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EDM advances spark new uses.

Once viewed as a specialty process for single-piece production, especially in mold making and platen industries, Electrical Discharge Machining (EDM) has undergone a true metamorphosis in the last 10 years.

Through the use of power-pulsed generators, metal, ceramic, and composite materials can now be burned away to create entirely new shapes and finishes to 0.2 [micro]Rmax to 0.3 [micro]Rmax which isn't very revolutionary news, except when details of how this is being accomplished are viewed in contrast to past practices.

Today, sinker EDMs are more precise and more productive, owing to several recent key developments.

Higher precision construction of the mechanical frame has improved machining accuracy, while economizing on space utilization. Ballscrew bearings in the bottom of the head-back have streamlined head motion, creating a more compact yet precise Z-axis motion in conjunction with an AC, smart servo, direct-drive motor.

A primary change in the control logic of sinker EDMs has been perhaps the biggest reason these machines can now be viewed as viable alternatives to traditional machining centers.

The latest generation of CNC con-. trollers feature far greater programming power for monitoring and controlling machining operations. RS232 communication and floppy disk output can allow faster, more reliable storage of programs and information about the machining process.

The controller itself has gone from simple electronic circuitry to the highly advanced 32-bit CPU digital response now found on new generation EDMs. This "fuzzy control" is adaptive to the particular conditions of an individual burn and will actually compensate machine variables of speed, distance, pulse, and power to achieve more consistent results, rather than simple replication of a series of machining steps.

A great deal of cooperative engineering has occurred between EDM and controller manufacturers to place greater emphasis on software. A 32-bit CNC controller with 32-bit processors is a faster and more adaptive tool, but only if the language it speaks to an operator enables the person to quickly interpret and react to data. Again, the best EDMs are those with advanced control logic and simplified operator interface.

Flexible Automatic Programming language, that of the CNC. controller, can frequently be learned in one day. Often, training is not needed if the operators have prior experience on sinker EDMs, because the language is nothing new to them.

Another benefit of the new controller enhancements found on today's sinker EDMs is better graphic displays and instantaneous call-up of key process data. Three-dimensional display terminals and remote control devices able to display the readouts of up to six axes (X, Y, Z, A, B, and C travel) to an operator make running a machine a simpler task. The combination of 32-bit processors, fuzzy control, and easy-to-learn program language is a must for high-production sinker EDM operation.

Simultaneously, the controller must be making good parts. Today's fuzzy control is capable of watching every spark, as many as a million-a-second, continuously evaluating the current condition against a preset goal and self-adjusting the operation to achieve an optimum end result. More sensors on the machine make this an even greater reality and will only improve operation of a sinker EDM in production runs of the future.

While it may seem a subtle change, fuzzy control differs greatly from adaptive control, which is unable to "read" a program in the true sense. Fuzzy control performs not only the on/off function of pulse generation, it controls the adjustment of the cycle, all the time. This means the controller perceives and reacts to conditions at a tremendous rate of speed, changing the rates of certain operation parameters in harmony with a predetermined ideal model. In a word, it thinks (or at least emulates human intelligence).

Another equally important development in sinker EDM technology has been the toolchangers and ancillary robotic handling devices for workpiece positioning. Tool changers on'the machine have meant different cavity sizes could be made on a workpiece without substantial downtime. Also, the presence of other workpieces in the work table area and the machine's ability to feed these into electrode contact made sinker EDMing a viable, cost-accountable method of high-volume production.

Clearly, future applications of sinker EDMs by old and new users will change dramatically as a direct result of these advances.

On an even higher plain of factory automation, sinker EDMs are being considered for workstation concepts, in which one operator will run multiple machines, conducting piecework, test, and quality checks simultaneously.

Not everything about EDM is in a state of flux. There have been only slight changes in electrode material used for sinker EDMing, though copper impregnated and micrograin materials pose interesting possibilities. Virtually all sinker applications today can be accommodated by one of the many graphite materials now available.

In the area of oils and other dielectrics, the safety, health, and odor considerations remain the manufacturers' chief concerns. Disposal of spent materials has launched some very creative thinking, including bum-off with fuel oil.

A reachable goal of maintenance free operation, except to change filters, dielectrics, and electrodes, has been established and seems likely within the next decade.

The small-hole devices, orbital and tilted capabilities, and multi*axis machining techniques being developed by sinker EDM companies will better facilitate side entries and undercuts, where sinker EDMs have marked advantages over traditional machining centers, as well as conventional EDMs.

Wire EDM

As a general trend in recent years, controversial remarks concerning speed and accuracy have been replaced by new thinking, resulting from the arrival of "smart" AC servomotors and CNC controllers on the scene. Overall finishing time has become a much more important criterion for evaluating EDM.

Less work has gone into programming development than has occurred in the evolution of the controller. This is especially true in wire EDMing because of the extremely tight tolerances involved, plus the attendant problems of residue and flushing.

Some wire EDM operators admittedly are intimidated by the new controllers, especially operators that came from a manual and tape-drive background. Most are not, and others should not be, because of the help they receive from the machine to perform their overall jobs better, faster, and with a greater utilization of their other skills.

CAD operators have welcomed the development of CNC controllers in wire EDMing because of the complex shaping requirements that invariably must be done off-line. Parts can now be designed and downloaded directly into the new generation of controllers. Design work translates more easily today from a CAD terminal to a CNC controller-driven wire EDM. Many piece-part cases require only a few variations of a standard program and can be done on the machine.

Expanding on the cutting speed controversy that has surrounded EDM for years, it matters more that an operator and the company purchasing wire EDMs concentrate on the concept of Total PartTime (TPT), comprising setup, programming, roughing, skimming, and finishing.

Setup still owes much to the 3'Rs: reading, 'riting, 'rithmetic. Only today, most of this work is done by them computer.

Programming is likewise directly affected by knowing how to use your CAD/CAM equipment.

Roughing is now, and always has been, strictly a factor of horsepower on the wire EDM.

Skim cuts and fine finishing are "where the action is" today. New TL circuit designs in the power supply have, as the goal, only two passes to achieve an acceptable fine finish. TL circuits reduce peak current, increase frequency, create a smaller spark, cause less wire vibration, effect more wall straightness, and thus produce a finer finish.

General construction: wire EDM

Years of customer input, classroom experimentation, and the sophisticated market tool of the focus group have collectively resulted in the new ergonomic designs of today's wire EDMs. The machines occupy less floor space, have more power, require less maintenance, and produce far less ambient heat output, all of which improve operator comfort.

Back in 1986, a major change had occurred in the power regeneration circuit, resulting in less heat and less power consumption. A new pump style arrived on the market, which put less heat into the air or water, thereby easing the operation of the machine fan, cooler, and chiller. Improvements here continue.

Machines today are quieter, owing to better overall construction. Linear motion bearings outperform old needle ways. Double-nutting ballscrew. construction eliminates the backlash problems of earlier machines. Drive motor technology has progressed dramatically on wire EDMs from AC stepper motors to DC servomotors to AC smart servos. Improved positioning capability from 1[micro] (micron) to 1/4[micro] to O.1[micro] has all occurred in the last six years. Likewise, harmonic drives gave way to split gear and finally, direct drives for marked improvements in positioning workpieces overall.

Controls: wire EDM

In 1985, dynamic random access memory (DRAM) logic was being used for control operations on wire EDMs. Bubble memory maintained program storage, was expensive and slow to access, and generated considerable heat. Today, 32-bit controllers feature static RAM, consume less energy, access more easily, and operate cooler. At the same time, they provide increased control, more memory, better retention of memory with NiCad batteries vs old acid batteries.

Maintenance: wire EDM

The newest machines are compact, lower for easy reach, and feature "face the front" designs for easy access to filters. Auto-threading, better filters, and significant improvements in power feeds, especially the new indexable versions, have made wire EDM operations easier and more consistent.

The new multitasking controllers make it possible for an operator to check programs while the machine is in operation.

The new coated wires are increasing conductivity to improve speed and surface finishing. EDM companies are also working with wire manufacturers to better match new power supplies to the right wire materials.

Future trends: wire EDM

Because of the speed and surface finishing improvements, production runs are the future for wire EDM.

All one need do is run an honest comparison between milling, grinding, and deburring time vs today's EDMing capability and, most often, the numbers tell the tale.

One company, for example, reported a reduction from 17 steps to six steps and an overall improvement of 30% in the production time for steam turbine components, with comparable finished-part quality.

A part for the medical industry, now made on a wire EDM, went from 23 rain to 5 min in production. Also, multi-fixturing and submerged operation allowed the company to go from a four-part setup to a 12-part totally unattended setup, resulting in significant savings and improved finish on the part.

Carbide work, as well as wire-cutting titanium, has reintroduced EDMing into the aircraft and aerospace industries after some earlier difficulties.

As broaching time gets too costly, wire EDM is becoming a viable alternative for hole making.

As an overall production tool, EDM allows faster turnaround from design to prototype to shop floor manufacturing. This scenario is allowing EDM to make headway, even in the formerly resistant automotive segment of the market.
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
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Title Annotation:electrical discharge machining
Author:Yamada, Atsushi; Zeman, Dan; Langenhorst, Greg
Publication:Tooling & Production
Article Type:Cover Story
Date:May 1, 1992
Previous Article:Measuring complex surfaces.
Next Article:Coldformer makes precise parts.

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