I got interested in these gadgets after an inquiry from a correspondent.
You can buy on-off magnetic bases *with* a cheapo (but functional) dial indicator from Enco for $24.95. Sometimes they're even less on sale. I now recall buying one of those for Kevin's bicycle-wheel truing stand. However, I don't think they have anywhere near the "grab" for given size that one built with a Neodymium-Boron (NdBoFe) supermagnet would have, and making one would be easy-peasy for a HSM with a VISA card (for the magnet), lathe, mill, and hacksaw (ungh!) or bandsaw. I played with some simulations today, one using a magnet 9/16" x1-1/4" x 1/8" ($2) and one 1" x 2" x 1/2" ($13). I did the sims with a simple but pretty accurate FEA program that does 2D modelling of magnetic fields.
The holding force I got with the little one was well over 100 lb, that with the larger one was 320 lb! This would vary some with specific construction and materials, of course. I modelled easy-to-make clearances of .005" and used a generic B-H (magnetization) curve for iron or steel. Making the clearances .010" wouldn't degrade performance much. . The little one would be 1-3/8" long with endplates, about 1" thick and 1" high if one used a 3/8" aluminum or brass top plate to provide threaded mounting holes to mount something to. The bigger one was2-1/8" long, maybe 1.5" and 1.5" on other dimensions -- I don't recall exactly but close enough. Many other variations are possible using various magnets available from
The little one would be about the size of an ice cube. You would*not* pluck this sucker off a milling machine table without a prybar when it's "on", while the "off" holding force looked like a pound or less. I've not built one. My simulations usually prove to be reasonably accurate but ultimate truth is always found at the bench.
If interested in a conceptual sketch and further drivel, see