| 主权项 |
1. A drill for drilling a hole, comprising:
a cutting edge (3) disposed on a front end of a body (1) as to extend from a rotation center of the drill to an outer periphery of the drill, the cutting edge being shaped symmetric with respect to the rotation center of the drill, the cutting edge including at least a rotation center side cutting edge portion (3a) and an outer peripheral side cutting edge portion (3c) connected to a diameter-direction outer end of the rotation center side cutting edge portion, point angles (θ) of the respective cutting edge portions reducing gradually from the rotation center side cutting edge portion (3a) to the outer peripheral side cutting edge portion (3c), wherein a small diameter portion (6) is provided on a front end side of the body (1), the small diameter portion having an outside diameter smaller than a diameter of the drill, the cutting edge (3) with the point angles varied is formed in the small diameter portion, and a flat cutting edge (8) having an inclination angle (β) of 30° or less with respect to a line perpendicular to an axis of the drill is provided in a step portion formed between the small diameter portion (6) and a large diameter portion (7), the large diameter portion being provided on a rear side of the small diameter portion and having an original outside diameter, and wherein a main helix angle (α2) of a helical flute (5) formed in the drill is set at an angle of 5° to 45°, and a helix angle (α1) in the small diameter portion (6) is set to be smaller than the main helix angle (α2) by varying a helix angle of the helical flute (5) at an axial-direction rear end of the small diameter portion (6). |
