Underreamer with cylindrical boss and socket hinge assembly for the cutter arm

Claim 1

An underreamer tool for enlarging an earth borehole, said tool comprising a tubular body having means for driving connection with a drill string; a plurality of cutter arms, including a cutter element, attached to said body by hinge means for movement between a retracted position wherein each arm and cutter element is disposed within an appropriately sized opening in said body and an outwardly extended position wherein said cutter element engages the borehole, said hinge means comprising: a hinge plate disposed in a recess in said body adjacent the upper end of said opening and defining a lower terminal end extending into said opening; the upper end of said arm and the lower terminal end of said plate defining cooperating cylindrical boss and socket engagement members with the axis of said cylindrical boss and socket of said members perpendicular to the plane of the hinged movement of the arm and: said socket member defining a cylindrical bore open along one side with the width of said opening being less than the diameter of said socket; and, a substantially cylindrical boss member of generally common radius and having a diameter greater than the width of said opening to provide a relatively large area of engagement between said engaged boss and socket member, with said boss member terminating in an integral reduced thickness area defining a neck portion projecting through said opening, said neck portion having a thickness less than the width of said opening to permit free hinged movement of said boss in said socket whereby all forces on said hinge means are supported at said large area of engagement between the cylindrical socket member and the cylindrical boss member; and, means for retaining said plate within said recess.