Ken Versprille Explained: The man who invented NURBS Modeling. part-1

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Ken Versprille Explained: The man who invented NURBS Modeling. part-1

by Kevin de Smet, Elite Student Group.


Q) What do you consider as major drawbacks of contemporary 3D modelers?
What are the challenges that exist in 3D modeling right now?

Ken Versprille:


In my opinion, the biggest drawback of contemporary 3D modelers today is the fact that they do not pay enough attention to the reality that we live in. Today, design is a multi-application, multi-CAD world and data created using one modeler will at some time need to be moved to a different modeler and be operated upon.
The issues that arise are due to many different reasons including the use of different tolerances in calculations, subtle undocumented implications in data formats, and different algorithmic approaches to common geometric calculations. For example, one of the most critical, if not the most critical, computations in a 3D modeler is the calculation of a surface-surface intersection. Other drawbacks, of course, include performance and accuracy.

Accuracy becomes a major issue whenever the results of a geometric calculation on a 3D model are less precise than needed for other downstream algorithms operating on the model.

Tolerant modeling approaches are important at the core of a 3D modeler.

Kevin De Smet (Pro CAD Modeler and member LearningAlias Elite student group)

Tolerances between different modelers is absolutely one of the biggest issues, but it’s a problem easier to fix in words than in action. Different modelers use different tolerances that will each have their positives and negatives, depending on what you are modeling. Thus, it would be wrong to say there is one “right way” of doing it and all the others ways should be banned. So, how will the industry vendors possibly take the decision to standardize on one tolerance scheme?

Take Pro/ENGINEER which uses a relative tolerance, meaning that if you have an aircraft wing the position tolerance might be 0.01mm whereas for an automotive door panel it might become 0.003mm, these numbers are fictive but it’s the point that matters here. The larger the model, the larger the tolerances become.

Then consider CATIA V5 and now you’re dealing with an absolute tolerance of 0.001mm, irregardless of the size of the part you are working on it will always be the same. (though CATIA does have settings for plant design and electronics, which do change this)

Now think about the millions of models that have been created at thousands of companies across industry. Suppose all modeling systems standardize on one tolerancing scheme, how will upwards compatibility be guaranteed? It wouldn’t.

I agree with Ken that a tolerant modeling scheme helps solve many of the most prevalent issues, it won’t catch every problem but it will catch most of them. Most systems today have some variation of tolerant modeling and they all achieve the same end goal which is why it tends to work, well, sort of… as we shall see



Let’s take an example


The positional tolerance, here between a trim edge on a surface (the surface in yellow) with a natural edge of another surface (the blue surface)  is defined in Alias construction options to be 0.005mm. The user adds degrees to the surface in order to give it enough flexibility to match the trim edge to within the tolerance, with the final result turning out around the 0.003mm mark.


Fig 1

Fig 1
Now let’s import this into CATIA V5

Fig 2

fig 2

As you can see it comes in as two surfaces because we have not Stitched the two surfaces together in Alias, before we exported

Fig 3

fig 3

In CATIA we can do a “Join” command to replicate the Alias “Stitch” function, notice how the default tolerance is 0.001mm

Fig 4

fig 4

Despite this the Join is successful? In Alias it is clearly seen this can not be the case

Fig 5

fig 5

And if we do an explicit check in CATIA we see it is indeed not to within 0.001mm

Fig 6

fig 6

CATIA V5 simply treats this local region as being “within tolerance” despite the global tolerance of 0.001mm in CATIA V5, it doesn’t care that it’s not to within 0.001mm and as such the “Join” worked. Dangerous, or helpful? I’ll let you decide on that one!
As such, tolerant modeling turns a model imported from “system A” into “system B” from being full of gaps and holes into one that is “adjusted” to suit system B, adjusted is not even the right wording as you can clearly tell from Fig 6 that the geometry is completely unaltered from the state it was in when it was exported from Alias.
It is just that CATIA thinks it’s “OK” and connected, this is important for shelling thickening and filleting commands in the solid modeling system. And all other modeling work to be done on the b-side of this surface-set.
In a perfect world you would correct the model in Alias to become positional to within 0.001mm. This is often time consuming, and pretty much even, unnecessary.
That is the power, of tolerant modeling!
But wait, we’re not out the woods… like some cheap horror movie, I’m announcing the return of the bad guy beyond the grave (in this case, our bad-guy is the “out-of-tolerance ghoul”) because when we go to thicken said geometry in CATIA V5

Fig 7

fig 7

So let’s try doing that whole “fix it in Alias” thing after all, by adding another degree we are within tolerance this time to 0.001mm

Fig 8

fig 8

And what do you know? It thickens just fine now

Fig 9

fig 9


Let’s try another route. We keep our 0.005mm pos. Tol. but decide to “stitch” our surfaces in Alias first, then export our .step file

Fig 10-11

fig 10


fig 11

Now select your shell by using “Pick > Object” and then go to “File > Export > Active As” and choose its options box, so you’re able to select your format. Let’s go over some of the options we’ll pick, and why (beyond the argument “because it was the default”)

Fig 12


First of all, you’ll see it is set to WIRE so use the drop-down and choose STEP. You may choose IGES or a flavour of IGES (VDAFS is the German IGES standard and SET is the French IGES standard, JAMAIS is the Japanese) but let’s face it: IGES is old news, IGES itself as well as VDAFS/SET/JAMAIS date back to the mid to late ’90s with little if any development since then. Today, STEP should be your preferred exchange format between styling and engineering.

Fig 13


The first thing to choose is the application protocol or “AP” this will determine what information Alias can write into the .STEP, and what information CATIA can read from the .STEP. There are many such APs for example: 209 for composites analysis, 212 for electrical harnesses and so on. 203 and 214 are the ones used in mechanical solid modeling, so you’ll want either one of those. 214 is newer and can hold color information for example, so I would advise to use 214. There is a 203 edition 2 which also can keep color, so it’s complicated. There is a new AP 242 in the works that is intended to be the successor to both 203 and 214. So expect to see it as an export option in Alias within the next few releases!

Fig 14


The model type will determine what will be exported from Alias and how, the default “Hybrid” will export whatever it finds in Alias. For example before we stitched our surfaces into a shell it would have exported “Surface Models” with the option “Hybrid”. Now (because we did Stitch our surfaces into one object) it will export “Manifold Shells” if we use the option “Hybrid”. Thusly, I would leave this on “Hybrid” and Alias will export whatever it happens to be in our Alias model.

Fig 15


The next option is a neat one, and only available with 214, if your surfaces weren’t stitched together yet – it will do so anyway upon export. This can be handy if you don’t want the shell objects in your Alias scene cluttering things up. However this will of course give you less manual control, it will stitch based purely on your “Construction Options” value for “Topology Distance” and may make mistakes!

Fig 16


The last option is a very important one as Ken Versprille has hinted at, I would suggest Parameter Space as per Ken’s recommendaton. But truth be told: this will depend upon your solid modeling system. You should experiment with some different models and import them into your solid modeling engineering system using both settings, and see which yields the best results for you. I have choosen Parameter Space, which is the default (i guess I just chose an option simply because it was “the default”!!)


If we now open this in CATIA V5 it comes in as a single “shell” surface object already, which thickens perfectly
Fig 17

fig 12

Let me conclude with these primary attention points

• If at all possible, make sure your Alias construction options are set to the tolerances the receiving system will expect. Despite the seemingly holy grail of tolerant modeling, you just can’t trust it one hundred percent. Do your homework and check what your engineering solid modeler tolerances are, then adhere to those in your Alias work! It will save you a lot of grief down the road!
I hear you though, this would mean modeling in Alias would be required to be done to 0.001mm. At least for engineering release models. Don’t shout at me! This was Dassault’s decision when they made CATIA V5!

• Rely on tolerant modeling if you wish, try to “Join” the geometry in CATIA V5, in theory this gives the receiving system and its user the most freedom to attempt to Join them with that system’s algorithms. Try perhaps, the “Healing Assistent” workbench in CATIA V5 to make the geometry possibly Thicken successfully. As we have seen, without it, CATIA V5 is dead-in-the-water

• Rely on tolerant modeling, but “Stitch” the geometry in Alias first so that CATIA V5 can import a resultant shell that is topologically connected (this is the key) as this topology information, made by the Stitch and kept in a format such as .step makes the model much more intelligent to a solid modeler, and gives the tolerant modeling in such a system a much better fighting chance at working. In our example, it absolutely did. I would consider this approach my choice of the three here.


Enjoy your Alias

Kevin de Smet

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