CBN tool material has been applied widely in the cutting of hardened steels, tool steels, difficult-to-cut materials, etc. due to its high temperature stability, hot hardness and low affinity to iron. In cutting hardened steels of different hardness by CBN tool, it was found that tool wear reduced first with the increase of the hardness of work material until a hardness of about HRc 50 where the wear started to increase abruptly. This phenomenon had been studied and unveiled by several investigators [1–4]. In cutting hardened steels, it was also noted that the life of CBN tools showed an increasing and then decreasing trend with increasing cutting speed [5–7]. This is quite different from that described by the traditional Taylor’s tool life equation where tool life is inversely proportional to cutting speed. Regarding CBN tool wear mechanism, Zimmermann et al. [8] showed that wear of the tool in the crater region was primarily tribochemical, and most likely chemical in the flank region.