Cannot create tangent constraint between offset ellipse and circle

[david.antliff]

I am looking to create a circle between an ellipse and another circle, such that this middle circle is tangential to both the ellipse and the circle. Normally, if I just create a simple circle and a simple ellipse, this works fine. And if I create a simple offset ellipse, I can also add the tangential constraint to the new ellipse also.

However in my drawing something has happened to my offset ellipse to prevent it from being used as a tangential constraint target - can anyone help me understand what might have happened to get it in this state? Note that I have trimmed the offset ellipse and in my experiments with simple shapes I was sometimes able to reproduce this issue, but I can't work out a reliable way to reproduce it.

Note that I am able to select the original elliptical arc (at the top left of the picture) as a tangent target, but it's the blue offset one I really want to apply the constraint to.

I've shared the design here: https://a360.co/2KRWvLG (v4, I've moved on since then).

Edit: I have no idea why that shared URL doesn't show anything.

Edit2: I was able to use a fillet to create the effect I was after, but the question remains as to what had happened to the offset elliptical arc to prevent it being used with a tangent constraint whereas the original arc is usable.

[etfrench]

Offsetting an ellipse doesn't create a new ellipse. @jeff_strater: Is this a bug? 

One of the outer lines is an ellipse, the other an offset of the inner ellipse.

@david.antliff: Can you start with the outer ellipse then offset in, or better just use two ellipses?

[david.antliff]

Actually, minor thing, I'm using the inner ellipse as the original and creating the outer as an offset.

I'd imagine that it's not a bug - offsetting the intersection of an ellipse by a constant amount on both the minor and major axis might not necessarily result in a curve that is also an ellipse. What puzzles me is that sometimes the result can be used as a tangent constraint target, other times it cannot, and that's what I can't explain.

I've moved on with my design for now, but it was quite easy to reproduce this issue in a fresh sketch by simply creating a circle, and ellipse, offsetting the ellipse, then using the 2-point circle to create a circle in-between, then try to apply the tangent constraint. Sometimes it works, sometimes it doesn't. Also, try cutting the ellipses with lines at various angles to split it into separate arcs - that seemed to make a difference, but I couldn't find a pattern.

[jeff_strater]

@etfrench, I'm not sure of the mathematics involved, but I think a true offset of an ellipse (meaning: the perpendicular distance at any point is the same) is not an ellipse.  So, Fusion approximates the offset with some kind of spline curve.  This is why you cannot select the offset of an ellipse as a tangent target - Fusion does not support tangent to these offset spline curves.

[TheCADWhisperer]

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@etfrench wrote:

Offsetting an ellipse doesn't create a new ellipse. @jeff_strater: Is this a bug? 

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Are you selecting the Ellipse for offset near the Major or Minor axis, or are you selecting the curve away from the axis?

Screencast will be displayed here after you click Post.

13503e70-c008-4f94-9546-63c42fb889c8

[david.antliff]

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@jeff_strater wrote:

@etfrench, I'm not sure of the mathematics involved, but I think a true offset of an ellipse (meaning: the perpendicular distance at any point is the same) is not an ellipse.  So, Fusion approximates the offset with some kind of spline curve.  This is why you cannot select the offset of an ellipse as a tangent target - Fusion does not support tangent to these offset spline curves.

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For it to remain an ellipse I think every point would need to be shifted towards the midpoint of the focii by a proportional amount, rather than a fixed (offset) amount.

Anyway, what you say makes sense, but what puzzles me is that sometimes you can create a tangent with the offset ellipse. It's only once you start cutting up the ellipse with intersecting lines that it no longer becomes possible. So maybe the approximation is sometimes conical, sometimes spline? It's a shame that a tangent to a spline isn't supported, since mathematically it should be possible to solve this, although maybe high curvature cases create ambiguous solutions.

[david.antliff]

@TheCADWhisperer thanks for the screencast - that's really interesting. Do you know how "close" to the two axes you need to be to get an ellipse vs a spline? I hadn't realised this about offset ellipses and it probably would account for the difference in behaviour I was seeing in my experiments, as I wouldn't have been controlling for offset start position.

How are these differences explained? Is the offset distance measured along a line extending from the start point to the geometric centre? However wouldn't that give a certain offset that could then be applied to the axes as well? I'm not sure I understand why you get two different results - isn't an offset simply defined as a constant perpendicular distance between two curves? I suppose it matters which curve you are casting the perpendicular from. Very interesting.

[jeff_strater]

@david.antliff - I have not been able to actually create a tangent constraint to an offset ellipse, but I admit I have not tried all that many cases.  I can, however, select an offset ellipse if I choose it first, when the tangent command is active.  I think that this is because you are supposed to be able to create an end-to-end tangent relationship between a spline and a line, but in my testing, I was not able to get that to work.  I think that's a bug.

I agree, though, that this limitation of not creating "mid-curve" tangencies is an unfortunate limitation of the sketch solver.  One day I hope we are able to fix that.

Thanks for the replies and for bringing this up!

[chrisplyler]

@TheCADWhisperer

Whoa! I consider that a failure of the user interface. An Offset of an elipse will not be another elipse. Anyone who thinks it will be just hasn't really thought it through all the way.

I think this is a case in which the Offset Pallet ought to be smartened up to recognize that the source is an elipse, and then either ONLY do a true offset (since the user chose the offset tool after all) or at least offer up a toggle option to the user.

[TheCADWhisperer]

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@david.antliff wrote:

... How do you know how "close" to the two axes you need to be to get an ellipse vs a spline? 

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There is a subtle indication, but it is easy to miss.

The following video should demonstrate how to know whether your Offset will result in an Ellipse curve or a spline curve.

[TheCADWhisperer · Accepted solution]

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@chrisplyler wrote:

Whoa! I consider that a failure of the user interface. ... 

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I guess they should find a way to make this more obvious the untrained user.

I assume this is considered a positive feature by Autodesk to give you the choice.

I exaggerated the aspect ratio of the Ellipse in my demonstration, but it isn't nearly so obvious when the difference between the minor axis and the major axis is not so great.

For most designs I would suggest that the Shell feature might be a better choice than Offsetting sketch curves.

[chrisplyler]

@TheCADWhisperer

Having the "offset" be a new elipse should not be part of the Offset tool in my opinion. I mean, if you enter a specific distance, which the Offset tool requires, WHERE exactly should that distance be?

If the user wants another elipse, the user should use the Elipse tool again.

[david.antliff]

@TheCADWhisperer thank you for explaining this. What I'm struggling to get my head around though is why where are even two possibilities for Offset. I would have thought that an offset was a set of points a fixed distance along the normal of the tangent of another set of points - i.e. take any point on the original ellipse, cast a normal, and move X along that line to locate the new point. It looks to me like selecting near an axis just creates a new ellipse with the offset only applied to the two axes - which suggests that it isn't really an offset at all. I suppose it's nice to have the choice, and now thanks to you I understand how to get both. Thank you.