I just saw your reply to Garry - its difficult to impossible to roll short ring sections, there is usually a length at each end that doesn't go through all the rollers or if the ends DO go through the rollers, they have a different curve. All metal has an amount of spring back when its bent; when you roll a full circle, you usually adjust the diameter by trimming the ends and then weld them. If the segments are made separately, it will be harder to get the curvature accurate.