Underreamer having variable arm extension

Claim 1

An expandable underreamer which comprises an elongate, generally cylindrical tool body adapted to be suspended from and rotatably driven by a drill string; roller cutter cones supported on respective cutter arms; said arms being peripherally spaced and arranged generally longitudinally of said body and being pivotally connected to the body adjacent their upper ends and supporting the respective cutter cones thereon adjacent their lower ends; said arms being pivotal between an inoperative position at which said cutter cones are retracted and wherein said arms are generally recessed within elongated slots in the body and an operative position at which said cutter cones are projected radially outwardly from said body wherein the arms incline downwardly and radially outwardly from their pivotal connections with the body, the pivot axes of said arms being substantially horizontally coplanar and said arms having substantially the same length, each of said arms including a cam follower plate and a radius limiting plate; said radius limiting plate being engageable in bearing contact with a load bearing shoulder on said tool body to limit the outward extension of the cutter assembly; said cam follower plate having a cam follower surface thereon; said radius limiting plate and cam follower plate being removable and replaceable; a vertically extending actuating member movable longitudinally downwardly within the tool body in response to fluid pressure; a cam member affixed to said movable actuating member having a cam surface in camming engagement with the cam plate of said cutter arm assembly; and the construction of said cam surface and cam follower surface being such as to move said cutter arm to a predetermined radius upon downward movement of said actuating member.