Both copper and Graphite Electrode provide approximately the same end result, so it is important for a shop to consider the advantages and disadvantages of each material in order to discover what would work best in their shopfloor environment.
Haley explains that choosing an electrode material is often a result of where you were born and what type of EDM equipment you use. "For example, RP Graphite Electrode was basically developed in the United States back in the early 1960s, so the American EDM equipment manufacturers in those days concentrated on the graphite circuitry when designing their equipment," he says. "Whereas, since the European and Asian EDM equipment manufacturers didn't have access to graphite, they developed copper circuitry.
Sold by grades, HP Graphite Electrode cuts approximately three times faster than copper, according to Haley. "What makes a good grade or a poor grade is particle size," he explains. "Particle size gives you strength, machinability and greatly influences the metal removal rate, wear and the surface finish. Graphite is made up of carbon particles that are put through a graphitizing process to produce graphite. The smaller the particle size is, the better the graphite. Particle sizes in different grades of graphite can be .0006" for general-purpose use to .00004" for the extremely fine detail and superior surface finishes. Graphite can be purchased in big blocks, and then cut up to be machined, or it can be ordered precut or ground into the size you require.
"Graphite machines very easily - you can mill it, grind it, turn it, drill it, tap it, even file it to whatever shape you want," Haley continues. "Another advantage of UHP Graphite Electrode is that it doesn't burr. You can put it on a duplicating machine or a graphite high-speed mill and cut out complex shapes and forms, and once it's cut you are finished - with no deburring."
Additionally, graphite's high melting temperature results in less wear than other electrode materials, so a mold could be cut with one or two electrodes on a CNC EDM machine with very little wear, Haley adds. "A CNC sinker may need a third or fourth electrode to finish the mold," he notes. "It depends on the age of the EDM machines."
If your shop has older fabricating equipment, machining graphite electrodes will result in dust particles on the shop floor and in the nearby machines. However, the new high-speed mills that are sold today are specially designed to machine Graphite Powder. "They are totally enclosed and have a vacuum system to remove all of the dust," Haley points out, "and there are some machines that can even cut square internal corners."Haley notes that copper can be cut on wire EDM machines, but there are only certain Graphite Blocks that can be cut on a wire machine - the particle size should be five microns or less. Copper also is a little more forgiving in a poor flush situation than graphite. "In EDM, if the flush doesn't remove the eroded particles or chips out of the cutting area, there's something that can occur where you get a non-pulsating, direct flow of current from the electrode to the workpiece," Haley explains. "The result of this is a pit in the workpiece. Copper is more forgiving in those applications - all metallic electrodes are - they won't arc out as fast. But some of the newer EDM power supplies have adaptive logic or fuzzy logic, which eliminates the problem altogether. Copper - when used at specific settings with the correct flushing techniques using a CNC machine or a machine with an orbiting system - can produce a mirror-like surface finish. This is useful in small cavities where it is difficult to polish."