PURPOSE: To perform the machining of metals speedily and surely, by conducting automatic operation with only necessary data inputted into a NC system by means of a computer-numerical control method.

CONSTITUTION: A grinding wheel 4 is opposed to a taper surface 2 of a tool 1 having a taper angle θ with a helical angle . Then, each of numerical values (X, θ, , small end O.D., large end O.D., tool length and rake angle) of tha tool 1 is inputted into a computer-numerical-control system (unillustrated herein). With this, the CNC system automatically calculates an infeed stroke Z- of the wheel 4 to a divided value X- and a turning angle A- of the tool 1 or the like and thereby controls rotation in both the tool 1 and the wheel 4, making equihelical machining set forward.