When it comes time to decide whether to use graphite or copper electrodes in your shop, it's important to look at the big picture. According to Stu Haley, regional manager of Madison Heights, MI-based Belmont Technologies, Inc., a provider of EDM supplies, tooling, accessories and machines, "To say which electrode works best is very difficult, it is totally applications-driven. So much depends on what you have to work with on your shop floor in the way of support equipment. Both copper and graphite provide approximately the same end result. The difference is time to EDM the work and electrode manufacturing time and cost."
Graphite
Advantages and Benefits
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."
Limitations
Another important point to keep in mind is that the finish on any electrode is the finish that will be put in the mold. "So, if you have a lot of cutters or grinding marks on the electrode, you will reproduce that in your mold," Haley says. "Normally, the finish on the graphite should be as good as you need in the mold."
Copper
Advantages and Benefits
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."
When you add tungsten to copper (copper tungsten), the result is an electrode material that has extremely good wear characteristics but is very difficult to machine, Haley notes. "When EDM'ing carbide, this is the best metallic electrode material to use. The best graphite material would be copper graphite, which is graphite impregnated with copper," he says.
