the knife 14 there is peripherally welded a nut 21 further a stationarily supported drive motor 17 is provided whose output shaft 17a is rotatable in either direction as indicated by the two directional arrow d the shaft 17a is supported in a stationary bearing 22 and has a terminal spindle 18 on which the nut 21 is threaded upon rotation of the shaft 17a the nut 21 the intermediate carrier 15 and the knife 14 are moved in the direction of the arrow b or c the drive motor 17 serves as a positioning drive that is it displaces the knife 14 and also immobilizes it in the desired position particularly in case of a small pitch of the thread of the spindle 18 and the nut 21 the knife 14 may be steplessly set with high precision and immobilized in the adjusted position by virtue of stopping the motor 17 expediently a d c motor the drive motor 17 may be actuated and stopped manually by operating a switch it will be understood that instead of an electric motor 17 a hydraulic cylinder unit may be used as the drive mechanism turning now to fig3 a and 3b the knife 14 is rotatable about an axis which coincides with the knife edge 14a the intermediate carrier 15 on which the knife 14 is mounted is thus pivotal through an angle u03b1 in the direction of the arrows f and g the knife carrier 15 has on either side angle.