I have a problem with the import of mesh from CAD software.
The OBJ import is correct but I do not get any custom normals. Its all smoothed 180°.
With FBX import it works but polygons like this in the image get ignored. But only in Blender, else it works with FBX. Also tried a better FBX importer… didn’t work better.
You may show a view from the other software to make a better comparison and maybe post this obj/fbx file ? (Maybe even mentioning the name of the other/better fbx importer?) FBX is proprietary and yes blender had/has it’s problems with FBX. (I believe there are also some OBJ import alternatives or even guides to import with older blender version but i don’t remember yet… )
Archiv.zip (174.6 KB)
Ok, was thinking of a known problem. Above the link to the files and a screenshot of how it looks in Blender.
Tested: OBJ, FBX, Collada, Alembic and USD. Either the polygon is missing or the normals are wrong. Tested with Blender 2.9 and 3.0 and 3.1. Also tested a better FBX importer add-on.
Exported from: Moi3D, Rhino3d, Modo, C4D. No difference in import. All these Apps display the mesh correctly. Correct polygons and normals.
It really is a problem to not get any geometry into Blender.
Have you tried importing a STEP file directly into Blender with the Stepper Addon?
Not yet, but I need to get mesh in anyway.
I had a quick look… also MeshLab has dificulties to show the concave n-gons:
Maybeyou should use some triabgulation on the n-gons in the original software??
Its a major bug for me, totally ruin Blender.
The question about need of custom normals is a joke or? Seriously, its an essential thing when working with mesh imports.
This is just a tiny part of a mesh.
I was asking about what i see or better dont’ see in this specific example. And yes i wonder why CAD data which defines the geometry (for example a 90 degree angle) do use custom normals ??? This is done in other 3D software to hide the fact that there isn’t enough geometry or there is no real bevel… Enlighten me.
And MeshLab also has problems with concave n-gons. I don’t understand why so much people blame blender for something which sometimes other software also have their problems with. Also the interaction between CAD and other 3D-program aren’t always easy…
Custom normals are the only way to have the same surface smoothing as you get them with NURBs surfaces. They are highly precise and absolute needed to display such surface perfectly. You need to understand how Nurbs work to know how important that is. Its simple impossible to get good looking mesh from Nurbs without having also baked normals.
Ahh i see. I forgot about nurbs (maybe because of the hardsurface example). So just subdividing a to a certain degree triangulated (or better quaded) surface is not enough… yes. Is there maybe any possibility to make the CAD system export something more like on the left in this image???:
Well I see no need for the exporter. It all works in nearly every App, except Blender. Not sure why no one fix that.
Especially OBJ support for normals is a standard since 25 years.
I just was playing around with insetting (some) of the problematic faces (sadly the faces overlap):
And i though about normals… does the enabled autosmooth under Object data Properties has a effect?? (i really can’t see one in the example but maybe in the render with you background image)…
So then i may be out of ideas… 
I have around 500 of such meshes in the scene, any attempt on modeling is overkill.
The only thing that would work is opening the file in another app, triangulating the mesh and exporting again as fbx. But then I can’t really adjust the mesh and also run into uses with wrong triangulation (causing black lines in rendering).
Autosmooth works only if you delete the custom data first in Blender. Its further down in the settings. Autosmooth however works on face normals and that will make surface with beveled edge lock wrong. It never worked right. It was one of the main reason I switched from 3dsmax to Maya 15 years ago. Damn 3dsmax can’t handle vertex normals (in blender custom normals) right.
If you look carefully, you will find a single edge matching several of the cylinders in the shapes…It was the original edge that was set to SWEEP the top of the cylinders…You just need to add a Screw modifier to that edge and match the number of edges on the cylinder…
Sure I can rebuild also everything in Blender. But I fear no one will give me money for a task that should only take 5 min.
If it is NURBS anyway why don’t you just let the stepper addon do the meshing?
Because its very very slow, buggy and loves to crash. And to be fair, I have tons of already imported CAD models that I like to reuse.
Beside this, there is no Nurbs to Polygon converted better than the one in Moi3d. Multicore support on Step loading and meshing.
Oh, and my Step files are often 800 MB size.
I keep hearing that. But not a single time has Moi3D worked for me.
It manages to load larger step files (about 600 MB) relatively fast but then everything becomes abyssmally slow, so that it is impossible to export the file because the UI won’t react to pressing buttons.
When I export smaller files, about 200MB, it will miss 50% of the objects and destroy the hierarchy.
Thats interesting. Never got that with Moi. I found the Mac version to run better since its 64bit then the Windows. But such cases are interesting. It might be a scaling problem. I think I had once a quite buggy file with some issues. Also you might not have joined surfaces. In such a case joining in Rhino is much faster.
For very tricky cases with not joined surfaces I found the one in C4D handling it sometimes really good with the heal function. But to be fair, knowing someone who got Solidworks or ProE helps even more 
Checked the bug database. The FBX import bug was not reported yet, might need to do that.
The missing normals on OBJ are reported in 2015. 7 years and nothing happened. Still open. Yeah seems Blenders development isn’t so fast at all…
