Abstract: A machine tool has a pair of rotary spindles which support opposite ends of workpiece to be machined. At least one of the spindles is driven and a ring having a plurality of arms is coaxially secured to this spindle for rotation therewith. The arms entrain an annular member which is essentially coaxial with the spindles and is provided with threads. An annular guide plate which is again essentially coaxial with the spindles supports a plurality of clamping jaws for movement radially of the spindles. The clamping jaws are provided with threads which mesh with the threads of the annular member so that rotation of the latter can cause movement of the clamping jaws. A plurality of rocker arms is mounted on the machine tool for pivotal movement between an inoperative position and an operative position in which the rocker arms engage the periphery of the guide plate to prevent rotation thereof.