Analytical model to determine the critical conditions for the modes of material removal in the milling process of brittle material

Brittle materials are prone to cleavage-based fracture during machining. In conventional scale machining of brittle material, crack-propagation is the dominant mechanism of material removal which results in a degraded machined surface. The challenge is to perform machining of brittle material such that the material removal occurs predominantly by chip formation rather than the characteristic brittle fracture. In this case, a high quality finish is achieved on the machined surface. Ductile-mode machining has emerged as a promising technique to finish a crack-free machined surface on macroscopically brittle materials. In the past, ductile-mode machining has mostly been performed by single-edge cutting process. This paper outlines an analytical model to determine the critical conditions for finishing a crack-free surface on brittle material by milling process. Four distinct modes of machining have been identified in the milling process of brittle material. In this model, the critical conditions for different modes of machining have been determined with respect to the relationship between the radial depth of cut and the depth of subsurface damage caused by the brittle fracture during machining. Verification tests were performed on tungsten carbide workpiece and the experimental results have validated the proposed machining model. It has been established that if the radial depth of cut is greater than the subsurface-damage depth in the milling process of brittle material, it is possible to finish a crack-free machined surface by removal of material through a combination of plastic deformation and brittle fracture. However, if the radial depth of cut is less than the subsurface damage depth, brittle fracture must be prevented in ductile-mode milling to finish a crack-free machined surface.

► This model determines critical conditions for ductile mode milling.
► Four modes of machining are possible in milling process of brittle material.
► At low radial depth of cut, critical feed per edge is variable.
► At high radial depth of cut, critical feed per edge is constant.