| 摘要 |
PROBLEM TO BE SOLVED: To control the form of chips, and to improve chip discharging performance by improving thinning form of a twist drill provided with an X-shaped thinning. SOLUTION: The X-shaped thinning is composed of a first thinning surface 6 positioned on the center part side of a drill tip, and a second thinning surface 7 disposed on the outer circumferential side of it, and for inclination angles of the thinning surfaces 6, 7, the inclination angleθ1 of the first thinning surface 6 is set at 40 deg.-60 deg., while the inclination angleθ2 of the second thinning surface 7 is set at 20 deg.-40 deg.. An opening angleβbetween the first thinning surface 6 and a rake face 8 of a thinning blade 5 is set to be 70 deg.-110 deg..
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