The polishing process of advanced ceramic balls using a novel eccentric lapping machine
Abstract The finishing process of advanced ceramic balls can be divided into two steps. The first step is lapping in which most of the stock from the ball is removed at a higher material removal rate. The second step is polishing in which the required ball surface roughness, roundness, and dimensional and geometric accuracy are achieved. In polishing, the abrasive particle size is 1 μm, and the load and speed are lower than lapping. A novel eccentric lapping machine is used for polishing hot isostatically pressed (HIPed) silicon nitride balls. In the initial polishing stage, the polishing load is demonstrated as being most influential in the reduction of surface roughness value Ra. However, in the later polishing stages, the erosive process played a major role in the further reduction of Ra, although the high roughness peaks cannot be removed by erosive process alone. Experimental results also show that, in order to achieve desired surface roughness value, the initial surface quality of the upper plate should be reasonably high, and deep scratches on the ball surface from previous lapping processes should be avoided. The best polishing results achieved were surface roughness values of Ra of 0.003 μm and r.m.s. (Rq) of 0.004 μm, ball roundness of 0.08–0.09 μm. This proves that the novel eccentric lapping machine is also suitable for polishing advanced ceramic balls.
A novel eccentric lapping machine for finishing advanced ceramic balls
Abstract Advanced ceramic balls are used extensively in hybrid precision ball bearings and show advantages in high speed, high temperature, high load and hostile environment. Finishing these balls with high quality, good efficiency and low cost is critical to their widespread application. A brief review of the methods for finishing ceramic balls is presented. The design of a novel eccentric lapping machine for finishing advanced ceramic balls is described. The kinematics of eccentric lapping is analysed and discussed, the symbolic expressions for the ball spin angular speed, ωc, ball spin angle, β, and ball circulation angular speed, ωc, are derived and numerical solutions are plotted. Two kinds of hot isostatically pressed (HIPed) silicon nitride ball blanks (13.25–13.50mm in diameter) were lapped and polished to 12.700 mm using this machine. A maximum material removal rate of 68 μm/h was achieved at the lapping step, which is much higher than by the traditional concentric lapping method. The polished ball surface roughness, Ra, value is 0.003 μm, and the ball roundness is 0.08–0.09 μm, which is above grade 5 and close to grade 3 of the precision bearing ball specification. This machine can be used as a prototype to develop a larger-scale machine for production.
Magnetic fluid grinding cell design
Magnetic fluid grinding is a process for the rapid grinding and polishing of ceramic ball bearings. Grinding grits are levitated in a ferrofluid contained above a bed of permanent magnets; and balls to be finished are driven through this medium by a shaft against which they are loaded, also by magnetic forces. This paper describes a theoretical and experimental study of the magnetic forces which exist in the process and discusses design of the magnetic hid grinding cell in terms of these. Shaft contact loads of the order of Newtons per ball are achievable and grits may be levitated tens of mm above the magnet bed.