to form an angle with the horizontal it is advisable to arrange the mounting extension 10 as shown in fig4 in such a manner that each resting edge 11 forms a well defined angle with the cutting edge 4 which faces it as seen in fig4 each resting edge 11 is at a distance a from the axis of symmetry 6 the knife holder 2 has a threaded hole 9 to receive the countersunk screw 7 as well as a stop edge 8 which also is spaced the above mentioned distance a from the center line of the threaded hole 9 the cutting plate 3 is secured to the knife holder by the screw 7 with a given resting edge 11 engaging the stop edge 8 of the knife holder 2 thus exposing for use the one of the cutting edges 4 which corresponds to the given resting edge 11 then by loosening the countersunk screw 7 a different cutting edge 4 which is still sharp can by suitable rotation of the turnable cutting plate 3 be brought into the position contemplated for dependable cutting by the upper knife 1 after the turnable cutting plate 3 is rotated it is again fastened by tightening the countersunk screw 7 since all cutting edges 4 of a given turnable cutting plate 3 are at the same distance from the axis of symmetry 6 no subsequent adjustment of the upper knife 1 is necessary in this way assurance is had that the operator can easily effect personally the replacement or rotation.