There are several methods of obtaining the smooth rotation of a piece of metal necessary for engraving on it. Many North West Coast American Indians use a hard leather pad and hand control to do it. One can install various chucking devices onto a heavy base as well. A standard machined engravers ball can cost $500 and more. This version costs less than $30.00.
In the version I came up with described here one uses plumbing parts from the hardware store and cement to produce the weighted, swivelling bottom unit. An aluminum tube from an extruded aluminum dealer is set into a large pipe floor flange and this forms the top rotating part. Pitch or dopping wax is used to fix the metal in place for engraving. Other arrangements could easily be used.
The top rotating unit slides and rotates over the bottom unit; a standard steel pipe section threaded into a second smaller floor flange which has bolts through its screw holes. Nuts on both sides of the flange hold them in place. The bottom floor flange unit is then set into a concrete hemisphere made by pouring cement into a stainless or plastic bowl with a diameter of at least 8 inches or 20 cm. The bottom unit is embedded into the concrete by gently twisting it into place and hung so that it sits level and flush on the surface of the concrete until it sets. Cement provides a smoother surface than concrete. I toss in steel junk or tire weights for extra weight (when the cement has half filled the molding bowl). This works as a method for obtaining pitch bowls as well.
Several top units can be made and work very quickly exchanged by changing top units-more quickly than with a standard engravers ball. If built correctly the rotation is as smooth and easy as a five hundred dollar version. This one should cost about twenty-five or so. The only tricky part is finding the aluminum tube for the top unit. It has to be of a size that is threadable so it screws into the top floor flange and yet has an inside diameter such that it fits smoothly onto the standard steel pipe section jutting up from the lower floor flange. The aluminum tube if filed down a bit will self-thread into the iron floor flange. I have made several of these and they work really well. When in use the concrete hemisphere is rotated and positioned in a sand bag or a ring of leather or wood. An old leather or canvas handbag filled with sand is great.