Abstract: A power chuck system is disclosed featuring a motor, a harmonic drive and a chuck. The harmonic drive is arranged with an input member driven by direct connection with the motor rotor, and an output member directly connected to a long stroke jaw actuation device, providing an integral unit which rotates together with the spindle during machining. During jaw adjustment, an electromagnetic brake releasably connects the motor stator with the spindle and chuck body. Gripping force adjustment is achieved by adjusting the motor torque. The inertial forces of the rotating components are overcome by controlling the motor torque and velocity as gripping ensues, and therefore both high and low gripping forces may be accurately achieved with high speed jaw adjustment. Jaw position and workpiece diameter measurement with a feedback capability are made by use of a rotary encoder.