Neil Cooke / August 24, 2017
Parametric feature-based solid modeling is almost 30 years old. In that time, not much has changed. Sure, there have been plenty of CAD systems with their own take on how to model in 3D, but the basic principles remain the same: Create a sketch → Create a feature → Repeat.
One of the main benefits of parametric modeling systems is being able to make changes quickly. Every sketch and every feature is driven by dimensions, so all you have to do to make a design change is to change the value of a dimension, right? Yes, in most cases, but if only it were that simple all of the time.
When you create a sketch, you are capturing design intent by adding dimensions, constraints and references to other model geometry. This design intent helps you predict how your models will update when changes occur. However, if you’re not careful, you can easily get yourself into a pickle. Making too many careless references to other geometry in your model can make your model very fragile. If your model has dozens of features, you could end up chasing errors for hours. A lot of this heartache can be avoided if you have a basic understanding of what’s going on behind the scenes.
Every geometric element created by a parametric modeling system has a unique internal ID number. This number is referenced by subsequent sketches, features, assembly mates, and drawings to work out where things should be placed relative to the rest of the model. So if that ID number no longer exists, guess what? That’s right, the feature doesn’t know what to do and fails. If a design change makes an edge or face disappear, it is likely that some downstream features will fail. It’s not the features themselves that are the problem, it’s the references you make between them. References can be your biggest ally, but also your biggest enemy.