PURPOSE: To enable reliable and accurate machining, by providing a lapping time calculator which reads a lapping time corresponding to a difference, compensates the read time on the basis of the sharpness of a lapping tool and sends out the compensated time.

CONSTITUTION: A flow rate Q is determined by the computing section 16 of a flow rate measuring circuit. A difference ΔQ determined by a difference detection circuit 18 is applied to the first calculating section 22 of a lapping time calculation circuit 21. Lapping times T corresponding to various differences ΔQ are previously stored in a programmable read-only memory 23. The read lapping time T is sent out and applied to the second calculating section 24 of the circuit 21. The second calculating section 24 compensates the lapping time on the basis of a cumulated lapping time applied from a cumulated lapping time detector 25 and sends out the compensated lapping time T', which is digitally displayed on an indicator 26.