Ultraprecision cutting of graphite with monocrystalline diamond tools

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Modern air bearings made of graphite depend on highest requirements concerning shape accuracies as and surface qualities. Air bearings are characterised by good sliding, try and emergency running properties, can reach high stiffness and show a high fatigue strength as well as a good shape resistance, which results in an increasing demand for industrial applications. Unfortunately, the machining of graphite shows a high breakout behaviour as well as a considerable tool wear. In order to machine the air bearings made of graphite concerning the high requirements, the ultra-precision machining represents an established manufacturing process and enables the fabrication of surface roughness values in the single-digit nanometer range and shape accuracies as in the submicrometer range. The cutting materials made of polycrystalline diamond (PCD) and chemical-vapour-deposition(CVD)-diamond, which were conventionally used show a considerable higher radius waviness WR, rounded cutting edge radius rβ and crack growth Cg compared to monocrystalline diamonds (MCD). Therefore, tools made of MCD are applied. Using the specific geometric properties of the MCD tools e.g. the rounded cutting edge radius rβ leads to different process conditions, which changes the stress conditions, the effect of the forces as well as the breakout behaviour. The aim of the research project is the identification of the separation mechanisms and the breakout behaviour during the machining of graphite with tools made of MCD. In order to reduce the breakout rate ξB and the tool wear, a transcrystalline cutting process is to be achieved.To realise the research project, the used graphite materials are characterised as well as the basic separation mechanisms will be obtained and detailed wear investigations of the MCD-tools are carried out. In order to analyse the breakout rate ξB and the tool wear, investigations concerning of ultra-precision turning are applied. Based on this, an explanatory model will be developed, which describes the mechanisms during the cutting process. In order to show the temperatures ϑ and the pressure loads during the cutting process, a particle-based simulation is carried out. This simulation model enables the time determination of the tribo-chemical tool wear. The results of the research project provide a fundamental knowledge of the cutting process and the breakout behaviour of graphite materials using MCD with a reduced tool wear.