Normals problem, flip when joining

This happens to me occasionally but first post about it.

Have a surface that I mirror and join. When joined, the surface normals flip to opposite directions i.e. one up one down.

Explode, and can get them going the same way. Join, and one surface flips.

What’s causing this?

See attached.

normals problem.3dm(371.6 KB)

Hi. See the attached view of your same file. Your surfaces overlap by about 50 times your tolerance, so the edges you expect to join do not match. The overlapping edges do match, and the only way to join them is to flip one of the surfaces.normals problemA.3dm(365.6 KB)

Actually I thought I got that edge from splitwithplane using the worldzx cplane and that overlap should have been impossible but I must have screwed up somehow. Thanks for this.

Ryan, if you are using the SplitWithPlane script, please send me the inputs, if they are different from the surfaces you posted…



Inputs? Apologies not familiar with the jargon. That’s the only surface I had.

I have the same problem.

I was able to fix it by selecting surfaces and joining one by one. As I did this if any of the resulting surfaces joined with flipped normals I would undo the join, remodel the problematic surface and then join again.

When you have many surfaces to join this is very time consuming. Is there a faster way to solve this issue?

Attached is the file I’ve had issues with. The problem happens with the surfaces in the dark navy blue layer.

Thanks for the help.

Best regards,

Leoflor1.3dm (11.2 MB)

Hi Leonardo - the surface points here:

should all come together exactly, and they do not - use ShrinkTrimmedSrf then turn on points (f11) and select the points, on both the upper and the vertical surfaces, that are meant to be coincident

and use SetPt to get them truly coincident:

Then I think your Join will work as you like.


1 Like

Thank you Pascal!