Twist revisited: Twist phenomena in single point incremental forming

Twist phenomena in incrementally formed parts have been observed both in the domain of single and two point incremental forming. In the reported experiments the resulting twist direction typically corresponds to the toolpath direction and can be explained by the monotonous tangential force component exerted on the workpiece when using unidirectional toolpaths. When processing parts with high drawing angles, however, twist deformations corresponding to in-plane shear in opposite toolpath direction have been observed by the authors. In this paper this phenomenon is documented and explained by means of strain measurements and FEA results. The role of asymmetric, cumulative, extended strain effects, resulting in severe thinning, and reinforced when stiff, semi-vertical rib features are present in the part geometry, is demonstrated by means of a detailed deformation analysis.