Development of grinding models for brittle materials

Accurate prediction of the onset of fracture during the grinding of brittle materials would enable parts to be made with little or no subsurface damage. The fracture mechanics of an indentation test provides a model for the behavior of brittle materials. During an indentation test, a critical depth exists where the energy of new surface formation becomes less than the energy of plastic deformation and the material begins to fracture. In grinding the critical infeed per revolution is an order of magnitude less than the critical indentation depth. For grinding, the indenters are the grit randomly dispersed in the grinding wheel. Determination of the individual depth of cut of the grit requires relating the distribution of the grit to the surface of the workpiece and the desired infeed. This research develops an analytic model which solves for the depth of cut using the statistics of a random distribution of grit cutting a surface. Two distinct cutting regimes can be identified. At extremely low infeeds the grain depth of cut is equal to the infeed, but as the infeed is increased the depth of cut becomes less than the infeed. Using this model as a baseline, the relation of the specific grinding energy {dollar}musb{lcub}rm g{rcub}{dollar} to the grit depth of cut was developed for ductile regime grinding with spherical grit. In the ductile regime, at nanometer level infeeds per revolution {dollar}musb{lcub}rm g{rcub}{dollar} was predicted to have a negative power law relation to the depth of cut. Plunge grinding experiments were conducted on BK7, CVD SiC, and silicon, which measured the cutting and normal forces in-situ and the workpiece surfaces after the tests. A method of determining {dollar}musb{lcub}rm g{rcub}{dollar} for each revolution of the wheel was developed. For each of the plunge grinding tests, {dollar}musb{lcub}rm g{rcub}{dollar} had a minimum value corresponding to the as-dressed wear state of the grit. The minimum {dollar}musb{lcub}rm g{rcub}{dollar} values confirmed the negative power law relation of {dollar}musb{lcub}rm g{rcub}{dollar} to the depth of cut in the ductile regime and found that {dollar}musb{lcub}rm g{rcub}{dollar} is no longer a function of the depth of cut in the brittle regime. The transitions infeed rates identified by minimum {dollar}musb{lcub}rm g{rcub}{dollar} were verified by optical and surface roughness measurements of the workpiece surfaces.