| 摘要 |
An inner bit (3) is provided with: a supply hole which is open at the front end of the inner bit (3); and a discharge groove (14) which is formed in the outer peripheral surface of the inner bit (3) and which extends in the axial direction. The supply hole is provided with: a front end blow hole (15) which is open at the front end surface of the front end of the inner bit (3); and an outer peripheral blow hole (16) which is open at the outer peripheral surface of the front end of the inner bit (3). The outer peripheral surface of the inner bit (3) has formed therein an outer peripheral groove (19) which connects the outer peripheral blow hole (16) and the discharge groove (14). The outer peripheral groove (19) is covered from the outside radially by a ring bit and extends from the outer peripheral blow hole (16) toward the discharge groove (14) so as to gradually tilt toward the axial base end side as the outer peripheral groove (19) extends about the axis. |
