Mesh to Nurb


I have an stl file that Rhino will not convert via the meshtonurb command, can anybody advise on what my options are to achieve this, its too hard to re model. The file is too big to upload here I’m afraid.

Many thanks in advance.

Read these:

Thanks, I am using a mac so a few of those aren’t options, not sure if rhino resurf can be used but no instruction to get that on to Rhino 5 only 6…still stuck

Like most people find, there is no “automagic” way to accurately convert a mesh to smooth NURBS surfaces.
Use the mesh as 3D reference, and remodel it using curves and surfaces.

I fear thats not an option, too complicated and this has to be deadly accurate as its an acoustic horn.

Maybe a more advanced Rhino user might be able to do it but I am a novice.

If it has to be “deadly accurate” you’re already off the mark as what you’re looking at is a visually smoothed faced mesh.
Click on the Viewport name and turn on Flat Shading to disable the display smoothing. Then you’ll see what you really have for geometry.

I can think of several ways to approach reverse engineering that so it’s of similar accuracy as your mesh model, but we would need the model file to see.

it’s too big to upload, any other way, happy to take direction

Maybe stage it up on a Dropbox/Google Drive/etc. location and post a link.

540hz 1.5 aspect…3dm (10.5 MB)

Here it is with some point draped over it.

thanks for your help John.

Just generally speaking, all those points look like they could be decimated, and degree 3 curves with an equal number of points, be interpolated through them. Those curves could probably be further simplified without distorting the shape too much. Then a surface could be created through the curves using the Loft command.
As a final step, check the surface using the Point Deviation tools comparing it to the original points.

A KEY POINT to remember with NURBS modeling, is you want to use the fewest possible number of points that still accurately represent the surface you’re after.

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Having some luck with this but loft and sweep2 won’t play ball but edge curve seems to. Heres a segment.540hz 1.5 aspect. copy.3dm (2.8 MB)

The suface produced it a little odd up close.

typical application of approximation of a single nurb surface to a set of points.
is the shape in fact symmetrical? would it suffice to approx a fourth of it and then close it by 2 mirror operations?horn-iCapp-approx-untrimmed.3dm (1.3 MB)

Thanks Stefan,

Well I have spent some time on this, the shape is indeed symmetrical so have attempted to create 1/4 of it…however the only command i am finding that works to surface the individual components is “edge curves”. So the picture shows the segment I have surfaced and the red line shows my split line which I have extruded in order to split through the whole surface.

I am trying to split or trim it to create an exact quarter but this just won’t work. I am wondering if anyone can help with a solution here. File attached.

540hz 1.5 aspect. EXPT…3dm (12.1 MB)

now you are in a much better situatution: youn can rebuild the curves, loft to a surface and use mirror operations to complete the horn. rebuild-loft-mirror.3dm (5.6 MB)

additionally, if you split the surface by isocurve near the long edge (shrink=yes) then with the command _Symmetry, you can make the shape symmetrical along the 2 axis
540hz 1.5 aspect. (7.5 MB)