What is a Bridgeport tracer mill?

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.

-- Ed Huntress

reproduction

Chuckle! Who the hell slipped those suckers in?

Harold

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 sensors. Bridgeport may have manufactured a 3-D tracer mill but I guess I never got to see one.

dennis in nca

Nice description. The machinery dealers here usually strip the heads and scrap the rest of the mill.

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

reproduction

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.

Harold

It sorta copies 2D part profiles from a master pattern. Worth about five cents a pound at the nearest scarp yard.

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).

Harold

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 standard BP. 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.

I'm wondering what makes the tool walk algorithmically all over the pattern in 2D or 3D. Do you have to walk it around by hand, or is there a step- and-repeat feed mechanism, or what?

Hey Al,

The wide head on the ram has two good features that a standard mill does not

1> It is parallel to the table T-slots; and 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)

and

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 double sine-vise.

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 X-axis hand-wheel. 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.

Take care.

Brian Lawson, Bothwell, Ontario.

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.

Jon