Edit trimmed surface edges -- flat to cPlane

I have a complex organic shape that I built with the Patch command. Basically it is a half dome with an outside profile. The “dome” is derived from several cross-section open curves that all touch on either end on a closed curve “rim”. Problem is that the resulting trimmed surface’s edge matches the “rim” in the x and y axis, but not z. I have something that from the top looks perfect, but when seen from the side looks like a Pringles potato chip. The “lip” is wavy. I need to have the edges all bound by the “rim” curve in all dimensions so that I can attach to a flat flange.

The Patch command creates a surface from the cross-sections and then trims it with the rim. I end up with no control points on the edge. How can I force the edge of the surface to conform to the rim in all three dimensions so that it will mate to a flat flange?

Method 1

  • Turn on control points
  • Select the CPs that you want to make flat
  • Use command _Setpt
  • Check only Z direction
  • Type 0

Method 2

  • Turn on control points
  • Use _ProjectoCplane command.

Unless I’m missing something, I don’t think there are control points along the edge of a trimmed surface. When I turn on control points they appear over the entire surface and ignore the trimed edges. Is there some way of creating control points along the edge?

Can you upload an example file?
Trimed surface still can do this…

Patch-Coplanar Rim example.3dm (90.9 KB)

That is correct. A patch is a trimmed surface and its control points will not correspond with the trimmed edges. It’s actually very difficult to try and fit the edges of a patch surface to something specific after it is created.

If you need the edges to be flat, my suggestion is to use the flat edge as an input curve together with the original elements and create a new patch surface, adjusting your parameters so that the patch edge passes through the curve within tolerance.


The only “good” way I came up with is

  • Draw two curves from elevation view and trime the surface.
  • Fill all open area.

The amount of work is unimaginable…

Patch is just an approximation, if it’s possible, starting from patch surface, extract curves and rebuild them. Making new untrimed surfaces…

I am not sure what you are suggesting. The patch was of cross-sections on a flat rim. The original curve that the cross-sections were bounded by is completely flat on the cPlane, and still I got this wavy edge. Are you suggesting using two coplanar curves in the x/y plane? Also, the patch command doesn’t have position/tangency/curvature/G3/G4 type controls. How would I adjust the parameters to force the patch to pass through the curve.

On a related note, if Patch doesn’t work for this how would I build an organic surface that does conform specifically to contours and perimeters? Basically I have a precise areal view of the perimeter and several precise cross-sections. The shape I am trying to create is something like an island in the sea. The coastline HAS to be flat, and has precise contours. I have several cross-sections that describe the elevations of the mountains slicing the island across its length. My object is not an island, but that is the best analogy I can think of.

Well, I didn’t see the generating curves in the posted file. If the outer trim curve was indeed flat and your cross section curves are not asking for a geometric impossibility like a sharp fold somewhere, then you should be able to get a surface that passes through your flat edge within some tolerance if you set the parameters correctly. It may take some playing with point count, stiffness and sampling. See the Help for more detail on what those parameters do.

Patch is an approximation, it tries to fit a surface through whatever you throw at it with the parameters given. It will never fit the edge exactly, but generally you can get it to within the file tolerance given the conditions stated above. A NURBS Patch surface is like a stretchy rubber sheet, depending on how flexible the rubber is and how hard you pull on it, it will deform more or less.

If you like, post the generating curves and someone will have a go at it.


Here is a model with the input curves. This is a little more like what I need than the first example with the exaggerated topography. The edge will need to be flat to mate to a flat flange/rim.

Patch-Coplanar Rim example.3dm (86.2 KB)

Well, it’s exactly as I said…

In this case the outer curve has sharp corners and kinks in it, which will make it very nearly impossible for it to be flat to the outer edge using the other profiles.

To get somewhat closer, I upped the spans to 30/40, lowered the stiffness to 0.1. The surface then gets to within about 0.02" of the edge curve - still far above the 0.001 file tolerance - and introducing the flexibility with additional spans and less stiffness makes a much less smooth surface. Can’t have your cake and eat it too…


Patch-2.3dm (152.4 KB)

Ok, but the first example had no kinks.

In any case, if this is not possible with Patch how should I construct these surfaces? There must be a way to construct organic bowl shapes with a flat rim even if the rim has kinks. Isn’t there? How would you model a police shield or violin? They are complex curves with flat rims with kinks.

I would normally use patch with input curves or point guides as simple as possible and then trim it later. dont try to finalize your form with patch but use it to generate a basic surface before further editing with trims, blends, rebuild and so on.
one more thing, not all shape can be made with just 1 NURBS surface, some shapes requires several NURBS srf “stitched” together with controlled continuity.