Joint spontaneously changes solution after clicking OK

[vex]

I have a simple spindle head design and the pulley and spindle were mated correctly. I then altered the pulley as I had more realestate to do so (added another level for additional torque/slower speeds). Then I ran in to this shinnanigins: The pictures below highlight the issue on a per-step basis.

This is the appropriate solution, and one that I would expect. However, after clicking OK...

What just happened?

Doesn't matter which component I select first. This happens. Any ideas?

[vex]

Work around: Rigid joint on centerpoint of circular reference. Alter angle and offset.

[TrippyLighting]

That's not a workaround. As a guidline its best to always use the simplest joint that provides you with a solution. That is particualary tru if you later want to do motion studies as it helps to speed up calculations.

Mathematically speaking your first joint has two alternate solutions. When you edit the part involved in the joint Fusion 360 re-computes the joint and uses the other solution.

You chane change that by editing the joint and use the "flip" button.

[vex]

The original joint was the simplest solution as it provided the rotation and offset information that was contained in the workaround. The error, as documented, shows that the resultant solution is altered from preview to final to be incorrect. The work around negates this by dictating an offset and angle to coincide with what would have been an identical joint without offset and angle.

[TrippyLighting]

Yes, the preview should defininetly match the outcome. Otherwise a preview would be somewhat superfluous 😉

That definitely sounds like a bug of sorts to me.

[jeff_strater]

The one caveat here is:  The joint command preview only shows the two components selected in their new orientation.  It does not do a complete assembly solve.  So, if you have a chain of joints (for example A is joined to B, B is joined to C, C is joined to D), and you create a new joint between another component and A, where you select A first, then the joint preview will only show A in its new position, not B, C, and D.  We do this for perfomance reasons.  Then, when you click OK, we do a full assembly solve.

So, there can be valid cases where the preview and final result differ.  I don't know, in this case, whether this is a bug or correct behavior.  If you can share the model with us, we can take a closer look.

thanks,

Jeff Strater (Fusion development)

[jeff_strater]

Thanks for sharing the design, @vex.  Yes, that is certainly a bug.  My theory about the partial solve vs the full solve doesn't apply in this case, as near as I can tell.

I will file this, and have the Joint expert take a look.

Jeff