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On Performance of Brazed Single-Layer CBN Wheel
On Performance of Brazed Single-Layer CBN Wheel A. K. Chattopadhyay (2),H. E. Hintermann (1) Received on December 16,1993
SUMkL4RY: In brazed single layer CBN wheel, high grit exposure but with strong grit-bond adhesion is known to be the most distinguished feature. The present investigation has, however, revealed that in spite of having a large grit protrusion, the consistency of performance of such a wheel may be affected by the manner how the density as well as the uniformity of grit distribution are regulated in the monolayer configuration. This is particularly true when a chip accommodation problem leading to wheei loading exists. Experiments with monolayer CBN wheel varying in grit density and under various grinding conditions have further revealed that pre-brazing placement and fixation of CBN grit on the tool substrate with an even distribution and adequate inter-grit spacing may remarkably enhance the flexibility of application of a high protrusion single-layer CBN wheel. KEY WORDS: Grinding, CBN, Loading 1.
Introduction
The remarkable properties of CBN such as high wear resistance andexcellent cutting edge stability are being used creatively to fabricate tools having different geomemes just with a single-layer of CBN grit brazed to a metal substrate. In comparison to a galvanically bonded tool, better grit retention, bond uniformity and higher crystal exposure are claimed to be the most distinguished features of a brazed tool (1,2,3).Even in such a brazed tool, the amount of crystal exposure is determined by the inherent grit size and accordingly the performance of the tool may be influenced by the size of the grit used (4).Brazed CBN tool can outperform a galvanically bonded CBN tool when a high bond level in the latter has an overriding influence leading to severe wheel loading (5). However, it has been revealed that control of the bond level and maintenance of its uniformity which substantially determine the initial topography of the working surface of the brazed tool depend on various constructional as well as process parameters of fabrication (6.7,8). Unlike a plated wheel where usually the grit concentration is very high, the working surface of a brazed tool may have a varying density (1,2,3).
Performance of a monolayer CBN tool even with high crystal protrusions can be strongly influenced not only by the density of the grit but also by its distribution pattern. The aim of the present work was to evaluate the dependance of performance of the brazed monolayer CBN wheel on the density of grit and the distribution pattern with particular reference to its capability to ensure free cutting action while grinding hardened and unhardened 100Cr6 steels under vanous grinding conditions. 2.
dynamometer from Vibro-Meter AG of Switzerland connected to a two channel recorder. SEM observations were made on the wheel surface before and after the grinding tests. Representative chip samples of the work materials were collected during grinding and were also observed under SEM. The work materials were as follows :hardened (hardness : 65.5 HRC) and unhardened (28.5 HRC) 100Cr6 steels. The workpieces were round bars of 12mm diameter, the end face of which was ground. The power of the machine was 1 KW. The grinding conditions were as follows :wheel speed, 30 m/s; table speed, 2 m/min; infeed, 10,20,30 and 40 pm; environment, dry and wet (5% soluble oil : Castrol Syntillo
I Deposition of a layer of1 1- 1 a braze material o n a metallurgically compatible substrate
surface
I
H I
Placement and temporary fixation of C8N grit in a monolayer
Control of braze layer thickness and its uniformity I
t
I
configuration
Brazing
Control of density of gri and distribution pattern
Determination of optimum brazing cycl
i Brazed single layer CBN tool
Experimental methods and conditions
Brazed monolayer CBN cup shaped grinding wheels were usedon a tool and cutter grinder to conduct experiments in the face grinding mode. The CBN grit used were of three sizes viz.Bl26 (125/106 pmmesh),B251(250/212pm mesh) and B301 (300/250 pm mesh). The diameter and the width of the working surface of the wheels were 75 mm and 5 mm, respectively. The normal and the tangential forces developed during grinding were recorded, using a two dimensional LVDT Annals of the ClRP Vol. 43/1/1994
Fig.1 Various steps in the manufacture of a brazed single layer CBNtool. DC 282). 3.
Results
Various steps of manufacture of a brazed single layer CBN tool are schematically shown in Fig.1. It can be well realized that the manner in which the abrasive crystals are set on the tool surface before brazing, determines the final
313
Wheel : Type A (distribution with high grit density)
(a) (b) Fig.4 Surface topography of wheel of Type A with B 126 grit size after dry grinding of (a) unhardened and (b) hardened 100Cr6 steels with 10 pm infeed for 60 passes.
Wheel : Type B (non-uniforni distribution with low grit density)
Wheel : Type C (even grit distribution) Fig.? Working surface of brazed single layer CBN wheels.
0
z o a*
c
Hardened (dry)
Unhardened (dry)
30 40 50 60 Number of grinding passes Fig.3 Grinding forces versus number of grinding passes for wheel of Type A in dry and wet grinding of hardened and unhardened 100Cr6 steels. 1
10
+
20
outcome so far as grit density and distribution pattern are concerned. Figure 2 shows the working surface of different CBN abrasive wheels before use. The arbitrary designation as well as the actual grit size ofeach tool are also indicated in the same figure. The wheel ofType Ain both B 126 and B25 1 grit sizes can be seenin amonolayer configuration but with a high grit density though the density was higher for finer grit size. 314
Fig.5 Types of chip produced during dry grinding of (a) unhardened and (b) hardened 100Cr6 steels with 10 pm infeed. The CBN grit can be seen to be virtually fully protruded over the tool face. For grit size B251 the grit protrusion was measured as 190-2IOpm while the finer grit (B126) showed 80-100 pm crystal exposure. The wheel of Type B may be characterized by the fact that the grit was distributed in a random manner to reduce its concentration. It is evident from Fig.2 that the average grit density of the wheel ofType B was less than that of the wheel of Type A. But, the fact which draws immediate attention was the existence of closely spaced grains forming acluster. This was true for both B126 and B251 grit sizes. From the investigation it was further established that the grit clustering was almost unavoidable when abrasive crystals were placed in arandommanner and the problem became aggravated with decreasing grit size. The same Fig.2 also shows the surface of wheels of Type C. In this case besides B126 and B251 size grit, CBN grit with B301 size was also included. With B301 grit size the crystal protrusion was found to be 235-260pm. In the wheel ofType C CBN particles were distributed more or less in an even manner. By this way of grit placement not only grit density can be regulated but also, the risk of grit clustering can be avoided. Figure 3 shows normal force and tangential force development by wheel of Type A (grit size :B126) during grinding of hardened and unhardened 100Cr6 steel for 60 grinding passes under dry and wet conditions. The most noticeable fact is that during dry grinding of unhardened steel, even after several passes of spark-in grinding, the tool did not reach asteady state; rather grinding forces continued to escalate and at the end of 60 passes quite high values of forces were
' O0 [
CBN wheel : Type A
P
Grit size : 8251
80
_+_
*-
work surface. Similarly, resistance to penetration even in the unhardened steel increased leading to rise of normal thrust force. Addition of cutting fluid immediately changed the situation for unhardened steel. Cleaning and lubricating action of the fluid can be recognized from the grinding forces which were reduced and became practically constant after a few passes of spark-in grinding as can be seen from Fig.3. Wet grinding also reduced forces for hardened steel though benefit of dry grinding of the same in allowing the grit to fracture in micro-level thus, maintaining its sharpness for long time has been reported (10). With 20 pm infeed the wheel also showed loading problem even in wet grinding of unhardened steel. The wheel showed its incapability too for dry grinding of hardened steel at 20pm infeed only because of chip accommodation problem. However, with application ofcutting fluid, this problem was minimized to agreatextent.
Hardened (dry) Unhardened (dry)
CBN wheel ofTypeA(grit size : B251) with 10 pm infeed 0
10
20
30
40
50
Infeed, pm
Fig.6 Grinding forces versus infeed for wheel of Type A with B251 grit size in dry and wet grinding of hardened and unhardened 100Cr6 steels. recorded. It can further be seen that for unhardened steel not only the tangential force was high but the normal force was also excessively large in comparison to that for hardened steel. Examination of the wheel surface at the end of 60 passes
(b) (a) Fig.7 Surface topography of wheel of Type A with B251 grit size after dry grinding of (a) unhardened and (b) hardened 100Cr6 steels with 20 pm infeed for 60 passes. clearly revealed large scale loading in dry grinding of the unhardened steel as shown in Fig.4(a). With lOpm infeed in dry grinding of hardened steel such wheel loading was totally absent as can be seen From Fig.4(b). However, the same figure indicates that the possibility of small scale loading may not be ruled out. The cause of severe wheel loading with soft steel can be better understood if one observes the types of chip produced shown in Fig.5. The chips produced out of the ductile steel were ribbon-like, mostly continuous and longer than those produced by the hardened steel. It was difficult to accommodate long chips in the storage space available ahead of the active grit and loading started in the initial phase of grinding. An earlier study (9)also has shown that it was easier to accommodate the same volume of smaller particles than large particles in a given accommodation volume. The loaded particles unfavourably changed the topography of the wheel surface which in turn increased tangential force because of rubbing with the
showed steady behaviour in dry grinding of both hardened and unhardened steels without any loading problem. But, with 20 pm infeed just after 60 grinding passes the wheel developed relatively high forces in comparison to what was recorded with 10 pm infeed. This is particularly true for unhardenedsteel asshowninFig.6.At thisstageexamination of the wheel surface undoubtedly established occurance of wheel loading as demonstrated in Fig.7(a). The wheel of Type Awith larger gritsize hadahigherprotrusion than that with a smaller grit size. The same figure shows that because of inadequate grit spacing long chips could not flow freely, instead were squeezed and entrapped in between grits followed by material building up with the number of grinding passes. The built up material started rubbing over the work surface increasing forces to high values. Figure 7(b) confirms that under identical conditions the CBN wheel was not free either from the material accumulation problem while grinding hardened steel. An attempt to increase infeed to 30 pm ended up with quick failure of the wheel due to severe wheel loading. It has been found that under a given grinding condition abrupt or disproportionate rise of forces at the early stage of grinding was always associated with chip accommodation problem. During wet grinding, the wheel showed improved performance with a reduction of the force, but limitation of the tool was again established at 40 pm infeed in grinding unhardened steel. Disproportionate rise of both normal and tangential forces can be seen in Fig.6 suggesting acute chip flow problem leading to wheel loading. In wet environment reduction of grinding forces could be observed for hardened steel as indicated in the same figure. In addition,grinding forces didnot show an abruptincreaseeven with 40 pm infeed suggesting considerable minimization of the chip storage problem. Wheel of Type B (grit size : B126) also showed its limitation in resisting wheel loading. This was again true in dry grinding of unhardened steel at 10 pm infeed. While at this infeed the tool worked satisfactorily in wet environment, it again faced chip storage problem only at 20 pm as revealed in Fig.8(a). An attempt to conduct grinding of hardened steel in dry environment did not succeed either. Figure 8(b) undoubtedly demonstrates that the chips, instead
315
60
a
1
CBN wheel : Type C Environment : Dry
-
8126
c
c 0' Fig.8 Surface topography of wheels ofType B (a) with B 126 grit size after wet grinding of unhardened 1 0 0 0 6 steel, (b) with B126 grit size after dry grinding of hardened 100Cr6 steel and (c) with B251 grit size after dry grinding of Unhardened 100Cr6 steel with 20 pm infeed for 60passes.
of being
cleared built up on the wheel surface. Interestingly, SEM micrographs in Fig.S(a) and (b) suggest that chip material built up in the region of grit cluster where grit density was high. Similar loading has also been found while grinding steels, varying in composition and hardness, at the clusters of abrasive grains in resin bonded CBN wheel (1 1). The same pictures also show that locations on the wheel surface with wide grit spacing did notaccumulate any chip
0
10
20 30 Infeed, urn
40
50
Fig.10 Grinding forces versus infeed for wheel of Type C with different grit sizes in dry grinding of unhardened 1 0 0 0 6 steel. that was recorded when using 10pm infeed. With introduction of grinding fluid forces were reduced and went up quite steadily with infeed upto 30 pm; at 40 pm infeed high rise of the same can be noted. This may be explained by enhanced interactions at the chip bond, and chip-work interfaces at higher infeed.
50 CBN wheel : Type B 40 Grit size : 8251 2 30 L
g 20 z:1 0 z
0
- 40
Q)
0
c -
30
-
.L 20 c
Fig.11 SEM micrograph showing resistance of wheel ofType C with B301 grit size to loading during dry grinding of unhardened 100Cr6 steel with 40 pm infeed for 60 passes.
%lo
K
2 0 0
10
20 30 Infeed, pm
40
50
Fig.9 Grinding forces versus infeed for wheel of Type B with B25 1 grit size in dry and wet grinding of unhardened 100Cr6 steel. material. It was also noted that the chip accumulation problem was significantly minimized for hardened steel even with 20pm infeed just by introduction ofgrinding fluid. CBN wheel of Type B with grit size B25 1 was also not free from wheel loading at grit cluster in dry grinding of soft steel as shown in Fig.S(c). Grinding was conducted with 20 prn infeed in dry environment only for 60 passes. Figure 9 again points to the fact that such loading resulted in a high rise of grinding forces when using20 pm infeed incomparison with 316
CBN wheel of Type C (grit size : B126) behaved quite steadily at 10 and 20 pm infeeds in dry grinding of unhardenedsteel unlike wheels ofType AandType B. Evenly distributed grit in the wheel showed a clear advantage in terms of easy chip accommodation. However, at 30 pm infeed there was a steep rise of grinding forces in comparison to what was recorded at lower infeeds as is evident in Fig.10. The same figure again shows how the limitation of small size grit could be overcome through use of large size grit. The wheel of Type C with grit sizes B25 1 and B301 could grind the unhardened steel at 30pm infeed in dry condition without any loading problem. The infeed could have been increased to 40 pm safely. Grinding forces increased in the very usual way with the infeed and there was no disproportionate escalation of the same unlike that always
happened with a loaded wheel. SEM micrograph in Fig.11 shows that the Wheel of Type C with B301 grit size was free of any kind of loading after dry grinding of unhardened steel with 40 pm infeed for 60 passes. At low infeeds, virtually there was no difference in the level of ginding forces developed by wheels with grit size B25 1 and B301, respectively. However, such difference in magnitude of forces is easily noticed at high infeeds. The difference in the magnitude of grinding forces may primarily be related to a smaller number of cutting points in CBN wheel with B301 grit size than that in the wheel with B251 grit size which interacted with the workpiece. However, apart from interaction at the grit-work interface leading to chip formation. interactions at chipbond, chip-work interfaces which also contribute to the generation of grinding forces in different degrees should be considered. The force components generated out of these interactions could be significantly high with the increase of the infeed. CBN wheel of Type C with grit size B301 had wider spacings and larger grit protrusions than the wheel with grit size B251 and this could have a positive influence in reducing interactions at the chip-bond and chip-work interfaces thereby decreasing bulk values of normal and tangential forces for CBN wheel with grit size B301:
4.
Conclusions
1. In comparison to a galvanically bonded single layer CBN tool, a brazed bonded counterpart may offer high crystal exposure but with a strong grit-bond adhesion. However, such high grit protrusion may not necessarily solve the chip accommodation problem when the brazed tool has a high grit density typical that of an electroplated tool. 2. Grinding in wet environment could improve the situation but, such improvement strongly depended on work material characteristics, material removal rate and size of CBN grain used in the tool. 3. Through random grit distribution, the density could not doubt be reduced rather easily but, this way of grit placement also resulted in grit clustering on the wheel surface which could again become the source of wheel loading. 4. Performance ofa brazedCBN wheelcould be remarkably improved by placing the grains in a regular manner. A brazed wheel with high grit protrusion and wide spacing exhibited remarkable resistance to loading even during dry grinding of a long chiping material like unhardened 100Cr6 steel. Such an aggressive tool could beequally efficient in dry and wet grinding of materials widely varying in terms of hardness, strength and ductility. 5. It is necessary to develop in industrial scale an economic process of grit placement in a regular manner on a tool surface having complex geometry possibly using handling and or setting processes as are commonly used in the watch industry.
5.
( 2 ) Peterman, L.M., 1985. Diamond Tooling in Non-Metallics, Superabrasive’85 Conference, Chicago, Illinois, 12/121. (3) AbrasiveTechnology Inc., 1980,Technical Bulletin on PBS Products. (4) Chattopadhyay, A.K., Chollet, L., Hintermann, H.E., 1990, On Performance of Chemically Bonded Single Layer CBN Grinding Wheel, Annals CIRP, 39/1:309-312. (5) Chattopadhyay, A.K., Chollet, L., Hintermann, H.E., 1992, Improved 1Monolayer CBN Wheel for Load Free Grinding, International Journal of Machine Tools and Manufacture, 3 3 4 5 7 1-581. (6) Chattopadhyay, A.K., Chollet, L., Hintermann, H.E., 1991, Experimental Investigation on Induction Brazing of Diamond with Ni-Cr Hardfacing Alloy under Argon Atmosphere, Journal of Materials Science, 265093-5100. (7) Chattopadhyay, A.K., Chollet, L., Hintermann, H.E., 1991, On Performance of Brazed Bonded Monolayer Diamond Grinding Wheel, Annals CIRP, 40/1:347-350. (8) Hintermann, H.E., Chattopadhyay, A.K., 1992, New Generation Superabrasive Tool with Monolayer Configuration, Diamond and Related Materials, 1:1131-1143. (9) Pai, D.M.,Ratterman, E.,Shaw, M.C., 1989, Grinding Swarf, Wear, 13k329-339. (10) Stokes,R.J., Valentine,T.J., 1984, Wear Mechanisms of ABN Abrasive, Industrial Diamond Review, 1/84:34-44. (1 1) Pecherer, E., Malkin, S., 1984, Grinding of Steels with Cubic Boron Nitride, Annals CRP, 33/1:211-216.
References
(1) Norton Company, 1990, Technical Information on MSL tools.
317
SUMkL4RY: In brazed single layer CBN wheel, high grit exposure but with strong grit-bond adhesion is known to be the most distinguished feature. The present investigation has, however, revealed that in spite of having a large grit protrusion, the consistency of performance of such a wheel may be affected by the manner how the density as well as the uniformity of grit distribution are regulated in the monolayer configuration. This is particularly true when a chip accommodation problem leading to wheei loading exists. Experiments with monolayer CBN wheel varying in grit density and under various grinding conditions have further revealed that pre-brazing placement and fixation of CBN grit on the tool substrate with an even distribution and adequate inter-grit spacing may remarkably enhance the flexibility of application of a high protrusion single-layer CBN wheel. KEY WORDS: Grinding, CBN, Loading 1.
Introduction
The remarkable properties of CBN such as high wear resistance andexcellent cutting edge stability are being used creatively to fabricate tools having different geomemes just with a single-layer of CBN grit brazed to a metal substrate. In comparison to a galvanically bonded tool, better grit retention, bond uniformity and higher crystal exposure are claimed to be the most distinguished features of a brazed tool (1,2,3).Even in such a brazed tool, the amount of crystal exposure is determined by the inherent grit size and accordingly the performance of the tool may be influenced by the size of the grit used (4).Brazed CBN tool can outperform a galvanically bonded CBN tool when a high bond level in the latter has an overriding influence leading to severe wheel loading (5). However, it has been revealed that control of the bond level and maintenance of its uniformity which substantially determine the initial topography of the working surface of the brazed tool depend on various constructional as well as process parameters of fabrication (6.7,8). Unlike a plated wheel where usually the grit concentration is very high, the working surface of a brazed tool may have a varying density (1,2,3).
Performance of a monolayer CBN tool even with high crystal protrusions can be strongly influenced not only by the density of the grit but also by its distribution pattern. The aim of the present work was to evaluate the dependance of performance of the brazed monolayer CBN wheel on the density of grit and the distribution pattern with particular reference to its capability to ensure free cutting action while grinding hardened and unhardened 100Cr6 steels under vanous grinding conditions. 2.
dynamometer from Vibro-Meter AG of Switzerland connected to a two channel recorder. SEM observations were made on the wheel surface before and after the grinding tests. Representative chip samples of the work materials were collected during grinding and were also observed under SEM. The work materials were as follows :hardened (hardness : 65.5 HRC) and unhardened (28.5 HRC) 100Cr6 steels. The workpieces were round bars of 12mm diameter, the end face of which was ground. The power of the machine was 1 KW. The grinding conditions were as follows :wheel speed, 30 m/s; table speed, 2 m/min; infeed, 10,20,30 and 40 pm; environment, dry and wet (5% soluble oil : Castrol Syntillo
I Deposition of a layer of1 1- 1 a braze material o n a metallurgically compatible substrate
surface
I
H I
Placement and temporary fixation of C8N grit in a monolayer
Control of braze layer thickness and its uniformity I
t
I
configuration
Brazing
Control of density of gri and distribution pattern
Determination of optimum brazing cycl
i Brazed single layer CBN tool
Experimental methods and conditions
Brazed monolayer CBN cup shaped grinding wheels were usedon a tool and cutter grinder to conduct experiments in the face grinding mode. The CBN grit used were of three sizes viz.Bl26 (125/106 pmmesh),B251(250/212pm mesh) and B301 (300/250 pm mesh). The diameter and the width of the working surface of the wheels were 75 mm and 5 mm, respectively. The normal and the tangential forces developed during grinding were recorded, using a two dimensional LVDT Annals of the ClRP Vol. 43/1/1994
Fig.1 Various steps in the manufacture of a brazed single layer CBNtool. DC 282). 3.
Results
Various steps of manufacture of a brazed single layer CBN tool are schematically shown in Fig.1. It can be well realized that the manner in which the abrasive crystals are set on the tool surface before brazing, determines the final
313
Wheel : Type A (distribution with high grit density)
(a) (b) Fig.4 Surface topography of wheel of Type A with B 126 grit size after dry grinding of (a) unhardened and (b) hardened 100Cr6 steels with 10 pm infeed for 60 passes.
Wheel : Type B (non-uniforni distribution with low grit density)
Wheel : Type C (even grit distribution) Fig.? Working surface of brazed single layer CBN wheels.
0
z o a*
c
Hardened (dry)
Unhardened (dry)
30 40 50 60 Number of grinding passes Fig.3 Grinding forces versus number of grinding passes for wheel of Type A in dry and wet grinding of hardened and unhardened 100Cr6 steels. 1
10
+
20
outcome so far as grit density and distribution pattern are concerned. Figure 2 shows the working surface of different CBN abrasive wheels before use. The arbitrary designation as well as the actual grit size ofeach tool are also indicated in the same figure. The wheel ofType Ain both B 126 and B25 1 grit sizes can be seenin amonolayer configuration but with a high grit density though the density was higher for finer grit size. 314
Fig.5 Types of chip produced during dry grinding of (a) unhardened and (b) hardened 100Cr6 steels with 10 pm infeed. The CBN grit can be seen to be virtually fully protruded over the tool face. For grit size B251 the grit protrusion was measured as 190-2IOpm while the finer grit (B126) showed 80-100 pm crystal exposure. The wheel of Type B may be characterized by the fact that the grit was distributed in a random manner to reduce its concentration. It is evident from Fig.2 that the average grit density of the wheel ofType B was less than that of the wheel of Type A. But, the fact which draws immediate attention was the existence of closely spaced grains forming acluster. This was true for both B126 and B251 grit sizes. From the investigation it was further established that the grit clustering was almost unavoidable when abrasive crystals were placed in arandommanner and the problem became aggravated with decreasing grit size. The same Fig.2 also shows the surface of wheels of Type C. In this case besides B126 and B251 size grit, CBN grit with B301 size was also included. With B301 grit size the crystal protrusion was found to be 235-260pm. In the wheel ofType C CBN particles were distributed more or less in an even manner. By this way of grit placement not only grit density can be regulated but also, the risk of grit clustering can be avoided. Figure 3 shows normal force and tangential force development by wheel of Type A (grit size :B126) during grinding of hardened and unhardened 100Cr6 steel for 60 grinding passes under dry and wet conditions. The most noticeable fact is that during dry grinding of unhardened steel, even after several passes of spark-in grinding, the tool did not reach asteady state; rather grinding forces continued to escalate and at the end of 60 passes quite high values of forces were
' O0 [
CBN wheel : Type A
P
Grit size : 8251
80
_+_
*-
work surface. Similarly, resistance to penetration even in the unhardened steel increased leading to rise of normal thrust force. Addition of cutting fluid immediately changed the situation for unhardened steel. Cleaning and lubricating action of the fluid can be recognized from the grinding forces which were reduced and became practically constant after a few passes of spark-in grinding as can be seen from Fig.3. Wet grinding also reduced forces for hardened steel though benefit of dry grinding of the same in allowing the grit to fracture in micro-level thus, maintaining its sharpness for long time has been reported (10). With 20 pm infeed the wheel also showed loading problem even in wet grinding of unhardened steel. The wheel showed its incapability too for dry grinding of hardened steel at 20pm infeed only because of chip accommodation problem. However, with application ofcutting fluid, this problem was minimized to agreatextent.
Hardened (dry) Unhardened (dry)
CBN wheel ofTypeA(grit size : B251) with 10 pm infeed 0
10
20
30
40
50
Infeed, pm
Fig.6 Grinding forces versus infeed for wheel of Type A with B251 grit size in dry and wet grinding of hardened and unhardened 100Cr6 steels. recorded. It can further be seen that for unhardened steel not only the tangential force was high but the normal force was also excessively large in comparison to that for hardened steel. Examination of the wheel surface at the end of 60 passes
(b) (a) Fig.7 Surface topography of wheel of Type A with B251 grit size after dry grinding of (a) unhardened and (b) hardened 100Cr6 steels with 20 pm infeed for 60 passes. clearly revealed large scale loading in dry grinding of the unhardened steel as shown in Fig.4(a). With lOpm infeed in dry grinding of hardened steel such wheel loading was totally absent as can be seen From Fig.4(b). However, the same figure indicates that the possibility of small scale loading may not be ruled out. The cause of severe wheel loading with soft steel can be better understood if one observes the types of chip produced shown in Fig.5. The chips produced out of the ductile steel were ribbon-like, mostly continuous and longer than those produced by the hardened steel. It was difficult to accommodate long chips in the storage space available ahead of the active grit and loading started in the initial phase of grinding. An earlier study (9)also has shown that it was easier to accommodate the same volume of smaller particles than large particles in a given accommodation volume. The loaded particles unfavourably changed the topography of the wheel surface which in turn increased tangential force because of rubbing with the
showed steady behaviour in dry grinding of both hardened and unhardened steels without any loading problem. But, with 20 pm infeed just after 60 grinding passes the wheel developed relatively high forces in comparison to what was recorded with 10 pm infeed. This is particularly true for unhardenedsteel asshowninFig.6.At thisstageexamination of the wheel surface undoubtedly established occurance of wheel loading as demonstrated in Fig.7(a). The wheel of Type Awith larger gritsize hadahigherprotrusion than that with a smaller grit size. The same figure shows that because of inadequate grit spacing long chips could not flow freely, instead were squeezed and entrapped in between grits followed by material building up with the number of grinding passes. The built up material started rubbing over the work surface increasing forces to high values. Figure 7(b) confirms that under identical conditions the CBN wheel was not free either from the material accumulation problem while grinding hardened steel. An attempt to increase infeed to 30 pm ended up with quick failure of the wheel due to severe wheel loading. It has been found that under a given grinding condition abrupt or disproportionate rise of forces at the early stage of grinding was always associated with chip accommodation problem. During wet grinding, the wheel showed improved performance with a reduction of the force, but limitation of the tool was again established at 40 pm infeed in grinding unhardened steel. Disproportionate rise of both normal and tangential forces can be seen in Fig.6 suggesting acute chip flow problem leading to wheel loading. In wet environment reduction of grinding forces could be observed for hardened steel as indicated in the same figure. In addition,grinding forces didnot show an abruptincreaseeven with 40 pm infeed suggesting considerable minimization of the chip storage problem. Wheel of Type B (grit size : B126) also showed its limitation in resisting wheel loading. This was again true in dry grinding of unhardened steel at 10 pm infeed. While at this infeed the tool worked satisfactorily in wet environment, it again faced chip storage problem only at 20 pm as revealed in Fig.8(a). An attempt to conduct grinding of hardened steel in dry environment did not succeed either. Figure 8(b) undoubtedly demonstrates that the chips, instead
315
60
a
1
CBN wheel : Type C Environment : Dry
-
8126
c
c 0' Fig.8 Surface topography of wheels ofType B (a) with B 126 grit size after wet grinding of unhardened 1 0 0 0 6 steel, (b) with B126 grit size after dry grinding of hardened 100Cr6 steel and (c) with B251 grit size after dry grinding of Unhardened 100Cr6 steel with 20 pm infeed for 60passes.
of being
cleared built up on the wheel surface. Interestingly, SEM micrographs in Fig.S(a) and (b) suggest that chip material built up in the region of grit cluster where grit density was high. Similar loading has also been found while grinding steels, varying in composition and hardness, at the clusters of abrasive grains in resin bonded CBN wheel (1 1). The same pictures also show that locations on the wheel surface with wide grit spacing did notaccumulate any chip
0
10
20 30 Infeed, urn
40
50
Fig.10 Grinding forces versus infeed for wheel of Type C with different grit sizes in dry grinding of unhardened 1 0 0 0 6 steel. that was recorded when using 10pm infeed. With introduction of grinding fluid forces were reduced and went up quite steadily with infeed upto 30 pm; at 40 pm infeed high rise of the same can be noted. This may be explained by enhanced interactions at the chip bond, and chip-work interfaces at higher infeed.
50 CBN wheel : Type B 40 Grit size : 8251 2 30 L
g 20 z:1 0 z
0
- 40
Q)
0
c -
30
-
.L 20 c
Fig.11 SEM micrograph showing resistance of wheel ofType C with B301 grit size to loading during dry grinding of unhardened 100Cr6 steel with 40 pm infeed for 60 passes.
%lo
K
2 0 0
10
20 30 Infeed, pm
40
50
Fig.9 Grinding forces versus infeed for wheel of Type B with B25 1 grit size in dry and wet grinding of unhardened 100Cr6 steel. material. It was also noted that the chip accumulation problem was significantly minimized for hardened steel even with 20pm infeed just by introduction ofgrinding fluid. CBN wheel of Type B with grit size B25 1 was also not free from wheel loading at grit cluster in dry grinding of soft steel as shown in Fig.S(c). Grinding was conducted with 20 prn infeed in dry environment only for 60 passes. Figure 9 again points to the fact that such loading resulted in a high rise of grinding forces when using20 pm infeed incomparison with 316
CBN wheel of Type C (grit size : B126) behaved quite steadily at 10 and 20 pm infeeds in dry grinding of unhardenedsteel unlike wheels ofType AandType B. Evenly distributed grit in the wheel showed a clear advantage in terms of easy chip accommodation. However, at 30 pm infeed there was a steep rise of grinding forces in comparison to what was recorded at lower infeeds as is evident in Fig.10. The same figure again shows how the limitation of small size grit could be overcome through use of large size grit. The wheel of Type C with grit sizes B25 1 and B301 could grind the unhardened steel at 30pm infeed in dry condition without any loading problem. The infeed could have been increased to 40 pm safely. Grinding forces increased in the very usual way with the infeed and there was no disproportionate escalation of the same unlike that always
happened with a loaded wheel. SEM micrograph in Fig.11 shows that the Wheel of Type C with B301 grit size was free of any kind of loading after dry grinding of unhardened steel with 40 pm infeed for 60 passes. At low infeeds, virtually there was no difference in the level of ginding forces developed by wheels with grit size B25 1 and B301, respectively. However, such difference in magnitude of forces is easily noticed at high infeeds. The difference in the magnitude of grinding forces may primarily be related to a smaller number of cutting points in CBN wheel with B301 grit size than that in the wheel with B251 grit size which interacted with the workpiece. However, apart from interaction at the grit-work interface leading to chip formation. interactions at chipbond, chip-work interfaces which also contribute to the generation of grinding forces in different degrees should be considered. The force components generated out of these interactions could be significantly high with the increase of the infeed. CBN wheel of Type C with grit size B301 had wider spacings and larger grit protrusions than the wheel with grit size B251 and this could have a positive influence in reducing interactions at the chip-bond and chip-work interfaces thereby decreasing bulk values of normal and tangential forces for CBN wheel with grit size B301:
4.
Conclusions
1. In comparison to a galvanically bonded single layer CBN tool, a brazed bonded counterpart may offer high crystal exposure but with a strong grit-bond adhesion. However, such high grit protrusion may not necessarily solve the chip accommodation problem when the brazed tool has a high grit density typical that of an electroplated tool. 2. Grinding in wet environment could improve the situation but, such improvement strongly depended on work material characteristics, material removal rate and size of CBN grain used in the tool. 3. Through random grit distribution, the density could not doubt be reduced rather easily but, this way of grit placement also resulted in grit clustering on the wheel surface which could again become the source of wheel loading. 4. Performance ofa brazedCBN wheelcould be remarkably improved by placing the grains in a regular manner. A brazed wheel with high grit protrusion and wide spacing exhibited remarkable resistance to loading even during dry grinding of a long chiping material like unhardened 100Cr6 steel. Such an aggressive tool could beequally efficient in dry and wet grinding of materials widely varying in terms of hardness, strength and ductility. 5. It is necessary to develop in industrial scale an economic process of grit placement in a regular manner on a tool surface having complex geometry possibly using handling and or setting processes as are commonly used in the watch industry.
5.
( 2 ) Peterman, L.M., 1985. Diamond Tooling in Non-Metallics, Superabrasive’85 Conference, Chicago, Illinois, 12/121. (3) AbrasiveTechnology Inc., 1980,Technical Bulletin on PBS Products. (4) Chattopadhyay, A.K., Chollet, L., Hintermann, H.E., 1990, On Performance of Chemically Bonded Single Layer CBN Grinding Wheel, Annals CIRP, 39/1:309-312. (5) Chattopadhyay, A.K., Chollet, L., Hintermann, H.E., 1992, Improved 1Monolayer CBN Wheel for Load Free Grinding, International Journal of Machine Tools and Manufacture, 3 3 4 5 7 1-581. (6) Chattopadhyay, A.K., Chollet, L., Hintermann, H.E., 1991, Experimental Investigation on Induction Brazing of Diamond with Ni-Cr Hardfacing Alloy under Argon Atmosphere, Journal of Materials Science, 265093-5100. (7) Chattopadhyay, A.K., Chollet, L., Hintermann, H.E., 1991, On Performance of Brazed Bonded Monolayer Diamond Grinding Wheel, Annals CIRP, 40/1:347-350. (8) Hintermann, H.E., Chattopadhyay, A.K., 1992, New Generation Superabrasive Tool with Monolayer Configuration, Diamond and Related Materials, 1:1131-1143. (9) Pai, D.M.,Ratterman, E.,Shaw, M.C., 1989, Grinding Swarf, Wear, 13k329-339. (10) Stokes,R.J., Valentine,T.J., 1984, Wear Mechanisms of ABN Abrasive, Industrial Diamond Review, 1/84:34-44. (1 1) Pecherer, E., Malkin, S., 1984, Grinding of Steels with Cubic Boron Nitride, Annals CRP, 33/1:211-216.
References
(1) Norton Company, 1990, Technical Information on MSL tools.
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