| 摘要 |
PROBLEM TO BE SOLVED: To drill a continuous hole without bending in a drill with a guide rod having a drilling rod 4 hammered by a hammering part formed at a drifter and the guide rod 7 parallel to the drilling rod and drilling the continuous hole H2 arranged continuously with an existing hole at a position adjacent to the existing hole by a drill bit 5 at the front end of the drilling rod while the guide rod 7 is inserted into the drilled existing hole H1. SOLUTION: The drill bit 5 comprises a master bit part 51 and a slave bit part 52 projecting forward of the master bit part, concentric to the master bit part, and smaller in diameter than the master bit part. The projecting length L of the slave bit part 52 from the master bit part 51 is longer than the hammering stroke S of the drilling bit 5. A difference between the outer diameters of the master bit part 51 and the slave bit part 52 is so set that a rock portion Wa left between a pilot hole H2a drilled by the slave bit part 52 and the existing hole H1 is not destroyed during the drilling. COPYRIGHT: (C)2009,JPO&INPIT
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