| 摘要 |
A bit connectible from the lower end of a rotatably powered drill stem for enlarging a preformed pilot hole includes a cutter carrier frame on which are mounted a plurality of roller cutters having peripheral cutting edges which sweep concentric circles upon rotation of the raise drilling bit. The circles swept by the cutters collectively define a generally dome-shaped cutting profile of constantly changing curvature which forms a segment of a spiral curve so that as the cutting profile extends radially outwardly from the drill stem, its curvature progressively decreases. The rotating cutter carrier frame structure drives a flywheel assembly at increased speed through the intermediacy of a step-up drive system composed of two planetary gear drives coupled in tandem. The flywheel assembly is mounted on the step-up drive in such a manner that if a sudden, large difference in acceleration occurs between the flywheel and the cutter carrier structure frame, the flywheel is free to continue to rotate to thereby prevent damage to the components of the step-up drive.
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