topic Re: sketch bug - invalid result when intersecting a patch surface in Fusion Support Forum

sketch bug - invalid result when intersecting a patch surface

mah56 — Wed, 20 Apr 2022 18:12:01 GMT

Simple problem, big consequences : when i intersect the body, there's an invalid result, specifically at patches borders.

In the image above, there's an invalid result at (1), at jonction between hood (loft) and the kind of windshield (patches based on surface borders and 3d splines).

Because of this problem, the sketch is not closed and cant give a profile. I also cant have a valid result with offseting.

Here's (1) zoomed :

The gap is 0.001mm

The problem also happens on (2) on another sketch, because the end of the tail is also a patch.

It happens in all directions. As you can see, at the bottom of windshield, there's an edge : this is a trim i have filled with a loft because the problem happened for the section i have made in the other direction

I replaced the border of the patch with a loft to avoid same problem at red cross.

Surfaces and patch are stitched of course. Patch's borders are tangent. I can thicken / offset it, looks like there's no topology defects. Isocurve / Zebra / section analysis shows nothing particular.

Known bug ?

ps : projecting a sketch line to surface to imitate the intersect gives the same result.

pps : i can't share the file (except privately to f360 team)

Re: sketch bug - invalid result when intersecting a patch surface

jeff_strater — Wed, 20 Apr 2022 20:57:52 GMT

With the caveat that you cannot share the model, can you describe where it comes from? Was it natively created in Fusion, or imported? If native, what tools were used to build these surfaces? I have seen cases where this happens, where the result that you describe comes from the fact that the surfaces are not precise, and there are actually small gaps in there. The surface/solid modeling kernel is OK with these gaps, but the problem of intersecting a plane with these "tolerant surfaces" is much harder to resolve. Here is a very old thread (2017) that discusses the underlying issue: intersect-sketch-accuracy-issue

Re: sketch bug - invalid result when intersecting a patch surface

mah56 — Wed, 20 Apr 2022 21:55:37 GMT

Thanks a lot for your answer.
The linked thread is very interesting.
I'm amazed we can face that kind of problem.

About your question, the model is completely designed into fusion.
Tools are the usual one, some sketch on offseted plane, mainly lofts with rails, 2 patch and thats all.
But i have understand now that there's a tolerance gap which creeps into the design, and why it happens.

Many thanks for your answer, i have nothing more to say except that i hope this issue would be resolved one day !