Thursday, April 30, 2009

Design Intent with History Free Modeling (another example)

As I have mentioned many times, combining parametric modeling capabilities with history free modeling to capture design intent is nothing new, as some CAD companies would indicate. There is certainly always room for improvement in how it is presented to the user, but the basic technology is fairly mature and robust. The term “parametric modeling” usually refers to capabilities in a CAD system that will allow users to get 3D geometry to behave as intended based on the design requirements, and this is typically done by capturing the steps (sketches, features, parameters) of the modeling process, i.e. the history tree. Many people understand the term “history-free” (or “direct” or “explicit”) then to imply that the capture of design intent is not possible. As this example will show, it is certainly possible and also can be easy to do.
In this example I am using PTC CoCreate Modeling to add intelligence to a non-intelligent assembly of a V8 crank assembly. You have probably seen examples like this many times, but I bet you have not seen one without a history tree.
In this case much of the geometry comes from a different CAD system through the STEP format. As you will see in the demo video, I add relationships (parameters/constraints) to control the assembly; how the parts relate to each other. I also add relationships to control the geometry of the crankshaft and other parts based on the stroke of the engine. It’s been a fun example as my son and I have recently finished building the engine for the Camero that we are restoring. In our case we took the stock 350 engine and stroked it to a 383. This is done by putting a 400 crank into a 350 block. When you do this, you have to use shorter rods and/or different pistons to accommodate the longer stroke. In this example I change the length of the rod based on the stroke.

As you may have noticed the change to the rod is somewhat simple from a geometry point-of-view. No topology change is required, only repositioning of some faces and stretching of others. The changes to the crank, on the other hand, are a bit more complex. There is still no topology changes required, but there are several tangencies, coincidences and blends/rounds that have to be maintained.
Direct history-free modeling systems may provide the capability to add (or assume) design intent within your models and assemblies, but that does not mean that the geometry kernel will have the power to solve the geometric problems driven through the parameters. In this case CoCreate Modeling was able to perform the required changes to the crankshaft and rod even though they were imported models with no history-tree.
If interested to learn more about direct history-free modeling, my team and I will be at PTC/User in Orlando in June. We will be presenting many topics related to history-free design, and also providing some free user training on CoCreate Modeling. Check it out at