| 摘要 |
A twist drill is provided to limit the occurrence of the burr and improve the centering performance of the initial drilling work by having the rake according to the projection of the web portion at the land portion. A twist drill comprises a mounting groove formed at the front end of a bag part; and an insert(20) fitted in the mounting groove. A web portion(25) is projected at a land portion(25) as the web portion containing a thinning edge(22) has a front end rake(Theta1) relatively different for the land portion containing a main cutting edge(26). A diameter(d) of the web portion is 25~35% of a diameter(D) of the insert, and a height(h) is decided by the front end rake, and the front end rake is normally chosen within the range of 130‹ ~150‹. A boundary line(27) of the web and land portions is rounding-processed.
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