EDM Shops | Development of Wire EDM

Mark Albert

High Performance EDM

Spectrum Manufacturing is clearly the leading pioneer of high performance wire electrical discharge machining (EDM). This Wheeling, Illinois, job shop routinely cuts two or three times faster than most shops using standard wire EDM units, even those with the latest generation of equipment. And it does so without compromising accuracy or surface finish.

Doubling, and in many cases, tripling cutting speed is a remarkable feat in itself, but that is not the most remarkable thing about Spectrum as a leading EDM user.

What is outstanding is how the shop has taken EDM in entirely new directions. Extremely small parts. Extremely large parts. The one-of-a-kind "impossible" parts. Parts by the hundreds of thousand. They are using wire EDM as an advanced tool that calls for new thinking about how many parts can now be manufactured.

High cutting speed is only part of this phenomenon. In fact, Bill Fricke, president of the company and one of four partners that make up Spectrum's management team, would rather talk about efficiency when discussing high performance EDM. "You can't think in terms of just speeding things up," he explains. "Programming, fixturing, power settings, flushing - every aspect of the EDM process - has to be part of a strategy to machine work-pieces at higher quality and lower cost than by any other method." In many cases, he points out, that strategy may include other technologies in creative combinations.

Spectrum has been innovative with laser cutting, multi-axis ram EDM, and computer-aided design/computer-aided manufacturing (CAD/CAM). Every one of its 25 ram and wire EDM units has computer numerical control, and all are linked to a shop-wide data network. "We are not afraid to experiment, to modify our equipment, explore every possibility, or develop our own systems and software." Mr. Fricke points out.

Not Afraid To Experiment

In fact, the development of high performance EDM sprang from Spectrum's willingness to change procedures and try new things. About four years ago, Spectrum was asked to take on a project for an aero-space contractor. It involved cutting 168 rectangular openings in tungsten alloy billets, 7.5 inches thick. Each opening had very precise geometry. The job was completed on a heavily modified wire machine. These modifications eventually became the basis for a high-speed cutting system that is now sold as a product line by a Spectrum subsidiary, T-Star Industrial Electronics (see box).

High Performance On The Market

Fast-Track is T-Star's family of enhancement modifications for various wire machines. This package is typically retrofitted in the field on existing equipment.

How big of a difference does it make? According to Tom Truty, T-Star's president, a two- to four-fold improvement in cutting performance is common for the various Fast-Track models. And, he notes, surface finish will be much improved because a patented feature allows smaller amounts of material to be removed in larger numbers, with less energy per spark.

In addition, T-Star and Spectrum have been conducting a joint development program with Charmilles Technologies Corp. The result of this project is FAST-CUT, an enhancement package tailored to Charmilles machines. This package is now standard on certain models.

T-Star's research and development efforts are continually producing a variety of electronic and mechanical modifications or inventions, all related to enhanced wire and ram EDM. It has numerous patents to its credit. Many of these products can be traced to experiments originating on Spectrum's shop floor.

The results of the original modifications hinted at their potential. Because the billets were made of a tungsten alloy that is extremely dense, cutting the openings was equivalent to cutting a steel plate almost 16 inches thick. To get the required accuracy at cutting speeds then available, it would have taken about nine months of machine time to complete each of these extraordinary work-pieces. After modification, however, each piece could be produced in three months of machine time.

Spectrum has since equipped all of its wire machines for high performance, where typical jobs are measured in hours or days. A similar three-to-one speed improvement is not uncommon.

Essentially, this system is a package of electronic, mechanical, and flushing enhancements. Together, they create a synergy of effects that makes high-speed wire cutting both possible and practical.

The chief electronic enhancement is patented circuitry that energizes the wire separately at upper and lower wire guides, effectively doubling the number of sparks that can be discharged at any given time. The servo system is also modified for greater performance.

The chief mechanical enhancements include redesigned wire guides that are mounted very close to the workpiece, interchangeable cartridge units for fast changeover of wire diameters, and passages for high-pressure flushing.

The chief flushing enhancement is a high-pressure pump and nozzle delivery system. Dielectric fluid is delivered to the spark gap at higher pressure but under greater control. Unwanted particles are removed faster and the fluid is ionized for the next spark sooner.

New Possibilities

Routinely doubling wire cutting speed has opened up a new world of possibilities - and a whole new set of challenges - for this job shop. Exploiting this new capability meant adopting new strategies for applications that were once beyond the scope of wire EDM. But efficiency, not speed, is the thrust of these strategies.

Bill Fricke stresses that efficiency must be multi-leveled. Metal removal by the wire, setup and workpiece fixturing, workflow and scheduling must all be efficient. It is Spectrum's ability to achieve efficiency on every level routinely that makes the difference.

Efficiency For High Production

A good example of an application that typifies Spectrum's multi-level approach to efficiency is an advanced computer component made of a special alloy. The geometry of this workpiece is both complex and tightly toleranced ([+ and or -]0.00025 inch). Wire EDM is the ideal method of machining such a workpiece, but the production run numbers in the hundreds of thousands. Wire EDM is only cost effective if production rates can meet the customer's time constraints.

To perform the work economically, Spectrum teamed the principle of double-head wire cutting with submerged machining in a small overflow tank as shown in Figure 1. The double-head concept uses a single wire that follows a U-shaped route (Figure 2). This unusual routing allows the wire to cut twice as many workpieces simultaneously in side-by-side setups. Workpieces can be ganged in multiples on special fixtures, as in the case of this computer component, or stacked so that numerous workpieces are cut at once by a single wire (the number depends on the thickness of each piece). Shuttle fixturing principles are typically used with this concept. Quick release clamps are also used at the machine to speed fixture exchange, thus minimizing downtime.

The small overflow tank fills and empties quickly for fast changeover, yet provides all of the advantages of submerged cutting. The front wall of the tank is transparent, allowing clear observation of the cutting process. The job runs on two machines around the clock, seven days a week.

Interestingly, automation isn't always useful in the pursuit of efficiency. For example, the double-head arrangement with manual threading yielded substantially more parts per shift than possible with one of the shop's single wire units with automatic wire rethreading.

Laser/Wire EDM Combination

Sometimes drawing on the strengths of several processes in one application is most efficient. A particularly promising combination is laser and wire EDM.

At one time, Spectrum concentrated its development efforts on laser cutting, producing precision high-speed cutting systems that were successfully marketed for several years. Although the company no longer sells laser systems, it still uses them extensively as a high-precision production tool. Expertise in laser cutting is particularly valuable for engineering high-production applications that take advantage of both technologies.

A family of parts produced for a well-known armaments manufacturer exploits the speed and accuracy of laser cutting as well as the speed, accuracy, and fine surface finish of the shop's wire EDM. Blanks are cut from steel strips, as shown in Figure 3, at 50 inches per minute using a synchronous pulsing [CO.sub.2] laser. Individual pieces remain attached to the parent strip by a small web. The laser brings the pieces to within 0.002 inch of final size, with an excellent edge by laser standards but not sufficient to meet the customer's requirements.

The strips are then stacked twelve high on the wire machine. Using a high-speed trim cutting mode and double heads, the stacks are recut to gain the accuracy and surface finish wire EDM can produce. The wire trims at 80 square inches per hour. A simple milling operation produces a beveled cut from end to end along the tab to complete the strip. Individual pieces are then broken away from the strip to finish.

In less than two and a half weeks, Spectrum produced 5,000 pieces. Laser alone could not meet the accuracy and finish specifications; wire EDM alone could not meet required production rates. Together, they were quite cost-effective.

Unique Fixturing

Production jobs are not the only area where unique fixturing and special modifications pay off. Spectrum cut the barrel-shaped workpiece shown in Figure 4 by modifying a relatively small wire unit. This workpiece, a large bearing component weighing 3500 lbs and standing 30 inches high, had to be cut into two identical halves for assembly around other components. Because the guide system developed for high performance is modular, it could be configured to clear the full height of this workpiece.

To cut this part effectively, special flushing fixtures also had to be designed and the electronics from its high performance system were modified for additional fine tuning.

Prototype Of The Future

Spectrum is one of a handful of job shops that might be considered prototypes of the future. Contract manufacturing is clearly undergoing a metamorphosis, and the shape of things to come can be glimpsed here. To prosper in the face of competition that often comes from around the world, shops will have to push the state of the art ahead for themselves. Like Spectrum, they will not only be expert users of some technology, but also inventors and developers of that technology, as well as leaders in innovative applications of their own discoveries.

Given the nature of wire and ram EDM at the time of its founding, Spectrum virtually had no choice but to pursue a course as both user and developer of this technology. Indeed, the prospect of being pioneers was a primary motive for the shop's co-founders.

As Bill Fricke puts it, "We could all see that EDM, especially wire, had tremendous potential. And we all recognized the technical barriers that were holding things back. But we didn't let those barriers give us preconceived notions about what it could or couldn't do. We didn't let them limit our imaginations."

While no shop can claim the future, it certainly appears that the future belongs to those with ingenuity and imagination.

PHOTO : The shop floor at Spectrum Manufacturing is both production center and "laboratory". Top management encourages its skilled employees to explore new ways of doing things, to experiment and tinker, with the goal of improving quality and efficiency.

PHOTO : Fig. 1 - Efficiency is more than high cutting speeds. By ganging workpieces, cutting with a double head, and using a small overflow tank, a special alloy computer component is produced by the hundreds of thousands.

PHOTO : Fig. 2 - The wire electrode feeds from top to bottom at the left, rounds two wheels, then feeds in the opposite direction at the right. This double head concept allows two workpieces or two groups of workpieces, to be cut at once.

PHOTO : Fig. 3 - This strip of break-away parts is cut on a laser (for speed), then finished on a wire EDM (for accuracy and fine finish).

PHOTO : Fig. 4 - A 30-inch high wire cut makes this application remarkable. Numerous modifications to the machine's electronics, flushing system, and wire guide were required.