At the local tool store, a 1/2" x 10TPI acme tap was $40. The cost was to much for a tool that will be used just a few times. Someone posting a message at CNCZone mentioned making their own tap using a piece of acme threaded rod. We purchased some rods and nuts from ENCO. The monthly sales flyer always has the three foot 1/2"x10 listed for $3.50 each. We found the matching acme nuts to be loose and has too much backlash for a CNC machine.
This is our homemade version of a acme tap, length is about 8 inches. The end was tapered on the lathe and four flutes were cut using the cnc micro-mill. A hole was drilled through and a 1/4" diameter piece of drill rod was used for a makeshift handle. The other end was also turned down so I can chuck the tap in the drill press and manually start the tapped hole straight into the work piece. We made a few nuts out of 1/2" thick delrin, they rotate smoothly with very little backlash.