motor 81 is increased the rotor 33 thereof even with the stator 32 rotating is rotated relative to the latter in conformity with the power that is supplied so that the adjustment member 20 is readjusted and hence the clamping force is increased the stator 32 of the rotary motor 31 is supported on the main spindle 3 of the machining tool 2 via the pulleys 41 and 42 as well as via the toothed belt 43 that connects these pulleys a reduction of the clamping force can be realized in a similar manner in the embodiment illustrated in fig5 the components that form the adjusting drive mechanism 120 are placed directly in a headstock 130 of the machining tool the stator 122 of the rotary motor 121 is connected to the adjustment member 20 39 and is rotatably mounted via antifriction bearings 125 in a holder 124 that is flanged to the headstock 130 in contrast the rotor 123 of the motor 121 is coupled with the main spindle 3 39 in this embodiment meshing sets of gear wheels 131 132 and 133 134 are provided as the gear means thus the rotor 123 of the rotary motor 121 is driven by the spindle 3 39 via the gear wheel 131 which is fixedly.