| 摘要 |
PROBLEM TO BE SOLVED: To prevent a drilling bit rotated to open at the time of drilling from falling to a storage position and being prohibited from drilling. SOLUTION: In the front end surface of a head ring 5 connected to a casing rod 2, a plurality of grooves 6 are radially formed. At a bottom part of each groove, an insertion hole 7 is formed, and a shaft 8 of the drilling bit 4 is rotatably and slidably inserted. Next to the groove, a stopper 12 is provided. As the drilling bit is set in a storage state over the groove as illustrated, the head ring 5 and the casing rod 2 are inserted into a propulsion pipe 1, and the drilling bit 4 is disposed ahead of the propulsion pipe. As the head ring 5 is rotated in a direction (1) for drilling, the drilling bit is rotated in a direction (3) to be inserted into the groove, and it is securely fixed to perform stable drilling. For replacement of blades, it is rotated in the other direction to be pulled off. |
