Shedding some light

 From:  Michael Gibson
4125.2 In reply to 4125.1 
Hi Burr, that's pretty freaky looking...

So I think this is basically related to the problem of having a fillet radius that is large that goes around a tight bend, where the fillet radius is actually larger than the radius of the bend itself.

That kind of thing has a natural tendency to produce a kind of bunching and self-intersection between the fillet's cross sections - a fillet is basically created through a set of cross sections similar to a Loft internally.

A radius larger than the bend it's going around produces cross-sections something like this:

The bunched up area of that surface where the cross-section pieces are colliding into one another will tend to make a bad quality surface there, it will tend to have strange wiggles and fold back on top of itself and stuff like that.

Then you get a weird result in your particular case here for the corner patch when the corner patch is trying to match up nicely and smoothly to that jumbled and chaotic area.

You can kind of get more of an idea of what's going on by looking at the result of some smaller radius values before you get to the 0.04 value:

Here's what it looks like at 0.01:

And at 0.02:

And at 0.03:

So note that as the fillet radius increases, the upper edge here becomes smaller and smaller and is starting to get somewhat awkwardly formed - that's due to something similar to the bunching problem as shown in that 2D illustration above.

- Michael