- Email: [email protected]
A New Preparation Method for Resinoid-Bonded CBN Wheels
A New Preparation Method for Resinoid-Bonded CBN Wheels I. lnasaki (2); Keio University, Yokohama/Japan Received on January 12,1990
Summary Truing and dressing for super-abrasive wheels are extremelv difficult because of their high degree o€ hardness. In this study, a new preparation method for resinoid bonded super-abrasive wheels, in which truing and dressing were integrated, were developed. By adopting the developed equipment, these two conditioning processes can be carried out simultaneously, and therefore, non-machining time can be reduced considerably. A metal bonded diamond wheel and steel wire brushes were combined to make up the preparation tool. Through a series of fundamental experiments, a recommended preparation condition has been established from the viewpoint of the truing a s well a s the dressing efficiency and wear resistance of the preparation tool. Key words: CBN wheels, dressing, truing, wire
brushes, metal bonded diamond wheel.
1. Introduction Super abrasive wheels, represented by diamond well as CBN wheels, are being widely applled for grinding of difficult-to-machine materials (1),(2). Truing and dressing for these super abrasive wheels are extremely diPficult and time consumlng processes because of thcir high degree of hardness. Some practical truing and dressing methods have been developed so far(3).(4). However, most of them are not, efficient enough for providing the wheel surface characteristic which can attain a good grinding performance. As a matter of fact, regardless of the several new developments in truing and dressing processes, what is now being applied generally in practical machine shops is a quite conventional method, i.e.. a n abrasive stick Is applied with hand pressure to the wheel face. Much more attentlon should be paid, therefore, to develop efficient truing and dressing methods to take full advantage of super abrasive wheels. The purpose of this study is to develop a new preparation or conditioning method for super abrasive wheels, in which truing and dressing processes are integrated. Truing, which is needed for shaping a wheel face and dressing. which is needed to clean and sharpen a wheel face, are carried out separately under ordinary circumstances for resinoid bonded super abrasive wheels. If these two conditioning processes could be done simultaneously. it would contribute a lot to rcduce non-machining time. The abrasive wheel chosen as the subject of the study is the resinoid bonded CBN wheel. This is due to the Pact that reslnoid bonded CBN wheels are now most widely used in industries, though the use of vitrified bonded CBN wheel has been gradually increased. as
2. Principle of proposed preparation method A special feature of the proposed preparation is that truing and dressing are done method The simultaneously for resinoid bonded CBN wheels. basic configuration of the equipment is shown in Fig. 1. T o meet the demand for shaping CBN wheels use efficiently, it is inevitably necessary to diamond grains which are harder than CBN grains. In this equipment, thercfore. a metal bonded diamond wheel is used for truing the CBN wheel. The grain of the diamond wheel is WlOO and its size concentration is 75. The diameter and the width of the truer are 80mn and 2mm. respectively. As shown in Fig.1, the truer has 6 equally spaced slits around the circumference, which can serve as pockets for the for removed material. They are also effective supplying coolant to the contact area. A sufficient amount o f coolant should be supplied to reduce the wear of the diamond truer. wire brushes as Dressing is performed with shown in Fig.1. The wire brushes applied in this study are the one which are commonly used for a dcbarring process. The flexible wire brushes are capable of removing the binder matcrlal of the wheel without directly attacking the CBN gralns. if the relative movement between the brush and the CBN wheel 1s given. The loaded swarf on the wheel surface can also be removed with the wire brushes. Chip pockets are, therefore, formed effectively around the grain cutting edges. Several different materials of wire brushes were tested. Thc bcst rcsult was obtaincd with the steel wire brush from the viewpoint of the wear resistance and the dressing ability. Therefore, most of the dressing test was carried out using steel wire brushes. The diameter of each steel wire Is 0.175mm and they are bundled to make a wire brush dressing tool of circular disk shape having a
Annals of the ClRP Vol. B/l/lswO
diameter 82mm and a thickness 5mm. Each steel wlre protrudes 7mm from a hub. The diamond truer is sandwiched with two wire brush dressers as shown in protrude Fig.1. The top ends of the wire brushes lmm from the diamond truer face after they are assembled. The above described preparation tool is attached to the spindle of the driving unit having a motor power of 0.75KW as shown in Fig.2. The size of the spindle and the flange for fixing the preparation tool was determined to attain a high rigidity. 3. Truing test First, the truing and dressing performances were evaluated separately. In order to quantitatively investigate the truing efficiency, the following parameter was introduced:
JO 96 where, Ei and Eo are eccentricities in the CBN wheel before and after truing. respectively. In the truing performance test, the eccentricity was intentionally introduced to the CBN wheel by using a clearance between the wheel and the flange. The eccentricity o f about 12pm was given before each truing test. This amount was confirmed with a n electric micrometer. The grinding machine used was the NC tool grinding machine (Makino Seiki CNJ-10). the main spindle power o f which was 0.75kw. The resinoid bonded CBN wheel used throughout the series of experiments had a grain size of #lo0 and a
CBN wheel
Wire brush Fig.1
Newly developed preparation tool for bonded CBN wheel
resinoid
317
Wire brush (t=Sl Diamond wheel (t=2)
Fig.3 and Fig.4 show the influence of the velocity ratio on the dccrense in the eccentricity ratio Dr. The reed ratc. in other words, the traverse rate of the trucr was lOmm/min. The nominal depth o f cut for one pass was 15 m and this sides o f amount of depth of cut was given at the CBN wheel. The total truing depth of cut was. therefore, 30pm. The time required for truing the CBN wheel is 1.8 min, which is calculatcd from the width o f CRN wheel, the width or truer and the feed rate. trulng (Fig.3). In the case of down-cut eccentricity can not be eliminnLcd in the Thls is neighborhood of velocity ratio +1.0. because of the zern w l n t i v e velocity bctwecn the CISN wheel and the truer, i.e., they are in the rolling contact condition. In the ranges of Rv<+0.8 and Rv>+1.2, however, 70%-90% of the eccentricity can be eliminated. In the case of up-cut truing. on the other hand, efficlcnt truing is performed over the wide range of the rclatlve velocity ratio(Fig.4). From these results, it is confirmed that the larger the relative veloclty. the larger the dccrease in the eccentricity. A similar result was obtained when the CBN wheel velocity was increased from 10.5m/s to 15.7m/s. From the viewpoint of the wear resistance of the truer, it can be recommended to apply down-cut truing. in which the relative velocity is low. For investigating the influence of the fced rate, the total trulng time was kept constant (1.8min). The number of the traversc strokes was. therefore, increased when the feed rate was increased. The influence of the fced rate was, however, not so slgniricant as the velocity ratio, though a slight improvement in the truing performance was observed when the feed rate was incrcased.
hot{
Configuration of the preparation device
Fig.2
concentration o f 75. The diameter and the width of the wheel were lOOmm and 7mm. respectively. One of the most important factors affecting the truing efficiency is a relative velocity between the CBN wheel and the diamond truer. Let the surface velocity of the CBN wheel be vs and that of the diamond truer be vt, the vclocity ratio Rv is defined as follows:
where, positive sign is for the down cut process and Keeping negative sign is for the up-cut process. other conditions constant, influence o f the velocity ratio on the trulng cfflcicncy was investlgated. During this series of experiments, the wire brushes for dressing were removed and not used.
* 1001
b
>r
1
4. Dressing test During the dressing test, the diamond truer was removed and only the wire brushes were mounted on the spindle of the equipment. The dressing efficiency was evaluated through the initial specific grinding energy and the cumulative material removcd during the wheel life(4). The workpicce material used in the grinding test was tungsten carbide. The tool life o f the CBN wheel was defined as the time when the specific grinding energy increased to 5.0 KN m/mm. The grinding depth of cut, the CBN wheel velocity. and the workpiece vclocity were 150pm. 20m/s and 30mm/min. respectively. Three different types of wire material i.e., steel, stainless steel(SUS303) and nylon, which contained A1 O3 abrasives, were tested and evaluated the in terms of the speciflc grlndlng energy. cumulative material removed during the wheel life and wear. The diameter of the steel wire and the stainless steel wire were 0.175mm and that o f the nylon wire was lmm. Abrasives contained in the nylon wire had a grid size of #80. The result is shown in Fig.5 and table 1. In addition to thrce different types o f wire brushes, a conventional abrasive stick (WA220G) was also tested to confirm the high efficiency of the proposed new method. The abrasive stick was applied to the CBN whecl by hand pressure for 5min. From Fig.5. It is clear that the steel wire brush attains the highest efficiency. From the wear resistance point of view. the steel wire brushes again show the best performance. The time required for dressing is important because the excessive dressing wastes not only the dresser but also the CRN wheel and time. Dressing was performed with the CBN wheel surface velocity 10.5m/s. the velocity ratio +0.84. and the feed rate of the wire brush 46mm/min. The initial specific and the cumulative material grinding energy E
80:oq r; I I
0
I
0
. I . -
60
401,
= 10:5m/s
,
vt
20 q0.6 Fig.3
>,
= lOmm/min
ut = 15pmx2
tt = 2min +0.8 +1.0 +1.2 +1.4 * +1.6 Velocity ratio RV ( Truer Wheel 1
Influence of velocity ratio efficiency (Down-cut truing)
€to./-
-
on
truing
-
Steel
a/ / / / / / /
Stainless steel
t
HI 6 0 -
rz 40-
g:
I 0
0)
20-
= 10.5m/s vf = lOmm/min at = 15pmx2 tt = 2min
vs
Abrasive stick (WA)
2.0 1.0 0 Specific grinding energy E kNm/mmS Fig.5
318
.
,
0 300 600 Cumulative material removed during tool life V mm'
Influence of wire brush matarials on efficiency
dressing
Table 1
I
Wear of wire brushes
Wire brush material
I
I
Steel
I
Wear
I
160pm
I
I
I
(a) A f t e r truing
Stainless steel (SUS303)
I
I
I
(b)
vs 10.5 m / s Rv +0.84
vf td
td
= 5 min
46mm/min 5 min
removed during the whecl ltfe are plotted against the dressing time in Fig.6. The longer the dressing time, the better the dressing performnnce. The initial specific grinding energy decreases with an increase in time, though it finally becomes nenrly constant. The cumulative mnterial removed is plotted against a dressing time of up to 10min. This is due to the fact that the occurrence of the chatter vibration made it difficult to carry out grinding when excessive dressing was pcrformed. In order to clarify the reason for the occurrence of chatter vtbration, the surface profile of the CBN wheel after dressing was investigated using an electric micromcter. FLg.7 shows the wheel surface profiles after truing and dressing. According to the profile records, i t is pointed out that the whecl surface is almost flat after truing. This means that the grain cuttlng edges are not protruding enough from the bond surface. With this U n
I
I
(c)
td
= 10 min vs : 10.5 m/s Rv +0.84 Vt : 46 mm/min
k
J
360'
(d)
td
Fig.7
= 15 min
Surface profiles of the CBN wheel
n
relative velocity. One of the typical results is shown in Table 2 . Wear o f the wire brush dressers was significant when up-cut dressing was carried out. For the same velocity ratio, wear of the wire brushes during up-cut dressing was about three times higher than in the case o f down-cut dressing. This is, o f course, due to the higher relative velocity in up-cut dressing. Influence of the feed rate on the dressing performance was also investigated. The result was signiflcant similar to the case of truing, i.e.. n o difference was observed as far as the total dressing time was kept constant.
Dressing time Fig.6
Influence efficiency
of
td
dressing
min time
Table 2 on
dressing
surface characteristic, good grinding ability can not be attaind. With an increase in the dresslng time, cutting edges start to protrude and chip pockets are formed. After a dressing time of 15mtn. however, waves generated on the wheel surface become significant. These wnves genernted during dressing must be the reason for the occurrence of the chatter vlbration. Non-uniformity of the wheel structure is considered to be the reason for the generation of waves. From the result shown in Fig. 6 , it can be said that the optimum dressing time exists from the viewpoint of restoring the grinding ability and avoiding the generation of waves. For the dressing condition adopted in this test, the optimum dressing time is 6-8min. When the stiffness of the wire brushes were increased by, for example, decreasing the protruded length of the wires from the hub, it was confirmed that the optimum dresslng time was decreased. With respect to the influence of the relative velocity on restoring the grinding ability. no significant difference was observed excepting that the dressing efficiency was extremely low in the neighborhood of Rv-+l.O. T o the contrary, wear of the wire brushes was markedly effected by the
Influence of velocity ratio on wear of brush
A Vf
wire
4 6 d m i n 9mm/min
-0. 8 4
850pm
+O. 8 4
160pm 190pm
vs
10.5m/s
,
td
630pm
Smin
5. Integrated test The metal bonded diamond truer and the wire brush dressers were integrated to make up a new preparation equipment as shown in Fig.1. and Fig.2. As the final step for the series of experiments. an integrated test was carricd out. First, truing efficiency was compared in two different cnses. i.e.. the case with wire brush dressers and the case without them. The truing depth of cut 15pm was given at both ends o f the CBN wheel. 30pm. At this Thus, the total depth of cut wns condition. the dressing depth of cut becomes about
319
lmm because the leading edge of the wire brushes protrudes 1mm from the truer face. When the feed rate is lOmm/mln. total time required for the preparation is about 4min. As in the result shown in Table 3, higher truing efficiency was attaind when the truer and the dresser were combincd. The front wire brush. which precedes the diamond truer, removes the binder material and, therefore, decreases the In other holding strength f o r abrasive grains. words, the front wire brush assists the truing process which is done by the diamond wheel. This mutual interaction is one o f the advantages of this proposed method. It was confirmed, to the contrary, that the dressing process was not assisted significantly by the diamond truers in regard to restoring the It is. sharpness of the grain cutting edges. however, possible to suppress the generation of waves on the CBN wheel due to excessive dressing. Through a series of separated and integrated tests, the following preparation condition could be recommended. The velocity ratio Rv should be set at tO.8 or r1.2 to minimize the wear of the wire brush dressers. This condition of low relative velocity must also be preferable to minimize the wear of the diamond truer. The feed rate does not have much effect on the truing and the dressing process as far as the total preparation time is kept constant. Table 3
Effect of integrated Preparation truing efficiency
Rv -1. 0 I
I
I I
truer only
+O. 8 4
VS
76. 7%
1 0 . 5m/s
1
Vf
on
t r u e r and dresser
88. 5%
I
tool
I 1% I
94.0%
I
I
89.
1 Omm/mi n
6. Conclusions A new preparation equipment for resinoid bonded CDN wheels was developed. The proposed method is capable of truing and dressing CDN wheels simultaneously. The preparation tool consists of the metal bonded diamond wheel and the steel wire brushes. Through a series of fundamental a s well a s field tests, availability of the developed equipment was confirmed. Influence o f the preparation condition on the truing and dressing efficiency was investigated in detail and the recommended condition was established. The velocity ratio of the diamond truer and the CBN wheel plays an important role in the prcporotion efficiency as well a s the wear o f the preparation tool. Down-cut truing and dressing, is giving relative velocity to some extent, recommended to minimize the wear of the preparation tool. It was confirmed through field tests that this method could reduce non-machining time considerably. References (1) Metzger. J.L..1986. Superabrasive grinding, Butterworths. London. Inasaki,I.. 1987. Grinding of hard and brittle materials, Annals of the CIRP. 36/2, 463-471 Salj6. E., Damlos. H.H., and Moehlen. H., 1985. Internal grinding of high strength ceramic workpiece materials with diamond grinding wheel. Annals of the CIRP, 34/1, 263-266. Inasaki.I., 1989. Dressing o f resinoid bonded diamond grinding wheels, Annals of the CIRP. 38/1. 315-318
Summary Truing and dressing for super-abrasive wheels are extremelv difficult because of their high degree o€ hardness. In this study, a new preparation method for resinoid bonded super-abrasive wheels, in which truing and dressing were integrated, were developed. By adopting the developed equipment, these two conditioning processes can be carried out simultaneously, and therefore, non-machining time can be reduced considerably. A metal bonded diamond wheel and steel wire brushes were combined to make up the preparation tool. Through a series of fundamental experiments, a recommended preparation condition has been established from the viewpoint of the truing a s well a s the dressing efficiency and wear resistance of the preparation tool. Key words: CBN wheels, dressing, truing, wire
brushes, metal bonded diamond wheel.
1. Introduction Super abrasive wheels, represented by diamond well as CBN wheels, are being widely applled for grinding of difficult-to-machine materials (1),(2). Truing and dressing for these super abrasive wheels are extremely diPficult and time consumlng processes because of thcir high degree of hardness. Some practical truing and dressing methods have been developed so far(3).(4). However, most of them are not, efficient enough for providing the wheel surface characteristic which can attain a good grinding performance. As a matter of fact, regardless of the several new developments in truing and dressing processes, what is now being applied generally in practical machine shops is a quite conventional method, i.e.. a n abrasive stick Is applied with hand pressure to the wheel face. Much more attentlon should be paid, therefore, to develop efficient truing and dressing methods to take full advantage of super abrasive wheels. The purpose of this study is to develop a new preparation or conditioning method for super abrasive wheels, in which truing and dressing processes are integrated. Truing, which is needed for shaping a wheel face and dressing. which is needed to clean and sharpen a wheel face, are carried out separately under ordinary circumstances for resinoid bonded super abrasive wheels. If these two conditioning processes could be done simultaneously. it would contribute a lot to rcduce non-machining time. The abrasive wheel chosen as the subject of the study is the resinoid bonded CBN wheel. This is due to the Pact that reslnoid bonded CBN wheels are now most widely used in industries, though the use of vitrified bonded CBN wheel has been gradually increased. as
2. Principle of proposed preparation method A special feature of the proposed preparation is that truing and dressing are done method The simultaneously for resinoid bonded CBN wheels. basic configuration of the equipment is shown in Fig. 1. T o meet the demand for shaping CBN wheels use efficiently, it is inevitably necessary to diamond grains which are harder than CBN grains. In this equipment, thercfore. a metal bonded diamond wheel is used for truing the CBN wheel. The grain of the diamond wheel is WlOO and its size concentration is 75. The diameter and the width of the truer are 80mn and 2mm. respectively. As shown in Fig.1, the truer has 6 equally spaced slits around the circumference, which can serve as pockets for the for removed material. They are also effective supplying coolant to the contact area. A sufficient amount o f coolant should be supplied to reduce the wear of the diamond truer. wire brushes as Dressing is performed with shown in Fig.1. The wire brushes applied in this study are the one which are commonly used for a dcbarring process. The flexible wire brushes are capable of removing the binder matcrlal of the wheel without directly attacking the CBN gralns. if the relative movement between the brush and the CBN wheel 1s given. The loaded swarf on the wheel surface can also be removed with the wire brushes. Chip pockets are, therefore, formed effectively around the grain cutting edges. Several different materials of wire brushes were tested. Thc bcst rcsult was obtaincd with the steel wire brush from the viewpoint of the wear resistance and the dressing ability. Therefore, most of the dressing test was carried out using steel wire brushes. The diameter of each steel wire Is 0.175mm and they are bundled to make a wire brush dressing tool of circular disk shape having a
Annals of the ClRP Vol. B/l/lswO
diameter 82mm and a thickness 5mm. Each steel wlre protrudes 7mm from a hub. The diamond truer is sandwiched with two wire brush dressers as shown in protrude Fig.1. The top ends of the wire brushes lmm from the diamond truer face after they are assembled. The above described preparation tool is attached to the spindle of the driving unit having a motor power of 0.75KW as shown in Fig.2. The size of the spindle and the flange for fixing the preparation tool was determined to attain a high rigidity. 3. Truing test First, the truing and dressing performances were evaluated separately. In order to quantitatively investigate the truing efficiency, the following parameter was introduced:
JO 96 where, Ei and Eo are eccentricities in the CBN wheel before and after truing. respectively. In the truing performance test, the eccentricity was intentionally introduced to the CBN wheel by using a clearance between the wheel and the flange. The eccentricity o f about 12pm was given before each truing test. This amount was confirmed with a n electric micrometer. The grinding machine used was the NC tool grinding machine (Makino Seiki CNJ-10). the main spindle power o f which was 0.75kw. The resinoid bonded CBN wheel used throughout the series of experiments had a grain size of #lo0 and a
CBN wheel
Wire brush Fig.1
Newly developed preparation tool for bonded CBN wheel
resinoid
317
Wire brush (t=Sl Diamond wheel (t=2)
Fig.3 and Fig.4 show the influence of the velocity ratio on the dccrense in the eccentricity ratio Dr. The reed ratc. in other words, the traverse rate of the trucr was lOmm/min. The nominal depth o f cut for one pass was 15 m and this sides o f amount of depth of cut was given at the CBN wheel. The total truing depth of cut was. therefore, 30pm. The time required for truing the CBN wheel is 1.8 min, which is calculatcd from the width o f CRN wheel, the width or truer and the feed rate. trulng (Fig.3). In the case of down-cut eccentricity can not be eliminnLcd in the Thls is neighborhood of velocity ratio +1.0. because of the zern w l n t i v e velocity bctwecn the CISN wheel and the truer, i.e., they are in the rolling contact condition. In the ranges of Rv<+0.8 and Rv>+1.2, however, 70%-90% of the eccentricity can be eliminated. In the case of up-cut truing. on the other hand, efficlcnt truing is performed over the wide range of the rclatlve velocity ratio(Fig.4). From these results, it is confirmed that the larger the relative veloclty. the larger the dccrease in the eccentricity. A similar result was obtained when the CBN wheel velocity was increased from 10.5m/s to 15.7m/s. From the viewpoint of the wear resistance of the truer, it can be recommended to apply down-cut truing. in which the relative velocity is low. For investigating the influence of the fced rate, the total trulng time was kept constant (1.8min). The number of the traversc strokes was. therefore, increased when the feed rate was increased. The influence of the fced rate was, however, not so slgniricant as the velocity ratio, though a slight improvement in the truing performance was observed when the feed rate was incrcased.
hot{
Configuration of the preparation device
Fig.2
concentration o f 75. The diameter and the width of the wheel were lOOmm and 7mm. respectively. One of the most important factors affecting the truing efficiency is a relative velocity between the CBN wheel and the diamond truer. Let the surface velocity of the CBN wheel be vs and that of the diamond truer be vt, the vclocity ratio Rv is defined as follows:
where, positive sign is for the down cut process and Keeping negative sign is for the up-cut process. other conditions constant, influence o f the velocity ratio on the trulng cfflcicncy was investlgated. During this series of experiments, the wire brushes for dressing were removed and not used.
* 1001
b
>r
1
4. Dressing test During the dressing test, the diamond truer was removed and only the wire brushes were mounted on the spindle of the equipment. The dressing efficiency was evaluated through the initial specific grinding energy and the cumulative material removcd during the wheel life(4). The workpicce material used in the grinding test was tungsten carbide. The tool life o f the CBN wheel was defined as the time when the specific grinding energy increased to 5.0 KN m/mm. The grinding depth of cut, the CBN wheel velocity. and the workpiece vclocity were 150pm. 20m/s and 30mm/min. respectively. Three different types of wire material i.e., steel, stainless steel(SUS303) and nylon, which contained A1 O3 abrasives, were tested and evaluated the in terms of the speciflc grlndlng energy. cumulative material removed during the wheel life and wear. The diameter of the steel wire and the stainless steel wire were 0.175mm and that o f the nylon wire was lmm. Abrasives contained in the nylon wire had a grid size of #80. The result is shown in Fig.5 and table 1. In addition to thrce different types o f wire brushes, a conventional abrasive stick (WA220G) was also tested to confirm the high efficiency of the proposed new method. The abrasive stick was applied to the CBN whecl by hand pressure for 5min. From Fig.5. It is clear that the steel wire brush attains the highest efficiency. From the wear resistance point of view. the steel wire brushes again show the best performance. The time required for dressing is important because the excessive dressing wastes not only the dresser but also the CRN wheel and time. Dressing was performed with the CBN wheel surface velocity 10.5m/s. the velocity ratio +0.84. and the feed rate of the wire brush 46mm/min. The initial specific and the cumulative material grinding energy E
80:oq r; I I
0
I
0
. I . -
60
401,
= 10:5m/s
,
vt
20 q0.6 Fig.3
>,
= lOmm/min
ut = 15pmx2
tt = 2min +0.8 +1.0 +1.2 +1.4 * +1.6 Velocity ratio RV ( Truer Wheel 1
Influence of velocity ratio efficiency (Down-cut truing)
€to./-
-
on
truing
-
Steel
a/ / / / / / /
Stainless steel
t
HI 6 0 -
rz 40-
g:
I 0
0)
20-
= 10.5m/s vf = lOmm/min at = 15pmx2 tt = 2min
vs
Abrasive stick (WA)
2.0 1.0 0 Specific grinding energy E kNm/mmS Fig.5
318
.
,
0 300 600 Cumulative material removed during tool life V mm'
Influence of wire brush matarials on efficiency
dressing
Table 1
I
Wear of wire brushes
Wire brush material
I
I
Steel
I
Wear
I
160pm
I
I
I
(a) A f t e r truing
Stainless steel (SUS303)
I
I
I
(b)
vs 10.5 m / s Rv +0.84
vf td
td
= 5 min
46mm/min 5 min
removed during the whecl ltfe are plotted against the dressing time in Fig.6. The longer the dressing time, the better the dressing performnnce. The initial specific grinding energy decreases with an increase in time, though it finally becomes nenrly constant. The cumulative mnterial removed is plotted against a dressing time of up to 10min. This is due to the fact that the occurrence of the chatter vibration made it difficult to carry out grinding when excessive dressing was pcrformed. In order to clarify the reason for the occurrence of chatter vtbration, the surface profile of the CBN wheel after dressing was investigated using an electric micromcter. FLg.7 shows the wheel surface profiles after truing and dressing. According to the profile records, i t is pointed out that the whecl surface is almost flat after truing. This means that the grain cuttlng edges are not protruding enough from the bond surface. With this U n
I
I
(c)
td
= 10 min vs : 10.5 m/s Rv +0.84 Vt : 46 mm/min
k
J
360'
(d)
td
Fig.7
= 15 min
Surface profiles of the CBN wheel
n
relative velocity. One of the typical results is shown in Table 2 . Wear o f the wire brush dressers was significant when up-cut dressing was carried out. For the same velocity ratio, wear of the wire brushes during up-cut dressing was about three times higher than in the case o f down-cut dressing. This is, o f course, due to the higher relative velocity in up-cut dressing. Influence of the feed rate on the dressing performance was also investigated. The result was signiflcant similar to the case of truing, i.e.. n o difference was observed as far as the total dressing time was kept constant.
Dressing time Fig.6
Influence efficiency
of
td
dressing
min time
Table 2 on
dressing
surface characteristic, good grinding ability can not be attaind. With an increase in the dresslng time, cutting edges start to protrude and chip pockets are formed. After a dressing time of 15mtn. however, waves generated on the wheel surface become significant. These wnves genernted during dressing must be the reason for the occurrence of the chatter vlbration. Non-uniformity of the wheel structure is considered to be the reason for the generation of waves. From the result shown in Fig. 6 , it can be said that the optimum dressing time exists from the viewpoint of restoring the grinding ability and avoiding the generation of waves. For the dressing condition adopted in this test, the optimum dressing time is 6-8min. When the stiffness of the wire brushes were increased by, for example, decreasing the protruded length of the wires from the hub, it was confirmed that the optimum dresslng time was decreased. With respect to the influence of the relative velocity on restoring the grinding ability. no significant difference was observed excepting that the dressing efficiency was extremely low in the neighborhood of Rv-+l.O. T o the contrary, wear of the wire brushes was markedly effected by the
Influence of velocity ratio on wear of brush
A Vf
wire
4 6 d m i n 9mm/min
-0. 8 4
850pm
+O. 8 4
160pm 190pm
vs
10.5m/s
,
td
630pm
Smin
5. Integrated test The metal bonded diamond truer and the wire brush dressers were integrated to make up a new preparation equipment as shown in Fig.1. and Fig.2. As the final step for the series of experiments. an integrated test was carricd out. First, truing efficiency was compared in two different cnses. i.e.. the case with wire brush dressers and the case without them. The truing depth of cut 15pm was given at both ends o f the CBN wheel. 30pm. At this Thus, the total depth of cut wns condition. the dressing depth of cut becomes about
319
lmm because the leading edge of the wire brushes protrudes 1mm from the truer face. When the feed rate is lOmm/mln. total time required for the preparation is about 4min. As in the result shown in Table 3, higher truing efficiency was attaind when the truer and the dresser were combincd. The front wire brush. which precedes the diamond truer, removes the binder material and, therefore, decreases the In other holding strength f o r abrasive grains. words, the front wire brush assists the truing process which is done by the diamond wheel. This mutual interaction is one o f the advantages of this proposed method. It was confirmed, to the contrary, that the dressing process was not assisted significantly by the diamond truers in regard to restoring the It is. sharpness of the grain cutting edges. however, possible to suppress the generation of waves on the CBN wheel due to excessive dressing. Through a series of separated and integrated tests, the following preparation condition could be recommended. The velocity ratio Rv should be set at tO.8 or r1.2 to minimize the wear of the wire brush dressers. This condition of low relative velocity must also be preferable to minimize the wear of the diamond truer. The feed rate does not have much effect on the truing and the dressing process as far as the total preparation time is kept constant. Table 3
Effect of integrated Preparation truing efficiency
Rv -1. 0 I
I
I I
truer only
+O. 8 4
VS
76. 7%
1 0 . 5m/s
1
Vf
on
t r u e r and dresser
88. 5%
I
tool
I 1% I
94.0%
I
I
89.
1 Omm/mi n
6. Conclusions A new preparation equipment for resinoid bonded CDN wheels was developed. The proposed method is capable of truing and dressing CDN wheels simultaneously. The preparation tool consists of the metal bonded diamond wheel and the steel wire brushes. Through a series of fundamental a s well a s field tests, availability of the developed equipment was confirmed. Influence o f the preparation condition on the truing and dressing efficiency was investigated in detail and the recommended condition was established. The velocity ratio of the diamond truer and the CBN wheel plays an important role in the prcporotion efficiency as well a s the wear o f the preparation tool. Down-cut truing and dressing, is giving relative velocity to some extent, recommended to minimize the wear of the preparation tool. It was confirmed through field tests that this method could reduce non-machining time considerably. References (1) Metzger. J.L..1986. Superabrasive grinding, Butterworths. London. Inasaki,I.. 1987. Grinding of hard and brittle materials, Annals of the CIRP. 36/2, 463-471 Salj6. E., Damlos. H.H., and Moehlen. H., 1985. Internal grinding of high strength ceramic workpiece materials with diamond grinding wheel. Annals of the CIRP, 34/1, 263-266. Inasaki.I., 1989. Dressing o f resinoid bonded diamond grinding wheels, Annals of the CIRP. 38/1. 315-318