The effects of lapping load in finishing advanced ceramic balls on a novel eccentric lapping machine

HIPed (Hot Isostatically Pressed) silicon nitride ball blanks were lapped from diameter 13.255 mm to diameter 12.7 mm by a novel eccentric lapping machine. A maximum material removal rate of 68 μm/hour has been achieved under a nominal lapping load of 43 N/ball. It was found that the material removal rate was increasing almost linearly with the lapping load within this load range. When the lapping load was higher than 43 N/ball, the material removal rate started to drop and the lapped ball roundness error started to increase. At the highest nominal lapping load of 107 N/ball, surface and subsurface damages were found on the lapped balls. Because of eccentric loading effect, the actual load on individual ball could be 25~28% higher than the nominal lapping load. The surface residual stresses of lapped balls under different lapping loads were measured, and it was found that the lapping load had less effect than previous HIP process. Rolling contact fatigue tests were conducted on balls lapped at nominal loads of 43N/ball and 107 N/ball. No failure occurred on the ball lapped at 43 N/ball after 138 million stress cycles. Ball lapped at 107 N/ball was failed after 13.3 million stress cycles with a shallow spall with flat bottom inside. This research suggests that the lapping load for advanced ceramic balls in conventional concentric lapping could be doubled from 20N/ball to 40 N/ball without degrading the surface quality of lapped balls