Hard turning of machine parts is a production process that holds considerable promise for the future since it is an effective means of increasing productivity. Instead of machining a product in the soft state, hardening the part by heat treatment, and then providing the required finish and dimensional accuracy by grinding the expensive grinding operation is eliminated. This is possible because of the availability of improved hard tool materials (polycrystalline cubic boron nitride and ceramics), and more rigid machine tools. The nature of chip formation in hard turning is quite different than in more conventional machining. As might be expected, fracture plays a much more important role in the machining of very hard materials. As in all facets of production engineering it is important to identify the key element involved in the operation and to give far less attention to secondary effects that tend to cloud an issue. The objective of this paper is to establish the key action involved in hard turning chip formation and then to develop the mechanics involved as far as possible at the moment.

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