Forces, form deviations and surface roughness in micro-milling of CoCr alloys for dental prostheses

Micro-milling, besides selective laser melting, is commonly used for the manufacture of dental prostheses. Demands to the application of these prostheses require hard and tough materials like cobalt-chrome alloys or ceramics. The cutting process is governed by static as well as dynamic interactions between the process and the structure of the workpiece. These interactions deteriorate the work result, especially the shape accuracy. The deflection of the milling tool is one effect, which has a significant influence. However, the deflection can be minimised by optimising the manufacturing strategy, by an adjustment of the process parameters or by optimising the tool path, which leads to an increased quality of the final product. The work presented in this paper aims to establish a connection between tool deflection and resulting form deviations during micro-milling of defined geometries in CoCr alloys. This is achieved by measuring the active forces while machining an defined test geometry with different feed rates and radial infeeds and a subsequent evaluation of the resulting form. Furthermore, the influence of the workpiece geometry on the forces and form deviations is analysed to serve as a foundation for future optimisation steps. In order to quantify the surface quality, white light interferometry is applied to measure the surface roughness Sa and Sq and the influence of feed rate vf and radial infeed ae on the surface roughness is derived.