Convert Smart Surface / B-spline to a Solid?

Convert Smart Surface / B-spline to a Solid?

I have managed to get a nice-looking helix to represent a threaded bolt. It’s identified as a smart surface from a B-spline. Now I want to merge this with the bolt rod and the hexagon head, both solids. Of course, “Unite solids” does not accept the helix, so I need to convert the helix to a solid first,

Is there a special command to achieve this? /Ingemar