now, i'll start by saying that i'm not at all familar with the particular unit that your working on, but if you say that the steppers are chattering, it almost sounds like thier cogging, or missing steps. My first thoughts are that the drive voltage/current isnt high enough, or that perhaps the steps are too fast... the only other thing that i can think of is if there is something thats keeping them from turning.. are the motors on or off of the machine?
Grizzly X3, CNC Fusion Ballscrew kit, 3 500oz-in bipolar steppers, 3 203v Gecko's, Linear power supply from Hubbard CNC, Mach 3, BOBcad Pro Art V22, Rhino.