The cutting performance of cBN tools for high-speed end milling of hardened steel is investigated. The cutting characteristics are evaluated by tool temperature at the flank face, cutting force, chip geometry and surface roughness at the change of cutting speed ν and workpiece hardness. The temperature of cutting tool is measured using a two-color pyrometer with an optical fiber. The tool flank temperature θα increases with the increase of cutting speed and workpiece hardness, and reaches approximately 850°C at ν=600 m/min and HRC60. In particular, the workpiece hardness has great influence on the tool flank temperature. On the other hand, the principal cutting forces does not change so much with workpiece hardness. Based on the chip geometry measured, the most likely explanation for these phenomena is that chip formation process changes with workpiece hardness. As the workpiece hardness increases, the cutting actions become stable so that the surface roughness decreases. In the case of the low cBN content tool, tool flank temperature increases by 20∼50°C or more than high content one, because of the low thermal conductivity.
