"Harold and Susan Vordos" wrote in message
...

"AL" wrote in message
...
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



Tracers are typically hydraulically operated machines that duplicate
desired
objects from a pattern. The link you provides shows a mill that has the
tracer head on the right. It follows the pattern with a stylus that is
the
same diameter as the cutter in use, thus yielding a faithful reproduction
of
the pattern. They were used for producing forms before CNC was
available,
and were made for various machines, including lathes. They are somewhat
more limited in ability than are modern CNC machines, but they opened the
door to making things that otherwise would have been difficult, if not
impossible, to machine. Large radii, windows in pieces, etc., are
easily
machined with a tracer, each of which would require considerable skill and
experience to accomplish by other means. There were various
configurations made for lathes, some with the template at the rear, others
with the template mounted in front. More complex tracers were made for
mills, offering a Z axis as well, so 3 D objects could be faithfully
machined. An example might be a cavity in a mold.

The broad mounting surface for the heads provides for spacing of the heads
to accommodate various sizes of parts. The heads are mounted rigidly and
do
not move while the machine is in operation. Hydraulic cylinders for the
saddle and table do the necessary moving, taking the place of screws.

Harold



Very nice explanation, Harold. Now, if you can get rid of some of those
extra spaces between sentences, I'd say you're ready to do some freelance
writing on the side, to keep you active in your retirement. g

--
Ed Huntress