Patent ID: 6886633

Claim:
An underreamer, comprising: a hollow body, with a longitudinal axis, the hollow body having an external wall and being arranged so that a pressurized drilling fluid can pass through the hollow body; at least two cylindrical bores provided through the external wall, wherein respective axes of the cylindrical bores are transverse to the longitudinal axis; in each of the bores, a cylindrical widening arm: which has a cutting end provided with blades for widening a bore hole; which can slide between an active position in which the cutting end is distant from the hollow body to effect a widening of the bore hole, and an inactive position in which the cutting end is at least close to, flush with, or retracted within an external periphery of the hollow body; and which has an internal end disposed inside the hollow body that is intended to receive from the pressurized drilling fluid a pressure capable of pushing the widening arm into the active position; and a locking member common to the widening arms: which can occupy a locking position in which the locking member locks each of the widening arms in the inactive position; which is immobilized in the hollow body, in a releasable manner, in the locking position, before use of the underreamer; and which can leave the locking position under a thrust of at least one of the widening arms subjected to the pressure if the pressure is capable of pushing the at least one of the widening arm into the active position.