The electro-tracer lathe: an alternative to clumsy hoses and costly CNC.
An alternative to clumsy hoses and costly CNC
When you think of a tracer lathe, you think of a typical hydraulic machine, Figure 1 or a conventional CNC turning center that can trace anything you can program as the next step up. That's a big step! Certainly CNC is the way of the future, the path to the automated factory, but is CNC for everyone? I think not.
There will always be plants that are not totally automated and don't want to be. There will always be low-volume jobs where changeover is infrequent and the machine tool is dedicated to a single part or family of parts and hardly requires the sophistication of CNC reprogramming. What's really needed is an inbetween system, one that retains the quick changeover of hydraulic tracer systems, while adding a measure of machining accuracy, particularly when taking heavy or variable-depth cuts.
The electrical/electronic (or E/E) tracer system of Figure 2 is nothing new. It's been around long enough for the patent to run out. It was never pushed by a major tracer-lathe manufacturer, it never caught on, but it's an excellent system.
It is controlled by a single-board programmable controller that is easily mounted in the electrical panel and removed for repair. The high-speed stepless unit has four programmable feed ranges with infinite adjustment within each range. Feed change, spindle-speed change, skip feed, start trace, stop trace, and home positions are fully programmable within the entire carriage operating envelope. It uses DC servodrives with pulse-width modulation to get sufficient torque for slide movement. Tracer drive motors are directly coupled to adjustable-preload ball screws. Drive speeds range to 200 ipm.
The tracer head is mounted to the microslide assembly and equipped with a breakaway stylus. The system is available for either 270-degree conventional two-plane tracing, or 360-degree under-cutting in any plane or direction.
E/E versus hydraulic tracers
Why does the E/E system represent an improvement over conventional hydraulic tracers? One major advantage is simplicity. One circuit board replaces complex hydraulic piping and sophisticated valving.
Another is temperature. Hydraulic system accuracy is affected by temperature. As the oil heats up, tool position in relation to template position changes, and adjustment for size must be made constantly to hold tolerances. The E/E system can provide consistent performance and accuracy throughout the shift.
Ease of maintenance is another important advantage. Should a control malfunction occur, the E/E's control board can be removed and replaced in 15 min. The hydraulic tracer has a much more sophisticated circuit and would require troubleshooting a number of large components at various locations, and a much more time-consuming remove-and-replace procedure.
Of course, initial cost for an E/E system is greater than for an equivalent hydraulic system, but this is offset by the E/E's improved accuracy and simpler maintenance.
E/E versus CNC
I don't intend in imply here that the E/E tracer system can replace CNC-level controls in sophisticated applications where on-line programming is required. Yet there are advantages of the E/E over the CNC.
The two major advantages are ease of changover and dimensional accuracy where heavy or variable-depth cuts are involved. We feel that if you're qualified to push a broom in a plant, you can change the setup to cut different parts on a tracer lathe equipped with an E/E system. Simply remove two bolts (one on some models), Figure 3, to change templates. Limits are programmable from the operator's push-button station; anyone can do it. A CNC system doesn't require a template, but it does require a trained person for programming changes.
In the dimensional accuracy area, the CNC system computer is programmed with the moves to be made by the cutting tool. The position-measuring mechanism is often a resolver on the rotary axis of the ball screw, counting the turns and translating this into specific distances along the screw. In theory, you know how many turns of the screw, you know where the carriage is, you know where the slide is, and therefore you know where the tool is in relation to the work.
But do you?
You really only know where the tool is supposed to be. In a heavy cut, the tool and slide assembly can be subjected to severe deflection. The gib on the slide and/or carriage may be loose. The ball screw may be loose, or have "grown' due to increased operating temperature. Or high-load torque on the ball screw can induce torsional movement and positioning error.
To combat these shortcomings, more recent CNC designs have done away with the resolver in favor of an optical encoder, the glass scale. This eliminates most of the ball-screw windup, thermal, and looseness problems, but the glass scale must be mounted at or near the back of the carriage or slide. Position inaccuracies still can result if gibs are loose or if some other form of machine-element deflection occurs. In a rear location, the glass scale can't read such deflection, and in fact, may amplify it.
Thus, CNC accuracy is great for light cuts, but little more than an approximation for cuts that are heavy or of variable depth. How is it that these same conditions are not problems for the E/E tracter system, Figure 4?
In CNC systems, the resolver at the end of the ball screw is the only positioning feedback sensor used. The E/E tracter system, on the other hand, uses resolvers for approximate positioning only. Fine-tuning feedback is from a template follower stylus mounted on the vertical slide the same distance out as the cutting edge of the tool. Because the stylus is directly above the tool, the system senses whenever the tool has been inhibited in its progress, deflected by a heavy cut, moved because a gib is loose, torsionally deflected, or dislocated because of ball-screw growth.
Thus, the bottom line is to know your requirements and be aware that there is a choice. CNC certainly has a future, but there is still a place for the E/E tracer lathe. If changeover can be accomplished by simply changing a template, why use CNC? If you don't need sophisticated on-line programming, why use CNC? If you're dealing with a family of parts, maybe you don't need CNC. If you know your requirements, you'll make the right choice. There is a choice!
For more information on the Phillips E/E tracer lathe, circle E32.
Photo: 1. The hydraulic hose maze that identifies the typical hydraulic tracer lathe.
Photo: 2. The J D Phillips Servo-Trace tracer lathe is an E/E system that provides an alternative to CNC sophistication.
Photo: 3. Tracer template is easy to change.
Photo: 4. The E/E tracer accuracy is fine tuned by the template stylus mounted on the same slide as the cutting tool so that it reflects any dimensional change from temperature, torsion, or ball-screw looseness.
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|Author:||Phillips, James D.|
|Publication:||Tooling & Production|
|Date:||Feb 1, 1984|
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