Fundamental Mechanisms in Orthogonal Cutting of Medical Grade Cobalt Chromium Alloy (ASTM F75)

Cobalt chromium (Co-Cr-Mo) alloys are sui generis materials for orthopaedic implants mainly due to the unique properties of biocompatibility and wear resistance in the demanding in vivo environments. Notwithstanding the importance of the machining processes, a review of literature in the public domain has identified a niche for research into the fundamental mechanisms in cutting of Co-Cr-Mo alloys. This paper reports on initial research into cutting of the biomedical grade cobalt chrome molybdenum (Co-Cr-Mo) alloy, ASTM F75. Following an initial review of the known micro-structural, physical and mechanical properties of the class of Co-Cr-Mo alloys, the results of a full factorial, orthogonal cutting experiment are presented. This involved measurement of force components (Ff and Ft) as a function of the undeformed chip thickness (h) and cutting speed (vc) which were varied over ranges from 20 to 140 µm and 20 to 60 m/min respectively. The results demonstrated an expected linear increase in force components with h at speeds of 20 and 60 m/min. However, at the intermediate speed of 40 m/min, there was a transition between about 60 and 80 µm indicating a discontinuous rather than continuous effect of speed. The results enabled determination of the cutting force coefficients Ktc, Kte, Kf c and Kf e, for the ranges examined as well as the coefficients, ki1.0.1 and mi0.1, of the Kienzle equations. These relations will enable macro-mechanic modelling of more complex cutting operations, such as milling, in the future.