Fundamental mechanisms in orthogonal cutting of medical grade cobalt chromium alloy (ASTM F75)

Abstract

Cobalt chromium alloys are sui generis materials for orthopaedic implants due mainly to unique properties of biocompatibility and wear resistance in the demanding in-vivo environment. Notwithstanding the importance of both defined and undefined edge cutting processes on form, finish and surface integrity of orthopaedic components, there has been minimal research reported in the public domain on the fundamental mechanisms in cutting of these alloys. Accordingly, this paper reports on initial research into cutting of the biomedical grade cobalt chrome molybdenum (Co-Cr-Mo) alloy, ASTM F75.Following a brief overview of physical and mechanical properties of this class of Co-Cr-Mo alloys, the results of a full factorial, orthogonal cutting experiment are presented. This involved measurement of force components (Ft and Ff) as a function of the undeformed chip thickness (h) and cutting speed (vc) which were varied over ranges from 20 to 140μm and 20 to 60m/min respectively. The results demonstrated an expected linear increase in force components with h at speeds of 20 and 60m/min. However, at the intermediate speed of 40m/min, there was a transition between about 60 and 80μm indicating a discontinuous rather than continuous effect of speed. The results also enabled determination of the cutting force coefficients Ktc, Kte, Kfc and Kfe, as well as 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.