topic Re: STL Export creates some reversed triangles in Fusion Support Forum

STL Export creates some reversed triangles

edkman — Thu, 25 May 2017 12:33:12 GMT

I have never seen an error like this from Fusion before. The STLs have all been clean up til now.

The export created an area of reversed triangles - the faces are flipped.

In Fusion, there is no visual indication of a problem with the model. It's fairly simple.

But the STL has a clear anomaly in an area where, as one of the last steps in the modeling, I made a small change.

Exporting with the history rolled back to just before the change results in a clean STL.

Attached are the project archive, and screen captures showing the problem.

Re: STL Export creates some reversed triangles

TheCADWhisperer — Thu, 25 May 2017 13:29:03 GMT

Your splines in Sketch1 look really questionable to me?

Do you really want non-tangent splines rather than lines and arcs.

They are not symmetrical either - which I would expect to be the design intent.

I would have created Sketch1 symmetrically about the Origin Center Point - Workplanes 1 and 2 would not be needed.

Re: STL Export creates some reversed triangles

edkman — Thu, 25 May 2017 13:39:29 GMT

Thanks for the insight. I'm weak on splines.

But the problem with the STL export comes only from a simple bore enlargement in the penultimate extrusion of the timeline.

Changing the bore size to 3.5mm instead of 3.25 mm results in a good STL. Something in the triangle generation code for STL export is very sensitive to the specifics.

Re: STL Export creates some reversed triangles

James.Youmatz — Thu, 25 May 2017 13:38:22 GMT

Hi @edkman,

Thanks for sharing. This definitely does seem like a bug. I am actually exploring something very similar now. Just to check - did the model you post include the bore enlargement? When I go to export your model, I don't see any issues in the preview - or an extrude command related to the bore command. It's early so perhaps I just need more coffee first!

Thanksl

Re: STL Export creates some reversed triangles

TheCADWhisperer — Thu, 25 May 2017 13:39:35 GMT

You could try a Presspull on that hole to enlarge rather than an Extuded-Cut.

Not sure if that will make any difference in the stl.

But I would want to go back and create clean geometry right from the foundation of the part.

Re: STL Export creates some reversed triangles

edkman — Thu, 25 May 2017 13:42:19 GMT

Yes. It will be the second-last extrusion, with sketch immediately preceding it.

The one flange ear that has the reversed triangles gets bored to 3.25 mm from 2.5 mm. (See my note above where I found that 3.5mm works just fine.)

I would guess that it's also sensitive to the specific export fineness settings I used - they're maxed out.