6 to a force which pulls the projecting portion 6 into the rotary cutter 1 thereby guaranteeing the secure fastening of the projecting portion 6 further as can be seen in fig3 6 the bottom surface 4 b of the projection portion 6 of the knife 4 rests on the bottom 7 b of the groove and extends in a direction that is substantially orthogonal to the knife edge 5 fig5 presents a variation on the design shown in fig4 in which the knife 4 is secured to the rotary cutter 1 by means of a notch 14 in the projecting portion 6 said notch 14 serving as an abutment 18 for this purpose the screw 15 is provided on its end surface with a corresponding conical portion which fits into the notch 14 and with the screw 15 tightened secures the knife 4 to the rotary cutter 1 as in the design shown in fig4 also in the design shown in fig5 the screw 15 is provided with a hexagonal recess into which it is possible to insert an appropriate key or wrench with which to tighten the screw 15 fig6 presents a variation on the design shown in fig5 in which the knife 4 is double edged in other words the knife 4 of fig6 has two l shaped portions 4 l this design has the advantage that when one of its cutting edges 5 is worn the knife 4 can simply be turned round by 180 u00b0 as a result of which a new cutting edge 5 comes into the region of the rotary.