Sheet Metal Tooling: Process Chain Completed!

Designing dies for body-in-white sheet metal in automotive or aerospace engineering, spring-back effects after tool retraction have to be determined and compensated. There's a long way from the original shapes in 3D solid models through meshing for Finite Elements Analysis to compensated moulding tools for class A surfaces or structure parts, including expensive experimental dies and time-consuming manual operations. The digital process chain leads to the dead end of incompatible data formats. Providing thinkcompensator, Munich-based think3 GmbH fills the gap between CAD systems and simulation software on the market.


Fig. 1: thinkcompensator atlas: 3D pressing mould, by courtesy of Atlas Tool Inc., Michigan

In industries like automotive engineering or aerospace even complex components are formed from high rigidity steel or aluminium alloy. Demands for design and surface quality do not only increase with visible parts, but in general. Even when form-finding is completed and perfect surfaces are defined in 3D CAD systems, OEM and systems suppliers for sheet metal forming or die makers and product managers have to bridge enormous gaps in the digital process chain on their way to functional pressing moulds. Missing or insufficient interfaces require long winded reverse engineering, time-consuming changes to the CAD model and finally dreaded changes to tool steel: they result in high costs for milling and eroding processes, delay specification test for weeks and are moreover hardly reproducible.


Fig. 2: 1_Dies: problem during sheet metal forming: the effect of spring-back ...

Spring-back Effect
Due to two absolutely welcome material properties, stiffness and elasticity, sheet metal dies can not exactly match the target surfaces: when pressing tools are approaching, the sheet metal board follows its form. But when tools are removed, the material springs back and in some cases considerably differs from the desired shape. Depending on the use of aluminium alloy or high rigidity steel and the degree of formation this effect appears to a stronger or weaker extent. Spring-back therefor must be compensated during each tool development based on surface models from 3D CAD systems. Nowadays expected spring-back is simulated via Finite Element Analysis using appropriate software packages. The CAD surface model is read into a simulation environment and meshed. After defining expected loads and conditions; a solver calculates their impacts. Several iteration loops are necessary until the target form of the sheet metal part meets the allowance.


Fig. 3: 3_Dies: ...means expensive and time-consuming change processes for designing and manufacturing tools

Interfaces affect data quality
That's where the digital process chain breaks: the FEA meshes can only be retransferred to CAD files at a loss. Using reverse engineering software this can be done in a little while, but control of tolerance as well as topology of the original model is lost. Especially with visible freeform surfaces - like the vehicle interior or the body-in-white - essentials of Class A quality are lost.

It is also possible to manually redraw surface models in the 3D CAD system: thus all surface areas manually have to be brought to coverage with the compensated target coordinates read in from the FEM system. The result is enormously time-consuming and depends on the user's experience. Neither approach enables users to retransfer all data to the CAD system in sufficient quality.

Process chain completed
With thinkcompensator think3 has created an automated process to match the initial CAD model with the final FEM springback results to get the compensated shape. For that purpose the surface model of the sheet metal part from any 3D CAD system has to be cross-linked for Finite Element Analysis. A first approach to the expected spring-back of the body sheet metal is calculated. The original CAD model as well as the initial and target meshes from the FEM system are sources for determining compensation.

thinkcompensator then calculates a first proposal for compensation. Depending on the number of points, the chosen accuracy and the calculation performance this takes several minutes. Then spring-back effect is calculated from this compensation in the FEM system again. This operation has to be repeated until the chosen range of tolerance will be reached. Normally this is the case after two or three automatic calculation loops.

Successful compensation of the spring-back is followed by the actual clou: thinkcompensator transfers the newly calculated surface geometry to the original CAD model. Due to Global Shape Modeling-Technology (GSM) developed by think3, topology (number of surfaces, boundaries, bendings and other characteristics) and data quality are completely conserved. Via GSM modifications of surface combinations can be performed globally without restriction to the quality of surface transitions. Time for changes depends on the size and complexity of the model and can be reduced from days and weeks to minutes. The new 3D surface model can be used in tool design or manufacturing immediately.


Fig. 4: thinkcompensator_Blech_GSM: Du to a new technology of Global Shape Modeling (GSM), the software thinkcompensator completes the digital process chain

Results in Practice
The software solution thinkcompensator bridges the gap between FEM calculation and CAD modelling of perfect pressing forms. Users are spared interface problems between different data files and the included quality losses as well as reverse engineering or manual redrawing of solids. An automotive manufacturer has compared the use of thinkcompensator to common practice: After two iteration loops results fitted specifications better than after having built an experimental die and two change loops in tool steel. Using thinkcompensator development time is shortened remarkably, tool manufacturing is accelerated and costs for real prototypes or several tool changes are eliminated. At the same time mistakes resulting from inconsistent data are avoided and enormous expenses saved!

Autor(en): Dr. Thomas Tosse

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