PURPOSE: To stably hold the relative position of the grinding stone face and the work finishing size face in such a way that output signals from a cross slide position measuring instrument at the place where the grinding stone is brought into cntact with the work are compensated.
CONSTITUTION: Infeed rates V1, V2 at the time of rough finishing and at the time of fine finishing are given as input from an input device 21. Then, a grinding stone spindle head 4 is moved ahead, a grinding stone 4a is inserted into the grinding hole of a work W, an in-process specified length device 8 is also inserted into the work, the former process size of the work is measured, to be memorized into a memory circuit 11. A cross slide is fed in by means of a servo motor 7. Then, when the difference between the former process size of the work and the output of the specified length device 8 is generated, trigger signals are generated in a first comparison circuit 13, then the position of the cross slide 6 is given as input to a memory circuit 15. In this case, the difference between the actual position of the cross slide 6 and arithmetically computed position is taken as the grinding stone wear rate, then output from an infeed position detector 9 is compensated in a third arithmetic circuit 18 using these values, and it is induced into an infeed control circuit 20.