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):
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.
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.
The first Bridgeport "Tracer" mills I remember seeing were strictly 2-D
units. They used a photoelectric tracing head in conjunction with a
document platform. I think it may have been possible to also add a
second machining head to this system.
Although we dealt with other, much larger, types of 3-D tracer mills
(I'm thinking Hydromill) these were equipped with hydraulic tracing
Bridgeport may have manufactured a 3-D tracer mill but I guess I never
got to see one.
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.
I was recently offered a Bridgeport tracer, equipped with three machining
heads, and a Tru-Trace hydraulic system (2-D). The length of the table
and arm on which the heads were mounted made the mill most unattractive for
home shop use. Way too large. Even with adequate space in the shop, I
decided against the machine. I didn't want the problems of transporting
such a top heavy machine, then tripping over it for the balance of my life.
I'd have likely taken the machine had it been equipped with a shorter table
and arm, with a single machining head. One of the additional features that
makes owning such a mill is the speed at which parts are generated.
Tracers, like CNC's, cut irregular surfaces as quickly as the cutter can
withstand the cut. They're fascinating to watch in operation.
I have a tracer for my Graziano lathe-------and wouldn't consider selling
it, let alone for a nickel a pound. While it's old technology, it's every
bit as functional and valuable to me as if it was a CNC.
For those of us that elected to stay away from new technology (read that CNC
or NC), for reasons best known to us, they're still very much a desirable
tool to own, and are capable of producing identical parts in volume with a
high degree of precision. I've turned out work that would have been
otherwise impossible. Example-------and it's the reason the tracer was
purchased many years ago, was to turn a 1" radius that blended into a 7
degree taper on more than 600 spacers made from 7075-T6 aluminum, for the
landing gear on a C-130. That part, for me, would have been impossible
to machine by any other means.
Bottom line-----don't discount the value of a tracer, be it for a mill or a
lathe. They open the door to many of us to what otherwise would be the
impossible. And they're cheap! (Thanks to guys that think they're worth
a nickel a pound).
Around 1982-83, I used a tracer kind of device. It was an add-on table that
we attached to a large Kent mill. The Kent was about 25% larger than a
The add-on table had a ~3-4' lever arm and it allowed you to move the raw
material freely under a rotating cutter (endmill) and maintain control
(because of long lever arm).
There was a stylus with a bearing that traced on a pattern. The bearing was
the same diameter as the cutter used unless you wanted to scale the pattern.
It was 2 axis only.
So, the raw material was bolted/clamped/vised... to the left side of the
floating table and the pattern was bolted to the right side. The
stylus/bearing center was some know distance from the endmill center and you
approximately had the pattern and raw material set the same distance.
Turn on the endmill and using the lever arm, rough out the shape you needed.
No worry about cutting too much since the pattern shape limited how far the
bearing could move and you were never able to gouge your part. ...of course
you could take a big fast cut and the endmill might deflect and climb into
the part, but basically you were safe. Stupidity shouldn't count. Think of
it as if you were able to use a router on aluminum, stainless etc. freehand!
I guess it would be the poor man's tracer since it didn't have hydraulic
drives or anything. It was strictly a manual device and it worked great for
repeating bizarre shapes, radii... without a CNC.
The wide head on the ram has two good features that a standard mill
1> It is parallel to the table T-slots;
2> The ram face and its T-slots are perpendicular to the table.
This provides the ability to either slide/relocate a single mill-head
along the X-axis and maintain both square and distance (good for long
work-pieces), or it can support multiple heads (3 Max) if you want to
do multiple duplicating. ( In fact, on the mill you show, the
back-mount for a second head is both shown in place to the left of the
head shown, and it is mentioned in the text.)
The two main drawbacks to these rams are:
1> You can't use one of the plain three-armed "spider" type quill
handles, but rather must use a single lever, preferably a
"quick-action" as shown for instance at
The reason is that quill feed handle cannot revolve more than 250
degrees (that provides 2 inches of quill travel), as it strikes the
ram face and will "jam" on the head if you are not careful when using
the power down-feed. When doing it manually, you do about a 1-1/2"
stroke, lock the quill lightly, release and bring the handle to the
start position, release the quill lock, and start down again. Sounds
worse than it is!! To me, these quill handles are superior to a
spider type anyway, even used on a standard/plain Bridgeport)
2> the mill-head can only rotate/swing, and not tip fore/aft
(up/down??). In an airplane, it would have the ability to roll, and
not pitch. I have never had any need to do a "compound" anyway,
besides which there are other ways to hold MOST work-pieces to do the
same thing anyway. Rotate the head and use a sine vise, or just use a
Neither "fault" is a deal-killer for most applications.
I am no authority on the many types made by Bridgeport, and you've
already had some other explanations, but just be clear about the one
you actually see there on Ebay......
As an "automatic" duplicator this is a 2-D, rather than a 3D, or at
best it is "two-and-a-half D"in CNC terms. Bear with me on
that........ You'll note the two hydraulic hoses running below the
table to the apparatus hanging on the left hand side of the table.
That "apparatus" is a hydraulic motor that can be engaged to turn the
There is a whole lot more to doing duplicating than what I'm going to
write here, and requires a fairly skilled operator.
In operation, the "model" (the thing to be duplicated) is fastened to
the table's right end, and the raw material is also suitably mounted,
but near the quill. Then the knee is brought up manually and the
X-axis is moved manually to where the stylus is close to touching the
right side of the model, and the Y-axis is hand-cranked to place the
stylus at the "front" edge of the model. (The stylus and control can
be seen in the pix on the far right hanging from the ram.) Then the
hydraulic pump is started, and the knee raises itself until the model
contacts the stylus. Then the mill is started, and the quill is
brought down manually until the end-mill cutter touches the raw stock,
then locked solid. Then that X-axis hydraulic motor mentioned above
is started up, and as the model moves to the right, the knee lifts up
and down to maintain the stylus just touching the model, and of course
the end-mill is "duplicating" that motion/cut in the raw stock. At
the end of the model, the X-axis motor is stopped, THE Y-AXIS IS MOVED
OUT BY HAND (each time), the X-axis motor is reversed, and the table
now travels left. This cycle is repeated over and over, with a whole
lot more operator input than I mentioned, until the raw stock now
looks like the model.
So, you can see that this requires quite a bit of manual input from
the operator, and the tracing is ONLY up and down.
The pump unit shown is of very good quality, and if you don't want a
duplicator can be removed and used for some other purpose or sold.
The ram face is a great place to hold "stuff" too.
oh...ps..... I bought my duplicator with-out the pump for $75Cdn, but
that one looks like it may be in a bit better shape..
looking at the 3rd picture of the eBay machine, there is an axis drive
clearly added to the Y axis, as WELL as the X axis. I can't tell for
sure if it is electrical, hydraulic or what. There certainly were
X-Y tracer machines, including some that used "electric eyes" to track
a black line drawn on a paper pattern.