| 摘要 |
A rotary drill includes a cylindrical shank 11 and a drill body 12. Drilling edges 18a, 18b and first and second reaming edges 19a, 20a are formed on a first reduced-diameter portion 8, a second reduced-diameter portion 9 and a third reduced- diameter portion 10 respectively at a foremost end of the drill body. Each of the reaming edges is inclined with respect to the longitudinal axis of the drill at an angle which is within a range of 5 DEG to 30 DEG . A hole is drilled by the drilling edges 18a, 18b and the drilled hole is then worked by the first reaming edges 19a and the second reaming edges 20a to provide a smooth inner wall. Since each of the roaming edges extends radially outward from the foremost end of the drill and inclines axially in the rearward direction at a relatively small angle, centering ability of the drill is greatly improved. <IMAGE> |
