To provide an increased life, in a drill to perform drilling on a drilling retarding material, such as a stainless steel.
A pair of chip discharge grooves 3 and 3 twisted around a central axis 0 of a drill body 1 are formed in the outer periphery of a cutting edge part 2 on the tip side of a drill body 1 and a pair of cutting edges 5 and 5 inclining to the rear end side of a drill body 1 as it is extended toward the outer peripheral side are formed symmetrically to each other on a basis of the axis 0 and at the ridge part on the tip side of a wall surface 3a pointing to the drill rotation direction T side of the chip discharge grooves 3 and 3. In a so formed twist drill, a web thickness W of a cutting part 2 is set to a value within a range of 0.1-0.25 D based on a drill diameter D, and a chisel 8 is formed on the axis 0 of the tip of the drill body 1. Length L of the chisel 8 in a direction orthogonal to the cutting edge 5 as seen from the tip side in the direction of the axis 0 is set to a value within a range of 0.3-0.7 W based on the web thickness W.
AOKI TAICHI
IKEUCHI HIROSHI
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