The physical properties of each grade of graphite determine the ranking within classifications. The properties that influence performance are particle size, flexural strength and shore hardness.

These properties along with a photomicrograph of the microstructure are the best tools for predicting graphite performance.

The best graphite in any classification has tightly packed particles with little variation in size. This kind of uniform material resists wear caused by the thermal nature of the EDM process. Particle size is generally stated as an average size. When particle size spans a small range, the microstructure of the material becomes more uniform with reduced porosity. The porosity in the graphite is boundary between particles. The particles are bound together by chemical or mechanical means and the failure of this system is what releases particles into the gap when EDMing. I

f the material’s particles are small, uniform in size and tightly packed, erosion of the electrode is minimal. Particle size has a bearing on the minimum surface finish that the material will produce. Since the electrode reproduces its structure in the cavity, fine surface finishes cannot be obtained with graphite grades that have large particle and non-uniform microstructure. The microstructure of the graphite grade is often the limiting factor determining EDM performance.

A non-uniform micro-structure with a wide range of particle and pore sizes can have soft spots that are large areas of porosity and/or hard spots which are conglomerates caused by inconsistent blending. Hard spots also can be caused by impregnating the open porosity of the material with pitch and then reprocessing the material giving the particles and pitch impregnated areas different hardness values. Since the unaided eye cannot see the microstructure, there is no way to detect these problems prior to the machining process. Identification of the cause of machining problems involves destructive testing and the examination of photomicrographs.