Tool wear mechanisms and tool life enhancement in ultra-precision machining of titanium

Titanium and its alloys are generally considered as difficult-to-machine materials due to their poor thermal conductivity and high strength, which is maintained at elevated temperatures. This paper examines the tool wear mechanisms involved in ultra-precision machining of titanium. In this study single-crystal diamond tools were used to machine commercial pure titanium (CP-Ti) and Ti-6Al-4V alloy. Industrial expectations for surface quality and tool life based on optical grade applications are presented. Results obtained from the characterization of the tool, chip and workpiece led to the identification of graphitization as the mechanism that initiates tool wear. As the cutting edge rounds-off due to graphitization the rate of adhesion of the workpiece material onto the tool increased, which caused the quality of the surface finish to deteriorate. To reduce this wear mechanism a protective barrier made of Perfluoropolyether (PFPE) polymer, was explored. Tribometer studies with PFPE coated diamond tools and titanium pins showed a reduction in the coefficient of friction (COF). Subsequent machining tests using PFPE coated diamond tools showed promising results in extending the tool life and enhancing the surface quality to a point where Ti can now be considered as a viable option for applications involving optical grade surfaces.