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Engineering Advantage

Direct vs. Feature Based Solid Modeling

April 24, 2015 By: Patrick Cunningham

When I began my career in the late eighties, most engineering companies were making the technology leap from drafting boards to Computer Aided Design systems. The CAD tools were 2D based and although faster and less dusty than a drafting board, were not very different in the design approach. In the early nineties "Feature Based" CAD systems were introduced and were a major leap forward in technology for design. Instead of creating plan views, the feature based tool allows you to construct a solid in 3D and then extract the required plan views and cross sections from it. In place of imagining what the design should look like in 2D, engineers and designers could visualize and create volumetric (solid) models. 

In the 3D solid modeling approach, features like Extrude, Revolve, and Sweep use 2D profiles to create 3D solids. The feature order often aligns closely with the steps that would be used to create physical parts so the design intent of the CAD model can take into consideration the manufacturing requirements. Additionally, the 2D sketch dimensions and 3D operation values are stored as parameters. The solid models can be altered by changing the parameter values and updating.

A parametric definition of the solid model is handy when the geometry is used to create analysis models. Design optimization studies can be automated using the solid model design parameters.  For more information on this approach you can review the CAEA e-Learning seminar "Parametric Modeling Best Practices".

However, what if the details that you are looking to modify in the analysis model do not align with the parameters used to generate the solid? This situation occurs often because the solid model design intent and the finite element analysis goals are often different. Without a driving parameter to modify the finite element model, it is difficult to automate the optimization study. 

The good news is that there's a relatively new approach available called "Direct Modeling" that allows the analysis folks to be less dependent on the CAD designer's intent. Direct Modeling tools do not require parameter driven regenerations of the original solid model. With a Direct Modeling tool, you import the solid and locally pull, press, stretch, and move the topology around as needed. This is well suited to manual design studies where you can tweak the geometry in just about any way and not be constrained by the feature history. No parameter? No problem! Just grab the surface or edge you want and pull on it. The offsets you create from the original geometry can also be used as the input parameters in an optimization run.

So, if you find yourself struggling with design studies and optimization runs because you do not have the parameters you need from your CAD model, consider the freedom of a Direct Modeling tool.  It's likely that the tool will pay for itself with the time you save and give you the added flexibility to explore the sensitivities of your design quickly and efficiently.