MAGNETIC FLUID GRINDING MECHANICS

Recently a new process has been developed in Japan known as magnetic fluid grinding which can remove material in the finishing of ceramic balls some fifty to one hundred times more rapidly than the lapping process that is conventionally used. Balls are driven round a cell by a rotating shaft in an arrangement similar to a thrust race but submerged in a magnetic fluid placed above permanent magnets. The magnets and fluid create buoyancy forces that levitate grinding grits in the fluid and also provide the loads for the process. The high removal rates occur when skidding motions are generated between the balls and drive shaft. This paper offers a first attempt to model the mechanics of the process, to predict the onset of skidding motions. It considers the force and moment equilibrium of the balls acted on by the forces and moments at the balls' contacts with the drive shaft and other surfaces and by fluid drag forces and moments. It presents measurements of the friction coefficients relevant to the process and of the viscosities of magnetic fluids containing grinding grits, for use in the theory. Predictions of skidding are qualitatively in accord with measurements but further developments are necessary for a good quantitative model.