“There were a few issues with ‘line of sight’ where the cameras simply couldn’t reach some of the undercuts where parts came together. Sure, existing 3D geometry would have given us a ‘heads up’ here, but instead we used the geometry processing capabilities of the GOM Inspect Suite software to fill in the gaps, and generate planar sections required to rebuild the clean geometry.”
Geometry Manipulation – Side Note
“The STL output from a scan (and the post-processed data) is made up of thousands or millions of points, all connected through a huge set of flat triangular faces which enclose a water-tight volume. If you’re looking at something fairly regular, with lots of flat faces, then the ‘facet body’ (as it’s known colloquially) will do. However, if you’ve got lots of features (radii, double-curved surfaces etc.) then a better definition is required.
Non-Uniform Rational B-Spline (NURBS) is a powerful mathematical description of geometry, to yield a far cleaner form than its STL facet body ‘little brother’. If you want any kind of smooth geometry that can be easily manipulated (add/removing features, extending solid faces), not to mention FEA-meshed, then NURBS geometry is a must-have. Fortunately, there have been many advances in this kind of geometry definition which make this 1) a reality and 2) accessible in a form that doesn’t take forever and break the bank at the same time.”
MSC Apex used a state-of-the-art Parasolid geometry kernel for FEA model development. Fundamental to this is the ability to create and manipulate geometry in many forms. There are tools in Apex which easily match or surpass the most powerful CAD software on the market today, and all within a CAE/FEA platform. One of these workflows is the ability to reverse engineer legacy FEA mesh data into a form that can be redefined for a new model, whether that’s to add additional structure, or to look at different analysis strategies, such as sub-modeling. A by-product of this ‘mesh to CAD’ workflow, is the ability to reverse engineer imported STL scan data into NURBS geometry.
Evotech CAE used these tools to re-build the geometry from the STL point cloud and the section curves that GOM Inspect Suite generated. The section curves were used as construction geometry for the standard NURBS primitives (planes, cylinders, spheres) for the machined regions, where it was easier to understand the design intent or shape in the structure. The more organic regions (historically more difficult to handle, especially at the weld interfaces), were represented with double-curved patches which were lofted using shape constraints into the machined region interfaces.
Once the geometry had been recreated, Evotech CAE were able to use GOM Inspect Suite to compare with the original STL definition, both in terms of visual overlay and enclosed volume. The overlay showed a high degree of accuracy, especially at previously critical structural features. The calculated volume of the NURBS CAD was within 1.7% of the original STL form. Engineers also checked the left/right comparison for a pair of pegs to understand any manufacturing variance, and its impact on the down-stream analysis. Any differences were shown to be tiny, and well within the ‘noise’ of the resultant FEA. A side benefit of the CAD geometry creation meant that Raptor also had reliable assembly CAD for their CAM and inspection processes.
‘Mesh to CAD’ for Generative Design/Topology Optimisation – Side Note
“One area where this STL/facet body to NURBS workflow is gaining traction is in the development of ‘clean’ geometry following a Generative Design/Topology Optimisation run. Historically, this optimisation workflow would have meant lots of semi-manual post-processing (and potential inaccuracy) in developing geometry from a mesh (I’ve suffered this pain many times – it’s not pretty!). These newer Parasolid-based ‘Mesh to CAD’ techniques mean that the smooth output from ‘level set’-type optimisation runs can be converted to NURBS CAD in a fraction of the time taken in the past” further comments Steffan.
Once generated, the NURBS geometry could be 3D meshed (using 10-noded Tet elements), with the appropriate automatic feature controls on fillets, faces, hole washers and cylinders. An iterative mesh convergence study was performed using the embedded Apex ‘generative update’ tools to give the appropriate balance between converged feature-based stresses and model size. Had the resulting model been too cumbersome, then engineers could have adopted a hybrid solid meshing approach using hex elements, but this was not the case.
“We applied unit static and fatigue loading to determine the load to failure in key directions from the material allowable strength. Once determined, this was assessed both in terms of non-linearity and sensitivity to load direction. Given that we had access to NURBS geometry, we were able to show the impact of design changes (such as thinning the discrete plates or changing weld sizes) from a mass and strength perspective stated Steffan.
“While the FEA aspects of this project were not particularly ground-breaking, the ‘part to CAD to FEA’ workflow was something pretty new to us. The ability to generate clean geometry in a timely manner, then witness the response, gives at least an order of magnitude of improvement in Raptor’s development process. And all using contemporary software that eased the whole process and made it commercially viable.” concludes Steffan.
So, what does this mean to Raptor? Well, they have a much better idea of their product’s performance, where the manufacturing and performance sensitivities lie, and what they can do with their next generation of foot-pegs, whether it’s looking at new bespoke geometries, manufacturing methods (including AM) or new markets. Reverse engineering has been around for a while, but the ability to use the latest optical scanning techniques, coupled with state of the art ‘mesh to CAD’ and FEA model generation, gives companies both large and small the edge they need in designing in the products of tomorrow.
For more information: www.evotechcae.com