I found this link to a YouTube clip on turning spherical knob ends in steel; there will be those here who turn metal as well and may well know of this, but the principle looks as if it could be transferred to the wood turning lathe.
http://www.youtube.com/watch?v=f4pwUw0G3RQ
Rob
http://www.youtube.com/watch?v=f4pwUw0G3RQ
Rob