| 摘要 |
Drill tools with eccentric bits for drilling a hole having a large diameter generally include a bit for swinging out to an eccentric position for drilling, and for swinging into a non-eccentric position for removal from a tubular casing. Relatively complicated structures are required for effecting bit movement, and it is often difficult to remove the bit for replacement or repair. A drill assembly offering a solution to both problems includes a mandrel for rotatable mounting in a tubular casing, a bit rotatably mounted in the mandrel, the bit and mandrel normally rotating together in one direction with the bit eccentric with respect to the mandrel, whereby the bit can bore a hole larger in diameter than the casing, a head on the bit for engaging a formation to stop rotation of the bit, shoulders on the bit and mandrel for retaining the bit in the eccentric drilling position, an inclined cam surface on the bit for engaging the bottom of the casing when the bit is moved upwardly to cause the bit to rotate in the other direction (opposite to the drilling direction) to a release position in which the bit is aligned with the casing for movement therethrough, and second shoulders on the mandrel and bit for limiting rotation of the bit to the release position. The shoulders on the mandrel and bit are in the forms of projections which overlap in the vertical direction, each projection defining approximately one-quarter of the cylinder for rotation between the bit extended and the bit retracted positions.
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