be made collectively to fig9 a and 9b there reference numeral 180 designates an l shaped base for supporting a scissor type cutting head sub assembly 182 at the forward portion thereof coupled to a double acting pneumatic piston 184 as shown the piston 184 includes a pneumatic rear and forward coupling 186 and 188 so that a piston rod 190 can be driven positively in both the forward and reverse direction upon selective application of a pneumatic driving force the piston rod 190 is attached to a clevis member 192 including a pivot pin 194 to which is attached a pair of diverging links 196 and 198 which respectively connect to a drive rod clamp lock 200 and a fixed pivot block 202 the pivot block 202 is secured to the base 180 at the forward end as shown the clamp block 200 is secured to a rod 204 which passes through a compression knife blade bias spring 206 which is located between a shaft collar 208 and a u shaped block 210 at the outer extremity of the rod 204 is attached an upper rotatable blade member 212 and an adjoining knife lubricating pad 214 associated with the upper blade member 212 is a fixed lower blade member 216 which is secured to the forward portion.