the clamping pressure on the pressure regulator 1 a can now be lowered to the desired level and the clamping pressure drops in lines a and b and as a result of the opened check valve 8 also drops in the clamping cylinder 9 the clamping force then remains at zero after completed switching back of the valve 4 b the pressure in the utility line b and on the relief side of the clamping cylinder 9 is reduced and the clamping force is built up again at a lower level on the clamping chuck 10 by the prevailing pressure in the utility line a in this way the indicated minimization of the clamping force is achieved the solution as shown in fig1 therefore uses two 4 2 directional control valves 4 a 4 b of which viewed in the direction of fig2 the output on the left leads into a blind connection furthermore the valves 4 a 4 b are connected to the respectively assignable back pressure block 5 connected to the tank line t respectively the outputs of the pressure sensors 6 a 6 b are connected to the machine control 12 shown as a black box for reducing the clamping pressure force for evaluation of the respective measurement signal the control 12 for stipulation of a pressure setpoint has a manual input which is not detailed or has a freely programmable flow diagram program if it should not be possible to ensure.