Eliciting knowledge about dimensional variability and influential parameters is crucial in competitive manufacturing. Moreover, this knowledge needs to be generalized and transferred into a practical tool box that design engineers can use as a decision basis in the early phases of product development processes when design flexibility is high and the ‘cost of learning’ is low. This paper introduces Flatness Limit Curves, a new Design For Manufacturing (DFM) tool, which is aimed at assessing cross-sectional distortions relative to dimensional tolerance requirements in bending operation. Analytical relationships are derived from the theoretical basis of continuum mechanics, applying the deformation theory of plasticity to plane stress sheet metal problems. The theoretical results are compared with experimental results obtained for hollow AA7xxx extrusions, which were formed in an industry-like rotary stretch bender in a controlled laboratory environment. The solutions are then structured and organized into a set of limit curves for assessing nominal flatness of the exterior of the cross section after bending. The associated variability may also be estimated by varying key input parameters to the tool (dimensional accuracy and material quality) within the capability ranges of the upstream process.

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