an elongated blade has a notch in its leading edge , with parallel chisel edges adapted to chop through the oriented fibers of uncured sheet material as the blade is reciprocated vertically at a high frequency relative to its forward velocity . the notch is about half as deep as the blade width and has a vertical extent of about 60 percent of the blade &# 39 ; s vertical stroke . the blade has a lower edge which forms a 135 degree angle with it &# 39 ; s leading edge , and which lower edge is spaced below the lower notch chisel edge by approximately the width of the blade .