Variations of tool life with cutting speed for the workpiece having hardness of HRc = 50 and 57 are shown in Fig. 6a and b, respectively. Both figures show that the CBN tool life is increased first with cutting speed. Once it reaches the highest tool life at a specific cutting speed (about 80 m/min), it starts to decrease thereafter. This trend is almost the opposite of the variation of friction force with cutting speed.