Mirroring sketch element causes element to become unconstrained?

james_aguilar160

https://cad.onshape.com/documents/b94f30bc54eecdcf0dcf8cd7/v/7cac3175a98a97c7647ad0f0/e/b578e14a3af7ac1b5415a883?renderMode=0&uiState=668aeb821bca2341b2ef2642



In this sketch, if I mirror the elliptical arc across the centerline of the left region, the original arc (and the mirrored one) become unconstrained. This doesn't match my understanding of sketches -- IIUC, adding a sketch element should never cause an already fully-constrained element to lose its constraints. Would someone please help correct my understanding?

glen_dewsbury

@james_aguilar160
I think it's the sequence used to generate the sketch. I drew the horizontal line and arc then mirrored about the horizontal plane. The vertical line are just join the dots.
Looks like your mirror center line is floating when it needs to be horizontal to the mid point of 22.5 vert line.
https://cad.onshape.com/documents/fa40a8e0adf37af8a2dd7498/w/91a671687060dc5bdeb87877/e/1b4e262915d554d744bfd005

james_aguilar160

In case it's relevant, here's the final sketch. I can't see hat these curves actually have any degrees of freedom. I have tried moving them around and nothing happens.

james_aguilar160

@glen_dewsbury

Thank you for your kind response. However, I'm not following. First off, the centerline I'm using for the mirror is not floating. It's black, as is its endpoint, which means it's fully constrained to my understanding. Secondly, even if it were floating, mirroring across it should only mean that the new mirrored sketch elements are unconstrained. The fully constrained inputs should still be fully constrained. They have no degrees of freedom and cannot possibly gain more with the addition of more sketch elements.

glen_dewsbury

Note that I have set the mid point of 8.5mm line horizontal to the origin. That constrains the the depth of the arc. That's the final constraint needed. When a dimension is added to set depth of arc it is driven(4.65 is grey not black and is ref only). 

james_aguilar160

@glen_dewsbury Once again, thanks for your kind and helpful responses. I can see that mirroring across the construction line as you've set it up does in fact fully constrain the sketch according to OnShape. What I don't understand is why. The depth of the arc (before mirroring) is already constrained by it being tangent to the construction lines that originate from the bottom and right portions of the arc. I don't understand why or how mirroring the arc unconstrains it.

wayne_sauder

@james_aguilar160

 I would guess it was a system glitch; I tried to reproduce the issue but could not, even after playing with several different drawing methods.

 I had played with it earlier and noticed that if a coincident constraint was added between the line and the arc's center point, it fully constrained everything. But that made no sense.  

S1mon

That's a quadrant of an ellipse, or an elliptical arc. Getting them fully constrained can be tricky, or at least more complex than a regular arc. The center, the two end points need to be constrained, and there needs to be one more constraint on the curve. When you sketch, this is typically a horizontal or vertical constraint which is automatically added. If you done some editing, that constraint might have gotten accidentally deleted. It's also possible that a tangent constraint to the horizontal or vertical centerline was automatically added, and later accidentally removed.

It's really important to Show constraints when trying to debug issues like this.

glen_dewsbury

james_aguilar160 said:

@glen_dewsbury Once again, thanks for your kind and helpful responses. I can see that mirroring across the construction line as you've set it up does in fact fully constrain the sketch according to OnShape. What I don't understand is why. The depth of the arc (before mirroring) is already constrained by it being tangent to the construction lines that originate from the bottom and right portions of the arc. I don't understand why or how mirroring the arc unconstrains it.

Looking at Your sketch, there s nothing to suggest the 8.5mm line is constrained vertically. Try adding a vert dimension to it.

glen_dewsbury

Hey @james_aguilar160
Can you make your document public and post a link to it? I have a couple thoughts as to what might be happening.
Much easier to resolve when looking at actual than just pics.

james_aguilar160

glen_dewsbury said:

james_aguilar160 said:

@glen_dewsbury Once again, thanks for your kind and helpful responses. I can see that mirroring across the construction line as you've set it up does in fact fully constrain the sketch according to OnShape. What I don't understand is why. The depth of the arc (before mirroring) is already constrained by it being tangent to the construction lines that originate from the bottom and right portions of the arc. I don't understand why or how mirroring the arc unconstrains it.

Looking at Your sketch, there s nothing to suggest the 8.5mm line is constrained vertically. Try adding a vert dimension to it.

This one?



@S1mon That is great advice to show constraints. See above. Interestingly enough, simply deleting the symmetrical constraint on the upper elliptical arc causes the upper arc to begin rendering as fully constrained. Adding it back again causes the arc to become unconstrained. This is a logical impossibility. I cannot but decide that this is a bug.

Thank you once again to all who have helped me with this. I have reported this as a bug to the OnShape dev team. A minor hiccough in the otherwise uniformly pleasant use of this wonderful tool.

james_aguilar160

@glen_dewsbury It is publicly shared in the first post at the very top. Here is the link again. (I am a free/hobby user so all my docs should be public.)

https://cad.onshape.com/documents/b94f30bc54eecdcf0dcf8cd7/w/892416c9e608e60ff904f71d/e/b578e14a3af7ac1b5415a883?renderMode=0&uiState=668cbd5f1bca2341b2f54ec7

glen_dewsbury

Did a quick scan of constraints and found a misalignment between these highlighted points. I put a vertical constraint between them and the whole sketch went black. The radius of the elliptical arc was not constrained.

james_aguilar160

@glen_dewsbury, thanks, I agree that marking those two points as vertical makes the sketch go black. However, it should not be necessary. The curvature of both endpoints of the arc are defined by tangent relationships with the horizontal and vertical construction lines. This implies the center of the arc. Also, it does not explain how the arc was constrained before the mirror and not after.

glen_dewsbury

When the center is not aligned correctly the curve will be skewed. When the center of ellipse is not fixed then neither is the radius fixed. If you want a dimension can be added to the implied center and that will leave a rise or dip in the ends of the arc. Don't think that's what you re looking for.
Where the curve passes the axis (or when projected to) it will be normal to the axis of the construction ellipse used to generate the curve.
Here's a sample pushed to an extreme.



You can use tangent constraints from construction lines, but it' not really needed. Extra steps.

james_aguilar160

@glen_dewsbury I understand that if you adjust the center point of the ellipse, the curvature would change. That is only possible when the center is not fully constrained though. In my sketch, it in fact is (by the tangent constraints on the bottom and right side of the ellipse). This can easily be shown by looking at what happens when you try to adjust the distance between the vertices the way you did in your first screenshot.



Attempting to dimension the distance between those top left vertices results in a conflict, which is exactly what you would expect since, although onshape is incorrectly rendering the ellipse as not-fully-constrained, it in fact is.

james_aguilar160

OnShape just got back to me acknowledging my report as a bug. The mystery is solved.