| 摘要 |
PROBLEM TO BE SOLVED: To provide a drill bit which has high drilling performance, hardly causes breakage of the external diameter end of a blade edge and enables high-roundness drilling. SOLUTION: The outer end 1a of a main cutting blade part 1A positions at the external diameter end of the drill bit A, the outer end 1b of a sub-cutting blade part 1B positions at more inner diameter direction than the outer end 1a, and the plurality of the main and sub-cutting blade parts 1A and 1B are formed radially in a peripheral direction. The drill bit A has a c bit tip portion 10 in which the joining ridge line 4 between a rake face 2 and a flank 3 formed in each cutting blade portion is made to be a cutting blades 4A or 4B, and the internal diameter ends of the cutting blades are joined at a central part O1 to have a steeple head 10A at a tip. The steeple head 10A is formed by joining the internal diameter ends of a plurality of the cutting blades 4B of the sub-cutting blade part 1B. The cutting blades 4B are extended from the tip of the steeple head 10A to a base end side and an external diameter direction at the prescribed pitch K1, K2, and the cutting blades 4A of the main cutting blade part 1A are also extended from the position near the base end as well as the external diameter spacing from the tip of the steeple head 10A to the base end side and the external diameter direction at a pitch K3 smaller than the prescribed pitch K1, K2. COPYRIGHT: (C)2008,JPO&INPIT
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