Abstract: A shredder structure and improvements therein for shredding or otherwise comminuting materials such as municipal and industrial waste, construction debris, automobile tires, and so forth, wherein there may be selectively installed structure which both delimits and constricts the travel path of incoming materials to the cutting portion of machine and which may also regulate the extent or degree of cut. Cutter bar segments disposed on the stationary cutter bars of the shredder can be inverted and/or turned end-for-end for prolonged use in the machine before edge sharpening or replacement of the segments becomes necessary, and this because only portions of the cutter bar segments will be utilized with any given segment installation, so that remaining portions and remaining edges are held in reserve for future use.