Very Basic Trim Problem

I had a problem trimming a surface, and when I isolated the problem, it is surprisingly basic (see attached file) – I have a simple revolved surface, and when I try to trim it with a plane, it fails in both Rhino 5 (Windows and Mac) and Rhino 6 (Mac).

Trim_Problem.3dm (3.0 MB) (Rhino 6 version)

Changing the tolerance, even to extreme values, doesn’t help.

Running Intersect makes the problem a bit clearer:

– Rhino is getting the intersection between these surfaces very wrong for some reason.

In this particular case I will probably just find a workaround, but it seems surprising that this would happen in such a simple situation. It would be good to understand exactly what is giving Rhino difficulties here, as it might help to understand more complicated trim / intersection problems in the future!

Hello - try InsertKnot > Automatic a couple of times in the U direction (knots go ‘around’). Then trim.

I’ll get this example onto the pile.


Hello , bobtato
The V direction is in degree 4>
If you rebuild your surface in degree 3 like in my exemple, you will be able to trim it.
I hope it help.


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Hello - keep in mind that Rebuild does not preserve the shape of the input, while InsertKnot does.


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Yes it s true i should have precise it.
It s because i have less problemes with degree 3 than with degree 2, 4.
I often rebuild before triming or booleen operations.
But InsertKnot is the precise way to do it.

Thanks @pascalInsertKnot resolved the problem!

It is useful to know that this command preserves the exact form of the surface – it isn’t important in the simplified example, but of course my real model is more complicated and involves trims and joins that would break if I used something like Rebuild