Symmetric constraint in Sketch mode

[Anonymous]

In sketch, if I have a line and want two other lines to be symmetric about it, I can use the symmetric constraint.

What if I have two lines and I want to constrain a third line symmetric to them?  Can I?  I keep thinking it is a work flow thing, but I cannot figure it out.

That comes up more in what I am working on - a lot of reverse engineering that requires me to find and build features off center lines that do not yet exist.

Thanks.

[mcgyvr]

The Symmetric constraint causes selected lines or curves to become proportionately constrained about a selected line.

So yes... 2 lines and a line in the middle.. select the first line (line on one side of the symmetric centerline), then the second (on the other side of the symmetric line) then finally select the "middle line" now the 2 outer lines are symmetric to the center line..

Its as simple as starting a sketch and trying for yourself to answer your own question.. 

and here is the help page on sketch constraints..

http://help.autodesk.com/view/INVNTOR/2018/ENU/?guid=GUID-A85A7A30-7D81-4C75-8769-CAD034EEA930

[TheCADWhisperer]

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@Anonymous wrote:

.... but I cannot figure it out...

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Can you attach an example file here - I feel like I must be missing something in the problem description that is unique to your file.

[Anonymous]

@mcgyvr were you able to understand what I wanted?  I know what the symmetric constraint does, but I want more.

The help does not show what I need to do. Looking at the help reference, I have lines 1 and 2 fixed and I want to create line 3 and constrain to be symmetric to lines 1 and 2.

I attached a part with a sketch.  The two fixed lines should not be moved and I want the un-constrained line to be symmetric to them.  I can construct geometry to make that happen, but wondered if I could constrain it.

[mcgyvr]

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@Anonymous wrote:

@mcgyvr were you able to understand what I wanted?  I know what the symmetric constraint does, but I want more.

The help does not show what I need to do. Looking at the help reference, I have lines 1 and 2 fixed and I want to create line 3 and constrain to be symmetric to lines 1 and 2.

I attached a part with a sketch.  The two fixed lines should not be moved and I want the un-constrained line to be symmetric to them.  I can construct geometry to make that happen, but wondered if I could constrain it.

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@Anonymous 

Your first sentence is entirely possible as thats the definition of symmetric..

Then your "what if" second sentence is just not possible..

How can a single line be symmetric to 2 lines.. That doesn't make any sense..  and does not meet the definition of symmetric.. But you can as you said add more geometry (a single line) and make them all symmetric to that..

[SBix26]

Seems pretty clear to me: you want to use the Symmetric constraint to constrain a bisector between two fixed lines.  I'd like it to work that way, too, but it doesn't.

It's easy to try, just project two part edges, or even two origin axes, create a line in between them and try to use the Symmetric constraint to pick the two projected lines and then the floating one— Inventor complains about constraining reference geometry.  But if you only use one projected line it works, even if the other line is fixed to projected geometry.

I'm guessing that removing that limitation wouldn't be too difficult.  Somebody want to create an Idea for improvement?

Sam B

Inventor Professional 2017.4
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[johnsonshiue · Accepted solution]

Hi Ronald,

Indeed, there isn't a constraint allowing you to find or constrain the bisector. It still can be done with the help from a circle and a point. You can create a circle (make it construction line), which is tangent to both lines on the side. then the bisector should pass the circle center and the intersection point of the two lines.

Many thanks!

[Anonymous]

See @johnsonshiue for a great description of how a line can be symmetric to two lines.  Or look at the help, in the end, line 3 is symmetric to lines 1 and 2.  Or are lines 1 and 2 symmetric about line 3?  Apparently that distinction is the difference between what Inventor can do and what SW can do.

[jtylerbc]

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@Anonymous wrote:

Or are lines 1 and 2 symmetric about line 3?

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This would be the correct way to state it.  Line 3 can't be symmetric to Lines 1 or 2 without first introducing a Line 4 to be the new symmetry line.

The issue, I think, is that Inventor's Symmetry constraint solves one-directionally.  By that, I mean that it can locate Line 1 and Line 2 if Line 3 is known.  But it can't solve the reverse situation (Line 1 and Line 2 fixed, find Line 3).  It might be possible in some cases to trick it into doing so (as @SBix26 indicated), but in general it doesn't want to do it.  This leaves you with construction geometry techniques like what @johnsonshiue described.

Similar limitations also apply in the 3D realm of the assembly Symmetry constraint.  You can use it to solve the location of two parts about a midplane, but you can't get it to move a plane to be centered between the two parts.  The last time I tried it, at least.

Would be nice if it could solve both cases, but unfortunately that's not currently possible.

[Anonymous]

I created an idea to increase the functionality of symmetry constraint to work in either direction.  Please upvote it!

https://forums.autodesk.com/t5/inventor-ideas/update-the-symmetric-constraint-in-a-part-sketch-to-co...