| 摘要 |
PROBLEM TO BE SOLVED: To provide a pre-slit jumbo capable of smoothly drilling without being affected by the state of a guide hole and installable easily. SOLUTION: This jumbo comprises a guide cell, a drifter sliding on the guide cell, a drilling rod installed extendedly from the drifter, a bit fitted to the tip of the drilling rod, a tip centralizer supported so that the drilling rod can be inserted therein, a guide tube rod installed extendedly forward of the centralizer, and a rod bracket for advancing the drilling rod in parallel with the guide tube rod. The guide tube rod is formed of a hollow pipe, comprises an enlarged part having a longitudinal slit on a side opposite to the drilling rod, and in which the other end of the rod bracket holding the drilling rod is invaded in the hollow pipe, and the rod bracket slides along the guide tube rod. The guide tube rod is larger in diameter at the tip part and the rear end part than at the intermediate part so as to be attached to and detached from the tip centralizer. COPYRIGHT: (C)2006,JPO&NCIPI
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