Underreamer with revolving diamond cutter elements

Claim 1

A borehole underreamer having a tubular body adapted at an upper end thereof for coaxial connection to a rotary drill string, said body forming a central axis therethrough, said body housing at least a pair of extendable cutter arms hinged at a first upper end to said tubular body, said cutter arms, at a second cutting end, forming bearing means for a rotary cutter element adapted for rotation thereon, said bearing means formed by said cutting end of said cutter arms comprises an aperture formed in said cutter end, said journal means formed by said cutter element comprises a shaft extending through the aperture, with the cutting face being at one end of the shaft, said body further containing means to extend said cutter arms, said underreamer comprising: at least a pair of cutter elements, said cutter elements forming journal means mounted for rotation relative to said bearing means extending from said cutter arms, said cutter element forming a cutting face, the cutting face being skewed at an angle from a radial line from said central axis of said tubular body so that the cutter element precesses as the cutting face contacts a borehole formation said cutting face is skewed so that the side nearest the central axis of the tubular body rotation extends further forward than the side remote from the central axis said cutter elements rotate when said cutter arms are extended so that said cutters move against a formation wall formed by said formation, said arms are actuated by said arm extension means contained within said tubular body.