STL file adds unwanted geometry to my design. How do I stop this?



I’ve made a model in SketchUp, and exported it as an .STL file. When I open that file in a program like TinkerCad so I can 3D print it, or KiriMoto so that I can CNC cut it, I find that there is new geometry on my designs that I didn’t put there. I would like to figure out how I can get a file from SketchUp to export correctly, so that what I designed is identical to what is exported. Below are images to better describe what is happening:


STL files by their nature make triangles of the geometry. You can’t prevent that But the tooth-like shape around the hole should be preventable. How did you create the hole in the first place? How large is it?

It looks like there are some internal faces that shouldn’t be there and it’s likely they are influencing this.

Can you share the SKP file?


it looks to me like your SU file has errors which TC has tried to repair for you…

can you upload the skp file…



Hi John and Dave–thanks so much for your lightning fast reply :slight_smile:

Here is the SKP file:
Camera_lens mount_d7_for_CNC_Dec14.skp (141.7 KB)


I can see a couple of problems with your file immediately. First, the internal faces around that hole. Second there are exposed reversed faces. These things can create issues with the STL file.

I’ll upload a repaired version shortly for you to try.


Ok, thanks for letting me know. Is there a way for me to see those problems too? (When I look at the file in SketchUp, I can’t see anything wrong with it.) Can I find the problems like you did?


Here’s the repaired file. The other part also had some reversed faces which I corrected.

Camera_lens mount_d7_for_CNC_Dec14.skp (160.0 KB)


I hid the top face in the group so I could see inside and access them to show you. The internal faces are actually visible in your first screen shot, though. To see the reversed faces, you need to change the face style to Monochrome.Adding materials or colors to the model mask those reversed faces.

If you approach the modeling correctly, you can avoid the internal and reversed faces.


dave did the hard bit, but I’ll add some observations…

‘Parallel Projection’ is best avoided, unless for a specific 2d ‘selection’ or when exporting 2D elevations…

don’t change the default colours, as it prevents some easy ‘debug’ methods…

e.g. view ‘monochrome’ for example cannot show your reverse faces as they are the same color as the front faces…



Thanks for helping out with this, and for fixing the file!

I’m completely self-taught so I imagine that I don’t do a lot of things the right way. Is there a command to use to change the orientation of a reversed face?


Very happy to be learning new things from you guys.

What does avoiding parallel projection help with?

And I’m still not sure I understand how Dave found the problem with the “the internal faces around that hole”.


Yes. You right click on the face and choose Reverse Faces from the Context menu.

I sent you a PM.


I’ve switched back to monochrome which which allowed me to see the reversed face near the circle. And I’ve found the reverse face command. Those two things make sense to me now.