the diamond segments for three types of sawblades is given in Table 1. The abrasive used was MBS-70t, a light yellow-green, tough and blocky cubo-octahedral crystal with predominantly smooth faces of M a n - M a d e t diamond widely used for sawing stone. The 40/50 diamond size corresponds to a diameter of 300-425 ~m. The porosity of the metal bond in all segments was controlled to the amount of 4 - 6 % . Figure 2 shows the micrographs of the diamond grains before and after being
@
FI6. 1. Photomicrograph of a diamond segment.
t Trademark of General Electric Company, U.S.A.
Diamond Grain Characteristics and Sawblade Wear
259
TABLE 1. DESCRIPTION OF THE DIAMONDSEGMENTS IN THE SAWBLADES
Sawblade no.
Type of diamond
Mesh size of diamond
Diamond concentration
Bond
Bond hardness, HRB:~
1 2 3
MBS-70t MBS-70 MBS-70
40/50 40/50 40/50
20 20 20
Metal Metal Metal
95 98 106
tTrademark of General Electric Company, U.S.A. ~:Average value.
FIG. 2. Micrographs of (a) virgin diamond, and (b) recovered diamond.
fabricated into the diamond segments. Macro- and micro-observations of the virgin diamond, and the diamond taken from these segments are shown in Figs 3 and 4, respectively. It can be seen that the basic morphology of the diamond in these segments was not affected significantly by the manufacturing environment; the sharp cutting points and edges of these grits were also maintained. The surface characteristics of the diamond grains taken from sawblade 1 (refer to Fig. 4(b)) show the greatest number of extensive etch pits among all sawblades, and looking black in appearance. A greyblack tinged with a little light yellow colour was observed for the diamond recovered from sawblade 2, and the diamond surface showed a relatively smaller number of pits (Fig. 4(c)). Tile diamond taken from sawblade 3 appeared light yellow and its surface contained the smallest number of pits (see Fig. 4(d)) of all the sawblades, but still, the pits can be seen very clearly. 3. RESULTS AND DISCUSSION
3.1. Observations of worn diamond particles A typical worn segment surface obtained after sawing with sawblade 1 is shown in Fig. 5 from which the wear behaviour and distribution of diamond particles in the working surface can be roughly determined. After examining several worn surfaces, the wear of the; diamond particles could be classified as good (without significant wear), polished (flattened), microfractured, macrofractured, and pulled-out. Figure 6 shows the appearanc,z of these classifications. The worn diamond particles of various classifications were counted, and the percentage variation of each classification plotted against the material sawn area for three types of sawblades under identical sawing conditions are shown in Fig. 7. The percentage of particles with no significant wear drops almost exponentially with increasing material sawn area for sawblade 1 (Fig. 7(a)); the curve obtained in the case of sawblade 2 shows less variation. It can be seen that the proportion of good diamond generated when using sawblade 3 is higher than those of sawblades 1 and 2. The difference in the
260
s.Y. Luo and Y. S. LIAO
FIG. 3. Macro-observations of etched features for (a) virgin diamond, (b) diamond in sawblade 1, (c) diamond in sawblade 2, and (d) diamond in sawblade 3. proportion of good diamond during sawing was attributed to diamond integrity. Because the diamonds in sawblade 3 contain a smaller incidence of etching than those of sawblades 1 and 2 (see Fig. 4), the diamond grains in sawblade 3 display better diamond integrity, and are less likely to be broken or fractured during sawing. Hence, sawblade 3 results in a higher proportion of good diamond. The amount of polished or flattened diamond for sawblade 1 increased with increasing sawn area to reach a quasi-constant value, and thereafter decreased until the end of sawing (Fig. 7(b)), while it increased parabolically for sawblade 2. Numerically, the amounts of polished diamond for sawblade 1 and 2 were higher than that in sawblade 3. When there were a sufficiently large number of polished or flattened diamond particles at the sawblade's working segment surface, the sawblade often had a glazed appearance. Figure 7(c) shows the variation of the microfractured diamond with the material sawn area. There was a rather large difference in the percentage of microfractured particles produced for sawblades 1 and 2 during the initial period of sawing. This may be attributed to dissimilar conditions of working surface after dressing, but, in the final period, both blades exhibited similar behaviour. Comparatively, sawblade 3 displayed the highest proportion of microfractured diamond in the final period of sawing. The reason why this occurred is that the diamond particles in sawblade 3 had a better crystal integrity, so that the points of the cutting edges were maintained for a longer period of time during sawing, which in turn caused sawblade 3 to have the highest proportion of microfractured grits. However, the presence of a greater number of surface etch pits on the diamond particles in sawblades 1 and 2 (see Fig. 4(b) and (c)) may have weakened grain integrity; hence, the cutting edges of microfractured grits in sawblades 1 and 2 during sawing lasted for a shorter period of time, therefore, leading to macrofracture of the cutting edges. For this reason, a lower proportion of microfractured particles was observed when using sawblades 1 and 2 than with sawblade 3. It should
Diamond Grain Characteristics and Sawblade Wear {10o} face
261
{ill}face
F16.4. Micro-observationsof etched features of the {100} face and {111} face for (a) virgin diamond, (b) diamond in sawblade 1, (c) diamond in sawblade 2, and (d) diamond in sawblade 3. be noted that microfractured grit is considered to be beneficial as new cutting edges are created so improving cutting ability. The percentage of the macrofractured diamond for sawblade 1 decreased in a nonlinear manner with the sawn area, finally reducing to a low value. Similar variation was observed for sawblade 2 (see Fig. 7(d)). The proportion of macrofractured diamond obtained during sawing was the highest for sawblade 1 and lowest for sawblade 3. The macrofractured diamond formed during sawing only protruded a little above the working surface (see F:ig. 6(d)), which significantly reduced the cutting ability. The percentage of the diamond particles pulled out from the bonding when using
262
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Fie. 5. Micrographof a worn segment surface after sawing. sawblade 3 tended to increase with an increase in sawn area (Fig. 7(e)). The proportion of pulled-out particles was slightly greater for sawblade 3 than for sawblades 1 and 2. Comparing Fig. 4 with Fig. 7(e), it is noted that roughening of the diamond surface texture seemed to improve the bonding strength between the diamond and the matrix bond and, hence, prevented excessive pull-out of diamond particles from the matrix bond. However, the degree of diamond particles pulled out during sawing depends mainly on the interracial bonding at the diamond-bond interface, the wear resistance of the bond and the state of stress induced. Figure 8 shows micrographs of the diamond-matrix interface of the worn surface for three types of sawblades. The worn surfaces at the diamond-matrix interface of sawblades 1 and 2 (Fig. 8(a) and (b)) show good interfacial bonding without cracks. Figure 8(c) shows the occurrence of cracks at the diamond-matrix interface of sawblade 3. In this case, the interfacial bonding is poorer and the diamond particles were more easily pulled out from the matrix bond. Generally speaking, it is desirable to control pull-out of the worn diamond particles since the bond will wear faster and new diamond grits will then be exposed to facilitate constant, efficient cutting; otherwise, the sawblade develops a glazed appearance. From the discussions above, two modes are proposed to describe the wear of diamond in the working segment surface during sawing. First, freshly exposed diamond particles containing a relatively greater area of rough surface become blunt or break down after initial engagement with the workpiece. These grits develop a higher proportion of macrofracture and wear fiats and are pulled out until the bond can no longer support the diamond. The other mode shows that the points of freshly exposed diamond, that contain a relatively smaller number of etch pits, break off as sawing progresses resulting in microfracture of the grits. After repeated microfracture, the particle is finally plucked from its bond. 3.2. Sawing forces and wear performance The sawing force is an important measurement for evaluating the sawability of material. The variation in sawing force and force ratio with sawn area for the up- and down-cutting mode are presented in Fig. 9. The results are related to those in Fig. 7. It can be seen that sawblade 3 seemed to have a lower sawing force than the other sawblades, which can also be implied from Fig. 7. This is primarily due to the existing polished and macrofractured diamond particles in the worn working segment surface. For sawblade 3, only small numbers of polished particles and macrofractured grits were generated during sawing, while a larger proportion of good particles and microfractured diamonds were present as sawing progressed. Hence, sawblade 3 exhibited freer cutting behaviour during sawing. Conversely, sawblades 1 and 2, which had a higher proportion of polished and macrofractured particles (see Fig. 7(b) and (d)) during sawing, resulted in higher sawing forces.
Diamond Grain Characteristics and Sawblade Wear
263
FIG. 6. Micrographs showing the various classificationsof worn diamond particles: (a) good, (b) polished, (c) microfractured, (d) macrofractured, (e) pulled-out. The sawing force and force ratio obtained when cutting in the up direction were higher than those in down-cutting. In up-cutting, the vertical sawing force was about 3-5 times greater than the horizontal force, while the vertical force was about 6-8 times greater than the horizontal force in down-cutting. In grinding 117], the force curves can be divided into three regions as grinding progresses. The normal force increases rapidly at the end of region II and grinding becomes progressively less efficient, hence, the wheel must be dressed at this point to restore its cutting ability. However, the sawblade is, in general, not dressed during sawing, and relies on the self-dressing action to maintain its free cutting ability. Hence, if the sawing force increases rapidly, it causes the sawblade to deviate from its path (especially for a larger diameter blade) and may, in extreme cases, lead to sawblade failure. The wear performance (defined previously in section 2.1) computed for three types of sawbtades is shown in Fig. 10. It indicates that the wear performance is the highest
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Diamond Grain Characteristics and Sawblade Wear
265
FIG. 8. SEM observations of the diamond-matrix interface for (a) sawblade 1, (b) sawblade 2, and (c) sawblade 3.
for sawblade 3, while it is the lowest for sawblade 1. Comparing Fig. 10 with Table 1, Fig. 10 with Fig. 3 and Fig. 10 with Fig. 9, it can be shown that the wear performance increases with higher bond hardness. This also indicates that the sawblade that contained diamond particles with fewer etch pits gave a higher wear performance and a lower sawing force. 4.
CONCLUSIONS
Based on the results of this investigation, it can be concluded that the segments that contain diamond particles whose surfaces are completely covered with small welldefined etch pits, lead to diamond particles that are dominantly microfractured with only a small number of macrofractured and polished particles on the worn working surface. This produces a relatively lower sawing force, and the sawblade displays a higher wear pertormance. Conversely, when using the segments containing diamonds whose surfaces contain a HTH 33:2-K
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large number of extensive pits, the resulting worn particles occur mainly as microfractured and pulled-out grits while the number of polished and macrofractured grits would be very high as well. The sawing force is characteristically higher and the wear performance o f the sawblade is worse. Acknowledgements--This work was sponsored by the Taiwan Diamond Industrial Company Ltd and General Electric Company. The authors are especially grateful to Dr C. C. Chou, General Manager, and Mr C. L. Chen, Manager, for their help and advice and for permission to publish the work. REFERENCES [1] [2] [3] [4] [5] [6] [7]
A. G. MAMALIS,R. SCHULZE and H. K. TGNSHOFF, Proc. 20th M T D R Conf. (1979). D. N. WRIGHT and H. WAPLER, Ann. C1RP 35, 239 (1986). M. W. BAILEY and G. I. BULLEN, Ind. Diamond Rev. 39, 56 (1979). W. ER1aNGSHAUSEN, Ind. Diamond Rev. 45, 254 (1985). A. B0rrNER, Ind. Diamond Rev. 34, 89 (1974). D. N. WRIGHT and V. B. CASSAm, Ind. Diamond Rev. 45, 84 (1985). S. J. PANDE and G. K. LAL, Int. J. Mach. Tool Des. Res. 16, 179 (1976).