How can I prevent a perpendicular constraint or plane from reversing orientation on model changes?

james_howard360

I'm relating sketches to the terminating point of a helix, including a tangential line, and I need the revolution count to be configurable, but the relations flip depending on the revolution count. If I model at 12 revolutions, then change to anything between 12 and -12.49 the relations hold. If I change to anything between 12.5 and 12.99, the direction of the sketched line flips due to the ending angle being greater than 180deg. I don't know which element is flipping (sketch plane or perpendicular line relation).

Is there a proper method to prevent this behavior, and is there a way to tell the difference between Side A and Side B of a plane to troubleshoot?
Sample document: test helix relations | Part Studio 1 (onshape.com)

S1mon

Again, I'd really have to fight with Onshape myself to check, but Mate Connectors have options to re-align the primary or secondary axis. If you use those, you might be able to use something that won't change with the helix.

glen_dewsbury

From your video, it looks like you've used the end of the helix to set sketch plane. I think if you used to start of the helix for your sketch, it won't rotate with the revolutions. Or start the helix at the other end.

S1mon

I’d have to try it myself, but perhaps a mate connector instead of a sketch plane would be more robust to rotational issues. 

james_howard360

@glen_dewsbury In my use case I want the sketch to rotate according to the helix angle. It's the flipping direction at over 180deg that's the issue.

@S1mon I just tried the mate connector, but unfortunately when defined the connectors have an attachment point and a fixed orientation, so a related sketch would always point in one direction regardless of the helix end angle.

That did give me an idea, though. I had been setting the tangent line perpendicular to the plane, and instead drew a new line from the cylinder axis to the helix endpoint and used that as the perpendicular reference; now it works as intended.

The plane must have been reversing its orientation re the helix end point or the sketch relation. As far as I can tell, planes have no defined front/back so the solver doesn't seem to care - that might also explain why there are so many issues with sketches that get flipped when redefining sketch planes.

Thanks

S1mon

Again, I'd really have to fight with Onshape myself to check, but Mate Connectors have options to re-align the primary or secondary axis. If you use those, you might be able to use something that won't change with the helix.

james_howard360

Good thought, that actually does work by using a sketched tangent line on the end of the cylinder and reorienting the mate connector primary axis to the line. A sketch plane defined from the mate connector then does not change direction.

S1mon

In case this isn’t obvious, you can use a mate connector as a sketch plane. You don’t need to add a plane through the MC unless you really like it that way.