(57) A web slitting machine (10) for cutting a web or roll of material includes an upper carriage assembly (16) joined to a lower blade holder assembly (18) by a selectively removable guide key (38) which is milled to provide a predetermined cant angle for the blade (20). The blade (20) is lowered into position by a piston (54, 58) which is rectangular and includes a torsion-resisting sleeve (60a, 60b) to prevent rotation of the piston about its vertical axis. A pneumatic control provides a plurality of control modes whereby the blade (20) may be raised and lowered with or without locking the upper carriage assembly (16) to its transverse bar (14) and vice-versa. A unique side shift adjustment is provided whereby the blade (20) may be shifted to a half-stroke position and the upper carriage (16) locked when the blade is positioned against a lower knife (22). This ensures that the pressure of the blade (20) against the knife (22) is the pressure exerted at the mid point of the stroke. A parallelogram linkage (210, 212) biased by a spring (224) can provide a shock absorber for maintaining blade/knife contact in the presence of webs moving at high speed.