| 摘要 |
The present invention addresses the problem of producing no cylindrical burrs at the outlet of a drilled hole of a metal layer even when the hole is drilled with the metal layer on bottom by a drill used for drilling in stacked FRP and metal plates, thereby making it possible to obtain the hole of high quality. A cutting edge (3) is configured from at least two portions of a rotation center-side edge portion (3a) and an outer periphery-side edge portion (3c), at the leading end of the body (1) of a twist drill in which the point angles of the respective edge portions become smaller in stages from the rotation center-side edge portion (3a) to the outer periphery-side edge portion (3c), a small-diameter portion (6) having an outer diameter smaller than a drill diameter is provided, the cutting edge (3) having a changing point angle is provided in the smaller-diameter portion, and a flat cutting edge (8) having an inclination angle (beta) of 30° or less with respect to a line normal to the axis is provided in a step portion formed between the small-diameter portion (6) and a large-diameter portion (7). |
