Are you doing a partial sphere, say, as in a high relief carving, or do you want to produce a complete sphere (a ball)?  

thanks for reply, I will be doing complete balls.
from 200mm(8") upwards.

i mess around with lapidary work as a hobby, and there is a machine known simply as a sphere grinder or sphere maker.  I owned one a number of years ago.  I don't remember exactly the precise angle of opposition, but it basically consists of 2 pipes set at approximately 30 degrees off of horizontal (roughly facing each other, and driven by a motor.  If I remember correctly, the pipes, or cups are rotated in the same direction.  The following link will give you an idea of the commercially available machines.  Good Luck!  http://www.kingsleynorth.com/sphere.html

Thanks again ornerynorsk, I know of kingleynorth and have their catalogue, they guy i spoke too didnt know how to do larger diamters, I know of the externally housed home made  ones in China, their work looks ok.It was in that line that I was thinking but needed to work out the detials, as the machine I saw was only a pic and i could not understand how they would get the accuracy??Anyway thanks again!

8 inch is generally not considered to be an extremely large sphere, but as you approach that size range, a 3 head machine might even do better for you.  I'm sure the accuracy comes from the phenomenon that as the part is rotated/tumbled in 3 planes, the high spots are automatically taken down into consistency with the rest of the surface.  Of course, you can't start out with a rough chunk, it needs to sawn and ground into a rough spherical form.  There is an online newsletter devoted to sphere making, I don't have the link handy, but I believe it is called "spheres to you".  If you ever get to Minneapolis, one of the downtown hotels close to the IDS Center has a granite sphere which is held in flotation by water pumped under its holder.  It rotates as the water cascades out, and is really awesome, considering its size.  It's got to be 6 feet in diameter.  Best of luck to you.