Patent ID: 8734067

Claim:
A drill, comprising: a pair of main cutting edges formed at opposite positions with respect to an axis of the drill; and a pair of auxiliary cutting edges formed at opposite positions with respect to the axis of the drill, the positions of the auxiliary cutting edges being different from the positions of the main cutting edges in a circumferential direction, wherein an axial formation range of each main cutting edge extends from a position close to a proximal end of the drill across a maximum diameter position toward a distal end of the drill, wherein an axial formation range of each auxiliary cutting edge extends from a position close to the proximal end of the drill across the maximum diameter position toward the distal end of the drill, wherein a terminal of the axial formation range of each auxiliary cutting edge located close to the distal end of the drill is closer to the proximal end of the drill than a terminal of the axial formation range of each main cutting edge located close to the distal end of the drill, wherein each auxiliary cutting edge has a ridgeline located within an angle range smaller than 90° from a ridgeline of a main cutting edge toward the rear in a rotation direction of the drill, and wherein the ridgelines of the main and auxiliary cutting edges have a point angle of 0° in a range from the maximum diameter position to a position located closer to the proximal end of the drill than the maximum diameter position, and the ridgelines of the main and auxiliary cutting edges have point angles which increase from the maximum diameter position toward the distal end of the drill, the point angle of the auxiliary cutting edges having a higher increasing rate than an increasing rate of the point angle of the main cutting edges in a predetermined range from the maximum diameter position, so that the ridgelines of the auxiliary cutting edges are located closer to the center of the drill than the ridgelines of the main cutting edges and the point angle of the auxiliary cutting edges is larger than the point angle of the main cutting edges in a range from the maximum diameter position toward the distal end of the drill.