Is this a bug in Fusion?

[Anonymous]

I made a design in Fusion and exported an STL of it.   It "sliced" without a complaint for my 3D printer but with its taking six days {!}to print I thought I'd check shapeways.   Shapeways kept telling me there was an error in the STL file!!  I contacted customer support at Shapeways and this is what they told me:

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It was a loose shell issue. So it contained about 5000+ shells. Loose shells can be any number of different issues- flipped triangles, bad mesh, etc. So it's not a bug issue but more that the files need to be checked after exporting. Sort of like when you touch up a photo in photoshop, you would need to preview it as it doesn't always look the same from the source. Meshmixer is a free tool you could use. So after you export a model from Fusion, you open it in meshmixer and make it a "solid Part".

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I know nothing about shells and flipped triangles and such.   Is this an actual bug in Fusion's STL generating machinery?   Since I have no way to "check" the STL, should I just start always running meshmixer as a matter of course after I export an STL?

[ryan.bales]

Maybe, have you had success with that file before?

[Anonymous]

I don't know how to answer that question. The STL file in question "sliced" just
fine and produced gcode for my printer, but since it takes 6+ days to print, which
is what led me to see if Shapeways would make sense [it doesn't: $500 to print it!
UGH] So all I can see is that my slicer [prusaslicer] didn't see any problem with
it, but Shapeways did and they fixed it by something strange about "loose shells"

/Bernie\

[ryan.bales]

Yeah i'm not sure what loose shells are either. If Fusion fails to export is one thing. If its exporting garbage is a complete other story. 

Testing it on other slicers might be worthwhile to see what is happening on them. I'm guessing shapeways is using a their own slicer and it is in fact erroring out. 

[jean.flower]

Hi bernieTB8UQ

I'm curious about this mesh quality issue and whether the STL export is somehow doing something bad.  I could take a closer look if you could make your data available to me (the f3d file, and a description about which body you were exporting?)  

thanks, Jean

[Anonymous]

It seems that when I reloaded Fusion onto my new win10 system, it isn't saving
the f3d files locally. It is probably someplace in the Autodesk cloud that I could
download. But I think it'd be huge.

I do have the "project" and the two STL files [my Fusion-produced one and the
Shapeways-fixed one. Mine is 17 megs and the Shapeways version is 14 megs].
As hinted at in the other reply, this could just be a bug/limitation in the
Shapeways slicer -- she did mention that my model had 5000+ shells and maybe
that's just beyond the limit of their slicer. Which, if true, then that'd be pretty
amusing.

What should I do to help someone who has a clue about this stuff figure out what
was "wrong" with my STL file. I'm content, now that I know what to do, to "fix"
my STL files if shapeways bounces them.. but if it is a bug in Fusion someone
might want to check.

[jean.flower]

Hi again.  I'm glad you're not blocked by this issue, but yes, it'd be great to look a bit closer to see if we can learn a bit more and identify whether this signals a bug in Fusion.  Are you able to share a link to your design like this? 

https://www.autodesk.com/products/fusion-360/blog/share-a-public-link-of-your-fusion-360-design/

[Anonymous]

This is very bizarre. The answer seems to be I can't!! The entire right-click menu
on my projects is gone. Now the only thing it shows is "open". Where'd the
public line, rename, delete, etc, etc options go. Does it have something to do with
my having changed from Startup to a personal account?

Never mind -- it said I have a new version of Fusion pending download, I
restarted Fusion and now it is working again. Phew!!

https://a360.co/36hr3y8

[jean.flower · Accepted solution]

Wow, that looks both beautiful and complicated like a minecraft fractal snowflake!  Thanks for sharing.  No wonder the files are large.

I have looked into the mesh output and I can see some geometry that could cause trouble for operations like slicers.  What looks at first glance like a solid actually contains many internal voids (hollow cubes).  Those voids meet the outer shell touching a vertex (a non-manifold vertex).  Here's an example, pictured after I cut through the mesh to take a look at the inside.

It's fine to build shapes like this in Fusion. The stl file that Fusion wrote is legitimately describing this shape, so I don't see something to fix in Fusion. But different applications which load in your stl (e.g. to drive printers) may or may not be able to handle this kind of geometry.

If these voids are not part of your intent, you might try revisiting the Fusion design to see if there's a different approach which creates a solid without the voids (then the mesh won't include the non-manifold vertices).   Alternatively, I think you already saw a way forward using a mesh tool to "fix" the mesh (probably fills the internal voids).

Good luck!

[Anonymous]

Other than the tiniest cubes, the whole thing has voids through it at every level -
small voids and big voids. The voids are part of the model. This one is a level-4
Mosely Snowflke and it is the highest level Fusion can deal with. Evern
operation at this level takes about 20 minutes {!} and so it is slow going, but
possible. To try to move up to the next level, a level-5 would be pretty much
impossible.

I don't know about outer shells and stuff, but there are no "fake" contacts. Every
unit in the snowflake is face-to-face connected to another {small} cube. It might
well be that although "legal" and Fusion produces a "correct" STL file, some
slicers might not be able to handle it. I don't understand what the 'fixed' version
of the STL file is like -- did they have to change the model to 'fix' it? I don't care
all that much about shells and contacts and such {which I mostly don't
understand} so long as it does add or remove things from the model itself.

Interesting observation: the slicer I use, prusaslicer, is the *first* program I've use
on my system that *fully* uses all 8 CPUs. When fusion is running it sits at 12%,
strictly monoprocessor {when I asked about that I was told that that's generally
the case with CAD programs}. But the slicer *pinned* my system [and got the
cooling fans running :o)]

Thanks for the help!!!

[Anonymous]

Wow -- that looks like an error in my model!!   It is so complicated it is hard to see what's going on. KUDOS for spotting that.  What part of the model is it?  I have it loaded into Fusion now [it took 10 minutes just to *load* it!] and I'll see if I can find the error.   By contrast, this Level 3  is its little brother and I *think* it doesn't have that problem.  Time to do investigation.  Thanks!!

[Anonymous]

Tiny data point, sorta.  I tried uploading the level-3 to Shapeways and it had no problem with it.   and I just double-checked the level-4 and everything looks correct and is aligned properly.  So I dunno where the problem with the shells arises -- could it still be a bun in Fusions STL-generator?

{I've searched the model for vertices that might have that apparently-slight offset problem and can't spot any.  But it is a pretty complicated mess.  :)}

[welbot]

The shells they are referring to would simply be internal objects. For an STL to be considered truly printable, it needs to have a "watertight" outer shell, and no internal geometry. Some STL exporters do a horrible job of exporting meshes, and while some slicers can cope with it, other will not. 

From past experience, I think if you have a bunch of separate objects in Fusion, and export the whole model as an STL, it will leave all the internal geometry in tact, rather than performing a "boolean" type operation to clean out the internals. Best bet for getting a nice clean mesh out, is to combine all your objects in to one single object prior to exporting. I understand that sometimes you might not want to, especially if you have hundreds of objects, but whether you do or not, it's always a good idea to load up the exported STL in either 3d builder, netfabb, or meshmixer to make sure the mesh is clean. All 3 can do it, they just call it different things. 3d Builder will inform you right away if there are issues, and you just click repair. Netfabb does checks automatically (the online one at least) and meshmixer you run a function called "Make Solid". They'll basically check all vertices are welded on the outer shell, and remove any internal geometry that could cause issues. 🙂 

[jean.flower]

Hmm tricky to guess what's going on in Shapeways. I see some parallels between level-3 and level-4.

Both have non-manifold connections where corners of internal voids meet corners of the outer shape.

If you consider the internal voids as separate shapes (some STL-loaders will work only off edge-connections not corner-connections), then the level-3 mesh falls into 325 parts and the level-4 mesh falls into 5833 parts.  Maybe these numbers for level-4 just got too big for Shapeways to say the mesh is OK to manufacture?

Did you ever try using the section-view tool?  That might give helpful insight into the internal parts of your bodies; here it is on the level-3 mesh.  Great to see how the slice changes as you drag the manipulator arrow.

I wish you a great weekend!

[Anonymous]

Didn't know about  section view.  I'll check it out.  I understand the structure of the snowflake well enough that I ought to be able to spot anomalies.  Thanks!

[Anonymous]

Note that the original project was called "level 4 -solid" -- I always made this complicated messes as "combine"d single objects.   Not because I understood the STL consequences [as I obviously do not] but because Fusion is too slow.  When I originally did this with components and joints and it was unwieldy  and very very slow, so I switched to objects and align and combine to make a single object [but with a lotta internal voids :)]

[Anonymous]

Thanks for all the comments and help.   I still don't exactly understand about meshes and shells and such, but I *think* I understand what happened, especially since Shapeways had no trouble with the level-3 model -- which is similar but less complicated.   I noted in the email I got from their support person that the level 4 had >5000 shells.  I'm betting that *that* was the problem -- however they process STL files barfs on that many shells and what meshmixer does is combine a bunch of shels [without, as far as I can tell, affecting the model].  My slicer had no trouble with it [but pinned *all*eight* cpus processing it  :)].   I can't get it printed... sniff --- it'll take 6 days on my Prusa Mk3 and Shapesways wants >$1000 to print it.  Alas...