| 摘要 |
PROBLEM TO BE SOLVED: To provide a drill hard to be broken, and drilling without machining prepared hole in advance. SOLUTION: The tip part 3 of a twist drill 1 is shaped quadrangular pyramidal, and among four ridge lines 5 to 8 formed at intervals of equal angle with a drill central axis 1a as a vertex, a pair of ridge lines 7, 8 are taken as the cutting blade ridge lines for machining a prepared hole. These cutting blade ridge lines 7, 8 bite into the surface to be machined to form a conical prepared hole, so that it is not necessary to form a prepared hole in a drilling position in advance using a center drill. Further, it is not necessary to make a helical flute 4 deep for reducing the chisel of the tip part 3, so that it is possible to achieve the drill, which has little sectional defective of the drill body 2 and is hard to break. COPYRIGHT: (C)2007,JPO&INPIT
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