CAD stands for Computer Aided Design. If you are using a 3D CAD tool in a product development company today, there’s about a 99% chance it is a history-based (AKA feature-based), parametric design tool.
VP of Engineering:
“Huh? Really? I’m not sure about that. I think we just use regular-old-CAD. We’re on Pro/Engineering.”
“Ok, whatever. Our design guys do that stuff for me, and they’re damn good at it.”
“Well, duh, all 3D CAD is parametric. That’s kinda the point.”
History-based CAD is so prevalent that most people in industry have no idea what the “history-based” part even means. “It’s just CAD, right?”
So, let’s chip away at the semantics. History-based refers to the set of steps most CAD tools use to define and create 3D objects on your screen. This works a bit like MS Excel macros. Every step the CAD Designer used to create that object gets “replayed” (in order) to redraw or “regenerate” the object each time you want to see it.
This term “design intent” gets tossed around a lot in CAD. In a history-based tool, the Designer needs to carefully think through how the object should be constructed before he begins constructing it. He needs to envision what changes people might want to make to this model in future revisions, and make sure the steps required to regenerate the model allow for those tweaks.
For example, a CAE analyst typically wants to de-feature a CAD model prior to simulation. He may want to simply remove an unnecessary hole… but if the next 2 steps in the “macro” of creating the part reference that hole, all hell breaks loose.
The model will “fail to regenerate.” The process of killing holes, fillets, and other noogies unnecessary for simulation can become a grueling game of JENGA. Remove the wrong piece in the wrong order, and the tower falls.
Direct modeling (AKA history-free modeling) offers a beautiful alternative for CAE analysts needing to prep manufacturing-ready models. Direct modeling doesn’t replay a macro to display a part on the screen. It just displays the part in its current state. You are free to slice, dice, move, push, pull, and manipulate the model with no fear of regeneration failures. All the unaffected parts stay where they are supposed to on the screen.
In this rough analogy, it’s a bit like playing magnetic JENGA in outer space.