When choosing an electrode material, you must consider many factors. The usual first decision is, should you choose a metallic or non-metallic electrode? Depending on the material to be removed, this can be as simple decision. Carbides and sintered metals require more heat, and the highly conductive metallic electrodes will allow for more current to reach the gap—and therefore perform better when machining these types of materials. When machining stainless steels, tool steels and mold steels, non-metallic electrodes such as graphite are a common and very acceptable choice.
First, a few properties of the metallic type. The most common metallic electrode material is copper, and like most metals, there are varying forms of hardness and alloy content. As a high-quality electrolytic, copper makes up a high percentage of metallic electrodes. Some of the requirements needed include free machining, high conductivity, and the ability to machine extremely fine detail without the worry of damage or breakage during the manufacturing process or the setup process. Also, because copper has a tight molecular structure, the ability to produce exceptionally fine finishes is another determining factor in choosing copper. Another commonly overlooked advantage would be the ability to reuse the electrode by re-machining or simply re-purposing the electrode as a completely different-shaped electrode. It should also be noted that it can be machined by the wire EDM process in fine detail and with little or no waste, and therefore has some economic influences in job costing.
Although copper has great potential for almost any application, it is unforgiving if the machining conditions or the flushing conditions are not selected properly. The machining conditions selected will vary from machine manufacturer to machine manufacturer, and in some cases, there may be advantages to certain types of power generation supplies. Copper requires a very stable and high machining temperature in order to control electrodes where ratio and surface finish. One of the most common mistakes with metallic electrodes is to over-flush the gap, causing a cooling effect on the electrode, stopping the re-plating process and causing high wear to the electrode. By simply using a Z-axis pulse or a light flow of dielectric flush through the gap, a copper electrode may experience virtually no wear, which will allow the electrode to be used for many cavities without variation in size or surface finish.
The most common non-metallic electrode material is graphite, and again, there are many variations in the types of graphite available. Most often, graphite will be picked by its grain size and hardness. The hardness will determine the machinability when forming the electrode, and the grain size can determine the finish on the workpiece. Although graphite electrodes are very forgiving when it is over-flushed, it is less forgiving when the power settings are not selected properly; this usually results in a short-circuit or direct current arc, which destroys both the workpiece and the electrode at the point of the short. This is commonly referred to as a “DC arc,” which comes from the visual similarity to a DC welder.
Another common form of electrode material is the combination of copper-infused graphite. Although there are some advantages to this material, if you carefully examine the technologies used in spark conditions and the effect that they have on the metallic versus the non-metallic electrode, you will notice that the current range required to keep the wear at a low and tolerable level when using the graphite electrode is offset by the high wear of copper. This disadvantage is usually offset by the machinability of exceptionally fine detail when manufacturing the electrode in a high-speed machining center. These facts about these two electrode types are supported by the technology that you will find in the machine manufacturers’ technology charts and manuals. Although different manufacturers may have similar work-arounds to utilize a more common electrode material and manufacturing process, the laws of physics dictate whether the electrode will wear or not. We find the technology in spark generation and machine movement has enhanced our ability to control the current flow by detection of abnormal discharges and by controlling the speed of the Z-axis to aid in flushing the gap during the machining process. So, the phrase most used circa 1970 was “there are three problems with EDM: flushing, flushing and flushing. This was one of the operators’ go-to excuses for decades, and now that we understand the electrode response to the material and the control of the spark generation, we now realize that the only issue with EDM is the operators’ understanding and education in Electrical Discharge Machining.