1. IN A PRECISION GRINDING MACHINE HAVING A WHEELHEAD, A DRIVE SHAFT CONNECTED TO THE WHEELHEAD FOR MOVEMENT THEREOF WHEN ROTATED, A PADDLE MOTOR RECEIVED ON THE DRIVE SHAFT AND OPERABLE FOR ROTATION THEREOF, AND A GEAR MECHANISM TO ROTATE THE MOTOR AND SHAFT IN UNISON, THE COMBINATION COMPRISING: (A) A SERVO-VALVE IN COMMUNICATION WITH SAID MOTOR AND HAVING A PAIR OF RELATIVELY MOVABLE MEMBERS THEREIN TO CONTROL THE OPERATION OF SAID MOTOR ONE WAY AND THE OTHER WHEN SHIFTED FROM A PREDETERMINED RELATIVE POSITION, (B) MEANS TO SHIFT ONE OF SAID CONTROL MEMBERS THROUGH A FIXED DISTANCE IN ONE DIRECTION FROM A STARTING POSITION AND THEREAFTER TO CONTINUE MOVEMENT THEREOF IN THE SAME DIRECTION AT A CONTROLLED RATE TO PRODUCE RAPID AND FEED RATE OPERATION OF SAID MOTOR IN A CORRESPONDING DIRECTION, SAID SHIFTING MEANS REVERSELY OPERABLE TO RETURN SAID ONE CONTROL MEMBER TO ITS STARTING POSITION TO REVERSELY OPERATE SAID MOTOR, (C) A FEEDBACK MECHANISM RESPONSIVE TO ROTATION OF SAID DRIVE SHAFT BY SAID MOTOR TO SHIFT SAID OTHER CONTROL MEMBER TO FOLLOW SAID ONE CONTROL MEMBER, SAID OTHER CONTROL MEMBER BLOCKING OPERATION OF SAID MOTOR WHEN IN SAID PREDETERMINED RELATIVE POSITION, AND (D) MEANS RESPONSIVE TO ROTATION OF THE MOTOR AND DRIVE SHAFT BY SAID GEAR MECHANISM TO RENDER SAID FEEDBACK MECHANISM INOPERATIVE TO SHIFT SAID OTHER CONTROL MEMBER.