EDM-ing thin ribs is a difficult application. High-speed machining of the electrodes has made it possible to fabricate these electrodes from grades that are not capable of maintaining the detail in the cavity. The ideal graphite grade for thin ribs should have a flexural strength above 700 kgf/cm². Materials that have these properties are normally found in the ultrafine classifications. Without adequate flexural strength, thin-ribbed electrodes can be deflected by flushing pressure or can break during orbiting. Graphite made of tightly packed small particles, will be able to resist erosion at the corners and edges of the electrode better than a material with large particles and pores. A thin ribbed electrode from material in the ultrafine classification can successfully complete the cavity, when the same electrode shape in a superfine or fine classification may crack or break during the cut.
Surface finish in the cavity is a mirror image of the surface of the electrode; therefore, grades with large particle and pore structure will not produce as fine a finish as small particle/pore size grades in the ultrafine classification. The operator can request a particular surface finish from the control menu and the operating parameters are adjusted to achieve that finish. If the graphite grade is not physically capable of producing the requested surface finish due to excessive porosity or particle size, the machine will continue to run without ever achieving the desired finish. When the proper grade of graphite is used for the electrode the desired surface finish can be obtained.
Decreased metal removal rates also can be a problem when the optimum grade of graphite is not used. Because the machine's sensors monitor the gap and adjust the machining parameters as necessary to maintain a stable cut, excessive or large particles in the gap will cause the ram to retract and advance slowly as cutting resumes. When there are excessive particles in the gap, it is due to high electrode wear.
Graphite selection is the key to achieving optimum performance from the equipment on the shop floor. The equipment cannot run any faster than the quality of the graphite material allows it to run. Ultrafine graphites with uniform microstructures tend to be high-performance materials that allow aggressive settings to be used. Using these high-performance materials can trim hours off each operation and eliminate the waste caused by a graphite that has excessive wear, slow machine speed or failure in the tank. Understanding the capabilities of the graphite grade chosen for an application is critical to the EDM performance.