Elsevier

CIRP Annals

Volume 61, Issue 1, 2012, Pages 251-254
CIRP Annals

Accumulative-DSIF strategy for enhancing process capabilities in incremental forming

https://doi.org/10.1016/j.cirp.2012.03.093Get rights and content

Abstract

This work proposes a novel Accumulative Double Sided Incremental Forming (ADSIF) strategy in which the forming begins at the location of the deepest feature and gradually shapes up the features by taking advantage of rigid-body motions. Compared to the conventional toolpath used in DSIF and SPIF, this strategy can dramatically improve geometric accuracy, increase formability, form components with desired thickness and create complex components. Furthermore, an examination of the forming forces shows that the dominant forces using this strategy are in the plane of the sheet resulting in a significant improvement in geometric accuracy.

Introduction

Incremental forming (IF) is a flexible sheet metal forming technique that uses simple generic tooling to locally deform sheet metal along a predefined toolpath, imparting the sheet a desired shape. Single Point Incremental Forming (SPIF) uses one tool on one side of the sheet to cause the deformation. SPIF is plagued by an inherent geometric inaccuracy due to non-local springback in the single point setup. Allwood et al. [1] attempted to improve the geometric accuracy by using partially cut out blanks along the periphery of the forming area. While the obtained geometric accuracy was better than that in regular SPIF, they commented that this technique was not useful in improving geometric accuracy in IF, especially in comparison to the significantly better geometric accuracy provided by a partial support in spite of the resultant loss in process flexibility. Allwood et al. [2] also used closed-loop feedback control to improve the geometric accuracy in SPIF by forming the component in a second iteration. Although the result obtained from the second iteration was better than the initial one, they mentioned that this strategy would be difficult to be implemented for freeform objects.

Variations of IF have been proposed to preserve its inherent process flexibility and to improve geometric accuracy, mainly die-based IF which uses a die below the sheet (DBIF in Fig. 1a) and double-sided IF which uses one tool on either side of the sheet (DSIF in Fig. 1b). In DBIF, for example, Tekkaya et al. [3] used generic sectional shapes to act as supports for the forming tool assisted with an analytical tool that calculates thinning to achieve a better geometric accuracy in IF. However, the strategy is limited to forming components on one side of the sheet only and requires process planning that is specific to the part geometry being formed.

An interesting alternative is the DSIF setup as demonstrated by Meier et al. [4] who used two tools on either side of the sheet, each tool mounted on a robot. Malhotra et al. [5] showed that using two identical tools on either side of the sheet with the gap between tools smaller than the sheet thickness, a so-called “squeezing toolpath”, can improve the geometric accuracy, particularly for forming tight radii or small fillets. However, they also pointed out that an accurate thickness prediction is critical in this toolpath, otherwise, due to loss of contact between the bottom tool and the sheet, DSIF will degenerate to SPIF. To maintain contacts of both tools with the sheet, Meier et al. [6] used a forming tool which was displacement controlled whereas the supporting tool used a combination of displacement and force control. They demonstrated that this strategy could ensure contact between the supporting tool and the sheet at all times, leading to greater formability. However, a drawback of this strategy is that the amount of force to be applied and a preset angular offset for the supporting tool have to be worked out by repetitive trials every time the component shape is changed. Furthermore, depending on the global shape of the component the force required will change. Therefore, to form a freeform shape the amount of force required will vary spatially and will have to be pre-determined by experimental iterations.

In past works on DSIF [5], [6], the conventional out-to-in toolpath has been employed for the forming tool. In this toolpath the forming begins from the outermost periphery of the component to be formed and travels all the way down to the actual component depth, while moving in the XY plane (Fig. 2a).

This work proposes a novel Accumulative Double Sided Incremental Forming (ADSIF) strategy for DSIF where both the forming tool and the supporting tool are purely displacement controlled. Contact between both tools and the sheet are maintained at all times during the forming process. Once the strategy is understood, it is surprisingly easy to generalize it for a freeform geometry since the toolpath can be decided completely a priori based on the CAD geometry.

In the following sections, the toolpath strategy for ADSIF will be detailed first followed by an experimental demonstration of forming components with features on both sides of the blank as well components with concavo-convex features, without flipping the sheet or changing the tooling in the forming process. The effects of ADSIF on geometric accuracy, formability, thickness distribution and forming forces will then be presented and analyzed.

Section snippets

Fundamentals of ADSIF

Malhotra et al. [5] demonstrated that the sine law provided an inaccurate prediction of the formed thickness in DSIF. Therefore, positioning the second tool based on the sine law in a conventional out-to-in toolpath leads to loss of contact and unsatisfactory geometric accuracy during DSIF. The proposed ADSIF strategy was originally conceived by co-authors of Ford [7] and has been enhanced here in this work and in the corresponding patent application [8]. This strategy prevents loss of contact

Experimental setup

A DSIF machine with one tool on either side of the sheet was custom-designed and fabricated at Northwestern (Fig. 5a), to investigate various toolpath strategies including the newly proposed ADSIF strategy. In this machine the X and Y axes of each tool are controlled using two motors on a double gantry system. The Z-axis of each tool is controlled by a single linear guide and motor. The two tools are controlled by a custom made DELTA-TAU controller. The velocity of the bottom tool is adjusted

Experimental results

This section addresses the effects of ADSIF on formed geometry, formability, thickness distribution and forming forces.

Conclusions and future work

DSIF is a relatively new process which enhances the geometric flexibility achievable in IF. A major challenge in DSIF is toolpath design that allows the process to achieve its full potential in terms of geometric accuracy and formability. This work proposes a novel ADSIF strategy for DSIF in which both the tools are under displacement control and the toolpath is generated completely a priori from the CAD geometry. No component specific process planning is required to use this generic strategy,

Acknowledgements

The authors would like to acknowledge the Department of Energy, USA DE-EE0003460, National Science Foundation, USA and Chinese Scholarship Council for their support.

References (8)

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