Method of roll forming cylindrical pipe

A method of forming a generally cylindrical section from a generally planar plate section comprising the steps of:

• pre-crimping the first and second edges of said plate section to a final desired radius of said cylindrical section,
• feeding the first edge of said plate section to a forming mill having an overhead flating mandrel with pressure applying means on one side thereof and bending means on the opposite side including a pair of circumferentially spaced, pivotally mounted rotatably driven bending roll assemblies such that said first edge may pass between the mandrel and the bending means;
• clamping said pre-crimped first edge of said plate section between the mandrel and one of said bending roll assemblies,
• forming said plate section into an arcuate shape by pressing the mandrel against said bending roll while the plate section advances therebetween in a first direction a given distance;
• continuing to form said plate section thereof into an arcuate shape by advancing the second edge thereof in a second direction, opposite to the first direction and pressing the mandrel against the bending roll assemblies;
• capturing the second edge with a springback roll and pressing the second edge against the mandrel so that the second edge can pass between the mandrel and the bending roll assembly;
• continuing to advance the plate in the second direction while applying pressure to the mandrel until the plate is formed into a generally cylindrical section;
• and
• applying pressure to said mandrel at points spaced longitudinally along the mandrel to bend the plate section during forming of the plate section between said mandrel and the pair of bending roll assemblies and to offset deflection of the mandrel and the bending roll assemblies throughout the length of the mandrel so that the plate section is uniformly bent along the length of said plate section to the desired final radius.