A method for forming a metal strip into a cylindrical core, comprising: passing an elongated metal strip having width and length and opposite longitudinal edges through a multiplicity of forming stands of forming rolls which form rectangular components and generally straight segments thereof in said strip and change the size and shape of the segments and components previously formed at said forming stands, wherein upon the completion of the corrugation forming process the strip has a rectangular ribbed profile that comprises longitudinally extending, alternating ridges and grooves having generally vertical spaced-apart sidewalls with the ridges further comprising longitudinally extending, flat, spaced-apart top surfaces extending between adjacent ridge sidewalls and the grooves further comprising longitudinally extending, flat, spaced-apart bottom surfaces extending between adjacent groove sidewalls, the flat ridges and flat grooves together forming a rectangular, flat ribbed cross sectional profile extending along the width of the strip transverse to the length thereof, wherein the multiplicity of forming stands are to initially form rectangular features and/or straight line segments of rectangular features and to progressively add rectangular features or straight line segments to the rectangular features or straight line segments and to refine the rectangular features and straight line segments to form the ridges and grooves, wherein each of the forming stands is to form rectangular components or generally straight segments thereof in said strip; forming the opposite longitudinal edges of the as-formed corrugated metal strip into flanges (16, 17) adapted for forming an interlocking seam; helically coiling the as-formed corrugated strip into a cylinder having a longitudinal axis and an outer surface defined by flat, spaced-apart surfaces selected from the top surfaces and the bottom surfaces; and compressing the opposite edge flanges (16, 17) together thereby forming an interlocking seam extending in a spiral around the cylinder along the longitudinal axis thereof. passing an elongated metal strip having width and length and opposite longitudinal edges through a multiplicity of forming stands of forming rolls which form rectangular components and generally straight segments thereof in said strip and change the size and shape of the segments and components previously formed at said forming stands, wherein upon the completion of the corrugation forming process the strip has a rectangular ribbed profile that comprises longitudinally extending, alternating ridges and grooves having generally vertical spaced-apart sidewalls with the ridges further comprising longitudinally extending, flat, spaced-apart top surfaces extending between adjacent ridge sidewalls and the grooves further comprising longitudinally extending, flat, spaced-apart bottom surfaces extending between adjacent groove sidewalls, the flat ridges and flat grooves together forming a rectangular, flat ribbed cross sectional profile extending along the width of the strip transverse to the length thereof, wherein the multiplicity of forming stands are to initially form rectangular features and/or straight line segments of rectangular features and to progressively add rectangular features or straight line segments to the rectangular features or straight line segments and to refine the rectangular features and straight line segments to form the ridges and grooves, wherein each of the forming stands is to form rectangular components or generally straight segments thereof in said strip; forming the opposite longitudinal edges of the as-formed corrugated metal strip into flanges (16, 17) adapted for forming an interlocking seam; helically coiling the as-formed corrugated strip into a cylinder having a longitudinal axis and an outer surface defined by flat, spaced-apart surfaces selected from the top surfaces and the bottom surfaces; and compressing the opposite edge flanges (16, 17) together thereby forming an interlocking seam extending in a spiral around the cylinder along the longitudinal axis thereof.