Ultra-precision grinding of hard steels

Abstract

Hardened bearing steel, M50, has been ultra-precision ground to produce an optical quality surface (<10 nm R a) using a novel ultra-stiff machine tool, Tetraform ‘C’. It has been shown that a repeatable surface finish of <10 nm R a can be produced using a 76 μm CBN grit and 500 μm wheel depth of cut. This represents a significant improvement over previous published work using conventional precision machine tools where nanometre surface finish can only be obtained at the expense of process efficiency. The development of optical quality surfaces is considered in terms of the processes occurring in the primary and secondary finishing zones of the cup-wheel, with the final surface finish enhanced by the burnishing action of worn CBN grits. It is shown that surface finish is limited by the pull-out of carbides in the secondary finishing zone. However, this can be overcome by using electrolytic in-process dressing (ELID), which maintains CBN grit protrusion and sharpness. This promotes cutting of the carbides at the ground surface and ensures a high level of surface integrity although the burnishing action of grits is reduced resulting in a slightly higher roughness for the steel matrix.