Patent ID: 7306411

Claim:
A drill bit comprising: a chip discharge groove twisted in the shape of a spiral formed in an outer periphery of a tip section of a roughly cylindrical drill body; a cutting edge formed on an intersecting ridgeline between a wall surface facing in a direction of drill rotation of the chip discharge groove and a tip flank of the drill body; wherein, the chip discharge groove includes, a narrow width section, formed along a direction of an axis of the drill bit, in which a helix angle relative to the axis of the drill bit is constant, and groove width is constant, a wide width section, formed along the direction of the axis of the drill bit, located farther towards a rear end of the drill bit than the narrow width section, in which the helix angle is equal to the helix angle of the narrow width section and in which the groove width is larger than the groove width of the narrow width section in the direction of drill rotation and towards the rear of the drill bit in the direction of drill rotation and the groove of the wide width section has a constant width, wherein an expanding width section is formed between the narrow width section and the wide width section in which the groove width of the chip discharge groove gradually becomes larger moving towards the rear end side, and the boundary between this expanding width section and the narrow width section is located within the range of 3-5×D relative to outer diameter D of the cutting edge from the outer peripheral edge of the cutting edge towards the rear end side in the axial direction.