PURPOSE: To enable high accuracy machining at high speed with high efficiency by forming tips in such a way that both side cutting edges of one tip protrude sideways from both side cutting edges of the other tip and that the peripheral diameter of the peripheral cutting edge of the protruded tip is smaller than the peripheral diameter of the peripheral cutting edge of the other tip.

CONSTITUTION: Eight cutout parts 2 are provided at the peripheral edge of a base metal 1. Tips 31, 32 made of diamond sintered bodies are rigidly fixed to the respective cutout parts 2. In the case of recessing the periphery of an aluminum alloy round bar with correct flute width in the axial direction but without flute bottom accuracy that cannot be obtained in this recessing, the tips 31, 32 are respectively arranged and fixed in such a way that the peripheral cutting edge 321 of the tip 32, for instance, is lower than the peripheral cutting edge 311 of the tip 31, that is, smaller in diameter by about 0.10mm in the radial direction and that the side faces of the side cutting edges 322 of the tip 32 protrude from both side cutting edges 312 of the tip 31 respectively by about 0.05mm. At the time of cutting, the cutting edges of the tip 31 precede to be in charge of most of actual cutting, and the tip 32 that is only one among eight tips is in charge of finish-machining.