| 摘要 |
PROBLEM TO BE SOLVED: To improve shortage of strength in the central part of the tip in the case of machining a large-diameter hole in a cemented carbide drill causing low work hardening, in which work hardening amount is comparatively smaller even in drilling a material easy to quench such as high carbon steel. SOLUTION: The tip angleα1 of a chamfer blade part 54 provided inside the central part of a drill tip ranges from 135°to 150°, and it is set larger than the tip angleα2 of the peripheral blade part 56, so that the strength of the central part is made higher without impairing the strength of a shoulder part 22, whereby even in a large-diameter cemented carbide drill for machining a large-diameter hole (e.g. a bore diameter 12 mm or more), which requires high load in drilling, loss of the cutting blade 52 of the tip central part, that is, the chamfer blade part 54 is restrained to obtain practically satisfactory durability. A connecting blade part 58 shaped like a projected circular arc with a radius R larger than 0.01 D and smaller than 0.4 D is provided in a boundary part between the chamfer blade part 54 and the peripheral blade part 56, thereby preventing loss of the boundary part and sudden progress of abrasion. COPYRIGHT: (C)2007,JPO&INPIT
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