| 摘要 |
It is an object to obtain high quality hole with no cylindrical burrs at the exit of a worked hole in a metal layer even if the hole is worked by a drill for drilling a hole in a laminated plate including FRP and metal with the metal layer disposed downward. A small diameter portion (6) having an outside diameter smaller than a diameter of the drill is formed on a front end side of a body (1) of the drill in which a cutting edge (3) includes at least two portions, that is, a rotation center side cutting edge portion (3a) and an outer peripheral side cutting edge portion (3c) and point angles of the respective cutting edge portions reducing gradually from the rotation center side cutting edge portion (3a) to the outer peripheral side cutting edge portion (3c). The cutting edge (3) with the point angles varied is formed in the small diameter portion. A flat cutting edge (8) having an inclination angle ² of 30° or less with respect to a line perpendicular to an axis of the drill is provided in a step portion formed between the small diameter portion (6) and a large diameter portion (7). |
