Sketch dimension says over-constrained but is not!

[r.moss]

I have just been editing a sketch.  I think it was getting unstable - I tried re-positioning the radius dimension on an arc and it just disappeared, but when I tried to recreate it it said the new dimension would over-constrain the sketch.  So I deleted the arc and 1-2 other lines, which left the sketch unconstrained, and thought I would add a dimension to re-constrain it before regenerating the deleted line.

The screencast shows me trying to add a dimension to constrain the sketch.  I can drag the blue line up and down, so I should be able to add a dimension to fix its vertical position - but when I try this, it says it will over-constrain the sketch.  Having then created this driven dimension, I find I can still drag the line (and the dimension varies) so it obviously hasn't actually over-constrained it.

I'm attaching the.f3z file.  (It's a bit of a mess as originally I created all the bodies and moved them into components at the end.  Then the timeline got longer as I imported nuts, bolts & other components; I created a few more bodies but could not bring them into the components along with all the others, so the timeline has got all fragmented (see my comments about components in the design forum).  So I'm half-way through regenerating all the extrudes as proper children of the 5 main components.  The finished assembly will look like this):

Finished article

 

[r.moss]

Playing with it a bit more, I can delete the two angle dimensions off this horizontal line, and the right-hand arc, and it still says over-constrained:

Constraining arc

I can still drag the blue line up and down.  If I delete the blue arc with the two tangent constraints I am then able to add a driving dimension to the horizontal line without getting the over-constrained message.

[intelinc]

I just ran into a similar issue in fusion 360 on my mac, beginning with the sketch instability (grabbing and moving sketch lines began to get choppy/twitchy). I had an unconstrained sketch that would no longer allow me to constrain it. I deleted some additional constraints so that it was even less constrained, but still could not create a new constraint on the line that I wanted to drive, even though the line itself was visually unconstrained and fully manipulable in the desired dimension (albeit in a hitching manner).

I have been having this happen with sketches more and more frequently over time (particularly since March/April updates). I used to assume the error was mine, and that I was not seeing some erroneous driven dimension -- and sometimes this is still true! 😉  But I've begun to realize that more often than not, the solver seems to start choking (sketching UI begins to slow down, sketch element selection becomes haphazard, CPU load goes up, etc) on some combination of parameters+constraints in the background that trip the app into an unstable/untrustable state where subsequent warnings it gives around parametric/driven elements are inaccurate. When I suspect this I save/restart, and sketches that were throwing errors are suddenly fine when reopened. I've learned not to bang my head against sketches that report over-constraint errors but are visually (based on line color and visible constraint icons) not fully-constrained -- especially if this is accompanied by a degradation in UI responsiveness.  Since spring this combination of add/change a parameter or constraint ==> solver throws questionable errors+UI slows down ==> restart required has been happening more often for me, and has begun to occur more frequently with parameter changes to solids, not just sketches. I quit, restart, and the same elements solve fine when reopened. 

I have tried to trace this error back to some combination of atomically identifiable/repeatable actions on my part, but no love as of yet. Frustrating.

[g-andresen]

Hi,

If you would share a sample file, there would be a chance to analyse the causes of the "misbehaviour".

günther