Walt, note that you can use a floating tap holder, as opposed to a true tapping head (which is more money). Maritool has some nice ones.
You just need to have some flex in the Z-axis since you spindle won't be precisely synchronized. Plus, you need a good handle on your VFD if you don't use a tapping head with a reversing clutch. If it doesn't reverse fast enough in a blind hole you're going to break the tap. Make sure you go slow enough to give it a chance.
Last thought: a butterfly impact wrench from Harbor Freight costs about $20 and will tap all day long with an Irwin tap adapter. Enco sells the latter to let you put a tapping chuck on any socket tool. It's not as nice as letting your CNC do the job, but it beats the heck out of a pure manual job.
A little more on the butterfly tapping here:
http://www.cnccookbook.com/CCTappingArm.htm
Make sure to use new high quality taps!
Cheers,
BW
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