I'm sure this question has been asked many times before so I'll keep it short and sweet. I have a new CNC Taig mini mill and want to be able to control the cutter speed with the G code (Mach3 software). I ran across a web site last week that had an add on kit but cant see to re-find it. I'm new to this so just point me in the right direction and I'll leave everyone alone.
Thanks
-John

That's an AC motor, so you may be stuck with addind a VFD (variable frequency drive). Tha's usually a pretty expensive option, which is why its pursued by the big boys and not hobbyists most of the time. A cheaper option might be finding a fast DC motor and controller then cutting a custom mounting plate option.

Either solution should have surplus options available, so I'd add the word 'surplus' to my initial searches in the interest of keeping the prices down.

HTH,
Kalo

http://www.campbelldesigns.com/mach-spindle-speed-control.php

Like Kalo said, the expensive part is the 3 phase spindle motor and variable frequency drive. You'll also need load reactor coils, maybe a power supply (for the control board, depending on your VFD), and an enclosure for everything.

homann designs (http://www.homanndesigns.com/)
You can retrofit a Sherline motor or any KB control and use Peter Homann's digispeed control board