| 摘要 |
PROBLEM TO BE SOLVED: To form a spot facing hole that secures fastening power by a bolt, improves cutting ability and chip partibility, and can form a bolt-hole in high accuracy. SOLUTION: Cutting edges 6 formed at a front side edge part of a drill rotational direction T of a tip flank 4 of a drill body 1 that is rotated around the axial line O includes: first concave-curved cutting edges 11, viewed from the drill rotational direction T, extending from an outer peripheral edge 6A of the cutting edges 6 toward an inner peripheral side to a rear end side while being bent, then extending to the tip end side; and second concave-curved cutting edges 12 merging into the first concave-curved cutting edges 11, then extending to the tip side while being bent; and a center cutting edge 13 crossing in concave bent with the second concave-curved cutting edge 12 in obtuse angle, straightly extending toward the inner peripheral side in the axial line O direction that is the tip side exceeding the outer peripheral edge 6A. The first and second concave-curved cutting edges 11, 12 also have a concave-curved shape when viewed from the tip in the axial line O direction. A curvature radius R1 of the first concave-curved cutting edge 11 is made larger than a curvature radius R2 of the second concave-curved cutting edge 12 when viewed from the tip in the axial line O direction. COPYRIGHT: (C)2011,JPO&INPIT |
