Improved design of a three roll tube bending process under geometrical uncertainties

In the tube bending industry a process is considered flexible if it allows the forming of different curvature radii, without the need for a machine setup or a tool change. This is possible by numerically controlling one or more moving dies or rolls which are able to produce different radii. Unlike conventional tube bending processes, where the tube is clamped at its end and bent around a fixed die, bending with variable radius generally requires that the tube is axially fed into the forming area. A flexible bending operation is traditionally operated by dividing it into an opening phase (the bending roll is moving) and a steady phase (the bending roll is on hold and the tube is axially fed). A technological limit of the process is its intrinsic variability, e.g. measured in terms of repeatability of the obtained bent angle. An FEM based sensitivity analysis is shown in the paper in order to verify which input parameters of the incoming tubes (dimensions, material properties, etc.) are more influential on the results in terms of repeatability. The presence of the two opening and steady phases, with different mechanical conditions is an obstacle to the production of an aesthetic tube with a constant, uniform curvature radius. As a result, the real curvature radius moving along the tube spine will have some variations, which may also transform into defects, such as wrinkling or bumps. A modification of the traditionally operated control curves is proposed in the paper in order to improve the uniformity of the obtained curvature radius. Finally, a method is proposed for optimizing the control curves, under the presence of noise factors.