| 摘要 |
A drill has a pair of cutting edges (4) which are arranged at positions that are substantially uniformly circumferentially spaced about its rotation axis. Each of the cutting edges (4) has a first linear cutting region (4a) and a second linear cutting region (4b) which are aligned with each other on the same straight line, and a curved cutting region (4c) which is located between the first and second linear cutting regions (4a, 4b). The curved cutting region (4c) has a third linear cutting region (4e, 4f) which extends in a direction intersecting with the second linear cutting region (4b) at a prescribed angle. With this structure, it is possible to prevent chip loading etc. which may be caused during perforation drilling operations. In a throw-away tipped drill, a notch (118a) is formed in a received portion of an insert (110) which is mounted on a shank (130). A holding portion (131a, 131b, 133a, 133b) of the shank (130) corresponding to the notch (118a) is provided with a through screw hole (138). A male screw (139) is inserted in the through screw hole (138), and a spherical pressing member (140) is interposed between the forward end of the male screw (139) and the surface of the notch (118a) of the insert (110). With this structure, it is possible to prevent separation of the insert (110) from the shank (130).
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