the blade assembly 120 is formed so that leading edges 121 are provided on opposite side ends of the leading portion thereof so as to pass through a target a pair of corresponding mounting slots 122 for the rotary blades are formed in a flat surface of the central portion thereof and a rotary pin receiving hole 123 is connected between the mounting slots unexplained reference numeral 124 in the drawings denotes an opening vertically passing through the flat surface of the blade assembly 120 the blade assembly 120 is securely fixed to the ferrule 110 by fitting it into the fitting slots 111 a of the ferrule 110 and then fixing it using fixing screws 114 and through holes 111 b and 125 the through hole 111 b of the ferrule body 111 which is coupled with the fixing screw 114 may be preferably provided with an internal screw instead of the screw type clamp means another clamping means such as a rivet or the like may be used to couple the blade assembly and the ferrule to each other in the mounting slots 122 for the rotary blades the respective circular rotary blades 130 are mounted such that they are rotatable about a rotary pin 115 fig4 a and 4b are a front view and a side view showing.