A new apparatus for finishing large size/large batch silicon nitride (Si3N4) balls for hybrid bearing applications by magnetic float polishing (MFP)

A new apparatus was designed and built for the finishing of large size/large batch silicon nitride (Si₃N₄) balls by magnetic float polishing (MFP) technology for hybrid bearing applications. The polishing chamber is so designed that during polishing it can self-align with the upper part of the polishing chamber connected to the spindle. In situ machining of the upper part of the chamber is performed on the machine tool in which the apparatus is located, in order to achieve high accuracy and geometric alignment of the system. The finishing methodology consists of mechanical polishing followed by chemo-mechanical polishing. Boron carbide (B₄C), silicon carbide (SiC), and cerium oxide (CeO₂) are the three abrasives used in this investigation. Three stages are involved in polishing, namely, 1. a roughing stage to remove maximum material without imparting any damage to the surface, 2. an intermediate stage of semi-finishing to control the size and improve sphericity, and 3. a final finishing stage to obtain best surface finish and sphericity while maintaining the final diameter. Taguchi method was applied for the roughing stage to optimize the polishing conditions for the best material removal rate. Level average response analysis has indicated that a load of 1.5 N/ball, an abrasive concentration of 20%, and a speed of 400 rpm would give a high material removal rate using B₄C (500 grit) abrasive. A groove is formed on the bevel of the upper part of the chamber which plays different roles, some beneficial and other not so beneficial, in each stage. In the roughing stage, it is preferable, though not essential, to machine the groove after each run to maintain high material removal rates. It is, however, necessary to remove the groove formed at the end of the roughing stage. In the intermediate or semifinishing stage, sphericity can be significantly improved by not machining the groove. Thus groove, in this case, facilitates in the improvement of sphericity. Before the beginning of 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 an average sphericity of 0.25 μm (best value of 0.15 μm) and an average surface finish, Ra of 8 nm (best value of 6.7 nm) with an actual polishing time of <30 h. This technology is easy to implement in industry and does not entile high capital investment.