| 摘要 |
Two thinned edges (6B) extend beyond intersections P with a chisel part (9), and the axial rake angles thereof are in the range of +5° to +15°. When seen from the tip side in the direction of an axial line (O), the percentage of the length (A) of a cutting edge in the extension direction of a main cutting edge (6A) thereof that is taken up by the length (B) of a respective thinned edge is in the range of 20% to 50%. With respect to the two thinned edges (6B), intersecting ridgelines (L), which are each between a respective tip flank face (4) and a respective thinned second wall (7B) oriented in the reverse direction to the drill rotation direction (T) and connect to a respective thinned edge (6B), are disposed at a distance from each other in an orthogonal direction to the intersecting ridgelines (L) or on one diameter line which runs through the axial line (O) when seen from the tip side in the direction of the axial line (O); alternatively, these intersecting ridgelines (L) are each disposed at positions beyond one diameter line running parallel to the intersecting ridgelines (L) and through the axial line (O) so that the ridgelines pass each other in an orthogonal direction to the intersecting ridgelines (L) by an amount equal to or less than 0.03xD where D is the diameter of the cutting edges (6). |
