Reference lost in simple project to surface scenario

[tg]

In the attached example, a sketch point is projected onto a sphere (using project to surface along vector). A plane is then created tangent to face at point. If the original point is moved (eg, just a little, no radical change to geometry), the projected points update correctly, but the plane always loses reference to its point. This seems like a scenario where dependency naming/matching should be robust. Why does it fail?

To repeat, load up the example, observe that there are no warnings, then open params window, change "P" from 30mm to, say 31mm, observe that the plane tangent to surface at point has lost ref to its point.

[rohit.bapat]

Hello @tg 

Thank you for posting the issue. I can reproduce the issue from scratch as well. I have conveyed the issue to the relevant team. But, since it is weekend I will be able to provide further details tomorrow (Monday)

[tg]

Any news @rohit.bapat? Thanks

[TrippyLighting]

What "news" are you looking for ?

[rohit.bapat]

Hello @tg 

I conveyed the issue to the development team that works around this area. We found that it's a legacy issue FUS-36316.

[tg]

Hi @rohit.bapat - thanks for the update. What is a "legacy issue"? I've tried searching for FUS-36316 to find more detail, but google knows nothing. Is the issue on any roadmap for being addressed? Many thanks.

[TrippyLighting]

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@tg wrote:

. Is the issue on any roadmap for being addressed? 

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That would indeed nice, but I doubt it is. At least it isn't on any public road map. I also don't remember an Autodesk Expert Elite meeting with a roadmap of which bugs are going to get fixed either.

If you can share a higher level description ow what you are trying to model, perhaps we can find another workflow so you can progress with your design

[tg]

@TrippyLighting - thanks for the offer of help. I have a parametrically positioned point P; I have a curved face F; I actually need to perform a cylindrical cut at a parametric angle where the centre of the circle passes through the projection of point P onto face F (along a specific vector which happens to be parallel to one of the origin planes). Using a plane tangent to the face was a compromise, as the angle is acceptable but not parametrically controlled. I've tried several other approaches, including sketches (both 2d and 3d) and projection of the body of face F - but all have proven to be referentially unstable (lost refs when changing parameters). This time around I've read the detailed doc here and am working on the design again from scratch, keeping as much as possible to the principles outlined in the doc. I'd love to compute the position of point P as projected onto face F so that I stay entirely within sketch geometry, but face F is formed between two non-coplanar parametric arcs, and the maths is way beyond me.

[TrippyLighting]

The curved face in your example is a sphere. I don't see any defined angles in your design.
Following what you tried to achieve in the design you attached can be achieved very easily.
I am assuming this is more complicated but if you ask narrow questions, you get narrow answers.

Perhaps you simply share a design that include face F, and then we go from there.
Given that your CAD skills AND the software failing you, perhaps share a hand sketch with what you are trying to achieve.

[tg · Accepted solution]

I've looked at this further. The fundamental limitation appears to be that standalone sketch points (eg, explicitly created sketch points, not sketch points that are implicitly generated at each end of a line) are a weak source for referencing. In the attached example, the standalone point from the original example has been replaced with a sketch line; that line is projected to surface; and then the point at one end of the projected line is used as the point for the plane tangent to face at point. The reference is then robust, and the plane updates as parameter P is changed. Note that the line must be projected to surface: projecting just one point at the end of the line to surface produces a projected point which can't be referenced robustly.

@TrippyLighting  The original example was intentionally simplified to get to the core principle of the issue without muddying it with the actual much more complex design. Having successfully applied the approach described in this post to my design, when designing for a specific angle I encountered another gotcha which I worked around, and which I intend to document with a screencast soon.