I'd hate to sacrifice (cannibalize?) decent malleable iron clamps, but ACME threads and foot pads make this a winner.
Tempts me to hunt up an ACME tap, 9/16 - 5/8" ~ 14mm or so.
BTW; trick for ACME threading. Single point the first few passes before running a tandem tap. Rather imperative for a single pass tap. Few shops have long enough tap handles to generate leverage needed, I don't care how far back they are chamfered.