When it comes to EDM electrode material, the copper versus graphite debate has raged on for decades. And during that time, the shift has come full circle. The use of copper versus graphite as an EDM electrode material has made a complete turnaround over the past few decades. During the 1960s, copper was used nearly 90 percent of the time for metallic electrode materials, while graphite was used 10 percent of the time. But today, the numbers are reversed and more than 90 percent of the EDM electrodes being used are graphite. Copper—once the dominant metallic electrode material—has nearly disappeared in favor of the ever-increasing popularity of graphite.

What caused this dramatic change? The domestic EDM industry can attest that this shift may be attributed to the advantages of graphite materials over copper. By examining the characteristics of these two materials in an EDM setting, a more educated decision can be achieved when selecting the proper electrode material for an application.

MACHINABILITY
Machinability is likely the most important factor in determining the use of an electrode material. If it can't be machined, then it can't be EDM'd.

In some circumstances, copper can take as much as five times longer to machine than graphite. This is because copper has a tendency to gall and tear during machining, requiring additional hand-working to remove burrs, nicks, etc. Because copper is a soft material, it also tends to gum up a grinding wheel, sometimes requiring redress after each electrode. In addition, when considering thin ribs or fine detail, copper again is not the material of choice because it does not have the strength to resist tool push, friction heat or rough handling.

Graphite, on the other hand, can consistently be machined two to three times faster than metals with no post process required, such as de-burring. When machining a graphite electrode, if a detail happens to "flex" because of tool push, the feature will rebound to its original shape. Graphite is easy to machine using standard machining procedures and can withstand a good deal of rough handling. Graphite does create dust during machining, but with the use of proper dust collection there is no reason a graphite shop cannot be as clean and safe as a shop machining metallic electrodes.

METAL REMOVAL RATE
The metal removal rate (MRR), more commonly known as the speed of the cut, refers to how fast metal is removed from the cavity. Since copper has a higher range of thermal conductivity, the heat from the EDM process will quickly spread through the electrode. As the temperature of the electrode increases, the electrical resistance also increases, and much of the energy needed to create the EDM spark is turned into heat within the electrode. On the other hand, graphite has a lower thermal conductivity than copper, helping it maintain fairly constant resistance at elevated temperatures. This ensures that the energy needed to create the spark is actually creating a more efficient spark. This difference in thermal conductivity gives graphite a big advantage over copper while achieving metal removal rates at approximately twice the speed.

ELECTRODE WEAR
Electrode wear is a concern of every EDM operator because excessive wear requires the use of more electrodes or requires redressing the electrodes more often. With the proper machine parameters, graphite can easily achieve less than one percent wear relative to the depth of cut, while copper has a tendency to create a greater wear percentage. The melting point of these two materials relates directly to wear percentage. Copper has a melting point of approximately 1,000-1,100°C. Most work metals have melting points higher than copper. Tool steel, for example, has a melting point of 1,500°C. As amperage is increased to achieve a high MRR, the copper electrode simply cannot take the heat generated to efficiently cut the workpiece. Graphite, though, has no melting point, but transforms from a solid to a gas while directly bypassing the liquid physical state. This process is called sublimation, and it usually occurs around 3,500°C. By virtue of this resistance to higher temperatures, graphite can transfer the energy of the spark to the workpiece, resulting in efficient speed and wear.

SURFACE FINISH
While copper materials give very good surface finishes, that alone does not justify the cost of the other determining factors, such as additional loss of materials and increased machining time. Fine grain graphite will deliver similar finishes much faster than copper, with less wear of the electrode. Finishes of 7 Ra (in), with orbiting patterns and that require no hand polishing, are easily achieved.

COST
It is easy to focus on the cost of the electrode material alone. In truth, though, that cost is a very small percentage of the total cost of the mold, which can also include the extra time required for machining or polishing, additional electrodes required due to higher wear rates, and more time spent on the EDM as a result of the slower MRRs. Such factors can greatly affect the bottom line. Thus, when the material cost is considered part of the total job and not as a stand-alone component, graphite again has advantages over copper materials.

So where does this lead us in our search for the "perfect electrode material?" While there is absolutely no perfect material for all EDM applications, if you consider the factors discussed here, graphite is the preferred material of the domestic industry. Time and again, the selection of graphite electrode material will pay off through increased productivity. Make the informed decision.

Source: Competitive Mold Maker, Volume 6, Number 2
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