Abstract
The early fracture is investigated experimentally that occurs in the turning of hardened steel with ceramic tools, and the occurrence condition is clarified. And a method of evaluation is propounded for the early fracture toughness. A turning test is carried out with hardened SUJ2 steel (HRC66) and ceramic tool. The critical cutting speed decided by one-minute criterion is found out, and the influences for the fracture of cutting conditions are clarified. The main findings are as follows : (1) Black ceramic tools show normal wear and long life at lower cutting speed. But they show suddenly early fracture at high speed. (2) The critical cutting speed becomes high according to the decreasing of feed rate, depth of cut and the increasing of corner radius. (3) The critical cutting speed is decided by the maximum undeformed chip thickness. (4) The early fracture is affected by rather the cutting temperature than the cutting force. (5) The early fracture toughness of various ceramics tools is evaluated by the maximum undeformed chip thickness and the critical cutting speed. (6) The early fracture is controlled by the coolant.
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