I did some more testing, it has to do with the angle between the spiral end and the straight line.

helix axis is 10mm, desired fillet is 2mm.

Hi Wim, curve filleting needs each curve to be on the same plane.

In your case here the helix is not a planar curve, that is why it fails.

A different technique that you could try is to trim out some empty space between the 2 curves and use Construct > Blend to put in a smooth blend curve.

- Michael

Now I am totally confused!

if what you say is true, then my intuition tells me it should be the other way around, then 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...

anyway, I found a way to make it work for me, just thought I should mention this. no big deal ;-)

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