how to keep a mirrored fully defined sketch symetrical ? what am i missing ?

[rumpelstielz]

trying to create an object of which i can control the shape via splinepoints defined by user parameters.

so i start by defining some points until everything is fully defined, i can now control overall width and height and the position of 3 points with user paramters :

and then draw a spline through the points:

.. still all looks black and fully constrained/defined so i mirror it, i made sure the spline has a tangent constraint to a horizontal line on the top and bottom:

.. all looks as intended, still all black and fully constrained/defined. BUT when i adjust the user parameters the symetry is lost:

I was kind of expecting the mirror to be .. well, a mirror .. shouldn't everything after the mirror be the same ... not sure why things go out of shape, what did i forget to constrain ?

[TheCADWhisperer]

Can you File>Export and the Attach the *.f3d file here?

I recommend that rather than mirror the spline - you mirror only the points and then create one closed spline connecting the points.

@rumpelstielz

Try dragging the Underconstrained Construction line in the attached file.

What to you observe?

As a general rule I (almost) never mirror a spline in any CAD software.

[TrippyLighting]

If you don't edit the original spline after mirroring, both the original and mirrored spline will be symmetric.

However if you edit the original spline, even if you applied all the necessary constraints will loose its symmetry.

I've cone across this before and the reason for that have been explained to me by @jeff_strater and IIRC it has to do with the fact that the scale of the tangent handles is calculated independently in both, the mirrored and the original spline. I believe this is a limitation in how splines are implemented in Fusion 360's sketch engine.

[rumpelstielz]

@TheCADWhisperertheres other things in the file so i can't share. but mirroring the points does work so many thx for that 🙂

here's a quick example i did which exhibits the same problem:

sketch is fully defined and i think i have constrained everything as i want it to behave when i change parameters, if you play around with the user paramters you'll see how it goes out of symetry.

http://a360.co/2Fn21Qo

[rumpelstielz]

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@TrippyLighting wrote:

If you don't edit the original spline after mirroring, both the original and mirrored spline will be symmetric.

However if you edit the original spline, even if you applied all the necessary constraints will loose its symmetry.

 

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that doesn't seem to make sense, in a way i'm changing the spline BEFORE i mirror it since the spline is generated with user paramters as coordinates. that's why i think it's odd that the splines are not the same once you change the inputs.

[TrippyLighting]

Sketches don't have a timeline, so it it irrelevant as to when you mirrored a sketch element.

By the same token user parameters are timeless. 

Yes, I would also prefer for that to just work, but it just does not. If you can, it is almost always better to mirror a solid or surface instead of a spline. That, however, does not work in every situation.

[chrisplyler]

Instead of mirroring the spline in the sketch, just make make sure that the HALF of the profile you desire is enclosed and Stop Sketch.

Create the Extrude or other feature you are after with the profile.

Then Mirror the feature or the resulting body.

In this way, only the single spline is calculated/recalculated, and the mirrored body always maintains symmetry with the generated body.

[chrisplyler]

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@TrippyLighting wrote:

If you don't edit the original spline after mirroring, both the original and mirrored spline will be symmetric.

However if you edit the original spline, even if you applied all the necessary constraints will loose its symmetry.

I've cone across this before and the reason for that have been explained to me by @jeff.strater and IIRC it has to do with the fact that the scale of the tangent handles is calculated independently in both, the mirrored and the original spline. I believe this is a limitation in how splines are implemented in Fusion 360's sketch engine.

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I don't think it's a limitation. I've done an experiment as follows:

If you just select the spline curve and mirror it in the sketch, it gets its own points having tangent control handles that are independent of the original, as you state. But if you select the spline curve AND manually select each tangent control handle also (so that both the curve and the tangent handles are all selected blue) and THEN mirror them all, the new tangent handles get symmetry constraints back to the original handles as well, and the whole thing works as you would expect.

Notice how the left half (result of mirror) has symmetry constraints on the endpoints of the tangent handles and not just the points themselves.

[TrippyLighting]

I've experimented with this extensively, even excessively and it does not work consistently. At some point in time after some editing the 2 sides will not be symmetric, by whatever means, unless, of course this has been somehow addressed in the code in the meantime.

Jeff's answer in this thread explains clearly why this does not work!

There's another thread where this is explained again by Jeff.

[chrisplyler]

Maybe so...

But after playing with the sketch I used in the Screencast for another twenty minutes, I am unable to get any failure. I can move a point to the opposite side of the mirror line, and the mirrored spline still works. I can twist a tangency handle 180 degrees to make a loop in the spline, and the mirrored spline still works. I've done everything I can think of, and it doesn't fail.

[TrippyLighting]

@chrisplyler I have to quote myself here "...unless, of course this has been somehow addressed in the code in the meantime."

That seems to be exactly what has happened. I tried several things that did not work in the past - e.g. using the mirror constraint on splines usually yielded disastrous results - and it seems to work quite fine now!

Thanks for insisting!

A lot of work has been done on the sketch engine since that thread and it seem we are seeing tangible results. Thanks @promm!