Hi Wim, I've been trying to fillet your same shape in some other CAD programs and they seem to have problems too getting a fillet radius of 1 unit there.

I think it's related to a bit of a tighter than you'd think amount of curvature in the bottom semi-circle shape (screenshots here from Rhino):



That semi-circle like shape is only an exact circular arc on this left part here:



On the right part it's a general curve and right near the end where the control points are kind of close together, the curvature of the curve in that end spot is actually 0.7 units, a fair amount more tightly curved than the rest of the curve:




I had to measure it with the Curvature command in Rhino to really see what the curvature at the end was. This tight curvature at the very end is what is likely limiting the fillet radius as well, because just in general if you try to offset something (fillets involve offset surfaces as part of their calculation) of a distance that is greater than the radius of curvature at a particular spot, it results in a bunching effect like this:



So anyway I think it's this that is causing your fillet difficulty, a kind of hard to see tightening of the curvature right on the back end of the bottom semi-circular shape.

So probably the easiest way to solve it is to make the bottom an exact semi-circle rather than half circle and half general curve, that will change the shape somewhat is that acceptable for what you need?

I have attached a 3DM file with a version where I have done that - replaced the bottom part with a revolved exact semi-circle, this version should fillet for you at radius = 1 without any problems. Again, note that this is a slightly different shape in the bottom rounded area than your original model. If that existing shape is important to you, moving that second control point in the current tightly curved end just downwards by 0.15 units is enough to make the curvature over 1 unit at the end of the curve so that would be a pretty minimal change.

- Michael