AN ANALYSIS OF SURFACE CRACKING DURING ORTHOGONAL MACHINING OF GLASS

It has been proposed that a rough-semi-finish-finish strategy may be possible in the machining of glass and other brittle materials to achieve higher productivity than is realized through either grinding or ductile-mode machining, both of which have been studied extensively to date. A previously presented experimental study of orthogonal glass cutting exhibited clear transitions in machining modes as the uncut chip thickness is increased. One of those modes involves ductile-mode chip formation combined with surface damage in the form of surface cracks that protrude down into the machined surface and ahead of the cutting edge. Here, a model is formulated and exercised to better understand this surface-cracking damage. The finite element method is used with a custom written re-meshing subroutine employed under a commercial software package. The analysis focuses on the crack depth and lead (ahead of the tool) as a function of the normalized process force and the fracture toughness of the work material. It is found that load ratio, the ratio of the cutting (surface-tangential) force to the thrust (surface-normal) force, plays a significant role in the crack growth problem, as does the manner in which the thrust load is distributed relative to the cutting load. It is shown that point-wise application of the loads produces results far off from the experimental results, whereas distributed loads can produce results well aligned with the experiments. Given that ductile-mode chip formation occurs during surface cracking, the load distributions found to work well exhibit qualitatively the same characteristics that one would expect based on extending the well-known mechanics of metal cutting.