Bending (Sheets)
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Bending (Sheets) Joost R. Duflou Department of Mechanical Engineering, KU Leuven, Leuven, Belgium
Synonyms Folding (for 180 bends in thin materials only)
Definition Bending is a forming process in which a blank is locally formed along a bending line as result of applying a bending moment. Starting from a flat blank, complex 3D parts can thus be formed, composed of flat flanges and semi-cylindrical bend connections. Below 15 mm thickness the process is known as sheet metal bending, while for blanks above 15 mm plate bending is the more common terminology.
Theory and Application Bending Methods Different methods can be applied to impose a bending moment on a sheet metal blank (Lange 1985, Benson 1997). The most common techniques consist of positioning the sheet to be
formed with the bend line coinciding with the centre line of a V-die and pressing a punch tool in the orthogonal direction (Fig. 1 right). The force exerted during this operation is typically delivered by a press brake (Fig. 1 left). Two main variants of this method can be distinguished. When the sheet is formed by imposing forces via line contacts at the shoulders of the die and the punch tip, the process is referred to as air bending, also known as three point bending (Fig. 2a). When the sheet to be formed is in full contact with both the punch and the die in de bending zone, the process is referred to as bottoming (Fig. 2b). Compared to bottoming, air bending offers a higher flexibility since different angles can be formed with the same tool set (a punch and die with a tool angle for the punch that is significantly higher than the V-die angle: Fig. 1 right) by controlling the Y axis displacement of the ram of the press brake. Bottoming offers better dimensional accuracy of the resulting parts since springback can be limited in this process variant, resulting in an achievable bend angle accuracy of 150 (Serruys 2006). In wiper bending (Fig. 2c) and swivel bending (Fig. 2d) one of the two flanges bordering a bend line is clamped and the bend is formed by respectively a translating movement of a wiper tool or a rotational movement around a fictitious bend centre line.
# CIRP 2018 The International Academy for Production Engineering et al. (eds.), CIRP Encyclopedia of Production Engineering, https://doi.org/10.1007/978-3-642-35950-7_16773-5
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Bending (Sheets)
Bending (Sheets), Fig. 1 Press brake layout and tooling
Stress: Strain In principle the different bending methods result in planar strain deformations. This planar strain deformation is only a correct assumption at sufficient distance from the ends of the bend lines (a multiple of the sheet thickness) where strains in the bend line direction are restricted by symmetrically behaving neighbouring material. The stress and strain components in this case are shown in Fig. 3. Near the ends of a bend line this assumption is not valid and the so called anticlastic effect can be observed (Fig. 4). During a bending operation the moment imposed by the tooling results in the inner part of the cross section of
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