Import stl with the same surface borders error

I want to modify a 3D model .stl file but after I import it I always get one surface borders error. If I fix it (see uploaded .skp file) and export as .stl than import it again the Solid Inspector2 always has the same surface border error! I tried to import with “model units”:meters but no change.

Another big question would be, as I know the .stl file is made up of many triangles, but after I import my .skp file it has lots of rectangles, why don’t I have just triangles, is my .stl file still valid ?(After I export it, will the triangles be restored??)

gear3.stl (109.5 KB)
gear.skp (161.9 KB)

I opened your .skp file and it shows as a solid component.

Did you change the import units in the Import Options panel? Changing the Model Units will have no effect on the import size.
Screenshot - 6_17_2024 , 1_15_17 PM

The importer merges planar faces and as a result deletes the unneeded triangulation or at least most of it.

Are the teeth on this gear supposed to be of different lengths?

  1. Yes, but if I export it as .stl then import the .stl file again, the Solid Inspector2 always has the same surface border error!


  1. Yes, I have changed it in the Stl import option. “No change” means after I import the .stl file, it has the same surface border error!


  1. So after I export it, will the triangles be restored??


  1. NO, this is the reason why I want to modify the .stl file, can you recommend a method to make all teeth the same ?

Thanks for your help!

I imported the .stl you supplied using meters as the import units and it came in as a solid. No issues with surface borders.

I don’t know why you’re still getting it if import units are set to meters. I’m not seeing it.

Yes. the .stl file is all triangles.

What are the teeth supposed to look like? If I were modeling this thing I would model it centered on the origin to use it as the center of the model. I would model one tooth and then use Rotate/Copy to make an array of identical teeth around the center.

I presume you intend to 3D print this thing. I would model it with units set to meters and make it 25 meters tall. When exporting the .stl set the export units to meters.

I would also avoid repeated exporting and importing the .stl file.

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Would this do what you want? Open it in your slicer and check.
proem gear.stl (287.2 KB)

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thank you for the nice explanation of all my problems :+1:


Why do I have to export it in meters when I need it in mm ? Probably here is my problem, I always import it in meters, then I export it in mm. If I export in meters how can I print it to real dimension?

Technically .stl files are unitless. If your model is created such than 1 meter in the SketchUp model is to equal 1 mm in the 3D print and you export the file with export units set to millimeters, the .stl will record the 1 meter length as 1000 long (no units, remember). Then you import the file into the slicer with units set to millimeters and that length is read as 1000 millimeters long.

If, you export with Export Units set to meters, that 1 meter length in SketchUp gets recorded in the .stl file as 1 long. In the slicer, then, it is 1 mm long.

Here’s an example using the same process I’ve been describing. Note the dimensions of the rod are in meters.

And here is the 3D print.
3D Printed Connecting Rod

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Finally my mistake has been found, thank you!

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Please tell me why in your model the teeth are together with the gear and I can’t get this in any way, I can’t escape of “this face”. How can I weld the teeth with the gear??
I used components because I have to modify to bottom of teeth like in new _gear2.skp

new_gear1.skp (37.6 KB)
new_gear2.skp (35.3 KB)

I modeled one tooth and copied it to make a radial array which I then made as a solid component. Then I modeled the body of the gear as a second solid component. These were then combined into one. I used the Union Tool in BoolTools2 because it respects components. You could also use the native Outer Shell or Union in Solid Tools.

combining them into a single solid isn’t really required but the body and the teeth must then be solids on their own.

In real time. A few guidelines for initial setup.

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Wow, it’s amazing :+1:

I tried to reproduce it, I got the two separate solids but after I made united_solid with UnionTool I don’t get solid component, Solid Inspector shows 96External Faces.

2solid.skp (76.2 KB)
united_solid.skp (83.6 KB)

You didn’t make the teeth long enough to fully pass through the side of the cylinder. Looking straight down on one tooth, you can see that it only touches at the apex.

On mine, I made sure to extrude the tooth far enough to pass through the side of the cylinder completely.

If I remember correctly, early on I suggested that you turn off Length Snapping but you haven’t do that. It’s not to blame for the lack of solidity in your model but Length Snapping can lead to imprecision which not something you want for this kind of modeling.
Screenshot - 6_28_2024 , 1_17_24 PM

Thank you, I redrew it and now it is solid :blush:.

I would just like to know if was there any way to modify the teeth component in the 2solid.skp slightly larger to pass through the side of the cylinder completely.

Open the tooth component for editing and use Push/Pull to pull the faces a little more to the center.

Should I push each twelve faces separately?

Push/Pull will only work on one face at a time so yes.

which grip must I move, no one does it in the right direction. . .
(if I can do it for one, then I delete the rest and will use Rotate/Copy to make identical teeth around the center.)

Why are you trying to use the Scale tool? I told you:

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Oh, it is late, I don’t know why I read Push/Pull and I use the Scale tool :sweat:. Problem resolved, thanks.