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Arc causing sketch to be overdefined - how?!?

tom_augertom_auger Member Posts: 92 ✭✭
Hi! I'm a bit stumped on this and hoping someone with an expert eye can spot the mistake. I'm constructing a sketch using a number of reference circles to drive the dimensions of my profile. All works out great until I add an arc and constrain its endpoints to opposite sides of my construction circles. With the arc in place, the sketch is solvable. The minute I do anything else to the sketch - move the "centerpoint" of the arc for instance, or add another constraint, the sketch can not be solved. 

In the first image you can see I've just drawn the arc and its endpoints are snapped to opposite sides of my construction circles. I believe it's actually snapped to the endpoints of the lines that are tangential to the circles and also vertically constrained to their centerpoints.




Now, here's a second screenshot where I've just adjusted the centerpoint of the arc slightly, and BAM overdefined.



Note that I have added no further constraints here.

Also note that if I delete the arc, the sketch can once again be solved. If I use a Circle instead of the arc, no problems at all. Unless I Trim the circle (turning it back into an arc) and the sketch can't be solved again.

https://cad.onshape.com/documents/2dd5eab6ad81749b5e236dff/w/374fb7f54571a1292c9618c9/e/bffabec7a9892f93e183e1aa if anyone cares to take a look at the doc.

Thank you so much in advance for your EYES!

Best Answers

Answers

  • NeilCookeNeilCooke Moderator, Onshape Employees Posts: 3,263
    It's because the center of the large arc has a vertical constraint with the origin - that centers the arc and therefore the rectangle which contradicts the dimensions on the circles in red. Delete the constraint.
  • tom_augertom_auger Member Posts: 92 ✭✭
    NeilCooke said:
    It's because the center of the large arc has a vertical constraint with the origin - that centers the arc and therefore the rectangle which contradicts the dimensions on the circles in red. Delete the constraint.
    Hi Neil, thanks for the quick answer!

    That indeed temporarily solves the issue, allowing the sketch to be solved but ultimately doesn't solve the problem for me which is that I NEED the center of the arc to be constrained to the center of the circle, as you can see in the following sketch which represents the final shape I'm trying to achieve:



    In the spirit of teaching me to fish, is there a reliable process you use to "debug" these kinds of conflicts (other than just sequentially deleting constraints and UNDOing until you find the one that's causing the conflict)?
  • romeograhamromeograham Member Posts: 435 PRO
    I think that @NeilCooke
    was getting at the external constraint (external constraints are blue) between the center of your arc (and the large construction circle) and the Origin of the Part Studio. It's likely the Blue Vertical constraint that's causing the trouble. You can see (when I hover over the blue vertical constraint) that it is in fact constrained to the origin:


    For me, when I have these "unsolvable" sketches, I eliminate the external constraints first as I debug. Usually there aren't too many of them, so it's a quick way to get started.

    Good luck!
  • steve_shubinsteve_shubin Member Posts: 350 ✭✭✭
    @tom_auger

    suppress inferencing before adjusting the arcs diameter




  • MBartlett21MBartlett21 Member, OS Professional, Developers Posts: 1,887 PRO
    If you delete one of the 1.5mm dimensions, it should work then.
    MB - I make FeatureScripts: View FeatureScripts
  • tom_augertom_auger Member Posts: 92 ✭✭
    I think that @NeilCooke
    was getting at the external constraint (external constraints are blue) between the center of your arc (and the large construction circle) and the Origin of the Part Studio. It's likely the Blue Vertical constraint that's causing the trouble. You can see (when I hover over the blue vertical constraint) that it is in fact constrained to the origin:


    Thanks Romeo for your response! But I WANT the whole thing constrained to that origin point. It might not appear important in this example, but the rest of the part is constrained to this origin across a number of different sketches and parts. I almost always constrain something in every one of my sketches to the Origin - is this a bad practice?
  • tom_augertom_auger Member Posts: 92 ✭✭
    If you delete one of the 1.5mm dimensions, it should work then.\
    Thanks for your suggestion!

    But those circles need to be that dimension as that measurement is derived from a real-world measurement. Does that make sense?
  • tom_augertom_auger Member Posts: 92 ✭✭
    edited August 26
    @tom_auger

    suppress inferencing before adjusting the arcs diameter

    Hi Steve thanks so much for answering and for introducing me to the fact you can turn off automatic constraint inferencing when needed! I'm sure that will come in very handy some time.

    The problem I'm having here is that I NEED that there to be a constraint - I want this arc to share a center point with the construction circle's center point, so it would need the constraint that seems to be causing all the trouble.

    I guess I'm trying to understand what is making this sketch unsolvable and why a contraint on the centerpoint of that arc is causing the overdefined state. I'm not seeing it, maybe you are?
  • tom_augertom_auger Member Posts: 92 ✭✭
    Thank you to everyone who responded. By combining a number of answers here, all technically correct, I have come to a better understanding of the issue. I think I need to get better at identifying potential situations where a sketch can become over-defined, and the automatic inferencing (which I love usually) may have made it a little harder to notice, so thanks for every comment – each one took a slightly different angle on the problem and I think I now have a better sense of where to look, not just in this case but in more general cases in the future.

    Grateful to have such a helpful and active community!
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