| 摘要 |
A drill bit (1) has a tapered cutting end part (2) terminating in a drill tip (3), a shank (4) and a body (4a) extending between the cutting end part (2) and the shank (4). A plurality of flutes (5) are formed in the drill bit (1) and generally helically extend along the body (4a) into the cutting end part (2) to adjacent the drill tip (3). Each flute (5) has a flute leading side wall (6) and a flute trailing side wall (7). A land (9) is defined on the body (4a) between each of the flutes (5) and extends to the cutting end part (2). Each land (9) defines a secondary edge (11) with the flute trailing side wall (7) of the leading adjacent flute (5). Each secondary edge (11) helically extends to the cutting end part (2) at a secondary edge helix angle. A plurality of tip faces (14) are defined on the cutting end part (2) and each extend from one of the lands (9) to the drill tip (3). The tip faces (14) are separated by the flutes (5) up to adjacent the drill tip (3). Each tip face (14) defines a primary cutting edge (21) with the flute trailing side wall (7) of the leading adjacent flute (5). Each primary cutting edge (21) extends from one of the secondary edges (11) in a variable conic helix type manner with a primary cutting edge helix angle that decreases from substantially equal to the secondary edge helix angle at the secondary edge (11) toward zero as it approaches the drill tip (3).
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