topic Re: Constraining to ellipses with tangent & coincident constraints (includes pic and demo .f3d file) in Fusion Design, Validate & Document Forum

Constraining to ellipses with tangent & coincident constraints (includes pic and demo .f3d file)

autodesk2HB53E — Tue, 23 Jun 2026 22:22:23 GMT

Hi,

So, I've got three solid cylinders. Their axes point in arbitrary directions. Conveniently, one cylinder happens to point straight along Z. But the other two are slanted arbitrarily. They are all sliced through by a plane (which happens to be the World XY plane), and my goal is to draw a profile that you might think of as a rubber band around them all on this plane, and I want this all to be fully parametric. So, I select the slice plane as my Sketch Plane and then choose Intersect to isolate the simple 2D underlying geometry - one perfect circle, two ellipses. I draw three lines and begin to add coincident and tangent constraints, like this:

But whatever I try, I can't get the last line to constrain to the ellipse indicated by the black arrow. I need to add tangent and coincident constraints. But Fusion 360 refuses to solve. Agh!

Please help. I have attached the actual demo .f3d file from which the pic was generated to this message.

Re: Constraining to ellipses with tangent & coincident constraints (includes pic and demo .f3d file)

TrippyLighting — Tue, 23 Jun 2026 22:47:34 GMT

Tangency to arbitrary points on a spline does not function in Fusion. If I remember correctly, the same applies to ellipses.

Re: Constraining to ellipses with tangent & coincident constraints (includes pic and demo .f3d file)

TheCADWhisperer — Tue, 23 Jun 2026 22:49:53 GMT

@autodesk2HB53E

You will have to test whether this proposed solution is fully parametric.

I did not understand why you created multiple work planes.

I would need to observe logic.

(view in My Videos)

Re: Constraining to ellipses with tangent & coincident constraints (includes pic and demo .f3d file)

kacper.suchomski — Tue, 23 Jun 2026 22:56:19 GMT

(view in My Videos)

Re: Constraining to ellipses with tangent & coincident constraints (includes pic and demo .f3d file)

TrippyLighting — Tue, 23 Jun 2026 23:45:28 GMT

@rohit.bapat, the behavior that the OP and the two posters show in their videos, which I find very easy to reproduce, is very buggy and is unlikely to create a reliable. PLease record this as yet another sketch bug.

@kacper.suchomski @TheCADWhisperer Posting workarounds is fine. Not reporting obvious bugs is negligent!

Re: Constraining to ellipses with tangent & coincident constraints (includes pic and demo .f3d file)

autodesk2HB53E — Wed, 24 Jun 2026 09:02:21 GMT

Sadly, neither of the proposed solutions result in reliable, properly parametric (updateable) models. Thank you both for trying. Autodesk, please take note of this bug.

Thanks for bringing this to our attention. We're investig...

Closed-Loop-Bot — Wed, 24 Jun 2026 14:29:06 GMT

Thanks for bringing this to our attention. We're investigating the issue, and you can reference ticket FUS-256555 when communicating with Autodesk to find out more. Once the status of this will change, there will be a follow-up forum post made.

Re: Constraining to ellipses with tangent & coincident constraints (includes pic and demo .f3d file)

rohit.bapat — Wed, 24 Jun 2026 14:30:41 GMT

Hello @TrippyLighting

It's a bad bug indeed. I have created ticket FUS-256555 for the team to take a look at. I was able to reproduce the issue very easily with the dataset.

Thank you

Thanks for bringing this to our attention. We're investig...

Closed-Loop-Bot — Tue, 14 Jul 2026 09:35:31 GMT

Thanks for bringing this to our attention. We're investigating the issue, and you can reference ticket FUS-261476 when communicating with Autodesk to find out more. Once the status of this will change, there will be a follow-up forum post made.