| 摘要 |
PROBLEM TO BE SOLVED: To provide a drilling bit which is capable of performing excavation only with the rotation without generating any vibration and noise even when ground is highly viscous, high in mud discharge efficiency, capable of preventing a lost bit from being detached by the reaction when the rotation is stopped, smooth in ejection of water and air, high in fluidization efficiency of the excavated earth, and capable of smoothly performing the boring only by the rotation. SOLUTION: An earth guide groove 8 for guiding the excavated earth from a blade tip to an outer pipe 4 is provided on an outer circumference of an inner bit 2, a key groove 15 extended spirally from a rear end of the lost bit and intersected with the earth guide groove 8 is provided on the lost bit, and a key projection 11 which can be fitted in this key groove from the rear end is provided on the inner bit in order that the lost bit 3 is fitted outside the outer pipe in an attachable/detachable manner to/from the outer circumference of the inner bit. The lost bit can be detached from the inner bit by rotating the inner bit in the direction opposite to the boring direction to allow the key projection to be drawn from the rear end of the key groove.
|
