An assembly positionable on the spindle of a machine tool for securing a workholding apparatus in position on the machine tool. The assembly comprises first and second end portions and an inner piston axially movable within an inner chamber formed by the first and second end portions when assembled together. The piston, first end portion and second end all comprise central openings which permit the draw bar of a workholding apparatus to pass therethrough and into engagement with a draw rod of a machine tool. The piston includes springs, to urge it in a retracting direction, and spaced apart lugs to engage similar lugs on the workholding apparatus for securing the workholding apparatus to the assembly and thus to the spindle. Retraction of the draw rod results in the springs urging the piston, and hence the lugs of the workholding apparatus, in the retracting direction to secure the workholding apparatus to the assembly. Further retracting movement of the draw rod pulls the draw bar to activate the clamping mechanism for securing a workpiece for machining. The draw bar is advanced by advancing movement of the draw rod to release a clamped workpiece. Further advancing movement of the draw rod produces contact with the piston to advance it thus overcoming the force exerted by the springs thereby loosening the workholding apparatus from its secured position and enabling disengagement of the lugs. The motion of a single machine tool draw rod can, therefore, accomplish tasks that formerly required two draw rods with their respective hydraulic or pneumatic systems.