THE MATERIAL REMOVAL MECHANISM IN MAGNETIC FLUID GRINDING OF CERAMIC BALL-BEARINGS

Recently a new process known as magnetic fluid grinding has been developed in Japan which can remove material in the finishing of ceramic balls some fifty times more rapidly than can the lapping processes that are conventionally used. Balls are driven round is 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, but it is not clear why the process is more effective than lapping. This paper reports on experimental studies of the grinding rates of silicon nitride balls in magnetic fluids loaded with diamond grits and of the motion of the balls through the fluids. The high removal rates occur when skidding occurs between the balls and drive shaft. A kinematic theory is developed to calculate sliding speeds and is used to deduce an abrasive wear coefficient for the process of 0.07 +/- 0.02, indicative of two-body abrasion. The high removal rates are a consequence of the large sliding speeds that can be developed, of several metres per second.