Perforating knives can puncture or cut various materials for a specific length. These knives cut vertical slots into materials at a given length and then keep your material together for another length. A perforating blades perf is measured by “cut and tie” where the cut is the size of the cut or perf, and tie is the length of the remaining material between cuts. Perforating knives often cut against a backing, like a metal cylinder or smooth blade, to form repeated cuts.