A variety of flexible and incremental forming processes exist which could be useful for forming sheet metal parts without the need for expensive part-specific toolilng. Automated versions of these have been the subject of considerable study in recent years. This study has been largely empirical, based on varying input parameters to improve forming depth and accuracy of the final geometry using a limited range of readily formable materials. Three questions arise from this study and are the basis of recent and current work here: How do these processes work? What other materials can they be applied to? How do we test materials to assess their suitability for incremental forming?
The class of processes known as paddle forming was first presented in 2006 to the CIRP in Paris. It uses a simple flat tool moving in sliding contact with a sheet metal work piece to create localised features incrementally. It was designed as a small scale process to be analogous to the incremental sheet forming process in terms of the mechanisms by which material is formed, but on a suitable scale for in depth analysis using finite element software. Details of on-going work on paddle forming can be found here. This allowed for the first time analysis to be done on this type of forming process with sufficient resolution to observe variation in strain through the thickness on the workpiece, leading to analyses of the influence of through thickness shear strain and increased hydrostatic pressure on formability. Details of this analysis were presented at SHEMET in 2007.
This formability analysis lead directly to a redesign of the forming limit diagram, which in its traditional form is insufficient to map forming limits for processes with non planar stresses. A theoretical analysis based on the 1067 Marciniak and Kuczynski model but allowing for through thickness shear and compression was performed, and example forming limit surfaces were generated showing increased formability for processes with higher hydrostatic pressure, and increased levels of shear. In experimental work to support this a preliminary experiment increased the extension at failure of a uniaxial tensile test sample from 8% to 300% by repeatedly running a smooth paddle-tool over the surface of the test region and reducing the tensile load. Both the formability analysis and the results of this experiment were reported in the International Journal of Plasticity in 2008.
Following on from this analysis, another new materials testing experiment has been devised to explore the region of the forming limit between pure shear and uniaxial tension, a region which has not been investigated before. This test makes use of modern computer controlled cutting technologies to create a range of cut-out geometries in rectangular samples. These cut-outs leave small test regions which deform with the desired strain ratios when the sample is pulled apart in uniaxial tension. The design of this sample was presented at ICTP in 2008
Current work is centred on the design and use of a new testing rig for the testing of plane stress combinations. This uses a tensile testing machine with an additional system for moving loaded tools over the surface of a sheet specimen during extension.
Paddle forming, so called because the shape of the tool is reminiscent of a canoeing paddle, is a new family of forming processes suited to creating pronounced features in sheet metal components. The processes are incremental in nature and are characterised by having contact between the tool and the workpiece along a short line segment. The processes lead to high through-thickness shear strains due to friction and local bending around the tool head which slides over the sheet surface with a small perpendicular load. This has been shown to increase the possible deformation before ductile instability occurs. Paddle forming tools, which are cheap and simple to manufacture, can be applied to the faces or edges of sheets, tubes and disks.
Simple configurations of paddle tools have been used for comparison with conventional punch forming to create axisymmetrical forms as part of a study into the mechanics of incremental processes, demonstrating increased formability over conventional stamping. The success of these experiments lead to a range of alternate geometries and applications some of which are pictured below, with videos available here. One of these is already being implemented by one of our industrial partners.