MergeSrf again

Hello everyone!
I have read and tested all I could but still don’t understand why those two surfaces won’t merge…
If I get the tolerance up to crazy numbers the surfaces get to merge with each other but the wrong edges meet…
Can anyone help?

The surfaces are untrimmed.
The edges are close enough to each other (max deviation close to null).

I need them one surface, join won’t do for my situation.

merge help.3dm (79.8 KB)
merge help.3dm (78.8 KB)

You can’t merge those two surfaces - they have a completely different shape and structure.

What situation is that?

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Thank you Helvetosaur.
What are the criterias then? How similar should they be?

My problem is that when I join them -> offsetsrf or extrudesrf -> fillet (any method) the given closed polysurface I get ugly results and kinks where the two surfaces meet.

I don’t understand why can’t you divide the surfaces at creases

If you need it to be a single surface what you need to do is something like this.

Replicate the surface structure of the 2 surfaces, and expand it to the same lenght.
Merge them after match, mirror to the center and merge again, then trim.

merge help_JL.3dm (87.4 KB)

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Thanks laborda.
I am not sure what you mean here by replicating the surface structure.
I just see it as two bits of paper and i just need tape :smile:

If you turn control points ON, (F10) you will see the structure of the surfaces.

The structure is the point count and degree of each surface, if the surfaces have the same structure in the edge you want to merge and also match the control points position over all the length of the surface, it will let rhino merge without problems and change in the structure.

Anyway although the 3dm I sent is what you are asking for, I don’t think this is the best practice for nurbs modeling. When you have crease I suggest having 2 surfaces.

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Thanks so much laborda!
It seems like I have a lot to learn still.
I’ll give it a go.

Hi Alric,

From the notes in the help file:

Surfaces can merge only if […] the surfaces share both edge endpoints.

Other than that, visually, it is clear that your larger surface has 6 edges (and the smaller one 5) whereas an untrimmed NURBS surface has 4. You have used some voodoo to make these surfaces into one single surface, but that’s likely to just cause problems down the road.


Thank you for your input Wim,
How would you go modelling those surfaces in a way that by the time it gets some thickness it can be cleanly filleted?
Because I just realised it’s still a mess even as an extruded single surface :partying_face:

Hello - please post those surfaces - the ones that mess up filleting- as well.


mergehelp_fillet.3dm (469.0 KB)

What I did is extrude the orange surface and filletedge.

Hello - I’d do this - DivideAlongCreases then DupBorder on your original… Extrude the border curve then copy the surfaces to the bottom of this extrusion. Join, then FlletEdge. Now… this makes clean fillets here so far but fails to trim them into the polysurface, apparantly due to the front to main edge transition here:

That edge is not quite tangent, nor is the one at the outer edge of the wing shape - but even cleaned up it fails to trim there. I see the front and back surfaces are not tangent there either, that is the very first thing to go back and fix, it all should work better if you make those nice and tangent.

mergehelp_fillet_PG.3dm (173.6 KB)


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Thanks so much for your time!
I think I see what you mean, I’ll try and go back to this then.

I rebuilt the whole wing as a loft surface making sure the transition is tangent between cambered and flat surfaces (if that makes any sense).
It looks cleaner but the render mode still shows some weird things happening, like untrimmed surfaces sticking out.

What do you think?
wing with fillets.3dm (1.2 MB)

being picky there is a tiny problen in the main surface, it needs to be smoother

no worries, any feedback appreciated!
is that using the curvature analysis?

removing two knots ( marked as x2 on the isocurve) with the tool marked, and adjusting those points selected, the surface becomes a bit cleaner and smooth

I’m using Emap analysis for this one.

thx Diego, I’ll try and play with this a little bit.