There has been a long-standing, unresolved debate over whether graphite or copper is a better EDM electrode material. The preference is most always the same in any one geographical region. In North America , the preferred electrode material has shifted from copper to graphite. In Europe and Asia, many argue that copper is the preferable material. Perhaps it’s time to take a closer look at both materials to identify the differences and help you select the one that is most appropriate for your EDM applications.
Electrode Material Considerations
Material variety. Graphite is produced with a wide range of material characteristics, enabling you to match the electrode material properties to the EDM application. Less-critical applications with electrode features such as a large radius, an open tolerance or minimal EDM requirements would use an economically-priced graphite material with large particles and lower strengths. However, a highly detailed electrode with critical features, extreme tolerances and stringent EDM requirements would entail a more premium graphite to fit the needs of this application.
On the other hand, due to the high purity value required for efficient EDMing, the types of copper available on the market and used in EDM applications are limited. This minimizes the ability to match material characteristics to the EDM application. The most commonly used types are electrolytic copper and tellurium copper, which vary slightly in elemental composition and are both cast as a solid.
Cost. Copper is often considered a commodity material and can be less costly than graphite in the bulk or blank stage. Many graphite materials are considered specialty materials and, as such, can be more costly. In addition, copper is recyclable whereas graphite is limited in its ability to be reclaimed. However, with the wide range of graphite materials available on the market today, it is possible to find some low-quality EDM grades that are more economical than copper. To establish the true cost of copper versus graphite, the value of machining the electrode must also be considered. Even with the more expensive graphite materials, the machining costs often offset any savings that are realized with the copper.
For example, a quote was recently solicited for a simple electrode blank with a ground finish on the top and bottom. The quote was received with copper at $4.68 per cubic inch, while a premium grade of graphite was quoted at $6.80 per cubic inch, or 45 percent more costly. However, when the cost of machining a simple, finished electrode was included, the story changed. In this case, the finished graphite electrode was quoted at $15.50 each, whereas the copper electrode (“oxygen free”) was quoted as $95 each. Due to the soft, ductile characteristic of copper, it is often gummy, and conventional machining practices (such as feeds and speeds) must be altered to accommodate. This can be mitigated with tellurium copper, but EDM performance may be jeopardized. On the other hand, graphite is not gummy and can be conventionally machined very easily. However, machining graphite generates significant dust, which must be collected through an effective vacuum system with HEPA-type filtration.
Detail. In EDM applications, the electrical current tends to concentrate at sharp edges and corners. Copper’s low melting point does not allow it to handle current density as effectively as graphite, so features on a single copper electrode must be similar in detail. It is not wise to EDM with varying details on the same copper electrode. This is because the higher current required to efficiently EDM larger electrode details exceeds the current allowable for the smaller detail.
With copper electrodes, high current density on smaller, more intricate detail increases the electrode wear. Also, when machining intricate detail or EDMing at high current densities, a copper electrode may move or flex as material stress is relieved. This limits the ability to maintain the very tight tolerances required on many of today’s molds. Graphite performs very well at a high current density even with complex geometry. Graphite electrodes allow for efficient EDMing, even with varying details on the same electrode. Therefore, the number of graphite electrodes required to perform a job can be significantly reduced.
EDM Performance
Metal removal rates. An electrode material’s thermophysical properties determine its ability to process the energy of the EDM cut and remove metal. In generating a spark, peak current is discharged only after the gap between the electrode and workpiece has been ionized. At this point, the electrode emits electrons that collide with the molecules of the dielectric fluid. As a result, the fluid is vaporized and an energy channel is formed that allows the spark to take place. For this to happen, the electrode material must be hot enough for electrons to absorb enough energy to escape and work to create the energy channel.
For copper electrodes to release electrons in the gap, the temperature must be high enough for these electrons to absorb sufficient energy.1 As a result, these high temperatures tend to burn some of the copper electrode away. In order to generate this heat, the on times for copper electrodes are generally much higher than for graphite electrodes. However, due to its carbon base, a graphite electrode is able to emit these electrons at much lower temperatures and does not require the extended on times for electrons to release and create the energy channel. Therefore, the time required to form the energy channel is considerably less. Since the graphite initializes the spark faster, significantly higher metal removal rates are the result.
Wear. Electrode wear is a constant concern because excessive wear results in adding electrodes or redressing electrodes more often. Graphite is able to achieve electrode wear of less than 1 percent in relation to the depth of cut at machine parameters much more aggressive than copper electrodes. This is achieved through a “replating” process during which molten particles from the workpiece may penetrate into the structure of the graphite electrode and reduce the amount of wear caused by the EDM process. The melting temperature of most standard work metals being EDMed is around 1,500°C. This exceeds that of copper, which is around 1,100°C. Any molten particles being ejected out of the EDM cut are likely to carry a thermoelectric charge and have temperatures that do not permit them to affix to the copper electrode. In this case, the molten particles may actually create secondary discharging and erode the copper electrode away.
This means that the high amperage and long on times of a roughing condition (and the process of replating) actually preserve the graphite electrode but is detrimental to the copper electrode which erodes away at these settings. On the contrary, in the finishing stages, with low amperage and short on-times, the graphite electrode has a tendency to wear at a faster rate than copper. However, since electrode wear is a ratio of the amount of material removed in the EDM cut, the actual wear on either a graphite or copper electrode is minimal in the finishing stage and sometimes immeasurable.
Surface finish. Because it is cast as a solid with no porosity, it goes without saying that copper electrodes provide very fine surface finishes. However, with the sophistication of today’s EDM sinker technology, the surface finish gap between graphite and copper has narrowed significantly. Fine-grain graphite electrodes are now able to deliver surface finishes similar to copper with comparable electrode wear. With the proper electrode material selection and machine parameters, graphite is able to achieve near mirror finishes without the use of a powder additive and mirror-like finishes with the additive.
Conducting test burns and tracking results will help determine the monetary impact that the electrode material has on an EDM application. Part 2 of this article will review the results of EDM testing conducted with both graphite and copper, and reveal the bottom-line cost for each. With all of this information, you will then be able make an informed decision on the electrode material type best suited for a specific EDM application.
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.
The selection of the most appropriate EDM electrode material is a key decision in the process plan for any job. I will examine the properties of graphite and metallic electrode materials. There are four main properties that effect EDM:
1. Mechanical Properties – The mechanical properties (tensile strength, grain size, hardness) will affect both the fabrication of the electrode as well as the operational performance of the electrode.
2. Structural Integrity – On a microscopic level the EDM process creates very violent sparks. The ability of the electrode material to withstand that stress, will determine the material’s performance regarding surface finish and wear.
3. Melting Point – EDM is a thermal process so the higher the melting point of the electrode material, the better the wear ratio will be between the electrode and work piece.
4. Electrical Conductivity – Since electric current works as a cutting tool, higher conductivity promotes more efficient cutting.
With the increase of the die precision, the quantity of many difficult material parts and the maturation of EDM technology, the EDM electrode of tungsten-copper composite material becomes more and more common. Even so, it is necessary to select the electrode materials reasonably in EDM since it is an important guarantee to ensure the machining quality, while it is also necessary for every technician engaged in EDM to deeply understand the properties of EDM electrode materials.
In a straightforward explanation, the EDM manufacturing process can achieve extremely high precision levels while removing material from substances that conduct electricity. It achieves this goal through the help of an electrode. You can compare how this electrode works to forming shapes out of soft materials. The electrode works by leaving behind a negative stamp on the workpiece you want to machine.
However, the physical process that takes place is indeed a bit more complicated. An electric discharge is let out in the minute gap between the electrode and the workpiece. Such discharges remove the material by melting or vaporizing it. Note that it is necessary for both the workpiece and electrode to remain immersed in a dielectric fluid for the process to function correctly.
The underlying principle which drives machining through electrodes is how you can erode material by exposing it to controlled sparks. During the EDM machining process, no actual physical contact between the electrode and the workpiece takes place. Instead, there a minute gap as thick as a single strand of hair of a human being. Indeed, the amount of material you can remove with a single spark is small and negligible. But the excellent discharge frequency helps to cut down the workpiece relatively quickly. For people fascinated with figures, the discharge takes place several hundreds of thousands of times every second.
One of main reasons people have historically used copper within Electrical Discharge Machining (EDM) is because it is cleaner in their working environment, but is that where the advantages of the material stop?
With over 70% of the global market choosing to use graphite electrode materials over copper today (totalling as high as 95% in the United States), the question is why choose graphite over copper in your EDM applications?
Here, Erodex takes a closer look at the differences between the two materials and outline five reasons why graphite is likely to be the most appropriate for your EDM application needs.
Electrode Cost
In terms of material cost, it is commonly assumed that copper is lower priced than graphite. This is often the result of comparing the price of copper material with more expensive grades of graphite available. Due to the wide range of graphite materials available, it is proven that some EDM grades are more economical than copper. Furthermore, most comparisons do not consider the cost of machining the electrode. Significant cost savings can be made choosing graphite over copper, generated through reduced machining times and speed of cut, less production time to create electrodes, faster EDM times and better throughput from EDM machines. For example, due to the soft ‘ductile’ characteristics of copper, the material is often ‘gummy’ and conventional machining practices, such as feeds and speeds must be altered to successfully machine this material. This results in longer machining times and increased costs. In comparison, graphite can be conventionally machined much more easily and quickly and even with more expensive graphite materials, the machining costs often offset any savings that are realised when choosing copper.
Material Variety
Graphite is produced with a wide range of material characteristics in order to allow matching the electrode material properties to the EDM application. Less critical applications with electrode features containing a large radius, an open tolerance or minimal EDM requirements would use an electrode with large particles, lower strength and economical price. A highly detailed EDM electrode however with critical features, extreme tolerance and stringent EDM requirements would entail a more premium graphite electrode to fit the needs of this application. On the other hand, the types of copper available on the market are few and minimize the ability to match the material characteristics to the EDM application, thus limiting optimum performance.
Electrode Detail
Copper does not have the ability to handle current density as effectively as graphite, which performs exceptionally well at a high current density even with complex geometry, allowing for various intricate machined details to be designed on the same electrode. The result is that the number of electrodes required to perform a job is significantly reduced.
it can carefully select the correct material for a specific job. Graphite can be chosen depending on required surface finish, electrode life, speed of cut or metallurgy of the job. When working with copper however, there is only one form and precisely selecting the optimum material specification is not possible.
EDM Performance
EDM operators know that excessive wear results in the use of extra electrodes or frequent redressing. Graphite is able to achieve electrode wear of less than 1% in relation to the depth of cut, while working to more aggressive machine parameters. This means, unlike copper, the high amperage and longer on-times actually preserve the graphite electrode.
In the aerospace sector when working with very thin, fine detail electrodes, copper is vulnerable to any rough handling and physical damage. Pressure applied to a thin section of copper will cause movement that can go unnoticed, causing further performance issues at a later stage. Graphite on the other hand is either in the right condition, or is clearly broken, vastly reducing the risk of the electrode being used in production.
Surface Finish
Copper electrodes do provide very fine surface finishes. With the sophistication of today’s EDM machine technology, the surface finish gap between graphite and copper has narrowed significantly. For example, fine grain graphite electrodes are able to deliver similar surface finishes to that of copper, while offering much faster speeds and vastly reduced electrode wear.
any material demon-strating good electrical conductivity can be used as EDM electrode. Materials with higher thermal conductivity are suitable as tool electrodes as well as materials with higher melting point and boiling point. Moreover, due to the high pressure and temperature present on the electrode during EDM machining, the electrode material must have accept-able mechanical strength and melting point to reduce tool wear and edge weakness. Furthermore, since the shape of the electrode defines the area in which spark erosion will occur, the dimensional accuracy of the produced part depends on the dimensional accuracy and the surface texture of the electrode.
The two significant factors to consider while choosing your electrode material in terms of both form and function are its resistance to erosion and its high-conductivity. Conductivity is critical. Electric current is the tool used by EDM equipment, and it depends on the cutting efficiency of the electrode significantly. On the other hand, the erosion resistance properties of a material depend on the combination of hardness, melting point, and structural integrity. Materials that resist erosion most have an extended service life. The determining factors that we mentioned, of course, vary from one material to another.
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.
Electrical discharge machining (EDM) makes it possible to work with metal for which traditional machining techniques are ineffective. It only works (except by specific design) with materials that are electrically conductive. Using recurring electric discharge, it is possible to cut small, odd-shaped angles and detailed contours or cavities in hardened steel as well as exotic metals such as titanium and carbide.
EDM electrode materials need to have properties that easily allow charge and yet resist the erosion that the EDM process encourages and stimulates in the metals it machines. Alloys have properties which provide different advantages based on the needs of the application.
Brass is an alloy of copper and zinc. Brass materials are used to form EDM wire and small tubular electrodes. Brass does not resist wear as well as copper or tungsten, but is much easier to machine and can be die-cast or extruded for specialized applications. EDM wire does not need to provide wear or arc erosion resistance since new wire is fed continuously during the EDM wiring cutting process.
Copper and copper alloys have better EDM wear resistance than brass, but are more difficult to machine than either brass or graphite. It is also more expensive than graphite. Copper is, however, a common base material because it is highly conductive and strong. It is useful in the EDM machining of tungsten carbide, or in applications requiring a fine finish.
Copper tungsten materials are composites of tungsten and copper. They are produced using powder metallurgy processes. Copper tungsten is very expensive compared to other electrode materials, but is useful for making deep slots under poor flushing conditions and in the EDM machining of tungsten carbide. Copper tungsten materials are also used in resistance welding electrodes and some circuit breaker applications.
Graphite provides a cleaning action at low speeds. Carbon graphite was one of the first brush material grades developed and is found in many older motors and generators. It has an amorphous structure.
Molybdenum is used for making EDM wire. It is the wire of choice for small slot work and for applications requiring exceptionally small corner radii. Molybdenum exhibits high tensile strength and good conductivity, making it ideal where small diameter wire is needed for demanding applications.
Silver tungsten material is tungsten carbide particles dispersed in a matrix of silver. Silver offers high electrical conductivity and tungsten provides excellent erosion resistance and good anti-welding characteristics in high-power applications. This composite is thus the perfect choice for EDM electrode applications where maximizing conductivity is crucial.
Tellurium copper is useful in EDM machining applications requiring a fine finish. Tellurium copper has a machinability that is similar to brass and better than pure copper.
In EDM, proper selection of electrode materials is an important factor in ensuring processing quality and increasing productivity. At the same time, since the electrodes are processed differently during use, it is difficult to standardize the tool electrodes, and the difference in size or shape is large. Therefore, the following principles should be followed when selecting electrode materials:
1.0 On the basis of analyzing the technical characteristics of the workpiece and the technical requirements (surface roughness, size, tolerance accuracy), the electrode material should be selected according to the workpiece material and technical requirements.
2.0 First consider the choice of materials with good thermal conductivity and high melting point as electrode materials.
3.0 Take into account the factors such as easy electrode forming and good processing performance to obtain good EDM. Although the electrode materials used in EDM are mainly copper, graphite and tungsten-copper electrodes, due to some properties of the high-performance electrode itself such as purity, viscosity, structural compactness, uniformity and physicochemical properties, When it comes to different processing objects and different processing requirements, the selected electrodes and processing parameters are completely different.