Evan Yares is a guy I have often quoted over the last several years. He’s now the General Manager of Nanosoft America, but he found time to finish up a series of articles he started to write for 3D CAD World entitled The Failed Promise of Parametric CAD. I take issue with a little bit of what he said but the overall message is something you need to pay attention to. The only problem with what he has to say is that the title of the piece, The Failed Promise of Parametric CAD implies that a system like Solid Edge is something other than parametric. It’s possible for a non-history based CAD system to be parametric. I would have called the articles The Failed Promise of History-Only CAD. You’ll get a lot more perspective on the whole issue if you go back to the beginning and read it all. It’s well worth the read, even beyond the topic at hand. Evan puts a lot of stuff in historical perspective, and digs in behind the hype in most cases.
Evan is talking about a method called Resilient Modeling System which Dick Gebhard has come up with. I saw his presentation at Solid Edge University last June, and wrote a review of the process on the Solid Edge Community Blog.
The real weakness of history-only systems is the parent-child relationships in feature-based CAD models are not very stable through changes. It is very easy to create changes that break the model’s “design intent”. This is not a weakness of a single CAD brand, but of the entire concept of history-based feature modelers. And it’s not news. If you’ve used systems like Pro/E, SolidWorks, and Solid Edge before Synchronous Technology, you’re undoubtedly familiar with failed features after changes.
Mr. Gebhard’s method is rather detailed, but to boil it down to its simplest form as implemented in Solid Edge, you create the basic prismatic form of a part as a history-free body, and then add details (such things as fillets, chamfers, extruded text) as ordered (history-based ) features.
The message here is not that history is a bad method, but rather that history should only be used for the types of things that it is well suited for. The best method for creating models that react well to changes is to create as many features as you can without parent/child relationships. This would be known as a Synchronous model in Solid Edge, and the Synchronous geometry is shown as gray in the image to the right.
Then add the detail features, usually fillets, as Ordered features, shown in red. If history’s deep dark secret is that it doesn’t handle parent/child relations very well, direct editing’s secret is that it can eliminate topology during changes just fine, but it cannot add topology during a change. This is essentially why we need both forms in a truly robust modeling system.
Any system that is not built with the capacity to create history-free bodies has already been obsolete for about 3 years. And yes, I’m looking at you SolidWorks. The “Move Face” feature does not excuse these parent/child limitations because it is after all, just another history-based feature that gets rebuilt every time the FeatureManager rebuilds, and is thus susceptible to the same failures.
This realization along with the fact that with Synchronous Technology, Siemens PLM has the best combination of tools, is what led me to switch camps. Regardless of what other flaws exist in companies or software, the fact is that history-only CAD is dead technology. The future for full-powered CAD is the combination of Synchronous and history.