| 摘要 |
PROBLEM TO BE SOLVED: To provide a safety drill method which is simple in mechanism, high in reliability, economical in cost, and safe to handle, and to provide a drill bit fit for the method. SOLUTION: The drill bit is formed of an outer bit, a small lost bit, and a fixing mechanism. The outer bit has a fitting groove formed at a front end thereof, a cutting edge attached to the front end, and an engaging portion engaging with a front end of a cylindrical drill rod, at a rear end of the outer bit. The lost bit has a fitting portion disengageably fitted into the fitting groove of the outer bit in a rotation-inhibited state, and a cutting edge at a front end thereof. The fixing mechanism fixes both the outer bit and the lost bit to each other when both the bits assume a fixed position. At the time of drilling, the fixture of the fixing mechanism is released due to the axial force and driving force of the drill rod, and by pulling out the drill rod in an excavated hole, the lost bit is disengaged from the outer bit. According to the safety drill method, drilling is carried out by using the drill rod which is connected to a drill head of an excavator, and has the drill bit attached thereto and a rock bolt inserted thereinto, and thereafter pulling out the drill rod, to thereby set the rock bolt in the excavated hole.
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