IronCAD

The model brittleness problem inherent with parametric feature-based modeling is a really big deal. And it’s something, honestly, that I don’t have a great answer for. I’ve even asked a few power users who I know, and their answers seemed to involve a bit of hand-waving, and a reference to having lots of experience.

While best practices are a potentially good step forward, they need to be straightforward enough that mere mortals (as opposed to power users) can follow them.

Around Christmas last year, I got a call from Richard Gebhard, an engineer’s engineer, who has made his living selling CAD, and training people to use it (including more than his fair share of power users), for longer than he would like me to admit. (I’m pretty sure I’ve been in the CAD industry longer than him, though.) Richard told me he had something he wanted to show me, and if I’d take the time to meet him, he’d buy me lunch.

What Richard showed me was a way of creating and structuring CAD models that made a lot of sense. It not only reduced parent-child dependencies, but it made them more predictable. And, more importantly, it made it a lot easier for a mere mortals to scan through the feature tree, and see if there were any grues (it’s a technical term. Feel free to look it up.)

Over the next several months, we had lunch several times. I made suggestions. He rejected some, accepted some, and thought about others. At the same time, he was bouncing his ideas off several of his best power users (including his son). By a couple of months ago, he had refined his system to the place where it would work impressively well with nearly any parametric feature-based CAD system. So, he went to work finalizing his presentation.

I had mentioned that Delphi, by patenting some of the elements of horizontal modeling, limited the number of people who could benefit from it. (Worse for them, they patented it, then filed bankruptcy. That didn’t help much.) Richard’s goal wasn’t to monetize his process. His goal was to evangelize it. To help CAD users—both power users and mere mortals—to get their jobs done better.

Richard and I had talked, over time, about what he should call this process. At first, I liked the word “robust.” In computer science, it is the ability of a system to cope with errors during execution. In economics, it is the ability of a model to remain valid under different assumptions, parameters and initial conditions. Those are good connotations. But, then I thought of one of my favorite examples of robustness. The first time I visited Russia, I noticed that the apartment buildings were built of thick poured concrete. Very robust. And nearly impossible to remodel.

Richard’s system wasn’t robust. It was resilient. So, he has named it the Resilient Modeling Strategy. RMS.

So far, I’ve written over 2,600 words, to provide some background on the problems of parametric modeling, and some of the solutions that have been offered over the years. But, after all that, I’m not going to tell you anything more about RMS. At least, not yet.

Tomorrow, Wednesday, June 26, Richard will present RMS for the first time ever, at Solid Edge University, in Cincinnati, Ohio. His presentation will start at 9:00AM local time, and will be in room 6 of the convention center. If you’re there, put it on your calendar. If not, you’ll need to wait until Richard gets back to Phoenix, and I publish a follow-up post.

RMS is not anything difficult, or fundamentally new. It’s just an elegant distillation of best practices, designed to work with nearly any parametric CAD system, and simple enough that it doesn’t get in the way.  It’ll help you make better CAD models faster.

In the early 90s, Ron Andrews, a senior product designer at Dephi’s Saginaw Steering Systems Division, became fed-up with the difficulties of editing parametric CAD models. So, he and a team of his colleagues, including Pravin Khurana, Kevin Marseilles, and Diane Landers, took on a challenge of trying to find a solution.

They came up with an interesting concept that they called horizontal modeling. Here’s a description of it from their patent abstract:

“Disclosed is a horizontal structure method of CAD/CAM manufacturing where a base feature is provided and one or more form features added to it to form a model. The form features are added in an associative relationship with the base feature, preferable a parent child relationship, but are added in a way as to have substantially no associative relationships with each other. The result is a horizontally-structured Master Process Model where any one form feature can be altered or deleted without affecting the rest of the model. Extracts are then made of the Master Process Model to show the construction of the model feature by feature over time. These extracts are then used to generate manufacturing instructions that are used to machine a real-world part from a blank shaped like the base feature.”

Here’s a picture that makes it clearer:

The simplest explanation I can give for it is this: You create a base feature, and bunch of datum (working) planes. You attach all the child features to those datum planes. Viola: no parent-child problems.

I admit that I’m not going to do justice to horizontal modeling in this conversation. There’s actually quite a bit to it, and it makes a lot of sense when coupled with computer-aided process planning (CAPP.)

Horizontal modeling has a handful of problems. First, it does a pretty good job of killing the possibility of having design intent expressed in the feature tree. Next, it works better with some CAD systems than others. (When horizontal modeling was in the news, SolidWorks had a problem managing the normals on datum planes, so it didn’t work too well.) The deadliest problem is that Delphi got a bunch of patents on the process, then licensed it to some training companies. From what I can see (and I may be wrong), none of these training centers offer horizontal modeling classes any more.

While, technically, you can’t use horizontal modeling without a patent license from Delphi, the concepts at its core are fairly similar to things that CAD users have been doing for years. A few years ago, Josh Mings posted on a couple of online forums that “Horizontal Modeling is just one word for it, you may also know it as Skeleton Modeling, Tier modeling, Sketch Assembly modeling, CAD
Neutral Modeling, or Body Modeling.” (It’s actually two words for it, but I get his point.)

Horizontal modeling is not a silver bullet solution for the problems inherent in parametric feature-based CAD. It’s just a best practice—a strategy for getting around the problems. It seems to be headed in the right direction, but it suffers from the complexity that comes from trying to fix too many problems at once.

Next: A Resilient Modeling Strategy

Direct modeling—a syncretic melding of concepts pioneered by CoCreate, Trispectives, Kubotek (and many others)–has shown the most promise to cure the parametric curse.

Direct modeling is today’s hot CAD technology. PTC, Autodesk, Siemens PLM, Dassault (CATIA, but not so much SolidWorks), IronCAD, Kubotek, Bricsys, SpaceClaim (and certainly some other companies I’ve forgotten) all have their own unique implementations of it.

The common thread in direct modeling is to use standard construction techniques when modeling, and feature inferencing (or recognition) when editing. It’s easier said than done. It’s taken about 35 years of industry research to get to the place we are today—where you can click on a face of a model, and the system will recognize that you’re pointing to a feature that has some semantic value. And that’s not even considering the tremendous amount of work that has been required by legions of PhD mathematicians to develop the math that lets you push or pull on a model face, and have the system actually edit the geometry it in a useful manner.

For the CAD software, figuring out which way to edit a selection is almost a mind reading trick: A user clicks and drags on a part of a model. What would they like to happen? In some cases it’s easy: Drag once face of a rectangular block, and the system will just make it longer or shorter. But if the block is full of holes, bosses, and blends, it becomes a lot more complicated. What should the system do if you drag a face so far back that it consumes another feature, and then pull it back to where it was? Should the consumed feature be lost forever, or should the system remember it in some way, so it can be restored?

There are no right answers. It seems that no two direct modeling systems handle the decision of what is a “sensible” edit in the same way.

While direct modeling absolutely solves the model brittleness problem inherent with parametrics, it does it by simply not using parametrics. Even with hybrid parametric/direct CAD systems, the answer to the parametric curse is still to not use parametrics when you don’t need to.

The solution of “use direct modeling when you can, and learn to live with parametric hassles when you can’t” just isn’t very satisfying to me.

Next: Going horizontal