Bug report: strange behaviour of projection/sketch after redefining sketch plane

[nigel76FS8]

I've encountered this before, but thought it was down to model complexity or some other issue of mine. This time, it's a very simple model so far, and is fully reproducible.

I have a tube, I've projected a sketch on the end face. I have then moved the part along the tube a bit, so I redefine the sketch plane, and it flips 180 and ruins things.

(I've just installed the Screencast software, I'll use that next time!)

[HughesTooling]

You are correct, I've found redefine sketch plane is a total waste of time! Pretty much every time I've used it it either rotates the sketch or flips it. Another nasty problem is you'll notice all projections are broken and changed to fixed geometry without warning! In your example it's quite obvious but on the odd occasion the sketch doesn't flip or rotate it can be very hard to see the projections are no longer linked. 

Really you need options to set the XYZ of the plan. This has come up a few times but just gets ignored by Autodesk.

Mark

[therealsamchaney]

Yes I think this problem is due to the same root cause as this post. I believe Fusion does not store data about the relative positions of sketch entities. In other words, it doesn't know up from down or left from right. If you project a horizontal line and sketch a circle above it and constrain it to be 5mm above the line, Fusion doesn't care if its 5mm above the line or 5mm below the line, as both cases technically still fulfill the constraint. It is only really constrained to be 5mm away from the line. This can cause entire sketches to flip upside down sometimes if you change something (or redefine the sketch plane in your case). Seems like a pretty fundamental issue as there are theoretically two possible solutions for any coincident constraint or dimension. Ambiguity like that causes instability and unexpected behavior.