I'm trying to put countersunk holes in a concave (cylindrical, very large radius) surface. When I place points arbitrarily on the surface, the holes are created at those points without a problem. But when I add relations - making the points coincident with the axes of the tapped holes in the mating part - the operation fails.

The error message is: this would result in zero-thickness geometry. I can see how the zero thickness would result from the top of the countersink cut being tangent to the surface, but then why does it work OK when I don't constrain the hole locations?

I've tried editing the countersink profile sketch so that it cuts above what it "thinks" is the surface, but that didn't work either.

Why must I lose so much valuable time teasing Solidworks into doing something that would be perfectly straightforward in real life?