Rediscovering "Good Modeling Practices"
Just recently I read a short article in Design News about the merits of properly choosing model references.
The article (scanned here for reference) talks about this "new" way of increasing
effectiveness of CAD. Although they emphasize the value of datum planes, in Pro/E the same is true using any
stable reference -- a base feature, datum, curve set, etc..
I chuckled as I read the article because, like a new revelation, the author was saying, in effect, that choices
in how a model is created make a difference in how easy it is to manipulate and extend its use.
It's a good piece and it says the truth. I chuckled only because PTC used to (very past
tense) emphasize the value of "good modeling practices". Now, (present tense) with
Wildfire, PTC has gone the other direction by making choices for you in selecting references and seem to care only
that you can make features with fewer mouse picks than with Solid Works. The truth is -- this type of modeling
is sloppy and like the article points out, reduces both efficiency and ROI (Return on Investment).
So, This months Tip (Or perhaps better stated, this months Reminder): Use modeling
techniques that enhance the usability and robustness of the model. As you work, think about how easy or
difficult it will be to manipulate for the next ECR (Engineering Change Request -- or whatever your
company calls it), and take time to choose good, stable references that reflect design intent.
There are no hard and fast rules for good modeling, rather a bunch of guidelines because there are always
circumstances that change the relevance of the rules.
For reference, here is a quick list of guidelines. The list is by no means complete, but contains some of the
big things that will make a difference in the usability and robustness of the typical model:
- Carefully choose references. This includes references for sketching planes, sketch orientation, sketcher references, edges or surfaces for rounds, etc..
- Choose references that follow the design intent. Though it's easy to make all your features reference only the base datum planes, the model won't follow when modifications are made. Choose references that allow the model "move" with the intent as changes are made.
(In Practice: Don't just accept Pro/E's automatic references (especially in Wildfire). Think about the design intent and choose references that are stable and reflect that intent. Wildfire is horrible about automatic references. Unless you're sketching on a default datum in the assumed orientation, automatic references are rarely applicable or appropriate. So much for minimizing mouse picks.)
- Choose references that won't disappear. References like edges that disappear when rounded, are not usually the best choice. Datums and planner surfaces are typically better. References from base features are typically more stable than those of later ones.
(In Practice: Pro/E provides different methods of selecting references that give similar results, and some are more stable than others. For example, surface selection by Surf & Bnd, or Loop Surfs when selecting for draft (and other times). Select using Intent Chain (May 2002) for edges. These are just a few examples. Choose the best method for your application.)
- Choose references sparingly. More references mean more feature interconnectivity which can make the model more difficult to work with. However, choose enough references to make the model follow design intent.
(In Practice: Some models are so tightly tied with references that the model fails with almost any modification. (August 2002) Users wonder why Pro/E is so hard to work with, but in fact, it's the way the model was constructed. Choices like Thru Next have no references.)
- When several features are to reference the same thing (like a planner surface or axis), create datums for control. (June 2001)
(In Practice: Key datums (planes, points, axes) are easier to find and select when they are named. Named features also denote significance for someone changing the model later. Name your important datums.)
- When several aspects of a part must interact (like cuts and protrusions to allow wall thickness and spacing) build control features like curve sets to manage the interactions, then reference them with the features.
- Create relations that associate features when direct references are not practical.
(In Practice: When writing relations, use comments for "what" and "why". If other people use the model, they'll be more impressed if you are specific in your comments.)
- Drafts and rounds are often best left to the end of the model. Although this is a good rule, there are times when they are required earlier. Care should be given to where the features are inserted in the model tree.
- Rounds should normally be inserted as round features rather than put in a sketch. Again, this is a good rule of thumb, but there are times where a round can't be created or a dimensioning scheme (design intent) requires the radius in a sketch. As above, carefully consider when. (The same is true for Drafts, Chamfers, etc.)
(In Practice: It is often good to build models without drafts and rounds, then go back using Insert Mode to put them in -- where possible clustering them with the parent features.)
- When the above guidelines don't make sense, carefully consider your options.
(In Practice: Good modeling practices are far more important early in the model than at the end when you're trying to put in the last round or draft.
Does all this seem silly when you're under the gun to complete a design?
I can see the reasoning. However, I can say from experience, it pays. I have done several projects
that, a year or two later came back for upgrades for the next generation. For one particular project, 6 weeks of
work went into the first generation, but it took only 4 days to completely re-build (with pretty drastic
changes) models for the next generation. For the most complex part, with the extent of the changes to the
first few features, I was amazed that the model would regenerate the next 740 features with so few areas having to be
rebuilt. Yes, there were rounds, drafts and other things to be rerouted or redefined, but most of the model
regenerated perfectly. Truly it paid off for that customer.
Make it pay off for you.
We wish for you a Merry Christmas!
Good luck in all you are doing!