| 摘要 |
PROBLEM TO BE SOLVED: To provide a twist drill having a cutting blade shape and a groove shape restraining abrasion and chipping of an outer peripheral corner part and chipping of a leading edge in drilling work not using cutting fluid and in drilling work to spray a small quantity of cutting fluid on a drill or a drill head end flank. SOLUTION: This drill has a chisel 1, a pair of central cutting blades 2 continued to an outer end part of the chisel 1, a pair of main cutting blades 3 continued to the outside of each of the central cutting blades 2 and an outer corner cutting blade 4 to retreat at an angle from an intermediate part of each of the main cutting blades 3. Length of the corner cutting blade 4 is 5-20% of a twist drill diameter, and an angleα1 made by the main cutting blades 3 and the corner cutting blade 4 is specified as 150-175 deg.. A rectilinear or curved chamfering part continued to the corner cutting blade 4 is provided on a leading edge 7 formed of a groove 6 forming the main cutting blades 3 and the corner cutting blade 4 and a margin 9.
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