Hi, I'm interested in making a graver's ball for myself. It is basically a 4, 6, or 8" sphere of solid mild steel with a wooden ring on bottom for it to rotate in, a vise stuck on top, and several bearings inside so that it can rotate at the equator. I'm fairly new to machining, so a 4" ball vise will be good practice if nothing else, and still useful for small items, although what I'm really interested in is machining a larger 6-9" diameter ball for myself. Most parts are fairly basic. But where can I get either a 6-9" ball (i.e., if anything on earth might use ball bearings of that size, which I could scavenge?) or 6-9" solid mild steel rod from which to machine the ball in separate hemispheres? I'm building it half for fun, but half to save money, so if it costs me $2+/lb for the steel it's not worth it. Any ideas on where to find stock, perhaps some sort of scrap, of such a dimension, and affordably? I live in Cambridge, MA.
thanks! -Bernard Arnest