Problems with fillet on brep edge

I am having problems adding a fillet in Grasshopper to a brep composed of the solid union of a couple extrusions.

After playing with it, I have reduced it down to a very simple example. The brep is a union of 2 flat sided solids - one an extruded rectangle, the other an extruded parallelogram.

I am trying to use fillet on some of the intersections. I found the correct edge indices, but fillet acts very strangely.

The parallelogram is 15 degrees off of vertical. There is one fillet between faces with an acute angle (75 degrees). It seems to work properly. The obtuse angle (105 degrees) seems to cause problems.

Strangely, the success of the fillet seems to depend on the length of the rectangular box!

For length = 459, (see uploaded file) the fillet works!

For length = 460, fillet fails.

For length = 450, fillet thinks it worked, but the preview shows the fillet extending off into space in the positive y-direction, but only on the fillet for the obtuse angle side.

Other values of length show it extending off in the negative y-direction.

I am new to Grasshopper, so maybe I did something wrong. But it must be almost right to work in some cases.


problem with (20.6 KB)

Your workflow would encounter problems even on rhino.
By having co-planar surfaces (right), a fillet would need that one of the surfaces extend to fill the hole… a mess!
Much better is to use a solid with a single big surface that avoid the problem from the start (left).
2020-04-23 00_48_23-Window
problem with (10.4 KB)

Attached a simpler solution that solve this matter during the “pre-extrusion” steps.

Advice: turn off preview of geometries, leave preview on only on geometries you are working at the moment.

Thanks a lot for the help!!

The reason that I did the extrusion of the parallelogram “fins” separately from the base is that the real ones are much more complicated than the simple parallelogram.

This was just the simple model to show the problem. However the more complicated versions are still co-planar with the sides of the base.

I can try to adapt some of the designs to this approach, but in general they are not simple extrusions.

Also, I have a lot of them, so I made one fin, and then use Move to make the rest.

So this is a good solution for some of my things I will apply it to, but not all of them.

Perhaps there would be a way to merge co-planar surfaces in the brep??

I think I never used this, and also i almost didn’t know it exsisted:
… it seems to be in vanilla gh (so no plugin needed).
Give it a try.

Merge faces works, at least in the simple version I have tried. Thanks again!!

Yes it works on coplanar faces only. The same command can be found in Rhino.

aka in case you don’t know. It’s MergeAllFaces in Rhino.