| 摘要 |
A downhole tool (114) is adapted to be connected to a drill string by provision of a connection atop the tool. The tool com- prises a mandrel (2) for connection with the drill string, the mandrel being rotatable within a body (5). A plurality of blades (27) are individually extendible radially from the body to engage the wall of a well bore, the radial position of the blades being adjus- table between a first retracted position and a second extended position to position the centre line of the mandrel at a desired posi- tion relative to the longitudinal axis of the well bore. Means are provided for holding each of the blades at the retracted position, at the extended position or at any intermediate position between the retract ed and extended positions.
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