The time sequence and spatial patterns of material deformations in the machining of ceramics are investigated through analysis of single point diamond turning of silicon and germanium model brittle materials. It was found that the key to fine surface finish without fracture is ductile or plastic deformation, but that only a small critical portion of the material removal need be ductile. Experiments based on the rapid withdrawal of the tool from the workpiece have shown that microfracture damage is a function of the effective chip thickness (as opposed to the nominal cutting depth). Damage created by the leading edge of the tool is removed several revolutions later by lower sections of the tool edge, where the effective cutting depth is less. It appears that a truly ductile cutting response can be achieved only when the effective chip thickness is less than about 200 nm (or about ten {dollar}mu{dollar}in), but that this restriction does not limit the cutting depth. Factors such as tool rake angle are significant in that they will affect the actual value of the critical chip thickness for transition from brittle to ductile response. It is concluded that the critical chip thickness is a powerful tool for measuring the effects of machining conditions on the ductility of the cut and for optimizing machining parameters and tool-workpiece geometry in both turning and grinding.
展开▼
