Overshot cutter with flexible release cage

Abstract

An overshot cutter includes concentric inner and outer members, the latter having a radially flexible portion thereon. A gripper for gripping a portion of a drill string is mounted on the flexible portion. The members have inclined surfaces which, when engaged, prevent radial flexing of the inner member as the overshot cutter is lowered over the drill string. When the gripper encounters an obstacle the outer member moves relative to the inner member to axially space apart the surfaces. The inner member and the gripper thereon thereby move radially outwardly to clear the obstruction. In a first embodiment, a spring imposes a biasing force on the inner member to maintain engagement between the surfaces. When an obstruction is encountered, continued downstring advancement of the outer with respect to the inner member disengages the surfaces to define the radial clearance and for shorten the spring to store energy to re-establish contact of the surfaces after the obstruction is passed. In a second embodiment a dowel on the outer member engages a groove on the inner member so that rotation of the outer relative to the inner member disposes the dowel in the groove to space the surfaces apart and define the radial clearance. In each embodiment, the surfaces brace the gripper into engagement with a distended portion of the drill string when it is desired to extend cutting elements provided.