| 摘要 |
PROBLEM TO BE SOLVED: To lock and fit a ring bit to the tip of a casing pipe on the tip side in the axial direction rotatably and very easily without complicating a structure in a drilling tool of the double tube type bit. SOLUTION: The ring bit 9 is fitted to the tip of the casing pipe 1 rotatably around the axis O, and an inner bit 10 is fitted to the tip of an inner rod 6 inserted into the casing pipe 1 couplably around the axis O with the ring bit 9. The casing pipe 1 and the ring bit 9 are coupled wit their inner and outer peripheries faced to each other, and annular grooves 11, 13 extending around the axis O are formed on the opposite inner and outer peripheries respectively. A locking member 15 elastically deformable in the radial direction relative to the axis O is inserted in an annular hole 14 formed by coinciding with the annular grooves 11, 13 each other.
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