High-speed machining centers have made it possible to machine intricate detail into almost any grade of graphite, a task that would have been impossible using manual or CNC machining equipment. The high-speed spindles, making multiple passes with smaller chip loads, allow thin ribs to be machined without putting pressure on the rib. Materials with low compressive or flexural strength tend to break and chip while being machined into thin ribs on conventional equipment.

These same materials can be machined on a high-speed machining center without problems. The ability of the machining center to put fine detail into almost any grade of graphite means the EDMer may have trouble producing the cavity if the appropriate graphite for the application was not used.

Graphites with inconsistent microstructures can produce problems that are more pronounced on a high-speed machining center. Hard spots in the graphite can cause the tool to deflect, resulting in an out-of-spec electrode. Material with hard spots is more likely to chip or break off during machining. When drilling there can be deflection off the surface of a hard spot and the hole may appear to be straight at the point of entry, but angled inside.

Soft spots, which are usually due to a very porous nature, can decrease the accuracy. The material may not have the strength to keep the tool straight. The cutter will often pull material out when it hits loose material. Chipping also is common. Machinists that regularly machine materials with this kind of microstructure learn to reduce feeds and speeds, thus increasing the machining time. All of these machining flaws can be detected on a CMM during inspection. If not corrected these flaws will be reproduced in the cavity.

Unlike metals that use the Rockwell method for measuring hardness, graphite hardness is reported by the Shore method. There is no cross-reference between the two measurement systems. Although hardness does not affect EDM performance, it will affect the machinability of the material. Graphites with a Shore hardness value of between 55 to 75 will be the most machinable materials.