Eliminating problems with EDM

High precision die-sink electrical discharge machining (EDM) is an unforgiving process and every aspect of it – from producing electrodes, to workpiece set-up, to maintaining a consistent machine temperature – must be controlled and optimised. Because when part tolerances run as tight as 2µm or less, even the slightest deviation at any stage of the process is magnified and will negatively impact the level of precision achieved.

For real high precision die-sink operations, especially in micro-applications, manufacturers must take a total process approach. This involves more than just the EDM machine, including processes for machining electrodes, transferring them to the EDM machine and setting up workpieces within the machine. Every step is critical to achieving true high precision.

Starting with initial steps in the die-sink process, manufacturers 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 accurate as the machine 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 machines to cut or grind the electrodes.

When manufacturers move the electrodes from their high precision milling or grinding machines to the EDM machines significant care must be taken when unclamping and re-clamping the electrodes for each process.

Going modular

Modular pallet systems can provide 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 also offer the same positioning benefits for the machine set-up stage as well.

Such workholding ensures the highest level of accuracy, especially when EDM machining very small parts. Manufacturers can quickly and easily clamp parts onto workholding pallets and then use a coordinate measurement machine (CMM) or optical measuring device to determine the exact workpiece location data that the EDM will use to calculate the appropriate offsets. The EDM machine can perform the same measuring operations if equipped to do so.

Die-sink 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 machine.

Die-sink EDM machines with on-board part measuring capability can undertake 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 consistent. An EDM machine 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 1 or 2µm.

To 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 as well. GF Machining Solutions uses a special polymer for its high-precision die-sink EDM bases and has developed a new machine design that incorporates cooling channels throughout the machines’ entire base, table, and upper head (C-axis) to control heat.

This internal thermal stabilisation system uses the machine's dielectric fluid/water as a coolant and 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.

In addition, for high precision applications, EDM machines should have an actual chiller as opposed to just a heat exchanger. This helps maintain dielectric water temperature, especially if the machine lacks any other thermal management system. While locating the machine in a climate controlled room helps keep heat generation in check, the dielectric water controls table and workpiece temperatures.

Getting in position

Besides temperature control, high precision die-sink machines must provide accurate motion control/positioning. Most machines achieve this through glass scales on all axes, and, often, closed-loop control systems drive these axes to maintain high precision.

Within its closed-loop system, GF Machining Solutions uses encoders on machine ballscrews and glass scales on the machine axes. The system first measures encoder resolution and then compares that to the absolute positioning of the glass scales. If there is discrepancy between the two, the machine will automatically adjust the difference.

Separate from upper head cooling, a liquid cooled C-axis is also critical for accurate die-sink machine motion. Cooling jackets that envelop 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-spindle function, which is standard on practically every die-sink machine, and which provides optimal flushing during the burn).

The proximity of a machine's C-axis drive motor also influences the positional accuracy of that axis. The further away the motor is from the spindle and chuck, the higher the risk of error. Machines that have these motors as close as possible to the chuck and spindle create not only a high accuracy-positional C- axis, they also result in extreme precision even if electrodes are slightly off centre.

For high precision operations and even more so during micro-machine applications, a die-sink machine’s generator has to continuously communicate with the machine's control. While many EDM machine builders are now incorporating fibre 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.

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 critical for micro-machining.

Spark adjustment

Most die-sink EDM machines can 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 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 eroding material, thus resulting in lower productivity. 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, only retracting the electrode if the generator is unable to adequately modify the gap or spark conditions, or if there is too much contamination.

EDM controls must process data and feedback at lightning speed for true high precision applications. In some instances, the EDM machine's CNC hardware runs in the background and a 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 processor becomes overloaded and, as a result, slow. This allows the control to quickly react and respond to all the cutting data being gathered.

To optimise programming for high precision operations, machines must be sophisticated and provide in-depth application descriptions. They need to offer as many parameters as possible to cater for different electrode materials, cavity shapes, and part materials as possible.

The key is being able to define the process accurately and determine the correct technology settings prior to generating the first spark. For instance, GF Machining Solutions machines offer numerous parameters for process optimisation and reducing electrode wear. If a manufactures is using a fine- rain 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 most accurate results.

With today's die-sink EDM technology, manufacturers can generate high quality surface finishes while holding part sizes to ±1 or 2µm but as more EDM machine builders make claims concerning the precision of their machines, it is important to remember that only those manufacturers that adopt a total precision process approach will achieve true high precision EDM results.

GF Machining Solutions www.gfms.com/uk