Abstract: For closing Pittsburgh-type rectilinear seams, as in conventional sheet metal heating and air conditioning ducts. The present machine, with only a single seaming roll, closes seams through 45.degree. to 90.degree. on an advancing stroke, and burnishes them on the return stroke. The machine functions without adjustment throughout substantially the entire range of sheet metal thicknesses normally utilized. Of particular advantage in manufacturing duct sections having integral end flanges, only an initial short length of seam need be peened manually before the duct elements to be seemed are clamped in place for completion. Workplace noise is greatly reduced.