| 摘要 |
PURPOSE: A structure is provided to minimize a stepped portion between a bit housing combiner and an enlarging bit combiner for preventing a bit from being damaged. CONSTITUTION: An enlarging bit(200) enlarges and shrinks an excavated hole in accordance with the forward and the inverse rotation of a bit housing(100) and a pilot bit(300) at 180 degrees. In case of the enlarging state, the bit excavates a hole. In case of the shrinking state, the bit housing is pulled out from a steel pipe. On pulling out the bit housing, an enlarging excavated face(203) is shrunk to be placed in the nearest position from an outer diameter by inversely rotating the bit housing.
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