Fully-constrained sketch elements become become unconstrained when a 3D element is added.

[piperVVYZ3]

I have a sketch that is fully-constrained.  It is based on projected geometry from multiple components, intersected with the sketch plane.  Midpoint constraints were used to create some new lines from that projected geometry.  It's all fully-constrained, everything rendered in black.

Then I turn on the "3D Sketch", and add a single line that leaves the sketch plane.  Somehow, magically, some of the other lines suddenly become not fully constrained, and render in blue.

It seems to be only midpoint constraints that suddenly stop working.  Geometry from coincident constraints stays fully-constrained.

Link to the screencast (sorry, inserting the screencast here doesn't seem to work... it keeps telling me it's Invalid HTML and removing it.)

I've tried to recreate this behaviour with a simpler design, but haven't been able to.https://autode.sk/3z

[jhackney1972]

Please attach your model so someone on the Forum can take a look.  If you do not know how to attach your Fusion 360 model follow these easy steps. Open the model in Fusion 360, select the File menu, then Export and save as a F3D or F3Z file to your hard drive. Then use the Attachments section of a forum post to attach it.

[piperVVYZ3]

Here it is.  Not my proudest work, I'm afraid.  The design is currently in a very broken state, as I've been replacing all instances of an imported component ("M2 Ball") with a different component ("M80 Ball"), and working back up the timeline trying to fix all the breakage.

But basically you can go straight to Sketch5 in the "BulletPack" component, edit that, and then try to create the 3D line like I did in the screencast.

(BTW, getting Fusion360 to produce geometry for packing a bunch of tapered cylinders together is really, really hard.)

[jhackney1972 · Accepted solution]

Those horizontal lines can float, as I show in the Screencast.  The best solution, in my opinion, is to create a new sketch for the 3D sketch and keep it separate.  If you do want to do that, then I would suggest what I did in the Screencast.

[piperVVYZ3]

I have observed that floating and snapping-back behaviour you're talking about.  But I don't understand why it's happening.  It seems like a bug to me.