Abstract: Apparatus for corrugating a length of strip metal to form a series of zig-zag folds in the strip has counter rotating forming dies for forming a series of equally spaced apart impressions in the strip and counter rotating corrugating dies for forming the zig-zag folds in the strip intermediate adjacent impressions. The apparatus includes a clutch for selectively disengaging the drive for the corrugating dies to permit adjustment in the phase relationship of the rotation of the corrugating dies relative to the forming dies while the apparatus is in operation. This permits adjustment while the machine is operating to compensate for variations in physical characteristics of the material feed.