fig3 are not shown separately they are integrated into the control devices 10 a 10 b and 14 reference numeral 15 indicates an operating and display unit for system control 14 and reference numerals 13 a 13 b indicate operating and display units for respective machine controls 10 a 10 b regulation of the grids is carried out by means of the motors 7 a 7 b which are controlled by the control 10 of machine 1 in dependence upon optimum adjustments established on one occasion for the various materials and stored in the control 10 these adjustments can be automatically produced again at any time when required all that is needed therefore is an entry indicating which material 17 is being processed when such a machine 1 is connected by way of a network 16 to a superordinate system control 14 see fig4 it is also possible for such data to be specified from there in fully automatic manner in such a case the optimum adjustments once determined are transferred from the machine 1 to the said control where they are stored in the event of a change of material a large number of adjustments including in accordance with the invention the positions of the grids 3 a 3 b are usually transferred from the system control 14 to the individual machines 1 analysis of the trash 19 u2033 removed which is to a very.