This paper describes a monitoring method of multi-edge tools by means of fluctuation signals of spindle rotational speed. The fluctuation signal can be measured from a tachometer generator or a pulse generator as easily as the spindle motor current which is widely used for the cutting state monitoring, but it shows more clear correspondence with the cutting torque variation induced by intermittent cutting with multi-edge tools. An algorithm is proposed for detecting tool breakage, in which the speed fluctuation pattern during one revolution of the spindle is extracted in the form of a cutting state vector and is compared with a reference vector. The effectiveness of the method is proved by the monitoring experiments in which the breakage of a cutting edge of a face mill cutter was clearly recognized.
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