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.
Graphite is by far the most popular material being used to produce EDM electrodes in the world today. Over 70% of the global market currently uses graphite electrodes for EDM and in the United States it is as high as 95%. With our extensive inventory of graphite materials, we can quickly produce electrodes from any grade required. The advantages of graphite over copper when producing EDM electrodes are many, these advantages include:
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.
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.
Graphite is produced with a wide range of material characteristics, enabling you to match the electrode material properties to the EDM application.
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.
The electrode material selection process should start with the consideration of five important factors:
Metal Removal Rate. This is the amount of material removed in a given period of time; most commonly measured in cubic millimetres per minute.
Resistance to Wear. This is one of the most important aspects of electrode material selection. There are four types of wear to consider: volumetric, corner, end and side wear, with corner wear usually being the greatest concern.
Surface Finish. The surface finish required can sometimes dictate the selection of the electrode material. For a fine finish using graphite, the electrode material should be dense. Mirror finishes are usually obtained with multiple metal electrodes or graphite electrodes and motion (orbiting or rotation).
Manufacturing Costs. This depends upon the complexity of the electrode. The more complex the electrode is, the higher the manufacturing costs will be
Raw Material Costs. This is usually only a small part of the total EDM time/cost evaluation. Unless it is an unusually large or high volume application, all four of the above points need to considered first before this becomes a major factor.
The next step is to consider some basic facts concerning the nature or properties of the work-piece and the electrode materials.
The approximate melting temperature of the work-piece material should be known. With all of the different alloys in use today, this can prove a difficult task, although material suppliers are a good source of information. Knowledge of the approximate melting temperature alone will usually be enough to start with, but it is also useful to know the material’s specific gravity or, in layman’s terms, its density. Knowing the specific gravity will give a good idea of how many molecules will have to be removed, and if they are big or small in molecular terms.
Electrical discharge machining is more popular than ever and has become an integral component of both the toolroom and the production floor. CNC and improved power supplies resulting in increased metal-removal rates have expanded the applications for EDM in the tool and die shop, as well as in the aerospace, plastic mold, and automotive industries.
And while tool and die work is still the mainstay of EDM, mass production applications, including transfer lines and FMS cells, are often taking advantage of today’s EDM technology. New techniques using multiple electrodes, automatic dressing, and sophisticated holding devices are further enhancing EDM’s burr-free, close-tolerance cutting.
For some shops, the expanded role of EDM introduces new challenges relative to selecting electrode materials. For instance, a carbide specialty shop, that invested in EDM for operating efficiencies will need to explore a wide range of new electrode material possibilities should it begin working in steel or aluminum.
Electrical discharge machining is more popular than ever and has become an integral component of both the toolroom and the production floor. CNC and improved power supplies resulting in increased metal-removal rates have expanded the applications for EDM in the tool and die shop, as well as in the aerospace, plastic mold, and automotive industries.
And while tool and die work is still the mainstay of EDM, mass production applications, including transfer lines and FMS cells, are often taking advantage of today’s EDM technology. New techniques using multiple electrodes, automatic dressing, and sophisticated holding devices are further enhancing EDM’s burr-free, close-tolerance cutting.
For some shops, the expanded role of EDM introduces new challenges relative to selecting electrode materials. For instance, a carbide specialty shop, that invested in EDM for operating efficiencies will need to explore a wide range of new electrode material possibilities should it begin working in steel or aluminum.
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.
how to choose graphite materials, there are mainly the four indicators that directly determine the performance of the materials:
1. Shore hardness of the material
Graphite is harder than metal materials, and the loss of the cutting tool is greater than that of the cutting metal. At the same time, the high hardness of graphite material in the discharge loss control is better.
2. The inherent resistivity of the material
The discharge rate of graphite material with high inherent resistivity will be slower than that with low resistivity. The higher the inherent resistivity, the smaller the electrode loss, but the higher the inherent resistivity, the stability of the discharge will be affected.
3. The bending strength of the material
The flexural strength of a material is a direct reflection of its strength, indicating the tightness of its internal structure. The material with high strength has relatively good discharge resistance performance. For the electrode with high precision, the material with good strength should be selected as far as possible.
4. Average particle diameter of the material
The average particle diameter of the material directly affects the discharge condition of the material. The smaller the average particle of graphite material is, the more uniform the discharge is, the more stable the discharge condition is, the better the surface quality is, and the less the loss is. The larger the average particle size is, the better removal rate can be obtained in rough machining, but the surface effect of finishing is poor and the electrode loss is large.
At present, there are many different grades of graphite available from graphite electrode suppliers. Generally according to the average particle diameter of graphite materials to be classified, particle diameter ≤ 4 m is defined as fine graphite, particles in 5~ 10 m is defined as medium graphite, particles in 10 m above is defined as coarse graphite. The smaller the particle diameter is, the more expensive the material is, the more suitable graphite material can be selected according to the requirements and cost of EDM.
How to choose the electrode material for “middle-sized or large-sized mold of plastic / die-casting”
(1) Characteristics of the mold
Molds of this field are represented by plastic molds for home electronics and automobiles. A large number of electrodes may be used and it may become around 400 of them for a single mold. In addition, the electrode sizes are widespread from a small one to a large one. Small electrodes are around 70% of the number of electrodes even for a large-sized mold, because it has a lot of small machining areas such as the reinforcement area (rib groove) above all.
Large number of electrodes
Extensive variety of electrode sizes (70% of electrodes are small ones.)
Emphasis on removal rate than the machining accuracy
(2) Elements required for the electrode material
The element made much of is machining efficiency at the time of the electrode production and removal rate at the time of the electric discharge machining. Graphite is used as the electrode material for this reason. Many kinds of graphite for electrodes are widely used. As for the high quality type of small graphite particle size, the finishing quality at the time of EDM is good. On the other hand, machining efficiency at the time of electrode production by milling is extremely bad. For this reason, average quality graphite of relatively large particle size is used for molds of the middle and large size. Also, graphite has an advantageous characteristic of less bending at EDM even for the thin electrode for reinforcement area.
As for a matter that requires attention on choosing graphite, an abnormal electric discharge is easy to be generated at the time of EDM, and when the electric discharge area at the time of starting EDM is small, or when EDM condition of small electrode wear is used, the risk increases in particular. For this reason, in case of using graphite, the EDM condition of a little larger electrode wear is suitable, and it is necessary to deal with it by increasing the number of electrodes for the same machining area.
The graphite has high machining efficiency at the time of the electrode production
As for the graphite, removal rate at the time of EDM is fast
How to choose the electrode material for “small mold of plastic / die-casting”
(1) Characteristics of the mold
Molds of this field are represented by plastic molds for such as cellular phones and digital cameras. In machining of the cavity surface, the surface finish will be required to be high quality of fine surface roughness which is easy to polish. On the other hand, in machining the core side, many small electrodes are necessary though importance of surface quality becomes small
High-quality surface finish cavity
The importance of surface quality of the core is small, but you need a large number of small electrodes
(2) Elements required for the electrode material
The important element is surface quality. For this reason, electrolytic copper is used as the electrode material. In EDM, electrode wear generally increases in the domain of fine surface roughness, but graphite has this tendency extremely much, and high quality graphite of small particle size is used for machining of small surface roughness. Still more electrode number is required than copper. On the other hand, the graphite which is high in machining efficiency at the time of electrode production is suitable for machining of core side because of a large number of electrodes.
Copper is suitable for high-quality machining
Graphite has high machining efficiency at the time of electrode production
How to choose the electrode material for “precision plastic molds”
(1) Characteristic of the mold
Mold of this field is represented by plastic mold for electronic parts and engineering plastics. Dimensional accuracy is required.
Making much of dimensional accuracy
(2) Elements required for the electrode material
The element made much of is machining accuracy. High machining accuracy is required at the time of electrode production and EDM. Generally, copper is used. And also, non-copper materials are used because of their advantages. For example, copper tungsten and graphite are used because of cutting performance at the time of electrode production and because of less thermal expansion at EDM, and graphite is used because of an advantage that an electrode is inflexible by heat at the time of EDM, and so on.
Copper has general appropriateness
Copper tungsten and graphite have an advantage of efficiency of cutting at the electrode production
Copper tungsten and graphite have an advantage with less thermal expansion at EDM
Graphite has an advantage that the electrode is inflexible by heat at EDM
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.