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Hi everybody:

I'm having some trouble with inconsistent chamfer behaviors in Spaceclaim. Here's a video:

In recap, I have three problems:

1. I need to be able to control complex chamfers beyond simple 45xd chamfers. Pivot edge *should* do this, but "Pivot" edge actually turns out to be a "translate edge" function when curves are too complex. I would have expected it to actually be more like "Offset Edge On Surface" or something. Oh well.

2. Chamfers seem to fail when confronted with anything but the most basic geometry, but in many cases fillets work! This baffles me: why would a fillet work, but a chamfer not? They should work the same way! Thoughts on this?



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Hey Adam,

I'll make sure R&D sees your enhancement requests. We currently only support equal setback chamfers. As you've discovered, pivot can help in some, but not all, cases. There are workarounds that I'll suggest after a brief digression:

Chamfers and rounds are not as related as many people might guess. Although they both are types of edge treatments, rounds are specified by radius and chamfers by setback. That means you only get a similar result only when treating edges where the faces are 90° separated. You can easily see the difference by creating an edge that varies in dihedral, such as this example that goes from 45° to 135°:

Observe that for the round to stay at a constant radius, it needs to consume more setback at acute angles than it does at obtuse ones. The chamfer, on the other hand, is always set back the same amount in the surface. Notice that the two rail edges intersect when the dihedral is 90°, as you might expect. (This property is only true when the cross sections are made of lines, as in this example. Expect different results when the surfaces are more curved.)

This means that 3D modeling software uses different algorithms for constructing rounds and chamfers, and that in some cases, one may be viable when the other isn't.

Need to create a funky, variable, unequal setback chamfer? It's pretty straightforward to roll you own. Here's the basic technique, which is a good basic approach for all sorts of custom edge treatments.

Now don't get too crazy...

Thanks, blake. Great explanation. You're assuming, of course, that we're using a "rolling ball" type fillet, and not a constant-offset round. I personally prefer the constant-offset in many cases. I'd love to see this implemented as well. But I can't have everything, I guess :) I'll play with it!
Actually, you can't really do that for any type of circular round. The setback of one side is derived from the other. It's not likely that you can construct a tangent circular arc between the two setback curves. If you construct a circle tangent to two curves and passing through a point, it is perfectly specified. Two tangent plus two point is too much information. This drawing might help you visualize it:

Now, if you just want to get close, generally you can eyeball it with a variable radius fillet.

Incidentally, we typically use the term "rolling ball" to refer to an advanced round:

If you were to roll a ball along the edge, those surfaces would be part of the swept volume.

Great explanation Mr. Blake

I had the same concept, but was a bit difficult for me to put it into words

I have uploaded a video, which compares the results for the same model in different CAD software

I have concluded that SpaceClaim has given an expected Output
It's great the SC makes a nice clean chamfer... when it works. The problem is that it fails way too often. That's my issue. I'd rather it make a "bad" chamfer than none at all.
Seems that some CAD systems have trouble with this simple example, so I thought I would try it in PTC CoCreate Modeling. Here's the video:

Here are the three files, if anybody wish to compare
This problem intrigued me, you always learn more from the stuff that doesn't work, and since I am still learning modeling in general and playing with new SC features, I thought I would hack at it and learn. I found one solution to what I think you are trying to do by deleting the entire fillet, which as you mentioned, works fine on the geometry. You can then translate the edges of the left over surfaces, then piece back in with blend via Blake's advice. After many 'mistakes', I learned quite a bit about cleaning up tiny gaps with the prepare toolset. FYI, don't let the program just fix the gaps all at once, it will make some unwanted surfaces, go at them one at a time so you can find the ones that are problematic, back up and use the simplify function around those gaps and then try filling again. Satisfying to see the solid pop in when you get that last gap filled...
Hope the video comes thru
I had the same solution with the tools available in SC
1) Deleting the Chamfer Surfaces
2) Using "Pull" on the edge, to reduce the size
3) applying blend between the edges of the two surfaces
4) finally applying the Repair tools for the Gaps, Missing Faces, Splits and Inexact edges

But I do really feel that Mr. Paul Hamilton, has displayed how cool does PTC Cocreate works in this case.

i believe that every CAD designer would really like to have that flexibility
But i wouldn't conclude that Cocreate is really good until I personally observe the tangency continuity of chamfered surfaces in that model.
Give me ANY CAD tool, and I'll do a variable chamfer manually. This is all well and good, but all I wanted was an EASY, quick prototyping method in SC. Since SC requires manual-building, I might as well do it in a better tool. The whole point of SC is that it's fast and easy, not that it makes class a quality surfaces (because believe me, it doesn't yet!).

So if I'm going to have to manually build my chamfers, even at a prototyping level, SC is not even worth using.
I personally do not have a CAD Swiss army knife of software to find the right tool for the job, so my 'help' is understandably one-dimensional. I see now that you actually didn't want a solution to this problem with the available SC tools, you want the SC programmers to solve your problems with it. To that end, I believe CoCreate has demonstrated a best case scenario to this problem, SC would do well to emulate.


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