PURPOSE: To evade the wear the root of cutting tip and contrive to prolong the service life and to enhance the cutting efficiency of the cutter blade concerned by a method wherein guide recesses, by which chips are discharged, are formed at the outer peripheral side end of a rotary board.

CONSTITUTION: A mounting hole 4 is formed at the center of a disc-like rotary board 2, at the outer periphery of which relief grooves 6 with radial depth to relieve chips are formed on the outer periphery of the rotary board at equal intervals. Cutting tips 8 made of diamond sintered body are fixed onto respective edge faces at its outer periphery. An annular hollow part 10 having a certain fixed radial width is formed to the outer periphery side of the board 2. A plurality of suction ports 12, through which air is taken in, are formed at equal intervals at positions, which are radially opposite to each other on the inner periphery side of the hollow part 10 on both side faces of the rotary board 2. When the rotary board 2 is rotated in the forwar direction or direction indicated with the arrow R, the air in the hollow part 10 is discharged from jets 14 with vigor by means of contrifugal force. Since the interior of the hollow part 10 is brought into negative pressure state as a result, air outside the rotary board 2 flows from the suction ports 12 in the hollow part 10 with vigor. The vigorous in-flowing and discharging of the air are maintained during the rotation of the rotary board.