Spindle encoder

Hello group,

Thanks for the several responses I got to my request for advice on the CNC
chucker auctioned on eBay last night. It went for $8085. with most of the
bids coming in the last few minutes. I droppped out of the bidding at
7200, at which point it crossed over the invisible "serious money" line.

I went to see the machine yesterday, and saw it running under power. It
was quite impressive, with the turret silently advancing to the next
position, the speeds and feeds adjustable for each turret postition,
carriage and slide moving smoothly back and forth. One feature which
seemed really slick was that it had a second motor mounted on the spindle,
a servo with an encoder, for use in threading. This leads to my question.

As I have mentioned before, I want to set up a vertical mill so that the
spindle acts as the fourth axis, so that I can do "hand" engraving and rose
engine work. I assumed that the servo motor attached to the spindle would
have to be disconnected when the spindle is running under power, but on the
Hardinge chucker, the servo shaft ran with a timing belt directly behind
the quick change collet mechanism. You can see it in the photo:

http://www.imagehost.com/91/5578-155.jpg

Or at least the belt guard for same.

If I set up a servo with a rotary encoder driven by a timing belt to the
spindle could it remain engaged when it is not in use? Doesn't a DC motor
act as a generator when driven? How would this work?

David M. Munro