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Finishing of Silicon Nitride (Si3N4) Balls for Advanced Bearing Applications by Large Batch Magnetic Float Polishing (MFP) Apparatus

Kirtane, Tejas Shrikant
This investigation deals with the development of a new apparatus for finishing large diameter and large batch of silicon nitride (Si3N4) balls (46 balls of 3/4 in. diameter) for hybrid bearing applications. Three stages are involved in magnetic float polishing of ceramic balls, namely, roughing, semi-finishing, and final finishing. Abrasives such as B4C, SiC, and CeO2 are used under low polishing loads (~ 1 N/ball) and high polishing speeds (~ 400 rpm for a 12.2 in. diameter polishing spindle). Taguchi method was applied in the roughing stage to optimize the material removal rate. In-situ machining of the polishing spindle is found to be essential for improving the accuracy and geometric alignment of the system. During polishing the chamber can be self-aligned with the spindle. A groove is formed on the bevel of the spindle, which plays an important role in each stage. Initially, in the roughing stage, it is preferable, though not essential, to machine the groove in order to obtain a high material removal rate. It is, however, necessary to maintain the groove formed at the end of roughing stage in order to improve sphericity. The sphericity can be significantly improved in the semi-finishing stage by not machining the groove. In the final finishing stage, machining the groove is necessary for rapid improvement in the surface finish. A batch of 46, 3/4 in. Si3N4 balls was finished to a final diameter of 0.7500 in. with a good sphericity ~ 0.25 m and surface finish, Ra ~ 8 nm in an actual polishing time of ~ 24 hours. This technology is easy to implement in industry and would not require high capital investment.