Folding of flat sheet to exact interior height

A folding apparatus and method for folding a flat sheet so that an exact interior height is provided between the two folded portions is disclosed. The flat sheet is folded about a shim having a folding edge which is rounded and which has a thickness corresponding to the desired interior height. The flat shim is positioned on the flat sheet with the edge of the shim located along the line where the fold in the sheet is to be made. Initially, the portion of the sheet away from the shim is folded approximately 90 DEG by tangential bending on the folding edge of the shim. Next, a die having a facing edge mating with the folding edge of the shim is pushed onto the folding edge of the shim. The portion of the sheet which was folded 90 DEG is thus folded onto the top of the shim to approximately 180 DEG . With this two step folding, an exact interior height adjacent the fold is provided. The method and apparatus of the present invention is conveniently used for forming flat jackets having an exact interior height. A plurality of these flat jackets can be formed by providing a plurality of forming devices on a turntable where each step of the forming process is sequentially performed.