Egonenglish:
I'm using a medical device drill. Used in surgery.
Is this a high speed medical drill? Cutting the power to one of those won't stop it immediately, as there will be some run-on, dependent on friction. Also, if this is to be used in a medical situation, it wouldn't be a good idea to use a carbon brush for contact, as bits of carbon could break off and get into the patient, and a carbon brush probably wouldn't withstand sterilisation.
The manufacturer has told me is 15v DC and I'm waiting to see if they'll answer my questions regarding current. If they don't tell me, I guess I might have to break it to get a multimeter in there.
Then how do you propose to turn it on and off? As long as you can get to the 15V leads you can measure the current with a meter.