[Feature Request] Concentric constraint in the Sketcher [SOLVED]

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NewJoker
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[Feature Request] Concentric constraint in the Sketcher [SOLVED]

Post by NewJoker »

Hi,

commercial CAD software typically features concentric constraint for sketches. It's similar to coincident constraint applied to circle/arc centers but has some advantages over it:
- it can be useful when arcs with large radii are involved and it's necessary to zoom out and look for their centers
- coincident constraint applied to circle/arc centers causes overconstraint when these centers lie on one of the sketcher's axes and then it's necessary to use vertical or horizontal constraint (depending on the axis) - concentric constraint would be more versatile
- it can be used when manually offsetting (due to the lack of offset tool) arcs, for example, to create curved slots discussed here: https://forum.freecadweb.org/viewtopic.php?f=8&t=72235

It's possible that I missed some potential use cases. I will be glad for mentioning them here.

What do you think about the implementation of such a constraint ? Do you agree that it could be useful ?
Last edited by NewJoker on Sat Nov 05, 2022 5:32 pm, edited 1 time in total.
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obelisk79
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Re: [Feature Request] Concentric constraint in the Sketcher

Post by obelisk79 »

I was recently doing some sketcher comparisons between FreeCAD and commercial software and I couldn't find any functional differences between coincident (or equivalent) and concentric. What would be the theoretical logic between the two constraints that differentiates them from a behavioral perspective?
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NewJoker
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Re: [Feature Request] Concentric constraint in the Sketcher

Post by NewJoker »

@obelisk79 To be honest, I'm a CAE guy and I don't use commercial CAD software too often. I will ask on their fora, maybe someone will provide a more detailed answer regarding the differences between those two types of constraints. From the user's perspective, it's just a matter of what you select (centers vs arcs/circles themselves) but I don't know how the concentric constraint works internally. However, it must be quite useful if it's included in most commercial programs.

Btw. can you share your observations regarding the differences that you've noticed when doing that comparison (maybe in a different thread) ? It might be interesting for future requests.
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obelisk79
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Re: [Feature Request] Concentric constraint in the Sketcher

Post by obelisk79 »

NewJoker wrote: Fri Sep 30, 2022 11:39 am Btw. can you share your observations regarding the differences that you've noticed when doing that comparison (maybe in a different thread) ? It might be interesting for future requests.
I was specifically comparing in relation to the coincident vs point on object constraints, but I had also noted that there was concentric and did some playing around with it as well. Come to think of it, I believe you are correct in the fact that concentric allows the selection of *either* the centerpoints or the arc/circle edge to create the constraint. Effectively though, both approaches make the centerpoints coincident. Perhaps the logic for arc/circle centerpoint based on edge selection could be incorporated directly into coincident instead. That functionality might be too obfuscated like that though.
adrianinsaval
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Re: [Feature Request] Concentric constraint in the Sketcher

Post by adrianinsaval »

Ideally when using coincident constraint and selecting two circles or a circle and vertex coincidence should be applied to the center point
chrisb
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Re: [Feature Request] Concentric constraint in the Sketcher

Post by chrisb »

NewJoker wrote: Fri Sep 30, 2022 11:23 am - coincident constraint applied to circle/arc centers causes overconstraint when these centers lie on one of the sketcher's axes and then it's necessary to use vertical or horizontal constraint (depending on the axis) - concentric constraint would be more versatile
I disagree. In my lecture linked below I describe the relationship between the constraints and the degrees of freedom (DOF). A concentric constraint would consume 2 DOF and thus cannot be equivalent to a vertical or horizontal constraint.

I think I agree with obelisk79, that coincidence of centers and concentric have the same semantics. Which is also what the translation from latin would say: they both have the same center.

The issue described is a different one, which is caused by the selection scheme and thus I agree that a concentric tool could be helpful. I have the feeling that it seems rather cumbersome, but the meaning of selecting two arcs/circles could be to make the centers coincident.
A Sketcher Lecture with in-depth information is available in English, auf Deutsch, en français, en español.
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NewJoker
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Re: [Feature Request] Concentric constraint in the Sketcher

Post by NewJoker »

One more important use case - when there are multiple arcs in a sketch, it might be hard to identify their centers.

Also, the difference between coincident and concentric constraints may become more visible in the case of 3D sketches but those are not available in FreeCAD's sketcher.
user1234
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Re: [Feature Request] Concentric constraint in the Sketcher

Post by user1234 »

I would more generalize the request, constraint line parallel (offset) curves. But this was already requested.

Greetings
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chrisb
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Re: [Feature Request] Concentric constraint in the Sketcher

Post by chrisb »

NewJoker wrote: Sat Oct 01, 2022 12:53 pm One more important use case - when there are multiple arcs in a sketch, it might be hard to identify their centers.
Good example!
Also, the difference between coincident and concentric constraints may become more visible in the case of 3D sketches but those are not available in FreeCAD's sketcher.
I still don't see any difference, albeit in 3D.
A Sketcher Lecture with in-depth information is available in English, auf Deutsch, en français, en español.
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NewJoker
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Re: [Feature Request] Concentric constraint in the Sketcher

Post by NewJoker »

chrisb wrote: Sat Oct 01, 2022 4:12 pm I still don't see any difference, albeit in 3D.
From what I've heard, concentric constraint in commercial CAD software makes the axes of arcs and circles colinear (coincident and parallel). In 3D coincident constraint just makes the centers coincident but doesn't make the axes parallel.
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