| 摘要 |
A drill bit which has no life ring, is easy in positioning, and can drill a drill hole high in roundness, wherein three cutting blade parts are formed on a cutting tip comprising a block body made of cemented carbide, equidistantly in circumferential direction, a connection line between a cutting surface (5) and a flank (6) formed on each of the cutting blade parts (4a, 4b, 4c) is used as a main cutting edge (8), a connection line between a connection surface (7) and the flank (6) is used as an auxiliary cutting edge (9), the main cutting edge (8) and the auxiliary cutting edge (9) are crossed with each other at a blade tip point (10), the blade tip point (10) is formed in a chisel edge-free sharp shape, the auxiliary cutting edge (9) is set on an extension line of the main cutting edges (8) of those cutting blade parts which are positioned on both sides of the blade tip point (10), and the main cutting edge (8), auxiliary cutting edge (9), and the connection line between the connection surface (7) and the cutting surface (5) crossed with each other at the blade tip point (10) are set in straight line shape.
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