PURPOSE: To prevent the working defect such as flaws and cracks by forming a recessed part at the top edge of a wall surface directed in the revolution direction of a groove part which communicates to an outer peripheral cutter, besides the outer peripheral cutter, and forming each preceding cutter at the ridge line part between the top edge flank and a wall surface directed in the revolution direction of the recessed part and in the ridge line part between the wall surface and the second flank, and gradually increasing the diameter of a worked hole.
CONSTITUTION: A recessed part 26 is formed at the top edge of a wall surface 21a directed to the revolution direction of an oil pouring groove 21 which communicates to the outer peripheral cutter 22 of a tool body 20. The wall surface 26 directed in the revolution direction of the recessed part 26 is set parallel to the wall surface 21a of the oil pouring groove 21, and each preceding cutter 27, 28 is formed at the ridge line part of the top edge flank 25 and the second flank 24 of each wall surface 26a. Each preceding cutter 27 retreats to the tool basic edge side in comparison with the tool top edge surface, and each preceding cutter 28 retreats to the tool center side in the radial direction in comparison with the outer peripheral cutter 22. Accordingly, the diameter of a ground member is gradually increased in the tool radial direction, accompanied with the feed of the tool. Accordingly, the working defects such as flaws and cracks are not left on the worked surface, and the quality and precision of the worked surface are improved.
TAKASAKI KAZUO
HOSONO HIDEJI
