a seaming mechanism for a machine of the type employed to secure a metallic head on a metallic drum by deforming and folding together adjacent , initially lapped , peripheral portions of the head and drum end ; such mechanism embodying a pair of seaming rolls positioned for sequential movement into working engagement with such peripheral portions of the head and drum end , and corresponding seaming roll supporting arm units -- actuated from a single , driven cam -- operative to impart such sequential movement to said seaming rolls .