Nov. 18, 2021
One of the main reasons people have historically used copper in electrical discharge machining (EDM) is because it is cleaner in their work environment, but is that where the material advantage lies?
With over 70% of today's global market choosing graphite electrode materials over copper, the question is why choose graphite over copper for your EDM application? This article examines the differences between the two materials and outlines the reasons why graphite is best suited for your EDM application needs.
Graphite has a wide range of material properties in order to match the electrode material characteristics to the EDM application. Less critical applications with electrode characteristics that include large radii, open tolerances or minimal EDM requirements will use electrodes with large grains, lower strengths and economical prices. However, highly detailed EDM electrodes with critical features, extreme tolerances and tight EDM requirements will require higher quality graphite electrodes to meet the needs of this application. On the other hand, the few types of copper available on the market and minimized the ability to match material properties to EDM applications, limiting optimal performance.
Copper cannot handle current densities as effectively as graphite, which performs well at high current densities even with complex geometries, allowing a variety of intricate machining details to be designed on the same electrode. The result is a significant reduction in the number of electrodes required to perform the job.
Graphite is available in many grades and it is possible to carefully select the right material for a specific job. We can select graphite according to the desired surface finish, electrode life, cutting speed or work metallurgy. However, when using copper, there is only one form and it is not possible to precisely select the best material specification.
Copper electrodes do offer a very fine surface finish. With today's advances in EDM machine technology, the gap in surface finish between graphite and copper has narrowed dramatically. For example, fine-grain graphite electrodes can provide a surface finish similar to copper while providing faster speeds and significantly less electrode wear.
In terms of material cost, copper is generally considered less expensive than graphite. This is usually the result of comparing the price of copper materials with the more expensive grades of graphite available. Due to the wide range of graphite materials available, some EDM grades have proven to be more economical than copper. In addition, most comparisons do not take into account the cost of processing the electrodes. Choosing graphite over copper can result in significant cost savings through reduced processing time and cutting speed, reduced production time for making electrodes, faster EDM times and increased throughput of EDM machines. For example, due to the soft "ductile" nature of copper, the material is often "sticky" and traditional machining methods require changes in feeds and speeds to successfully machine this material. This results in longer processing times and increased costs. In contrast, graphite can be routinely machined more easily and quickly, even with the more expensive graphite material, and the machining costs often offset any savings realized when choosing copper.
EDM operators know that excessive wear can lead to the use of additional electrodes or frequent repair. Graphite is capable of achieving less than 1% electrode wear relative to the depth of cut, while being suitable for more aggressive machine parameters. This means that, unlike copper, high currents and longer conduction times can actually protect graphite electrodes.
In aerospace, copper is susceptible to any rough handling and physical damage when very thin, fine electrodes are used. The pressure applied to the thin copper sheet can cause movement that may be overlooked, which can lead to further performance problems later on. Graphite, on the other hand, is either in the correct condition or visibly broken, thus greatly reducing the risk of the electrode being used in production.
Some companies wire etch their own electrodes and are misled into believing that you cannot wire etch graphite. In contrast, we tested wire corrosion on graphite and had no breakage and a speed time comparable to copper.