Controlling the electrical-discharge power supply

Effectively controlling the spark energy of an EDM’s power supply is critical to meeting cycle-time and part-accuracy requirements for all sinker EDM applications. But it’s especially important when micro-sinker-EDMing because a lack of control causes more electrode wear, and—in the fine-feature micro world—when you lose a little, you lose a lot.

To achieve that control, the ramp phase, or initial spike, of the electrical discharge’s on-time needs to be short, ranging from 0.5 to 4,000 microseconds, noted Takuro Sato, applications engineer for EDM and milling machine builder Sodick Inc., Schaumburg, Ill. When the power supply applies a significant amount of energy into the spark in a short time, “it is easier for the spark to ignite, which improves machining speed and stability,” he said.

Sato added that “the degree of difficulty for electrodes to discharge varies depending on the material type. The more difficult examples are carbide, titanium and nickel-base alloys.” The faster ramp phase of the on-time can have a dramatic effect on the machine’s performance with these materials.

With the “Arc-less” electrical- discharge circuit from Sodick, the company reports that an end user has significantly reduced sinker EDMing cycle time and consumes fewer electrodes, such as this 0.04"×0.98" one with 1.0° side tapers (top). Workpiece is below. Images courtesy Sodick.

As a further development of its power-supply technology, Sodick recently introduced the TMM3 circuit, which controls how the pulse wave ramps up and controls the energy within the on-time phase to a much finer degree, according to Sodick President Dave Thomas. “We bring the maximum energy of that particular on-pulse in a much shorter time,” he said. “The ramp phase of the on-time is much faster than the previous generation.”

The main difference between Sodick’s older and new power supplies is the latter does not use resistors, Sato noted. “So the discharging can be controlled to a finer level.”

Electrodes used for micro-EDMing applications are small. But it’s not uncommon for an electrode to have a high depth-to-size ratio, and its shape must remain consistent to achieve the required accuracy from part to part. Sodick’s latest power-supply circuit minimizes electrode wear to less than 0.01 percent per part and has increased machining speed up to 40 percent, according to Thomas.

He noted that two types of electrode wear primarily occur when sinker EDMing: surface and corner loss. “In micro-EDMing, control of the corners is critical,” he said. Electrodes are primarily made of copper-tungsten for microscale applications, Thomas added, because they hold corner definition better than pure copper or graphite.

Multiple enhancements

Similar to other types of machining operations, effective micro-sinker-EDMing requires a “systems” approach. Incorporating one enhanced technology on its own is not likely to produce the desired results. Sato pointed out that Sodick’s TMM3 circuit works in combination with the company’s “Arc-less” technology and SVC voltage-control circuit. Collectively, he noted, they monitor the discharge gap and automatically adjust cutting conditions and on-time ramping to optimize performance and avoid arcs, which damage the electrode and part, while machining at up to four times faster than conventional EDMs.

In addition, a linear-motor drive system and the company’s motion controller eliminate the need to flush debris from the electrode gap with side-mounted auxiliary flushing lines, Sodick reports. With an axis speed up to 1,440 ipm and a 1.2G Z-axis acceleration, the electrode moves at such a high rate that it creates its own flushing action. According to Thomas, if someone tried to achieve that highly repeatable pumping action with a ballscrew drive, the ballscrew would need to be replaced every 3 to 6 months because too much heat would be generated in the ballscrew and nut, causing premature wear. As soon as a ballscrew wears, the electrode will not return to the same position each time, he added. In contrast, a noncontact linear motor’s mechanism allows for permanent maintenance of accurate axis movement without backlash.

A workpiece shows a part feature before and after sinker EDMing (the basic shape was milled prior to EDMing). By using an electrical-discharge power supply to achieve the highest possible on-time, electrode wear is virtually eliminated, according to Sodick. Photo courtesy Sodick.

“If we didn’t have linear motors, we couldn’t apply all of our technology because we wouldn’t be able to move and control the electrode accurately enough,” Thomas said.

EDM vs. HSM

High-speed milling advances have replaced many sinker EDM applications over the last couple of decades because HSM provides a faster material-removal rate.

However, Thomas sees growth in the micro-EDMing market, especially for Japanese manufacturers because of their expertise in microelectronics and other microparts. And, in some cases, HSM complements sinker EDMing. An example is when an end user mills the basic part features and then finishes them by EDMing sharp corners. “More and more manufacturers want to machine smaller and smaller features, so improvements in EDMs must be made,” he said. “Electrode wear and machining speed are key areas that our new technology has improved.” µ

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

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