| 摘要 |
A drill bit is provided with a pair of cutting edges which are positioned so that a narrow slot is formed therebetween in an area including the rotational axis of the drill bit and so that the lines joining the rotational axis of the drill bit and the innermost points of the respective cutting edges are at an angle not larger than about 160 DEG with each other as viewed in the axial direction of the drill bit. |
