| 摘要 |
An eccentric drilling tool comprises a guide member (10) rotatably centered by the mouth of a casing tube (11) that follows the tool downhole during drilling. A pilot bit (23) on the guide member (10) is provided with axially oriented hard metal pilot button inserts (57, 57I-57III) peripherally thereon directed to define the diameter of the pilot hole to be cut during drilling. A laterally extending eccentric bit (22) behind said pilot bit (23) has axially oriented hard metal reaming button inserts (48-51) at the eccentric top portion thereof directed to ream up the pilot hole to a maximum diameter. The eccentric bit (22) is movably supported relative to said guide member (10) between a drilling position, in which the eccentric bit (22) projects laterally beyond said casing tube (11) so as to drill jointly with said pilot bit (23) a hole larger than said casing tube (11), and a retracted position, in which the drill bit can be retracted or lowered through the casing tube (11). In order to counteract radial deviation of the pilot bit (23) during drilling, the frontal quadrant (between 57I,57) thereof, which lies diametrically opposite to the reaming inserts (48,51) of the eccentric bit (22) that lead in the rotational direction (8) of the tool, is left void of peripheral diameter cutting pilot buttons. |
