Modeling of ductile-mode machining of brittle materials for endmilling

Abstract

Brittle materials like glass and silicon are considered difficult-to-machine because of their high tendency towards brittle fracture during machining. In conventional scale machining of brittle material, crack-propagation is the typical mechanism of material removal which produces a degraded machined surface. The technological challenge in machining such brittle materials is to achieve material removal by plastic deformation rather than the characteristic brittle fracture. Ductile-mode machining has emerged as a promising technique to finish a crack-free machined surface on typically brittle materials. In ductile-mode machining of brittle material, the material removal takes place predominantly by plastic deformation and hence a crack-free finish is achieved on the machined surface. In the past, ductile-mode machining has mostly been performed by single-edge cutting process which has limited capability for machining complex shapes on the workpiece. The complex shapes on brittle material are typically fabricated by non-traditional processes such as photo-lithography, chemical etching, ultrasonic machining etc. These non-traditional processes have certain limitations in terms of material selection, machined surface characteristics and low material removal rate. Furthermore, these non-traditional processes employ highly specialized equipment which involves high capital investment and hence are not feasible for producing few parts or prototypes. It is therefore highly desired to develop an economically feasible process for rapid production of discrete parts or low-volume high-mix batches of brittle components with complex shapes and 2? dimensional features. Mechanical micro-milling is suitable process for low volume production and is capable of machining complex shapes and slots on the workpiece. The objective of this research is to perform ductile-mode machining by milling process such that crack-free features can be machined efficiently. This thesis presents theoretical and experimental study on ductile-mode machining of brittle materials by end-milling. Analytical models have been developed to determine the critical chip thickness, critical feed per edge and the effect of cutter diameter on the critical conditions for ductile-brittle transition in milling process of brittle materials. The experimental results have validated the proposed analytical models. It has been established that material can be removed by a combination of plastic deformation and brittle fracture in milling process of brittle material to finish a crack-free optical surface directly on the brittle workpiece without requiring secondary finishing process. The research work has potential applications in production of micro-molds, biomedical slides, and microfluidic devices from brittle work materials.