Abstract: A knife which is moveable vertically by a mover cooperates during the cutting process with a cutting plate which is mounted in a throat plate. A cutting edge of the knife is aligned parallel to an upper edge of the cutting plate so that these two cutting bodies momentarily contact each other over the full surface. In order to achieve a predetermined depth of penetration of the cutting edge into the cutting plate, which is made of visco-elastic plastic, the knife is first brought into an upper position by the mover. Then it is adjusted by a displacement member mounted in the mover. Its vertical position with respect to the cutting plate is displaced by a desired amount, while maintaining the parallel relationship of the cutting edge and the upper edge, regardless of their instantaneous position.