CAD Forum

Hi,

I wonder what the differences are between concentric and coincident (applied to centers of circles/arcs) constraint in sketches. Both in terms of applications and the way they work. When it comes to applications, I'm looking for cases when the concentric constraint is superior to coincident. One example that comes to my mind is when arcs have large radii and it's necessary to zoom out and look for their centers to apply the coincident constraint. I can't think of any other concrete examples. When it comes to the way they work, it seems that in practice the only difference is that concentric constraint is applied directly to circles/arcs while the coincident constraint is applied to their centers. However, I don't know how it works internally and if it's not the case that concentric constraint merges the centers as well. The point is to justify the existence of a separate concentric constraint when a more universal coincident one is available. Can you think of any other examples ? When do you use concentric constraints in sketches ?

Thank you in advance for your help.

Well, some people buy six eggs and some buy a half-dozen.

Concentric is very handy for circular features. It is typically far easier to pick a cylindrical surface than it is to grab its axis. Concentric constraints work with cylindrical, conical, and spherical surfaces as well as sketch elements of circles and arcs. A concentric mate in an assembly restrains multiple degrees of freedom. Concentric mates between the barrel of a screw and the wall of a hole not only make them concentric, but they also align their axes, whereas if you made the centers or the circles of those two features coincident their axes would not be aligned.

Coincident has its place as well. As you state, coincident mates can be used for circle/arc centers in a sketch and with axes of 3D feature. But I use coincident for many more things, such as a circle center to a line or a vertex in a sketch.

Really I don't know what your question is. They both work in a multitude of ways. One is not necessarily better than the other. There are many times when a person could use either to accomplish their intent; it just depends on the situation. Be glad you have multiple ways to accomplish your goals.

Thank you for the reply.

DennisD wrote: ↑Fri Sep 30, 2022 10:32 am Really I don't know what your question is.

I guess that I should explain what the source of this question is. I'm helping with the development of open-source CAD software. It already features a coincident constraint tool in the sketcher but concentric constraints are missing. So I suggested adding them and I'm looking for arguments explaining why they are worth adding when coincidence is available.

Besides, I was just wondering when concentricity becomes useful in sketches. I'm a FEA engineer and I don't spend that much time in CAD software so I wanted to know what more experienced people think about the usefulness of this constraint.

Concentric of Circle or Arc means they have same center. Or coincident of center point.

How would you define "Coincident" Circle or Arc?
If two lines are coincident, they're collinear.
So "Coincident" Circle or Arc should have same center and radius.

You can consider Concentric as Parallel to lines.

Coincident is a jack of all trades relationship type that works to make any point type on any element type coincident with any other point/element.

Concentric is a dedicated relationship type only working with circles and arcs requiring the axis' to be both parallel and coincident (or "colinear" if you like).

So you could use Coincident to make the center points of two circles coincident but they would not be concentric if they did not have parallel axis' in the case of a 3D sketch. Using the Concentric relationship would force both of their axis' to be parallel and coincident.

All I'll add since @KennyG, @Frederick_Law, and @DennisD all gave great answers is....between the two relations, I always use concentric for arcs and circles because it conveys more readily at a glance what's going on in the sketch. Especially when things get real busy in complicated sketches.

Same applies to mates. I could use coincident or a 0 value distance mate. But then looking through dozens of mates to figure things becomes harder if all coincident conditions are distance mates.

FEAnalyst wrote: ↑Fri Sep 30, 2022 12:28 pm Thank you for the reply.



I guess that I should explain what the source of this question is. I'm helping with the development of open-source CAD software. It already features a coincident constraint tool in the sketcher but concentric constraints are missing. So I suggested adding them and I'm looking for arguments explaining why they are worth adding when coincidence is available.

Besides, I was just wondering when concentricity becomes useful in sketches. I'm a FEA engineer and I don't spend that much time in CAD software so I wanted to know what more experienced people think about the usefulness of this constraint.

Just my opinions:

Coincidence is the catch all constraint but should only be used when other, more descriptive, constraints will not work. Design intent should be inherent in the sketch/model whenever possible.

Coincident behavior should be (again, just my opinion):
> point to point: obvious
> point to line/arc/curve/axis: the point must be on the curve and can move along it.
> point to surface: point is constrained to the bound surface.
> line/axis to line/axis: same as co-linear (should use co-linear instead, unless that isn't available)
> line/axis to arc/curve: line is tangent to arc/curve, does not constrain where on the curve (should use tangent instead)
> line/axis to surface: planer, the line must be on surface; non-planar the line is tangent to surface but where is no defined.
> arc to arc: same as co-radial.
> surface to surface: planar they are co-planar, planar or non-planar to non-planar they are tangent at some point.

Concentric is a fun one and seems to be perceived differently in mathematics and modeling and architecture. My thought is that concentric is making the axis (implicit or not) of the two objects co-linear. For the sake of flipping mates/relationships I'd like to take it a step further and define the axis as a "Ray" not a segment or line for some objects. What kinds of objects?
> arc/circle/2D spiral: the axis is a ray normal to plane.
> conic section: axis is ray extending out the open end of the cone and terminating at the convergence point of the cone.
> helix/cylinder/torus: axis is ray extending from the plane the extruded sketch is on or extending in the direction according to right hand rule given the direction of revolution.

if the object does not have an implicit axis, then the concentric relation should not be available.

I'm accused of over thinking things though...

When dealing with 2D sketches, they are very similar (but not quite the same).

In a 3D sketch, though, they are very different. Especially with the case of 2 2D circles in a 3D sketch (a sketch where you are not bound by a sketch plane), concentric is not the same as coincident.

Coincident between the 2 center points will still allow for planar rotation of the 2 circles. Whereas making them concentric controls both position of the center point AND rotation of the circle itself.

FEAnalyst wrote: ↑Fri Sep 30, 2022 12:28 pm I'm helping with the development of open-source CAD software.

Make sure it can do multithread processing.
Most, if not all CAD now are suck in single thread world.

Thank you for all the replies, they helped me convinced others that coincidence can be helpful.

Frederick_Law wrote: ↑Tue Oct 04, 2022 5:20 pm Make sure it can do multithread processing.
Most, if not all CAD now are suck in single thread world.

It's FreeCAD, it supports multithreading partially.

FEAnalyst wrote: ↑Wed Oct 05, 2022 3:28 am It's FreeCAD, it supports multithreading partially.

Most multithread partially.
On UI and higher level calculation.
I think it's the "kernel" where it calculate all the 2D 3D that need multithreading.
Might need a mathematician.

FEAnalyst wrote: ↑Wed Oct 05, 2022 3:28 am Thank you for all the replies, they helped me convinced others that coincidence can be helpful.



It's FreeCAD, it supports multithreading partially.

I kinda like FreeCAD. Looking back at what we've spent we could have hired a couple of programmers to work on a fork to implement what we need and accomplished the file management we needed. Bonus if the community would have merged the bits that they wanted back into master.

bnemec wrote: ↑Wed Oct 05, 2022 10:21 am I kinda like FreeCAD. Looking back at what we've spent we could have hired a couple of programmers to work on a fork to implement what we need and accomplished the file management we needed. Bonus if the community would have merged the bits that they wanted back into master.

Not to mention that successfully deploying a free, open-source software in a multi-user production environment would be a major notch in your belt. It would be so big you might not have much belt left...

Something else to consider regarding more functionality (maybe even outside of circles):

Coincident of the center might be important for patterns as a reference - pattern the circle/arc around the point, rotate
Concentric of the arc/circle might be useful for profile-alignments (align closed profiles to each other)

mike miller wrote: ↑Wed Oct 05, 2022 2:03 pm Not to mention that successfully deploying a free, open-source software in a multi-user production environment would be a major notch in your belt. It would be so big you might not have much belt left...

LOL. I'm not sure if that sarcasm or not. I don't consider free software == $0 cost. It's free to take the codebase and do almost as you like with it. For example, I think that open source is better suited for extensions. Extensions are optional functionality so they're not forced on the entire user base. All the discussions on here about a feature implemented for one group that couldn't live without it, how it trashed the program for another group that now hate the new functionality because of the unexpected (untested use case) side effect.

As for the notch in my belt, there's only one but it's been stressed so badly it has become a slot. Point is it wouldn't be me that implemented anything, the trick would be finding and hiring the right people that could actually pull it off.

We're way off topic...

I think others have covered it pretty amazingly, one of the reasons why I luv the SW community.

One thing to also help define...
- A point cannot be concentric and an arc/circle cannot be coincident.
- Arc/Circles can be more than just concentric (coradial or tangent), and point can only be coincident or Pierced (which is a much more "powerful" constraint), and Horizontal/Vertical.

One thing to really consider is much more beyond just the 2D/3D Sketching and how also to consider how these exact same relationships translate to working in the Assembly when it comes to adding Mates. It's the same language which helps to keep things a bit more universal when it comes to adding these constraint types through the software. This may seem like a silly thing but it's one of the reasons that helps to make the software "easier" to use.

Thank you guys for your help. FreeCAD now features concentric constraint. Not as a separate constraint type but they added the possibility to select circles, arcs and ellipses when applying coincident constraint. FreeCAD's sketcher supports only 2D sketches so this solution should be sufficient for now.