A thermo-mechanical force model for machining hardened steel

The machining of hardened steels is becoming a viable technology. At the inception of the present research, this technology for milling processes was in its infancy. Advancements in cutting tool materials such as poly-crystalline cubic boron nitride (PCBN) have enhanced the ability to machine these difficult to cut alloys.; In this thesis, an experimental investigation is presented regarding the optimal process parameters that make high speed hard machining a viable technology. The experimental investigation shows that hard machining of AISI H13 (55 HRc) is possible, and is an extremely effective technology if the proper conditions are used. Tests were also performed on hardened AISI D2 tool steel (62 HRc), which showed that this material in its hardened state challenges the ability to machine this material in the fully hardened state. The primary tool failure modes are outlined and a detailed analysis of the chip formation mechanisms is reviewed.; Owing to the difficulty associated when machining hardened AISI D2 tool steel, the development of an analytic force model was attempted. The modelling methodology required a correlation of the flow stress (the mechanical response of the material) with the cutting conditions in the form of kinematic parameters derived from chip morphology. The hardened material was characterised using high strain rate ballistic impact tests (using the compressive split Hopkinson pressure bar) in a punching shear configuration.; Having a correlation of the flow stress of the fully hardened material, the force model was derived using the chip morphology and chip formation kinematics to represent the governing strain and strain rate conditions during machining. The resulting formulations allowed for a time domain orthogonal machining simulation represented by specific inputs such as cutting speed and feed rate. The orthogonal formulation was verified against experimental data and showed good correlation with observation.; The orthogonal formulation was extended to the ball milling process (an oblique cutting configuration) to test the validity of the force model. Good correlation was realized between the experimental and predicted results. The ball milling process challenged the validity of the force model by applying the modelling strategy to small chip loads and low cutting speeds. The predicted results also rationalized the tool failure mode that was observed in the ball milling investigation.