Getting the maximum from micro sinker EDMing

Micro sinker EDMing requires part manufacturers to optimize a host of areas to achieve success, such as applying new technology, developing new machining parameters, mastering electrode production and managing electrode wear.

For example, machining electrodes for micro sinker EDMing is difficult because part features, such as thin ribs, must withstand the rigors of being cut, noted Jason Kaczmarek, sales and applications engineer for Belmont Equipment & Technologies, Madison Heights, Mich., a supplier of EDMs and EDM consumables, and a producer of machined electrodes.

Round electrodes can be dressed on an EDM when producing microholes. All images courtesy of Sodick.

Machine advancements

Advancements in EDM equipment technology have played a major role in helping micro sinker EDMing shops optimize their operations, according to Dave Thomas, the president of EDM builder Sodick Inc., Schaumburg, Ill. Thomas cited the following recent developments:

• Linear motors. Conventional ballscrews are known for creating backlash and excessive wear, whereas linear motors eliminate those mechanical-drive problems because they are electronically driven. Linear motors also move an electrode in and out of the burn significantly faster than a ballscrew, enabling automatic and balanced flushing of the debris from the workpiece instead of manually forcing it from one side of a cavity to another, which allows the debris to build up in the cavity. “That minimizes electrode wear and improves the workpiece surface finish and cutting speed,” Thomas said.
• Automation. The level of micro sinker EDM automation ranges from a simple robot to load and unload workpieces and electrodes to integrating that automatic feeding of the EDM with a high-speed milling machine that produces the electrodes and automatically transfers them to the robot. “We’re seeing more and more customers wanting to run machines 24/7 with minimal operator input,” Thomas said.
• Electrode dressing systems. Although users must dress complex electrodes on a milling machine, systems are available for dressing round electrodes and the bottoms of flat electrodes on a sinker EDM. Dressing round electrodes is often done by rotating them with a 2,000-rpm head. (Round electrodes are often used to produce microholes from 0.004" to 0.0002".)
• Micro sinker EDM controls. Advances in EDM circuitry enable finer control of the spark and its waveform (see EDM Notes, May/June 2011). That enables holding finer corner definition on an electrode, which is critical when EDMing microparts. In addition, because modern EDMs are closed-loop, the CNC monitors the spark and, if needed, takes corrective action, such as moving the servo or changing the voltage, with response times measured in nanoseconds.

Electrode material options

In addition to equipment options, a variety of electrode materials are available for micro sinker EDMing, including graphite, copper tungsten and silver tungsten. When maximizing electrical conductivity is crucial, the high conductivity of silver makes silver tungsten an ideal material for EDM electrodes, according to Mi-Tech Metals Inc., an Indianapolis-based supplier of material blanks. Bob Wilson, sales manager for Mi-Tech, recommends a material with 65 percent tungsten and 35 percent silver, which has an electrical conductivity of 53 percent International Annealed Copper Standard (IACS). According to the standard, the conductivity of annealed copper is 100 percent IACS.

When using copper tungsten to make an electrode, Wilson suggests a grade with 75 percent tungsten and 25 percent copper for enhanced wear resistance. That material has an electrical conductivity of 48 percent IACS and a hardness of 94 HRB.

Metallic electrodes are able to impart a finer surface finish than graphite ones because graphite’s relatively porous grain structure is imparted to the workpiece surface. However, Kaczmarek noted that about 97 percent of the electrode material Belmont sells for micro EDMing is graphite. “Graphite is a much more machinable electrode material,” he said.

In addition to being difficult to machine, electrode material containing tungsten costs more than graphite and may not be as readily available, according to Jerry Mercer, applications manager for graphite producer Poco Graphite Inc., Decatur, Texas.

Mercer pointed out that graphite is available in a variety of grades, with the premium grades being comprised of smaller and more uniformly sized particles than intermediate grades. For example, a premium 5µm-particle material will have a tighter range in particle size and may vary only by a few microns. That microstructure assists burn stability, eliminating problems such as pitting and EDM arcing, he said. “As the particle size decreases, strength increases,” Mercer said, noting that Poco offers graphite with particles as small as 1µm.

With current EDM machine technology, even with its porosity, graphite electrodes can impart a surface finish as fine as 2 to 3 R_max, Mercer noted. “A mirror finish, I should say.”

Wear considerations

Controlling electrode wear adds to the complexity when micro sinker EDMing. That’s because EDMs must burn small, delicate features into parts at low amperages and with short on-times to achieve tolerance requirements, but it is the longer on-times that reduce wear through “a replating action” from the workpiece to the electrode, Mercer explained.

Delicate electrode features, such as these ribs, must be able to withstand the rigors of being milled.

Miles D. Holder, commercial manager of ultrafine materials for graphite manufacturer GrafTech International Holdings Inc., Emporium, Pa., noted that with any sinker EDM electrode there are three primary areas of wear: side, end and corner. When micro EDMing, corner wear is the most critical because it’s more likely to cause out-of-tolerance parts, he added.

“You’re going to have wear concentrating at sharp corners prior to any other area,” concurred Poco’s Mercer.

A graphite electrode for sinker EDMing microscale honeycomb features.

Holder noted that to EDM an acceptable part, multiple electrodes might be needed to bring it near its final specified shape before a final electrode completes the job. “The part is then as sharp, crisp, clean and as close to the final tolerances as it can possibly be,” he said.

GrafTech, headquartered in Parma, Ohio, manufactures Micron “bundled technology” graphite. Holder explained that, in addition to the bonds between individual graphite particles generated during the mixing and heating of the material, the company engineers those particles into high-density bundles, which are also bonded together.

An electrode wears because the EDM process breaks the bonds in the material. “By having more than one molecular bond in the microstructure of the material, both bonds would have to be broken for wear to occur, and that’s really the unique aspect of the product,” he said. “Because the graphite wears less, it holds the integrity of the shape that’s been machined into it longer.”

Holder added that wear occurs as individual particles break from the electrode rather than through removal of bundles.

GrafTech offers a family of bundled graphite materials, with the Micron E-970 grade having the smallest particles, which measure around 4µm. That material has a density of 1.85 grams/cc, which is at the high end of available graphite for EDM electrodes, according to Holder.

Although the finer the grade the higher the material cost, Holder noted that GrafTech’s prices are comparable to similar grades. However, cost isn’t a significant consideration when the requirements are demanding and the amount of graphite consumed is relatively small. “Especially in the micromachining segment of the market, the cost of the electrode material becomes extremely negligible,” he said.

For higher-performance requirements, GrafTech offers bundled copper-impregnated graphite. That material is suitable when flushing is a challenge, such as when EDMing deep cavities, according to Holder. “All things being equal, copper tends to flush more easily than graphite,” he said.

Holder added that copper-impregnated graphite, which costs twice as much or more than standard graphite, is also appropriate for producing very thin features, where electrode chipping and breakage is a concern, and for inexperienced EDM operators. “The copper impregnation tends to make the graphite more rugged and forgiving.”

Multiple methods are available for machining microparts, but micro sinker EDMing can be the most-effective—if not the only way—according to Poco’s Mercer. “EDMing gets you into features where cutting tools can’t,” he said. µ