Rolled steel plate is press-fabricated into cylindrical UO pipe with a large wall thickness, by the steps of crimping, U-ing and O-ing. In the U-ing process, a collapsible U-ing punch, including a center block carrying paired side punches on both sides and a bottom punch at the bottom, is used. The U-ing punch insures that the O-shaped workpiece has the desired butt gap by decreasing the ratio of its height to its width when the gap proves to be smaller than the desired butt edge gap, and vice versa when the gap proves to be larger than the desired butt edge gap.