High speed (up to 127 m/s) and high material removal rate (up to 10 mm{sup 3}/s/mm) grinding experiments using a vitreous bond CBN wheel were conducted to investigate the effects of material removal rate, wheel speed, dwell time and truing speed ratio on cylindrical grinding of silicon nitride and zirconia. Experimental results show that the high grinding wheel surface speed can reduce the effective chip thickness, lower grinding forces, enable high material removal rate grinding and achieve a higher G-ratio. The radial feed rate was increased to as high as 0.34 {micro}m/s for zirconia and 0.25 {micro}m/s for silicon nitride grinding to explore the advantage of using high wheel speed for cost-effective high material removal rate grinding of ceramics

Grant, M. B.

McSpadden, S. B.

Morris, T. O.

Shih, A. J.

Yonushonis, T. M.

UNT Digital Library

c~F-q%/aao- -  Vitreous Bond C8N High Speed and High Material Removal Rate Grinding of Ceramics I A. J. Shih, M. B. Grant, and T. M. Yonushonis, Cummins Engine Company, Inc. Columbus, IN 47@ 1.0. Moms and S. B. McSpadden, Oak Ridge National Laboratory, Oak Ridge, TN 37831 7 Abstract High speed (up to 127 d s )  and high materiai removal rate (up to IO mm3/s/mm) srindins experiments using a vitreous bond CBN wheel were conducted to investigate the effects of material removal rate, wheel speed, dwell time and truing speed ratio on cylindrical grinding of silicon nitride and Zirconia. Experimental results show that the high grinding wheel surface speed can reduce the effective chip thickness, lower grinding forces, enable high material removal rate grinding and achieve a higher G-ratio. The radial feed rate was increased to as high as 0.34 cun/s for Zirconia and 0.25 cun/s for silicon nitride grinding to explore the advantage of using high wheel speed for costeffective high material removal rate grindins of ceramics. 1. Introduction The high speed cylindrical grin- experiments presented by Shih, et al. (1998) are extended to study the CBN grinding of silicon nitride and to explore the high and silicon nitride. The trutng process which generates the lowest grinding f o m  was used in the high material removal rate grinding of zirconia and silicon nitride. The effect of dwell time was investigated to identify the improvement in surface finish and roundness. Silicon nitride and zirconia have been 1.ecogwed as difficult to machine materials. The CBN abrasive was selected because it can be txued to the pm-level precision form by a rotary diamond wheel and it has the flexibility to grind both d c s  and hardened steels. Table 1 compares the properties of the three work-materials, silicon nitride, Zirconia and hardened M2 steel, the three work-materials used in this and previous high speed grinding tests (Shih et al., 1998). It can be seen that silicon nitride has signilicant higher hardness and elastic modulus than the other two materials. The ceramic grindability test method developed by Chand and Guo (1 996) was used to quanti@ the gnndability of Zirconia and silicon nitride. Test results showed that the grindability index, Go, for zirwnia and silicon nitride were 31.%104 and 1 0 . 6 ~ 1 0 ~  mm3/Nm, respectively. Comparing to the Gc of other ceramics, these two work- materials have relatively low G,, indicating that both materials are difhult to grind speed, high material removal rate grindins of zirconia ~ a b k  1. Cornparism of material properties of the zirconia, silicon nitride, and hardened Wsteei. I I I Go. Grindabiiihrlndex I 31.9 I 10.6 I N/A The experiments were conduoted on a Weldon AGNS CNC cylindrical grinding machine. The gzindhg wheel was slowed down during truing and increased to the desired speedduring grinding. This technique has been proven useful to reduce the grinding forces. Detailed process parameters have been documented in SI& e4d. (1 998). some critical parameters were: ooolant flow rate was 62.5 Itmin, workhead speed was 600 rpm, width of pluuge grinding was 6.35 mm and grinding w h l  was the Cincinnati h4ifacron 2BN 320-Ql64-vMB. The next gnnding of silicon nitride and high material removal rate g r i d k g  of both silicon nitride and p'uwwia. TaMe 2 selection of the sixteen grinding tests. two sections SUIMnarize the results on high speed x: selected for the test 2. Hgh speed grinding of silicon nitride The siZaeen grinding tests (Shih et al., 1998), as listed in Table 2, were used to test the effects of two setup parameters, grinding wheel surface speed (v,) and truing speed ratio (Q), on silicon nitride grinding. Tmhg speed ratio is &fined as the ratio of the tmhg wheel surface speed (v,) to v,, i.e., qpv,/vp is the surface speed of the truing disk and qm,h,. Figure 1 shows the results of grinding tests for silicon nitide. 2.1. Specific normal and tangential grinding forces, f, and f i ,  and grinding force ratio This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or servia by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, m m -  mmdation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Y 9 3  ~ 2 l  . . .  : . . . : . . . : . . . i . . . (  Fg. 1. speciricgrindhgfweeagrirdifocceFp1D*8pBcr(cgrjndihg energy,surfacsfm~,ondrwnd~dCBNgmdingd~nlride. Note that lowercase letters are used to distqpsh specific force (force per unit length) firom force, which is shown in uppercase. Compared to the grinding results for zirconia and M2 (Shih, et al., 1998) using the same machine and CBN grinding wheel,f, andf, are much higher and FJFI is about the same for grinding silicon nitride. Effecz ofv,, Similar to the trend observed before, lower grinding forces and a highex grinding force ratio were generated at higher v, When &dmg silicon nitride, the lowest& and5 could be achieved at high v, (=I27 m/s) a d  high q d  (4.661). They are 8 and 0.8 N/mm, respectively, which are very high compared to the values grinding zirconia and M2 steel. Such high grinding forces will adversely affect the wheel wear rate. 30 50 70 90 110 130 -lno-s-spard(mh) FJF,, the normal-to-tangential force ratio, represents the tribological behavior between the grinding wheel and workpiece. FJF, remains low, below 10, for silicon nitride grinding at high coolant flow rate. This is due to the relatively high F, in silicon nitride grinding. The trend is similar for Zirconia grindmg. FJFt haeases at higher v, i.e., the F, decreases at a faster rate than F, or the fiiction coefficient decreases at high v, Effect of ad Truing at the highest qd (4.661) is beneficial because grinding forces are reduced However, unlike the four distinctly Mkrent levels of grinding forces at different q d  for zirconia and h4.2 steel, the o d  effect of q d  on fn, J, and FJFI is not obvious for silicon nitride grinding. 2.2. Specific Grinding Energy, u Silicon nitride sublimates at about 1900OC. The specific energy reqwred to transform silicon nitride finnn the solid state at room temperature to the gaseous state at 1900°C is only 4.2 J/mm3. The calculated u for silicon nitride ginding obtained h this experiment was much higher, exceeds 70 J/mm3. It is also higher than the lowest calculated u for h n i a  and M2 grinding, which equals 1 1.6 and 18.9 J h m 3 ,  respectiwly. Effect ofv,, The high v, does not result lower u. This is differentf?omthetrendinzinx>niagrinding. Efkct qf Truing at the highest q d  (4.661) yields the lowest tl during grinding. However, the overall e&t of q d o n  P is not as clear as in the zirconia and M2 steel grinding, 2.3. Surface finish, Ra Effect of v,, A trend of improving m i k e  f'mish (lower R,) at higher v,, could be seen for silicon nittide grinding, which is Merent h the Zirconia and M2 steel grinding, At vw=127 ds, the R, of ground parts wereallbetterthan0.15 pm. Effect of Q The effect of qd on surface finish was not significant. The four plots of surface finish at dif€erent qd were roughly parallel. 2.4. Roundness Unlike the trend observed for surface Gnish, higher Y,, does not always generate better roundness. The roundness at v,,=127 m/s and qfl.661 is quite high (4.6 p). 'The effect of q d  on roundness cannot be seen. 3. High Material Removal Rate Grinding of Zirconia and Silicon Nitride At high speed, more aggressive feed rates can be applied to achieve high material removal rate, reduce grinding time and possibly create savings in production. One of the objectives in this study is to investigate the effects of feed rate and dwell time on the high speed gnnding of ceramics. Three setup parameters in this test were: feed rate, dwell time and workmaterial. As shown in Table 3, five feed rates were tested. When grinding the silicon nitride at the highest radial feed rate, 0.34 &s, the high grindug forces stalled the workpiece due to the lack of power in the chuck motor. Tests for silicon nitride Table 3. Radial feed rates used in this study and their corresponding grinding time, specific material removal rate, material removal per revolution, and equivalent chip thickness. (for 9.525 mn da silicon nitride at start of the grinding) v+61 mls grinding at such high feed rate were abandoned. Two levelsofdwelltime,3and lOs,wereaddedtostudyits effects on surf& &ish and roundness. In summary, a total of 18 tests, 10 for zirconia and 8 for silicon nitride, werecarriedout. At high grinding wheel surface speed, the equivalent chip thickness can be estimated using the following formula (Mayer and Fang, 1997). 2 3 4 6 a 7 e e 10 SpocWk Yato~hl  Removal Rat., 0 ' (mm'hchnm) Fig. 2. The grinding forces, grinding force ratio, and specifii grinding energy v8. specifii material, ~emvai rate for cylindrical grinding of zirconia and silicon nitride. the grinding wheel. C ( ~ 3 8  g r i t s b 2 )  is the number of active cutting points per Unit area on the wheel surhce. v, is the surface speed of the workpiece. r (=lo) (Mayer and Fang, 1997) is the ratio of chip width to average The material removal per revolution and equivalent chip thickness, h,, at four Werent feed rates are listed in Table 3. The equivalent chip thicirness at 127 m/s wheel speed is about half of that at 35 d s  wheel speed Table 3 also shows the grinding time far a 5.613 mm diametrical material. It takes only 7.91 sec at 0.34 cun/s radial feed rate to grind thezirumia part h n  9to 3.387 vs. 9.525 mm), the specific material removal rates am slightly different for Ziroonia and silicon nitride. In the high feed rate tests, v,,=127 m/s and qd.661, which generate the lowest gnndin& were used. Figure 2 shows the effects of feed rate, dwell time and work- material on grinding forces v, andfj), epinding force ratio (FJF,) and specific grinding energy, u. The change of dwell time cbges.the surface &.ish and roundness, which are illustrated in Fig. 3. The linear regression analysis was applied to find the best-fit line to model the experhentall results, 5 data points for Zirconia and 4 data points for sficon mtride, against both Q,,' and a. The regression d t s  were shown in Figs. 2 and 3, along with the best-fit lines. 3.1. Specif= normal and tangential grinding forces, f., and f i ,  and grinding force ratio Eflect of Ow: As the feed rate or Qw ' inmead, the grinding time is s h o r t d  and higher J, and f, are generated. At highex Q,. ', the downward trend of FJFt illustrates that increasing iiiction occurs in both zirconia and silicon nitride grinding. Effect of Work-Material The rate of increase of& andJ for silicon nitride is h i g h  than that of zirconia. The trend of increasingfi for silicon nitride can explain why the test at 0.34 pmls feed rate was failed,due to the lack of motor power for the chuck. 3.2. Specifk grinding energy, u For silicon nitride, high Qw ' signtficanty duces the u h m  75 to 45 J/mm3. For zirconia, the u remains at about 10 J / m 3  across five material removal rates. The u mdqformed chip thicl<ness for Ceramic grinding. mm diameter. Due to the difference inpaxt diameters (9 3.0 2 3 4 5 6 7 6 9 Specrk Material Removal Rate, 0 , ' (mm'lsec Fig. 3. The roundness and suface finish vs. specific ma removal mte at 3 and 10 seconds dwU time for cylind grinding of zirconia and silicon n W e .  for silicon nitride is considerably higher than that of zirconia. 3.3. Roundness Figure 3 shows the roundness and surhce fhish vs. specific material removal rate (Qw 3 at 3 and 10 seconds dwell time. Effect of 0,l The effects of Qw' on roundness are different in silicon nitride and Zirconia grinding. For silicon nitride, the roundness deteriorates at higher Qw'. It may cause by the rapid increase in grinding forces. In contrast, the roundness of zirconia parts improves as the Qw ' increases h m  2.4 to 3.5 mm3/s/mm. Effict of Work-Material Silicon nitride parts, under the same truing and grinding conditions, have better surface fkishthanthezin;onia parts. Effect of Dwell Time Longer dwell time improves the roundness. For silicon nitride, the improvement of roundness is more sigmticant at high Qw '. . 3.4. Surface Finish, R, -UEL For zirconia, R. remains in the 0.18 to 0.2 pm level at five Merent Qw'. For silicon nitride, the overall trend shows the deterioration of surface finish at higher Qw '. Effect of Work-jUaterial Under the same process conditions, better roundness can be found on silicon Effect of Dwell Time Better surface finish can be seen on longer dwell time. nitrideparts. 4. Conciuding Remarks In this paper, experimental resuits of high speed, high zirconia ceramics weze presented The benefits of high &dmg wheel speed and high truing speed ratio were demonstrated to be beneficial to reduce the grinding forces and to increase the material removal rates. The increase in dwell time was also shown to be beneficial in terms of roundness and slpface finish improvement. The high grinding wheel surface speed not only generated lower grinding f- but also reduced the wear or improved the G-ratio when grinding ceramics. The follow-up experiments, d t s  not included in this paper, showed that the G-ratio improves as the griedins wheel s m h x  speed inoreaseS. Overall, this study showed that high speed grinding is beneficial to the Acknowledgment Partioas of this research were spansored by the High Temperaaae Materials Lab User Rogram, Oak Ridge N a t i d  Lab for the U.S. DepWent of Energy under contract number DE-ACO5-96OR22464. material removal rate grinding of silium nitride and ginding of ceramios. References Chand, R and Guo, C. (1996) "A New Concept in CoSt-Effedve Machinin&" The American ceramic society Bulletit& pp. 58-59. Maya, J. E. and Fang, G. P. (1997) ''Optimum GTinding of Structural Ceramics," SME 2nd International Machining & Grinding C o d m e ,  Sep. 8- ll,DearbOrn,MI. Shih, A.J., Grant,h.B., Yonushds, T.M., Monis, T.O., and McSpadden, S.B. (1998), "Vitreous Band CBN Wheel for High Sped Grinding of Ceramic and Steel," Transaction of the North American Mant&duing Research Institution of SME, Vol. 26, p ~ .  195-200. 

Vitreous bond CBN high speed and high material removal rate grinding of ceramics

Abstract

Similar works

Full text

Available Versions

UNT Digital Library