There's special fixtures you can buy just for this.

For occasional use, I mount a tool bit right over the center of the
compound. Then slightly loosen the compound holding bolts. Attach a lever
arm to the compound, I just use a Kant Twist clamp. Radius of your ball is
set by adjusting the compound screw. Turn in the amount of your cut and grab
the lever to rotate your cut around. Both concave and convex shapes can be
done this way.

--
(©¿©) An apple a day keeps the doctor away.
Two apples a day gets the doctor's OK.
Five a day makes you a fruit grower like me.