Observations on the Polishing of Metals

We present some observations on traditional (nonchemical) methods of lapping and polishing metals. Both relatively hard, nickel and hardened stainless steel, and relatively soft, copper and aluminum, materials will be discussed. Lapping and polishing techniques for the more easily polished hard materials yield poor to disastrous results for the softer metals. We further observe that metals require techniques different from glass due to the generally crystalline structure of metals. Glass being amorphous has a uniformly hard surface and responds uniformly to a variety of (sometimes rather brutal) polishing techniques. Metals on the other hand vary in hardness on the microscopic level due to crystal orientation and grain boundaries. Worse yet, they may also be stressed or work hardened from previous machining operations. Finally, the apparent hardness is affected by abrasives being embedded in the surface during lapping and initial polishing processes. Prior to obtaining a satisfactory finish, all these surface defect areas must be polished out using a lap and polishing compound designed to remove the harder material as easily as the softer. These considerations lead to diamond being considered a natural choice for polishing compound.