3-BLADE DRILL

摘要

Each cutting blade (12) has an outer peripheral side cutting blade portion (12a) shaped like a convex curve and an inner peripheral side cutting blade portion (12b) shaped like a concave curve. On the cross section perpendicular to the axis, the first convex curve (CL1) corresponding to the convex curve shaped cutting blade portion (12a) crosses the first concave curve (CL2) corresponding to the concave curve shaped cutting blade portion (12b) at a cross point (A). Therefore, the range of the first convex curve (CL1) can be reduced and the first concave curve (CL2) can be enlarged to the outer peripheral side, and the concavity amount (LF) of the first concave curve (CL2) can be increased. As a result, the generated chips are curled appropriately and become easy to break in the first concave curve (CL2) portion of a chip discharge groove (18). Since the chips have relatively short curl shape free of needle-shaped bumps, the chips can be discharged smoothly so that the chip discharge performance can be improved. For the 3-blade drill (10) with the chip discharge groove (18) having relatively small groove width and cross-sectional area, jamming caused by chips can be restrained, and the service life of the tool can be further improved.