High speed machining of hardened steels is the recent preferred manufacturing technology in die and mold manufacturing technology. P20 tool steel is the most widely used material in injection molding dies, and its high speed-high productivity machining is the focus of this thesis. P20 has an average hardness of 35 Rc. High speed cutting of P20 is constrained by chatter vibrations, accelerated tool wear and chipping of cutting tool edges. The chamfered cutting tools are used in machining hardened steels due to their increased strength of cutting edges. An analytical model, which allows the evaluation of cutting forces, stress and temperature distribution for cutting tools with chamfered edges, is studied in this thesis. The cutting process is modeled at three distinct zones by extending the slip line field proposed by Oxley et al. [36].

The primary and secondary deformation zones are treated similar to the work of Oxley, but the flow stress characteristics of the work material are calibrated from orthogonal cutting tests, as opposed to high-speed compression or tensile tests. The chamfer zone is modeled by assuming dead metal trapped over the chamfered edge. The trapped metal is assumed to be stationary and the metal flows around it similar to the extrusion process. The contact between the rake face and chip is assumed to have equal sticking and sliding lengths, and the total contact length is measured experimentally. The flow stress of the material in all three zones are expressed as a function of temperature, strain and strain rate. The deformation and friction energy in all three zones are evaluated individually, and summed to find the total energy consumed in forming the chip. By applying the minimum energy principle to total energy consumed, an average shear angle in the primary shear deformation zone is predicted. The overall analytical model allows evaluation of stress, temperature and cutting forces contributed in each deformation zone for a given set of cutting conditions and chamfered cutting tool geometry. The predicated and experimental results obtained from orthogonal turning of P20 steel with ISO S10 carbide and Cubic Boron Nitride (CBN) tools agreed well. The model and experimental results indicate that the optimal chamfer angle is about -15 degrees, and optimal cutting speeds are about 240 m/min and 500 m/min for ISO S10 carbide and CBN tools, respectively. The model predicts a rake face temperature, which is just under the diffusion limit of binding materials for S10 and CBN tools at the optimal cutting speeds and chamfer angle.