Hi Wim,

re:
> the line that goes straight down, perpendicular to the last segment of the
> spiral, lies in the same plane, and should work, the angled one is definitely
> not in the same plane and yet that one works...

Neither line is in the same plane as the helix, because the helix itself isn't on
a single plane. It's a non-planar curve.

However it is close to being coplanar if you stay in the general area near
the end of the helix.

That is enough for the fillet solver to get some solutions if it is lucky. But the
arc generated in these cases is not exactly tangent to the neighboring curves.

- Michael