Fully Defined Spline Showing Up as Undefined

[Anonymous]

Hi all,

I'm having an issue with a sketch in Fusion 360. I've been parametrically modeling because I expect my geometry to change and have been careful about working out dimensions. However, there is a spline in my sketch that is showing up as undefined even though I don't see how it could be.

A spline is defined by (1) defining the locations of all points and (2) defining the tangency vector for each point. The Attached screenshot is quite cluttered, but here is how the spline is fully constrained:

• 4 points:
  • Very top: Location is defined to be coincident to the endpoint of a line which is fully defined
  • Remaining Points: Defined with distance from origin and angle from fixed object. 2 DoF => 2 constraints
• Tangency vectors:
  • Lengths are defined
  • Angle to various fixed objects defined
  • These definitions combined with the fact that each vector must pass through its corresponding point fully defined the vector. 3 DoF (angle, magnitude, location) => 3 constraints.

As an added bonus, all of those straight lines coming off the edge of the circle are now showing up as undefined (blue) even though they are also definitely defined and were showing up as such previously.

Does anyone know what's going on? Should I just move on if the sketch is mathematically defined and Fusion just doesn't realize it, or will I run into problems later because fusion thinks it's undefined and therefore won't let me perform certain operations.

Thanks

[brink.gregory]

Hi folks,

I'm having the same problem so I made a simple sketch to illustrate the issue so we might gain some additional insight.  The attached PDF has a set of screen shots that show that a sketch that was fully constrained can indicate that it is not fully constrained simply by adding a line and then deleting it without changing any of the constraints on the spline.  I was able to get the sketch fully constrained again by deleting most of the constraints on the spline and then re-adding them.  That was quick in this case but can be a big pain for a more complicated parametric sketch.  It seems that something about other sketch elements interacting with the spline cause the spline to latch the unconstrained status even though all of the elements appear sufficiently nailed down.

I hope this helps lead to the solution.

[brink.gregory]

A quick update:  Even adding a line to the sketch that doesn't touch the spline but simply has dimensions or constraints referenced to the origin breaks the spline constraints when the line is deleted.

[jhackney1972]

Splines are inherently hard to "fully" constrain.  If you feel you have it locked down enough to preserve your design, move on.  If you are really worried about it, place a Fix sketch constrain on the spline.  That will hold if for good.