Abstract: A cutter for sheet materials, packing or cushioning materials, and the like, comprises an elongate, hollow cutter bar or housing having a spring blade pivotally mounted therein, with the upper edge of the blade positioned in a longitudinal slot in the bar. A circular knife is slidably carried on the bar, and includes a cutting edge which engages the blade to shear the sheet therebetween as the circular knife is moved along the bar. The blade is normally biased into a safety position, wherein the upper edge of the blade is retracted into the bar slot. Engagement between the circular knife and the blade automatically shifts the adjacent portion of the blade into a cutting position with the knife, wherein the upper edge of the blade is extended from the retracted, storage position to a cutting position, flush with the upper surface of the cutter bar to cleanly and evenly shear the material between the blade and the knife.