The tube trick is a good one. I first saw it at the 1981 M E Exhibition, then George H Thomas wrote a chapter on it in "The Model Engineer's Workshop Manual". It can be used on steel, but it works best on brass- turn your ball blank to its eventual length, turn the o/d plus a thou or two, and rough to a ball-ish sort of shape. Mark the o/d with a permanent marker. Then take a piece of round steel, not much bigger than the ball you want, face it, drill and ream it to a size smaller than the ball you want, and case harden it.
Euclid- Any section through a sphere is a circle...
Spin the lathe up, and present your new tool to the work with your hands. Roll it around the blank, and the excess material will be shaved off. Roll it around between your fingers as you go.
***This is a golden opportunity to slip and punch the chuck, so be careful!***
When the marker ink is shaved off, you've reached finished size, and a perfect sphere. A quick rub with fine scotchbrite (read- worn out scouring pad) and you're done.
Unbelievably, I can find no reference to this method on the internet; just lots of people selling expensive attachments.
For bigger radii, I use two other methods too- I'll explain them when you need 'em.
-Andrew UK
