Here's a ball turning tool I made with my lathe and mill. Everything is made of scrap, with the exception of the screw and the carbide bit. The handle was given to me from a co-worker. It came off a surface grinding machine that he used to own. I milled the surface flat, milled a 1/2" slot in it, used the existing tapped hole and made 3/4-16" bolt to fit in it to hold the tool-post. The base has a bearing press fit into it that is a press fit into a dovetail that I made of scrap steel that engages into the carriage where the top slide fits. The tool-post was made from 1/2" steel plate that I rough cut out with a hacksaw and finished shaping on the mill. All you have to do is drill and tap a hole in some scrap aluminum/brass/steel, and turn it in the lathe on a rod that is threaded. When it's done, I use a pair of pliers with leather padding to cushion the ball while I unscrew it. The bearings really make it smooth to turn and easy to move.
(I vacuumed up the aluminum and steel shavings to make the pictures more presentable.)
Here's a pic of the ball attached to the bridgeport mill. This is my homemade quill feed handle that replaces the bubba'd welded/brazed and bent handle they had on it.
I made another ball to go on my improved ram handle today as well. The handle was rust blued with "Andy's Rust Blue". I am very happy it turned out well and the ability to turn little nubs and cutoffs into balls is very handy. Eventually, I'll mill the other end for another insert so I can turn inside radii, but that's not needed at the moment.
This is the one that I saw on youtube that gave me inspiration to make it. Watch the video and you'll understand how they work better:
He put a lot of thought into his design.