Ultraprecision machining of nitrocarburized steels

Abstract

Ultra precision machining processes generate surfaces in optical quality and with sub-micron form accuracies. These specifications can be realised by applying single crystal diamond tools with nanometric edge sharpness. Typical workpiece materials are non-ferrous metals which can be machined without significant tool wear. But for optical mould making these materials have disadvantages regarding tool life in injection moulding of plastics. Alternatively diamond cutting of thermo-chemically treated steel is a new way to machine hardened steel moulds. This paper presents results from the machining of two thermo-chemically treated steel alloys. Analyzing and evaluating the machining results regarding surface integrity will lead to recommendations for the ultra precision machining of nitrocarburized steels. The influence of the thermo-chemically generated compound layer composition on surface quality and tool wear has been investigated. Therefore, diamond turning experiments have been carried out on a five axes ultra precision lathe in different depth beneath the surface. Here, both steels can be machined in optical surface quality with a surface roughness Sa < 10 nm, but the achievable surface quality strongly depends on the depth beneath the surface in which the machining takes place. The results show that with increasing depth beneath the surface the roughness values increase as well. Therefore, diamond machining at the edge between compound layer and diffusion layer has to be avoided to gain the best possible surface quality.