| 摘要 |
PURPOSE:To obtain a stable quality, by constituting a wall surface, containing a thinning edge in a thinning part, of tilt surfaces in plural stages so that a crossing part prevents the generation of chipping and the separation of cutting chips with the thinning part being easily machined. CONSTITUTION:A drill changes a radial angle in stages from a thinning edge 17a to a thinning edge 17b and from the thinning edge 17b to a cutting edge 12, and each crossing part of these edges forms a large obtuse angle. Accordingly, the generation of chipping and the separation of cutting chips in the crossing part can be prevented. While the drill, enabling tilt surfaces 16a, 16b to be easily formed by machine work, enables the stable quality to be attained. Further the drill, which forms the tilt surface 16b in the peripheral side into a spiral curved surface along a twist groove 13, generates no change of length of the thinning edge 17b in the peripheral side and its rake angle even if a flank 11 is polished. Accordingly, the drill, when it is repolished, is required for polishing only a bottom surface 15 of a thinning part 14, enabling the repolishing to be easily performed.
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