Hardened die mold steels have been conventionally made by electric discharge machining. Cutting these hardened steels would have large effects, allowing speeder delivery and lower production cost of die molds. In this study, therefore, a ball end mill has been developed and conditions for cutting hardened steels have been investigated.

First, the change in cutting condition and the damage to cutting edges have been examined by cutting the die mold steels, which have been hardened from 40 HRC to 60 HRC.

Next, the tough sintered CBN was selected for a cutting tool material, and a ball end mill was produced by the way of trial. This tool produced had removed end on the rotation axis and cutting edges strengthened by chamfering.
As a result of the endurance test, the life of the ball end mill was found to be shorter at the cutting plane due to its chipping. The damaged cutting edge was then analyzed by EPMA, which revealed that the steels of the workpiece adhered to the cutting edge.

For the prevention of the failure, the cutting zone was lubricated with the mist oil which gave a little thermal shock to the cutting edge. Spraying the mist of the high lubricant lengthened the life of the cutting tool. Under the condition of the high cutting speed, the roughness and the change in hardness were smaller than those of the discharge-machined surface, thus producing the higher quality finished surface.
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