PURPOSE:To enable to automatically confirm the abnormality such a wear, break down and the like of the tip of the tool by a method wherein electric signals from a piezo-electric sensing element imbedded in the tool are detected. CONSTITUTION:The piezo-electric sensing element 4 is imbedded in the tool 1. During cutting operation, the tip 3 abuts against a work and is given vibration, which is converted to electric signals by means of the piezo-electric sensing element 4. If abnormality occurs in the tip 3, the magnitude of the amplitudes of said electric signals changes and is automatically detected, resulting in causing to stop the cutting operation or actuate an annunciator.
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