topic Re: STL export mesh is too dense in Fusion Design, Validate & Document Forum

STL export mesh is too dense

n8bot — Sun, 14 Jan 2018 01:11:03 GMT

Hi. I've encountered this problem before, and now it has popped up again so I need to share it.

When I export STL, I use a value of "5" for normal deviation, and the smallest possible value for surface deviation. This has worked very well, until recently. This problem seems to have appeared after an update to Fusion that occurred in the last few months.

The problem is that some areas, usually compound curves, receive FAR too many triangles, which causes problems for 3D printing.

Notice that a (Nearly?) identical surface is not affected on the mirror side of the problem area.

Re: STL export mesh is too dense

n8bot — Sun, 14 Jan 2018 01:16:17 GMT

Oddly enough, if I split the body in half, keep the "good half" mirror it, combine it and try exporting as STL, the mesh is perfectly as expected and mirrored.

Something is odd about these curves that is causing too much density -- but I did not model these, so I have no idea what they may be. Any thoughts?

Re: STL export mesh is too dense

I_Forge_KC — Sun, 14 Jan 2018 04:30:55 GMT

Sounds like there is some topology wonkiness on the "bad side" that is causing the tessellation engine to fill with a lot of triangles. For kicks and giggles, have you looked at the surface and zebra analysis of both sides to see if they are identical?

Re: STL export mesh is too dense

TrippyLighting — Sun, 14 Jan 2018 13:36:48 GMT

Is this a model you can share ?

Re: STL export mesh is too dense

n8bot — Sun, 14 Jan 2018 16:40:22 GMT

I did not look at the part under zebra analysis yet, but I did mirror the body and compare the volumes and they seem essentially identical.

Trippy_Lighting, thank you for your post. It helped me figure out even more to this problem. It's not my model, it's not a fusion design, it's an imported STEP file. I didn't want to immediately share the entire model, so I went to slice off the offending bits and export just that as a new STEP file- Lo and behold, the new step file does not exhibit the poor meshing behaviour.

Now, the REALLY odd part, is that if I simply export the original unmodified STEP file into a new STEP file, import that as a new design from file, the problem disappears.

I think that these STEP files are exported from SolidWorks -- it appears that simply exporting them from Fusion and re-importing them solves whatever problem there was.

Re: STL export mesh is too dense

TrippyLighting — Mon, 15 Jan 2018 12:28:10 GMT

No actually, that's not so odd. I've use STEP exports to fix problems with Fusion 360's horrible surfaces. I mean surfaces created with lofting in Fusion 360. I had created a model for a client, but could not shell it. Using the curvature map I could see the bad surface immediately. I also use a higher end CAD software and the intention was to simply take the STEP into it, remove the offending surfaces there and recreate them, but I would have had to log out of macOS and into Windows.

So I decided to try re-impoting the STEP. Ans so the offending bit was gone and shelling worked fine. Still not a great surface but workable for a concept model.

Most mainstream and midstream CAD packages, when reading STEP only read the boundary information of NURBS surfaces, in essence the boundary curves that define a surface. They don't read the actual surface information e.g. the NURBS control mesh.Then they re-generate the surface form that boundary information. When doing so I guess they modify data. My guess wold be that Inventor does the same thing as both share the same geometric modeling kernel.

Also, most common CAD solid modelers only can represent 3rd degree curves/surfaces. Higher order surfaces as used in real Class-A surfacing software such as Alias are approximated with 3rd order curves/surfaces also resulting in a loss of precision.

So this does not surprise me a bit1

Re: STL export mesh is too dense

n8bot — Mon, 15 Jan 2018 16:19:37 GMT

Interesting, thanks for the info!

I had a hunch that that having Fusion "re-generate" the exact same geometry would help -- this problem only seems to occur when importing STEP files from SolidWorks users. (90% sure the offending STEP was from SolidWorks)