Method for manufacturing metallic ribs on sheet stock

The process of forming a sheet at low pressure to produce therein a longitudinal corrugation having a major length of large cross-section and having ends of minor length which gradually taper from the large cross-section of the major length into the plane of the sheet, the outermost ends of the tapered ends stoping substantially short of the adjacent ends of said sheet, said process comprising:

• forming the major length and the two tapered minor lengths of said longitudinal corrugation between matched male and female die members by closing and pressing said die about said sheet;
• whereby excess sheet material tends to be present in the region of the sheet longitudinally extending from the ends of the longitudinal corrugation to the adjacent edge of the sheet and whereby said excess material in said region tends to produce wrinkles;
• forming a plurality of parallel mini-corrugations in said region with the length of each mini-corrugation being parallel to the length of the longitudinal corrugation and each mini-corrugation being located so that it intersects one of said tapered ends;
• whereby said mini-corrugations absorb said excess material while protruding from the plane of said sheet only a minor fraction of the corresponding protrusion of said longitudinal corrugation;
• and
• whereby the corrugated sheet produced by the process does not wrinkle from excess material in said region, and will, with the corrugation protruding upwardly from the plane of the sheet, lie flat.