Pressure resistant sheet metal end closure

A method of forming a pressure resistant sheet metal closure comprising in sequence the steps of

• forming a sheet metal closure having a central panel having a predetermined circumference and a center portion, a downwardly projecting countersink around the central panel and having a junction therewith, a generally frustoconical chuckwall around the countersink and a curved attachment flange around the chuckwall, said closure having surface area and said central panel disposed above said countersink,
• coining the metal in the closure near the junction of said central panel and said countersink around a major portion of the circumference of the central panel and thereby increasing the surface area of metal in the closure, and
• reforming said central panel by substantially increasing the height of said center portion thereof spaced inwardly of said countersink to utilize the increased surface area of metal produced by said coining and place the metal in the closure in tension.

Apparatus for forming a sheet metal closure having a central panel which includes a center portion, an undersurface, and an upper surface, a panel radius around the central panel, a countersink around and below the panel radius, a chuckwall around the countersink, and a curved flange around the chuckwall comprising

• means for coining the panel radius in said closure around at least a portion of the circumference of the central panel, and
• means for reforming the central panel subsequent to said coining by substantially increasing the height of said center portion thereof spaced inwardly of the countersink and forming a downwardly stepped portion between the central panel and the panel radius.