I have seen turners rough turn the outside diameter of the sphere you want and then use a hole saw of the correct outside diameter with the teeth ground off, and the resulting straight edge ground like a scraper/chisel to finalize the outside surface.
You have to make a couple of concave holders to fit into the chuck on your headstock and the live centre in your tailstock to hold the sphere while you rough-turn and scrape/finalize the parts where the piece was held to turn the rough shape.
I’m not sure if there are any YouTube videos on this process but it was neat to see it work, and the spheres were perfectly round.
Hope this helps;
Cricket, from Canada
