Wear mechanism of single crystal diamonds in ultra-precision cutting of graphite for air bearing applications

In order to meet the continuously increasing function-determining requirements of complex precision components, advances in machine tool components are necessary in addition to technological developments. In this context, motorized spindles in particular show great potential for enabling further increases in accuracy. To achieve high stiffness and damping behaviour of spindle systems, aerostatic bearings made of graphite with bearing gaps Sg < 10 μm are gaining more importance in ultra-precision machine tools. To ensure the function of these bearing components, a low surface roughness Ra ≤ 10 nm and shape accuracies aS ≤ 1 μm are necessary. Regarding the state of the art, uncoated tungsten carbide tools, diamond coated carbide tools and thick film chemical vapour deposition (CVD) diamonds are used for cutting graphite materials. Cutting with these tools leads to asymmetric breakout behaviour of the graphite grains as well as high tool wear during machining, resulting in insufficient shape accuracies and surface qualities. A promising approach to overcome the current challenges for aerostatic bearing components made of graphite is the use of single crystal diamonds (SCD) with characteristic low rounded cutting edge radii rß ≤ 20 nm as well as cutting-edge waviness wa ≤ 200 nm. Besides the material-specific properties of the SCD, tool wear occurs during the cutting of graphite. For this purpose, the abrasive wear mechanism and surface attrition of SCD were analysed concerning specific blasting tests with defined time intervals of 60 s ≤ tb ≤ 3,000 s and different process conditions. Furthermore, two types of graphite granulate with grain sizes in a range of 200 μm ≤ gs ≤ 400 μm were used. First results show a dimensional loss of material on the investigated specimens made of SCD depending on the process conditions, whereby an influence of the surface attrition could be proven.