is fed through the coil 9 thus the coil 9 and the rotor 8 constitute a rotary motor 17 i e rotary driving means a pressure rod 10 is disposed above the collet chuck 2 the pressure rod 10 is movable within the rotor shaft 1 along the axis o the housing 4 is fixed to a base 16 the housing 4 and the base 16 constitute a body portion of the unit the vertical connecting shaft 6 is connected with a movable element 12 by means of bolts 11 the vertical connecting shaft 6 and the movable element 12 constitute the holding means disposed on the side of the movable element 12 is a magnet 14 that constitutes a linear motor 18 together with a coil 13 the coil 13 is disposed in the base 16 facing to the magnet 14 the movable element 12 moves along the axis o when electricity is supplied to the coil 13 it is noted that while the linear motor 18 constitutes the linear driving means not only the linear motor 18 but also other linear driving means such as a hydraulic cylinder may be used bearings 15 support the movable element 12 so as to be movable within the base 16 along the axis o the movable element 12 is connected in a body with a link plate 20 by fixing a flange 12 f provided at the upper end of the movable element 12 to the link plate 20 by means of bolts 21 a spring 22 i e resilient means is interposed between the link plate 20 and the base 16 to force the link plate 20 upward in the figure a cylinder.