A New Angle On Wire EDM

"The Cutting Edge." That is how Applegate Wire EDM, Ltd., styles itself on letterhead and in company publications, and it fits—not only because it's a catchy cachet for EDM, but because it also describes the nature of much of Applegate's work.

Wayne Applegate's Dallas-based job shop specializes in the fabrication of very high taper pieces in stainless steel and aluminum. Often involved in larger profile work, Mr. Applegate faced a challenge not uncommon in EDM: the tendency for some cutting wires to break where taper angles exceed seven degrees on machines with non-tilting upper and lower guides such as when working on a recent project, a 2.5" aluminum transitional die.

In the GFAGIECHilles machines Applegate uses, wires with a tensile strength of 100,000 psi range or more, simply did not like to bend at the tangent point of the wire guide and so they often broke. A critical consideration when selecting a wire for higher tapers is to choose a wire with ample stretch or elongation. Typically, greater than 30 percent elongation is required for tapers to 45 degrees. But further complicating the wire selection challenge, auto-threading machines like the GFAGIECHilles and others usually work best with higher tensile strength wires that have less than three percent elongation.

Aside from being just plain annoying, a break in the EDM cutting wire can sometimes leave a mark in the workpiece and, of greater impact to costs, instantly change a machine from a money-maker into an unproductive investment.

Production quality is negatively impacted by the wrong choice of wire as well. Harder, high tensile strength wires, though often better able to resist deflection under cutting forces, can cause vibration as the wire's "memory" fights the machine's round guide profile geometry and high-taper setting. The result: intolerable lines on the workpiece, and more lost material and production time.

Since the effect of wire choice on profitable production is so critical that it can make or break a shop, extra time and experimentation with test cuts must be figured into the decision making process when experience is lacking. The ideal, of course, would be an EDM wire with sufficient tensile strength to resist breaking at significant taper angles, plus adequate elongation for the required taper, plus reasonably high conductivity for greater cutting speed.

The answer for Applegate EDM was Gisco's Cobra Cut B 0.0.10", a CuMg (copper magnesium) wire with a treated Zn (zinc oxide) coating (Gisco Equipment, Hauppauge, New York).

How Applegate Wire EDM came to use the Cobra Cut B is a story of cooperation among three geographically diverse wire EDM companies: Applegate Wire EDM, Wiretech EDM (Alamitos, California), and Automated EDM (Minneapolis-St.Paul, Minnesota). The three similar job shops have backed each other up with process information and additional capacity in peak production times.

Applegate manager Terry Click was once closely associated with Wiretech owner, Jack Beggs, who specifically recommended the wire to Mr. Click. Mr. Beggs had been working big tapers since 1989, cutting up to 30 degrees and spent 18 months experimenting with various wires until his west coast supplier recommended the Gisco product.

Mr. Beggs solution was exactly what Applegate needed. But Mr. Beggs cites another plus. With its Zn coating, the Cobra Cut B is said to cut 30 percent faster and its slightly rougher finish helps carry EDM residue out of the slot.

"On balance, the three times greater cost of coated product is by far outweighed by the advantage of getting almost a third more work from our machines," Mr. Beggs observes. He also points out the Cobra Cut B CuMg alloy has unusually good conductivity for its softness--62 percent IACS and that makes it an optimum choice for aluminum, especially for deep cuts.

Another of Wayne Applegate's associates, Automated EDM co-owner Wally Pelto, is equally enthusiastic about the wire solution. "One of the most important elements of automated wire EDM is a reliable auto-threading system," Mr. Pelto acknowledges, "but an auto-threading system is only as good as the wires you use..."

Mr. Pelto's partner, Don Zoubek, added: "In the past, we have tried other brands and you may have get several good spools and then you get one bad one. Inevitably its the spool you put on before what should be a long, unattended burn."

So, three successful, widely separate wire EDM job shops have become convinced of the production benefits of coated wire because of its application-specific configuration, but most of all, because of its consistency and reliability for fast, trouble-free EDM operation. MMS