In the roofing industry it is common to use sheet metal flashing for sealing and finishing various roof edges. The flashing members typically comprise elongate metal sheets in varying lengths and widths which are interlocked with one another at respective ends by a slip lock or S-lock joint. The general profile of an S-lock joint is essentially a flattened S such that the sheet member forms a main portion of the flashing member, an intermediate portion folded back over top of the main portion, and a free end portion extending back across the intermediate portion in the longitudinal direction of the flashing member beyond the end of the main portion. The free end portion is typically fastened to the roof structure. A subsequent flashing member can then be inserted into the space between the end portion and the intermediate portion of the already fastened flashing member. As presently practiced in the industry, slip locks can be formed using conventional sheet metal bending equipment but the process involves many steps and is therefore very labour intensive and expensive.
U.S. Pat. No. 4,803,879 by Crawford describes a slip lock forming apparatus using two dies about which a sheet metal panel is folded for forming the S-lock joint. By using only two dies, which are forced together, the sheet metal involves some relative sliding against the surfaces of the die during the forming process which produces scratches in the finished product and provides limited control over the precise path which the sheet metal is bent to produce the S-lock joint.