blade 118 past the stationary blade to shear the slat in the embodiment shown the redirecting means include the cam face 126 on the cam bar 122 the redirecting means also include mating cam follower or roller means in the form of a wheel 140 attached to the blade driver 120 in operation before the collar 136 engages the boss 138 the force on the cam bar 122 drives the cutting assembly forward only because less force is required to do that than to push the cam bar 122 forward relative to the wheel 140 the cam bar 122 can move forward only by forcing the wheel 140 to roll upward along the inclined forward cam face 126 the wheel 140 however is mounted to the blade driver 120 which is in turn biased downward by a tensioned spring 142 therefore the wheel can roll up the cam surface 126 only if the force applied to the cam bar 122 exceeds a threshold value determined by the spring tension the angle of the cam surface 126 and friction in certain parts of the mechanism the applied force cannot exceed that value as long as the cutting assembly can move forward after the limit means 134 138 come into engagement however the full force of the cylinder or solenoid 121 is available to cam the wheel 140 and the blade driver 120 upward against the spring action accordingly as the force applying means then continue to move the cam bar 122 the wheel 140 rolls up the inclined slope of the cam face 126 the cam follower wheel 140 continues to rise along the cam face 126 at least until the blade driver 120 has raised the movable blade 118 sufficiently toward or beyond the height of the.