PURPOSE: To improve a contact detector in detection sensitivity of the contact condition between a tool and a workpiece by placing a winding for electromagnetic induction in the looped secondary circuit formed by the contact in order to raise induced voltage.
CONSTITUTION: In contact detection, when a main spindle 15 is rotated at a high speed, contacts 29 are closed. When the tool T and the workpiece W are relatively moved to contact with each other, the secondary circuit loop having the secondary winding 25 is formed. The secondary circuit loop has the secondary winding 25 with more windings than a conventional loop which passes through a ring iron core only once. Consequently, the induced voltage is increased, and if the contact resistance of a brush 21 increases, a large current can be supplied through the secondary circuit loop, so that the detection sensitivity of a detection circuit 28 for contact detection can be improved. Moreover, even if the contact resistance of the brush 21 varies a little, the contact detection can be reliably performed, so that safety can be improved.
YOKOE ISAMU
OWA KAORU
