http://www.woodcarvingguru.com/ball-pein/
What is the ball for on a ball – pein hammer?
Just wondered whathe ball is used for (traditionaly perhaps) on a ball pein hammer.
Thanks,
Matty 
If you get a chance, watch American Chopper when they take a piece of flat sheet metal and place it on the English Wheel. Usually when they make seats of are forming gas tanks. The English wheel is a very fast mechanical hammer that smacks the sheet metal many times per second. After pushing the sheet metal back & forth thru the wheel, the sheet steel panel becomes arched or curved. The ball on the ball peen hammer does the same, but manually & is a lot slower process.