| 摘要 |
The present invention provides a drill to stably bore workpieces at high precision to metal members and fiber-reinforced resin composite materials with a long service life. In the drill (a) wherein a cutting blade is axially divided to have two or more steps, first-stage cutting blades (10) from a front end has less cutting blades, a wider point angle, or a wider relief angle with respect to the cutting blades (20, 30) after a second stage; thus a cutting thrust resistance is less when compared with the same cutting blades, point angle, and relief angle as the cutting blades after the second stage. The first stage has two blades, the second stage has four blades, and the third stage has four blades. The point angle is &thgr;_1 > &thgr;_2 > &thgr;_3 and the relief angle is γ_1 >= γ_2 >= γ_3. The point angle of the first and second stages is constant, and the point angle of the third stage is gradually reduced towards a rear end of the drill to make a ridge of the blade edges curved smoothly. |
