Dielectric fluids for micro-EDMing

Theoretically, any insulating liquid can be used as a dielectric fluid when EDMing to allow the spark to cross the gap between the electrode and workpiece, and vaporize material.

When micro-sinker-EDMing, however, the ideal dielectric fluid has a high dielectric strength and a low viscosity. Those characteristics enable a fluid to withstand a high level of electric stress, while maintaining its insulating properties, and to flow through narrow gaps to effectively flush the debris.

An oil-based dielectric fluid is generally used in conventional sinker EDMing, and the same holds true when eroding microscale parts and features. That’s primarily because water doesn’t provide the quenching properties needed to achieve the accuracy and surface finish needed for micro-EDMing.

IonoPlus 3000 ET dielectric fluid flows in an EDM tank at Milco Wire EDM. Courtesy Milco Wire EDM.

Numerous dielectric oils are available that can be used for micro-EDMing. One is IonoPlus 3000 ET synthetic dielectric oil from Hirschmann Engineering USA Inc. Schaumburg, Ill. Produced by oelheld GmbH, Stuttgart, Germany, IonoPlus 3000 ET is for fine operations with small spark gaps. The fluorescent green fluid has a viscosity of 1.90 centiStokes (cSt) at 68° F and 1.40 cSt at 104° F, Hirschmann reports. That compares with a viscosity of 3.80 cSt at 68° F and 2.50 cSt at 104° F for IonoPlus 3000, which is for macroscale applications.

“The main difference between IonoPlus 3000 ET and the normal dielectric is the viscosity,” said Peter Knowles, president of Hirschmann. “The ET is geared for micro; it’s too thin for standard apps.”

Steve Miller, owner of Milco Wire EDM Inc., Huntington Beach, Calif., uses IonoPlus 3000 ET for micro-sinker- EDMing, such as producing 0.008"-wide slots in tungsten satellite components.

EDMing features smaller than half that size becomes problematic because the oil is likely too thick, and the electrical discharge process disintegrates the delicate electrode unless the power is at a level so low that cycle time is unacceptable, Miller noted. “I haven’t been successful EDMing anything smaller than a 0.004" slot because my overburn in the rough cut is probably going to be 0.001" per side, and that would make my electrode 0.002" wide,” he explained.

Regarding feature size, Dave Thomas, president of Sodick Inc., indicated that the machine builder’s sinker EDMs can produce a slot smaller than 10µm (0.0004") wide and a 5µm-dia. hole when using the company’s low-viscosity Vitol dielectric oil.

Although deionized water is the most common dielectric when wire EDMing, Thomas noted that Sodick introduced oil as a dielectric for wire applications about 25 years ago because users couldn’t impart the specified surface finishes with water. “Oil allows us to get a much finer surface finish,” he said, noting that finer than 2.0 rms is possible.

In addition, water can cause cobalt depletion when machining carbide, resulting in die failure because of microcracking. Wire EDMing with oil also improves performance when machining with wire diameters below 0.002". “The EDM spark is not as ‘hot’ in oil as it is in water, which ensures better protection of the wire and, therefore, a much more stable process,” Thomas said. Oil also enables a wire to cut a smaller kerf because the spark gap is about half the size achievable with water, he added.

The traditional downside is that wire EDMing in oil typically took about twice as long vs. water, Thomas noted. To address that, Sodick developed Eco Cut O, a combination of software and enhanced machine power supply characteristics to optimize the erosion process with oil. “With the technology, we can machine faster in oil than we can in water, depending on the target finish,” he said.

Despite most micro-sinker-EDMing operations being performed with oil, the use of deionized water as the dielectric fluid is growing because it reduces cycle time and extends electrode life by up to 100 times, noted Jerry Mraz, general manager of SmalTec International, Lisle, Ill. “We find ourselves using water more these days than not,” he said, adding that one end user he knows is evaluating using water to micro-sinker-EDM 300,000 platinum components annually. “The new dielectric of water has opened up micromanufacturing to high-speed, high-volume manufacturing—not just molding or R&D.”

Because water has a lighter viscosity (about 1.0 cSt at 68° F) than any petroleum-based product, Mraz explained, water is able to more effectively flush the minute particles from the spark gap, which is as small as 0.4µm on SmalTec equipment. However, not just any deionized water will do. It must have a high purity level, and the higher the resistivity, the higher the required purity. “We have a strict resistivity requirement that 16 megaohms per square centimeter is the very minimum,” he said. “If you have any impurities in the water, they’re going to wreak havoc on your spark.”

The water must also continually flow across the part. “If you keep it in a bath, you’re constantly ionizing that deionized water, and there goes your effectiveness,” Mraz said. He noted that effectiveness is reduced because water purity is compromised, and it’s the purity of the water that creates an environment in which to effectively form a consistent, small spark. “Without a good environment, the spark can be erratic in size and shape,” Mraz said. “This, then, directly affects the quality of the workpiece.” To avoid that water ionization scenario, the electrode can’t be larger than 0.5mm.

Workpiece materials also influence the choice of dielectric. Some materials are not suitable for being processed with water. They include titanium, which can cause some uncontrollable sparking based on material resistivity, and tool steels and aluminum because deionized water causes oxidation problems.

On the other hand, all stainless steel grades can be effectively EDMed with water, according to Mraz. “Water and stainless steel is the best match out there,” he said, adding that other materials can also benefit from water. µ

About the author: Alan Richter is senior editor of MICROmanufacturing. Telephone: (847) 714-0175. E-mail: alanr@jwr.com.