PURPOSE: To execute the precise finishing of the flank face of a spiral groove by controlling the relative parallel movements of a work and tool.

CONSTITUTION: The distance X to a groove flank face 1F from the groove center face 10 of a spiral groove on a horizontal reference surface ST can be expressed by the total of the horizontal distance S between the groove center face 10 and tool rotation Ok, the horizontal distance (d/z)cosβ, between the tool rotating shaft Ok and a point P, and the horizontal distance (θ/2π) L between the intersection Po where the reference face ST is crossed with the groove flank face 1F. If a tool diameter (d), lead L or S be given as constant by deciding by the value of a groove width l in the effective diameter of a spiral groove 1, the value of X is found by relating to a relative parallel movement V. Namely, if the distance Xo at the outer diameter position, the distance Xi at the root diameter and the distance Xm at effective diameter position be found how they vary according to the movement V, the inclination of the spiral groove flank face can optionally be controlled by the movement V.