| 摘要 |
PURPOSE:To assure the straight movement of the bit of a drill, by protruding a tip portion of the cutting part of the drill at the bit. CONSTITUTION:The tip portion 13 of a cutting part 12 provided at the bit 1 of a drill is protruded nearly in the form of a trapezoid. Cutting reaction forces act to protruded edges 13a, 13b, 13c. As a result, one of guide pads 9a, 9b is prevented from receiving the cutting reaction forces concentratedly. The protruded edges 13a, 13b, 13c cut into a workpiece so as to perform a restricting action for keeping the drill bit 1 from being displaced in the radial direction. The straight movement of the drill bit 1 is thus assured. |
