AL wrote:
Over the past few years, I've seen numerous Bridgeport "tracer mills" on
ebay. They appear similar in appearance to regular Bridgeports, but it
appears that their heads can slide sideways in either direction. Could
someone explain what purpose this serves?

Here is an example (note that I am NOT affiliated with the seller):

http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7568256090



The multiple heads (which this one doesn't have) can be set up to make
multiple identical parts at the same time.

This particular tracer seems to have manual override, so could be used
as a manual mill. Some of the tracers were hydraulic only, and would be
practically useless for manual use - no leadscrews, etc.

Jon