| 摘要 |
The drill assembly includes a mandrel for rotatable mounting in a tubular casing and a bit rotatably mounted in the mandrel. The bit and mandrel normally rotates together in one direction with the bit eccentric with the mandrel, whereby the bit can bore a hole larger in diameter than the casing. There is 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, and inclined cam surface on the bit for engaging the bottom of the casing when the bit is moved upwardly to cause the bit torotate in the other direction (opposite to the drilling direction) to a release position in which the bit is aligned with the casing for movement through it. There are 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 approx. one-quarter of the cylinder for rotation between the bit extended and the bit retracted positions. |
