If you want a little head start, keep an eye open for 'tap disintegrators' on CL and used machine sales places. Used to bust up taps that have broken and stuck in workpieces, but they're apparently baby EDM machines with somewhat looser precision. If you're retrofitting some position control, you could get most of what you need for not too much.
If you want the serviceability or challenge of starting from scratch, it's been done quite a few times. You might browse CNCZone for some play-by-play.
Chip
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