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Tuesday, July 29, 2014

High-Precision Sinker EDM Means Zero Margin for Error

Credit: GF AgieCharmilles

Credit: GF Machining Solutions

Extremely high-precision sinker-style electrical discharge machining (EDM) is an unforgiving process, and every aspect of it — from producing electrodes, to workpiece set up, to maintaining consistent machine temperature — must be as close to perfect as possible. Because when part tolerances run as tight as two microns or less, even the slightest of inaccuracies at any stage are magnified and will negatively impact the level of achievable precision.

For true high-precision sinker-EDM operations, especially in micro applications, shops must take a total process approach. This involves more than just a sinker EDM with the special capabilities needed for high-precision operations. Shops must also consider their processes for machining electrodes, transferring them to the EDM, and setting up workpieces within the machine. Every step is critical to achieving true high precision.

Starting with initial steps of the sinker-EDM process, shops should first eliminate any inaccuracies in their electrode manufacturing operations. Electrodes must be machined to exacting sizes — even more so when burning micro-sized parts — and be as equally high precision as the sinker EDMs that will use them. For maximum accuracy as well as superior part surface finishes, EDM machine builders often recommend metallic electrodes, such as those machined from copper tungsten, as well as using high-precision equipment to cut or grind the electrodes.

When shops move the electrodes from their high-precision milling or grinding machines to the sinker EDMs, they must take  significant care in unclamping and re-clamping the electrodes for each process. Today’s modular pallet systems provide this all-important consistency and repeatability. In most cases, the pallet systems repeat well within a micron and prevent positioning errors or stacked tolerances as electrodes move from one machine to the next.

Modular pallet systems offer the same positioning benefits for the machine setup stage as well. Such workholding ensures the highest level of accuracy, especially when EDMing very small parts. Shops can quickly and easily clamp parts onto workholding pallets and then use a coordinate measurement machine or optical measuring device to determine the exact workpiece location data that the EDM will use to calculate the appropriate offsets. Or the EDM can do the measuring if equipped so.

Sinker EDM machines can be equipped with an optical measuring device mounted to the C-axis, allowing them to autonomously determine exact part positions. This capability eliminates the risk of error involved with the transfer of workpieces from separate measurement equipment to the EDM. Sinker EDMs with on-board part measuring are capable of actual high-precision machining because they provide extremely accurate axis movement. And the key to achieving that on a consistent basis is temperature control.

Ambient room temperature and that of the machine must remain stable and unchanging. A sinker EDM that is unable to consistently maintain optimal operating temperature to within 1°F will fail to hold positioning accuracy, not to mention demanding part tolerances of one or two microns.

To efficiently control heat generation, many of today’s EDM machine builders will not only construct machine bases and frames from other materials, they will incorporate some type of cooling system. For instance, GF Machining Solutions uses a special polymer for its high-precision sinker EDM bases and has developed a new machine design that incorporates cooling channels throughout its entire base, table, and upper head (C axis) to control heat.

This internal thermal stabilization system uses the machine’s dielectric fluid/water, which is kept at an operating temperature of 68°F. The dielectric water circulates through all the internal cooling channels and keeps the machine temperature to within +/- 1°F of the set temperature to virtually eliminate any thermal machine growth that could negatively affect positioning accuracy.

In addition, for high-precision applications, EDM machines should have an actual chiller, as opposed to just a heat exchanger. This efficiently and accurately maintains dielectric water temperature, especially if the machine lacks any other thermal management system. While housing the machine in a climate-controlled room helps keep heat in check at the outside mechanics of the machine, the dielectric water controls table and workpiece temperatures. However, other systems and pumps on the machine generate heat that warms the dielectric water.

Besides temperature control, high-precision sinker EDMs must provide accurate motion control/positioning. Most machines achieve this through glass scales on all axes, and, often, closed-loop control systems physically drive those axes for extremely high precision.

Credit: GF AgieCharmilles

Credit: GF Machining Solutions

Within its closed-loop system, GF Machining Solutions uses encoders on machine ball screws and glass scales on the axes. The system first measures encoder resolution and then compares that to the absolute positioning of the glass scales. For instance, if the encoder indicates it has performed 10 revolutions, then the axis should be at the 10-inch position. However, if the glass scales indicate otherwise, the machine will then automatically adjust for the difference.

Separate from upper-head cooling, a liquid-cooled C axis is also critical for accurate sinker-EDM machine motion. Cooling jackets that envelope the rotating C axis for temperature control can be incorporated for high-precision machines. The C-axis cooling system is in no way part of the coolant-through-the-spindle function, which is standard on practically every sinker machine to provide optimal flushing during the burn.

Proximity of a machine’s C-axis drive motor also influences the positioning accuracy of that axis. The further away the motor is from the spindle and chuck, the higher the risk of error. When machine designs place these motors as close as possible to the chuck and spindle, they create not only a high-accuracy-positioning C axis, they result in extreme precision even if electrodes are a bit off-center of the axis.

For high-precision operations, and even more so during micro part applications, a sinker EDM’s generator has to continuously communicate with the machine’s control. While many EDM machine builders are just now incorporating fiber optics to enhance this critical communication link and to speed data flow, GF Machining Solutions has been using the technology for well over 20 years to quicken the response time of its generators.

Constant feedback — originating from sensors monitoring the spark gap and moving to the control and generator — allows machines to adjust generator settings for maintaining optimal spark and precision cutting conditions. This capability, especially in making very fine adjustments, is extremely critical for micro EDMing, due to the relatively small sizes of the workpieces, the electrodes and the gap between them, as well as the spark that is generated for burning.

Most sinker EDMs can effectively generate sparks, but what many of them lack is the capability to automatically adjust those sparks as cutting conditions change. To address changing gap conditions, these machines will apply faster Z-axis speeds to improve flushing and attempt to keep a clean gap. In reality, a spark adjustment is needed to overcome the poor cutting conditions. Instead of approaching the issue at the front (spark) of the operation, a large number of machines address it from the back end (flushing).

Unfortunately, the latter approach greatly reduces the amount of time the electrode is engaged in the cavity and removing material, thus resulting in lower machine production levels. Parameter off-times are longer, which means less spark generation, and the electrode is extracted out of the cavity more often. The key should be to stay in the cavity and only retract the electrode if the generator is unable to adequately modify the gap and/or spark conditions or if there is too much contamination.

Sinker EDM controls must process data and feedback at lightning speed for true high-precision applications. In some instances, the sinker machine’s CNC hardware runs in the background and an HMI (human machine interface) operates on top of, or in front of, the hardware. For example, GF Machining Solutions’ HMI works in a fashion similar to how a computer uses an operating system. It helps manage information to improve processing time. Also, various functionalities are divided among and dedicated to multiple processor technologies, so no individual one is overloaded and slow. This allows the control to quickly react to all the cutting data being gathered.

To optimize programming for high-precision operations, machines must provide for more in-depth application descriptions. They need to offer as many parameters for as many combinations of various electrode materials, cavity shapes, and part materials as possible.

The key is being able to define the process and determine the proper technology settings prior to generating the first spark. For instance, GF Machining Solutions machines offer numerous parameters for process optimization and reducing electrode wear. If a shop is using a fine-grain graphite electrode on a titanium workpiece, operators enter the electrode type, required surface finish, and other desired performance conditions, and the machine will then determine all of the necessary parameters for the best and consistently accurate results.

With today’s sinker EDM technology, shops can currently generate high-quality surface finishes while holding part sizes to +/- 1 or 2 microns. But as more EDM machine builders tout the micron capabilities of their machines, only those shops that take a more encompassing total-precision approach will achieve true high-precision sinker EDM operations.

 

Gisbert Ledvon is director of business development for GF Machining Solutions (formerly GF AgieCharmilles), a manufacturer of milling (including clamping and palletization systems), EDM, laser texturing, and automation equipment and solutions for the mold and die making industry and precision component makers. Its FORM 200 mS sinker EDM features onboard part measuring and a thermostabilization system. It recently introduced the AC CUT HMI for wire EDM, adding to its AC FORM HMI for sinker EDM. GF Machining Solutions is based in Switzerland and part of the Georg Fischer Group. For more, visit www.gfms.com.

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