Planar Surface for CNC

I have an issue trying to make planar faces out of a trapézoïdal box.
When I extrude my curve using the gun-ball then use the cap command it create a non planar face on the top.
If i try to make two planar faces and fillet them it make also non planar faces.

The end of it is to make individual faces to cut on wood panel with miter cut.

Do you have any ideas how i can do this ?

Hi @edwin3
Look at your range for the gaussian curvature. I can’t see the whole number, but I’m guessing it’s a VERY small numbers; 3.3e-“something” - probably way below tolerance. Also, the curvature analysis is based on the analysis mesh. The default settings for the analysis mesh aren’t always the best, so click “Ajuster le maillage” and optimise the analysis mesh to fit your obejcts better.
If you want to be absolutely positive that all your surfaces are flat, you can explode your boxes and test for planarity (eg. using SelPlanarSrf or the What command) and the rejoin them.
If it’s still causing you trouble, post your 3dm file, and I’m sure someone will take a look :slight_smile:
HTH, Jakob

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The Cap command should not produce non-planar (within file tolerance) surfaces. Something else is going on. Please post an example file.

Thank you all for your responses

@Normand I know that the tolerance is ridiculously small but the supplier for cnc need “absolute” flat surface.
I useWhere I have a strange response is that when i use selplanarSrf it select two object that appears in the surface analysis non-planar …

I tried to make a solid by extruding a curve again but with the command “extrude close planar curve” and it work.
The question then is why when I am using the gumball to extrude then cap it create a non planar face ?

Also i notice that the command "extrude closed planar curve " can extrude non-planar curve where the command make face from planar curve doesn’t work for the same close curve. is it normal ?

Here is my 3dm if you could have a look

Thank’s again
test planar box.3dm (248.0 KB)

Planar is assumed to be within the file tolerances you set. If your cnc supplier needs ridiculously low tolerances, you will need to set your file tolerances accordingly (or find a better supplier).

That being said, in checking the file the surfaces are planar out to seven decimal places after the zero. If your cnc suppier finds them non-planar, then it’s really time for you to change suppliers.

As I said, Cap only creates planar faces. It if caps, it’s planar.

Yes. Extrude is a combined command, if you set Solid=Yes, it will extrude a closed planar curve and cap it if the curve is really planar, but it will still extrude the curve without capping it if the curve is non-planar.

Rhino defines your surface as a “Plane surface”

    Valid surface.
    trimmed surface.
      Plane Surface

Here are the 4 corner point coordinates of your box #1 upper surface:

Point in world coordinates = -744.9332608,-17672.0354563,150.0000000
Point in world coordinates =  399.2958265,-17505.0356324,150.0000000
Point in world coordinates =  149.6979771,-15794.8714377,150.0000000
Point in world coordinates = -994.5311102,-15961.8712616,150.0000000

You are pretty far from the origin - your grid is a bit misleading because it’s huge - that might affect the quality of analysis mesh you are using, but the absolute coordinates of the NURBS surface are not affected by that.


Be very careful judging deviation from a perfect plan using Gaussian curvature. Gaussian curvature does not correlate with of the deviation from absolutely planar.

Of course a Gaussian curvature of exactly zero everywhere means a perfect plane. But how small does Gaussian curvature need to be to for the object to be “close enough”? Is 0.1 small enough? How about 0.001 - that seems like a small number? Or should it be even smaller, perhaps 0.000001?

Gaussian curvature has units of 1/length^2. If millimeters are the length unit then Gaussian curvature has units of 1/mm^2. If meters are used then Gaussian curvature has units of 1/m^2. So the numeric value for the Gaussian curvature of a surface will be 1,000,000 larger if meters are used rather than millimeters. A Guassian curvature of 0.1 if meters are the length units use is the same as 0.009 if feet are used, the same as 0.00065 if inches are used, and the same as and 0.000001 if millimeters are used.

My reasoning was different and does not involve curvature analysis at all…

The surface is a simple degree 1 NURBS surface and it has only four control points. Degree 1 means that the edges are linear, i.e. straight lines connect the control points. Therefore if all four control points have the same Z value to within 0.0000001 mm (probably zero further out than that, but that’s the limit of Rhino’s display), the surface is dead planar - it mathematically can’t be anything else anywhere else in the surface domain.

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Hello @Helvetosaur and @davidcockey

Thank you for your help

If i understand right, then it mislead me to check my faces with the analysis tools because it’s too precise but still why it show différents colors if cap make an absolute flat plane ?
On this file from my supplier the face of the trapezoidal box are completely flat i think regardind the analysis tool again that’s the reason i try to understand this
I will come back to you if I have some other problem related to this during my construction

Thank’s again
test planar box 2.3dm (235.4 KB)

Math on computers is done with a finite number of digits, equivalent to 15 to 17 decimal digits. There may rounding/truncation errors in the last couple of digits. This accuracy is more than sufficient with very few exceptions. The Gaussian curvature calculations are probably displaying the effects of the these errors. The CurvatureAnalysis display auto scales so even when the deviation in curvature is extremely small it may show colors.