Hello,

I'm an experienced 3d modeler, but new to nurbs and Moi and is currently using the free version to try it out.
Liking it so far but once in a while I run into problems such as the one I'm about to describe..with images.

Basic model and edges to fillet: https://drive.google.com/open?id=0B4W2M6Gk9EM8X3BYVEd6X1RzNlU

Results: https://drive.google.com/open?id=0B4W2M6Gk9EM8SDNiNVdaR09jb0U and https://drive.google.com/open?id=0B4W2M6Gk9EM8ZkdFbUNOT2NrNlU

And when i tried a workaround by insetting and using blend I got this https://drive.google.com/open?id=0B4W2M6Gk9EM8X3JMeGE0UFFONjA

The model seems pretty simple to me...but that chamfered edge seems to cause it?
Anyway some insight would be appriciated.

Hi Octo, could you also post a link to the 3DM model so I could take a look?

But it looks like MoI is having some difficulty intersecting the fillets with each other - the first result you show might be salvagable by some direct editing of the fillet surfaces that were able to be generated despite the intersection error, I could try to show you how to do that.

Or also if you were also to include the chamfer edges as part of what was being filleted it would probably make it easier for the fillet to work, because edges that meet up smoothly with one another also have fillet segments made along them to also meet up smoothly as well. When there is a sharp corner between 2 edges that are being filleted, it means the fillet surfaces do not just naturally meet up end-to-end with each other and the fillet engine has to figure out how to extend them and cut them back and that's a more complex thing than it may initially seem.

Blend is more focused on smooth things - it's focused on making a smooth connection between 2 surfaces but when you have a sharp corner between 2 adjacent surfaces, that means there isn't really any similarity of surface shape at that spot and so the 2 blends will be done separately there and you'll have gaps like you show. In the future I think I'm going to try and do something like filleting does and try to extend the blends horizontally and intersect them with each other but just like the fillet case failed that can be a fairly difficult operation.

If you wanted to use Blend you'd also probably need to fillet the chamfer edges so the things you were trying to blend between all had a common surface normal all the way around the blend. Then a tangent blend would be able to work.


> The model seems pretty simple to me...but that chamfered edge seems to cause it?

Yeah, well that makes for a sharp corner there and just fundamentally sharp corners tend to introduce a lot of complexity to fillet calculations.

- Michael

Thanks for the quick reply.
Unfortunately I'm using the trial/free version with saving disabled.
On the first result the fillets were created as separate surfaces..I could just delete them, and the original was still below it unchanged.
What I really wanted to do was a much wider fillet but higher values produced worse, or no visual results..I just used smaller values to have something to show for troubleshooting.
I guess I mainly need to learn what to avoid. Could I for instance have done the fillet first and the chamfer afterwards?
Or lofted/revolved that angle cut at an early stage to produce similar results but avoiding chamfer?
Or is technique irrelevant and the issue is with the "topology" itself and soft and hard edges are simply not friends in some cases?
It sounds like it's a technical issue with nurbs and not just me doing something wrong which is too bad, having a nurbs modeler in my toolset would be great.

Hi octo, I don't think you'd be able to do the fillet first before the chamfer because that would give you a result like this:



There you won't have the right topology to get the chamfer you want. Usually you have to apply larger features like a wide chamfer or fillet first before you apply a smaller radius fillet.

Yeah I guess it would be correct to say it's a technical problem with the fillet engine in the geometry library that MoI currently uses, it can sometimes have difficulty with this particular case of fillets meeting up across a sharp boundary with the surfaces being at different angles (90 degree angles is a special easier case). It's more difficult than it first appears because the correct result would look like this, note how the fillets are of different widths where they run into each other:



That's probably not exactly what you're expecting but it's how fillets in CAD programs are meant to work - the fillet surfaces there are all of a constant radius meaning they are arcs that are portions of the same radius circle but when the surfaces are at different angles to each other some will get a longer arc from that circle and some will get a shorter arc from that same radius circle.

The geometry library that MoI uses is not too robust in this particular situation but some other CAD programs can handle it better, you might check out OnShape or ViaCAD if you need to get that particular fillet done, the geometry libraries they use have been more highly developed for these kinds of fillet cases.

It is an area that I'm hoping to improve on in MoI in the future as well.

- Michael

Hi Octo,

FWIW, I typically jump from MoI to OnShape a few times during the production of a model. It's typically about a 5 minute round trip:

1. Export to STEP making sure dimensions are SET to working model size (I typically choose mm - it's in the PREFS)
2. Import STEP into OnShape
3. Apply FILLET
4. Export STEP
5. Import back into MoI

It may sound like a lot, but the overall workflows and ease-of-use while working in MoI offset the occasional roundtrip to OnShape. Furthermore, there have been times when OnShape wouldn't fillet either, which points to a flaw in my model-- which I can typically fix in MoI. HTH