EMC and threading on mill

["Roger Swift" <vrsculptor-at-hotmail.com>]

I was trying to understand how to tap small holes (1/4-20) using EMC.

Hardware: Servo mill,  a tap holder that is essentially a collet mill holder that telescopes and a 4HP spindle  controlled by a +/- 10 volt servo board and a a tach. There is also a low resolution single channel spindle encoder. The spindle speed is currently manual (set with a wall wart and a 5K pot).

How I think it should work: Start spindle at low speed, feed quill to engage tap to start treading, rotate spindle 20 turns clockwise, tool self feeds on telescoping holder, lift quill a bit and rotate spindle counter clockwise to disengage. I'm pretty sure this is how the original controller (long gone, never witnessed) handled it. 

Any thoughts on how to configure this? Is this possible with any other PC based G-code drivers?

       Roger