Tool-wear mechanisms in hard turning with polycrystalline cubic boron nitride tools

Hard turning is a turning operation performed on high strength alloy steels (45 < HRC < 65) in order to reach surface roughness close to those obtained in grinding (R_a ˜ 0.1 μm). Extensive research being conducted on hard turning has so far addressed several fundamental questions concerning chip formation mechanisms, tool-wear, surface integrity and geometric accuracy of the machined components. The major consideration for the user of this relatively newer technology is the quality of the parts produced. A notable observation from this research is that flank wear of the cutting tool has a large impact on the quality of the machined parts (surface finish, geometric accuracy and surface integrity). For components with surface, dimensional and geometric requirements (e.g. bearing surfaces), hard turning technology is often not economical compared with grinding because tool-life is limeted by the tolerances required (i.e. high flank wear rate). The aim of this paper is to present the various modes of wear and damage of the polycrystalline cubic boron nitrides (PCBN) cutting tool under different loading conditions, in order to establish a reliable wear modeling. Flank wear has a large impact on the quality of the parts produced and the wear mechanisms have to be understood to improve the performance of the tool material, namely by reducing the flank wear rate. The wear mechanisms depend not only on the chemical composition of the PCBN, and the nature of the binder phase, but also on the hardness value and above all on the microstructure (percentage of martensite, type, size, composition of the hard phases, etc.) of the machining work material. The proposed modeling is in a generalized form of the extended Taylor's law allowing to prediction of the tool-life as a function of the cutting parameters and of the workpiece hardness. The effects of these factors on tool-wear, tool-life and cutting forces are discussed in the paper.