Abstract: A guide provides a straight edge along which a cutting tool may be guided to form V grooves in fiber glass sheet material. The guide has indicia and line markings at spaced locations thereon, selectively alignable with the edge of the last previous groove cut in the sheet or panel. When the guide is so located, according to the transverse dimensions of the several walls of the duct that is to be formed from the fiber glass panel, it automatically accounts for the width of the cutting tool that is to be moved along the straight edge of the guide, to assure that the next V groove will be formed at an exact distance from the groove that was last cut.