Self-centering work holding and positioning fixture for machine tools

A self-centering workpiece-positioning fixture for a machine tool, especially for a lathe, of the kind comprising a body attachable to a rotary spindle of a machine tool and haveing a pair of opposed jaws supported for rotation about a common axis which intersects the axis of rotation of the said body, a first servo control means for moving one of the said jaws along the said common axis, a second servo control means controlling a clutch for selectively connecting the other jaw to the body of the fixture, and a third servo control means for rotating the said other jaw about said common axis via a transmission means including a sprocket which is coaxial with and rotatable relative to said other jaw, wherein the said other jaw is mounted in the body of the fixture in such a manner that it is always at a constant distance from the axis of rotation of the said body and the said clutch includes an intermediate member mounted upon the said other jaw for rotation therewith, at least part of the said intermediate member being movable axially relative to the said other jaw to connect said other jaw to said body of the fixture in a first position and to connect said other jaw to said sprocket in a second position for effecting different rotational displacements of the other jaw about the common axis upon rotation of the sprocket.