PURPOSE: To reduce the biting phenomenon of a cutting edge and to relieve chattering vibration by setting up the twists of an edge and of an outer peripheral edge as a left twist, and smoothly connecting an end cutting edge to the outer peripheral edge along the twist to provide them in a circular arc-shape.

CONSTITUTION: The depth of cut is given from the upper face of a workpiece within the limit of the value less than the radius of a circular arc-shaped end cutting edge 5, which is rotated clockwise round its axis for machining a groove by feeding of a table. In the case where a tapered groove is finished as it is expanded in width by vanishing action, each edge 3 of the hexagon section of a tapered part comes smoothly into contact with a groove side face because of its spiral-shaped twist to the left, and the viration of the end cutting edge 5 can therefore be controlled. The circular arc-shaped end cutting edge 5 is smoothly connected to the outer peripheral edge, and moreover since the end cutting edge 5 is finished by grinding so that its face may form a continuous flat face, the end cutting edge 5 produces a continuous uniform chip, and forms a fine slope being excellent in surface roughness and dimensional accuracy.