CAD Masters opinion on re-creating trimmed surfaces

Hi forum, if I receive a file with multiple surfaces that have been trimmed to make poly-surfaces. I can explode the poly-surfaces to get to just surfaces. Then turn on points to each surface and see the points that go beyond the trimmed surface. Does anyone know how to use the points to make a full un- trimmed surface that is (exact) close to that original surface before it was trimmed? I am unable to provide an example of the trimmed surfaces. I can say that the point count goes beyond the trimmed surface by 6 to 10 points beyond the trimmed surface.
If I can make an example , then I will send it in. Thanks to any advice, Mark

Hi Mark - in the special case where all the trims are on surface isocurves, ShrinkTrimmedSrf will in effect make an untrimmed surface. That command can, depending on the non-isocurve trims, also remove at least some of the ‘extra’ points but there is no way to make an arbitrarily trimmed surface into an untrimmed one, you’ll have to re-create it somehow, and the success or viability of that depends on the particular surface…


Thanks Pascal, I will try to come up with that example in a couple days. In the mean time , Thanks
What I am trying to do is learn the structure on those surfaces and create them so I can trim them and make ( what I see as) very clean and accurate surfaces.—-Mark

Do you think passing an interpolated curve through the outside points on each edge would give a good result that represents the original surface Richard? I will experiment some. I might be wishful thinking . It’s worth experimenting. Thanks guys

What points would you use to create the interpolated curve?

First step is to think about how the untrimmed surfaces might be arranged.

First rule of NURBS surfaces - all untrimmed surfaces have 4 edges, but 1, 2 or 3 edges can have zero length. Zero length edges can cause problems but surfaces with 1 or 2 zero length edges are very useful in some situations.

Hi Mark @markintheozarks

I might be misunderstanding what you are trying to do, but from your description, it sounds as if you are looking for the untrimmed surfaces, not how to re-create them based on the points. If yes, just explode the polysurface, select any given (or all) surfaces and use the untrim/untrimholes/untrimall command. Once you have the untrimmed surfaces, you can - if needed - use rebuild to simplify them or re-construct them from scratch to suit your goals.

HTH, Jakob

Thanks Normand, It finally dawned on me to use the method you described.
The surfaces came into Rhino from NX software. They are so clean. The point distribution is very even throughout the surfaces. I used a VSR tool called Deviation and made some surfaces in Rhino and got to within .007”. —-Mark