Hi,
The stepper driver is an a4988, attached to one of these:
The cnc shield sits on top of my uno. It has separate 12vdc inputs, and with no power applied to these it does not drive the motor. For power, I split the + and - outputs from a 12vdc 5a wall wart, and plugged them into 1) the arduino barrel jack and 2) the power inputs on the cnc shield.
I'm not sure how to tell whether the stepper driver/cnc shield is handling the power, though it seems like that should be the case?
Thanks again for your help
Lee