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Sketch can't actually obey "coincident" constraint
rehevkor5
Member Posts: 17 ✭
So, here's what I did:
- created a sketch with a spline
- approximated that spline using Edit Curve to simplify it
- converted the resulting 3d curve back into 2d by "Use"-ing it in another sketch
- used "Offset" on the resulting sketch curve
- added various lines which intersect with the offset curve
What I'm observing is that Onshape doesn't create sketch regions as it should. Despite using "coincident" constraints between my lines and the offset curve, Onshape doesn't recognize them as actually intersecting. Similarly, if I zoom in, Onshape doesn't draw the lines' endpoints in the expected places: there's a gap between the lines and the curve.
There appears to be no way to fix this… deleting & re-applying the constraints has no effect.
How do people work around this? I guess I could approximate the offset curve, too?
Shouldn't Onshape mark the coincident constraint as "error" status, when it can't actually achieve a coincident point? The current behavior is misleading.
What else could Onshape do to fix issues like this? For example could they add a project-level or sketch-level setting which tells it to use approximations for operations like "offset" rather than winding itself up into a mathematical ball of impossibility? In general, it seems like Onshape's documentation could do a better job of explaining why these things happen, and what to do about them…
Answers
Offset splines are twitchy in the Sketcher. You might have better results doing the offset as a feature (either extrude a surface and offset, or use the offset curve feature) instead of a sketch entity.
If you can share a public document it might be easier for people to help.
I think steps 1-3 are a bit problematic: why not create a "good" spline in the sketch in the first place? The "use" to create a sketch spline from a curve is probably negating a lot of the advantages of the "Edit curve" step and because it wasn't generated as a sketch spline it's going to be even more problematic to offset.
As @S1mon said, you should generally avoid using offset splines in sketches and if you can share more details about what end result you are after we can provide better options for how to achieve that.
That said, I would submit a ticket so that they can look at the constraints/regions situation.