Now that we know generally how graphite is made, how do we choose the correct grade? In order to maintain cost effectiveness, an EDM shop needs to choose EDM graphite that provides dependable quality at a good price.

The incorrect selection of consumables can cause the operation to take longer than planned or cost more than what was budgeted. If an improper grade was chosen, the customer may experience undesired results or pay too much for a grade that they did not require.

When looking at roughing grades they generally have a particle size around 10 to 8 microns. They are used for applications that require no detail and no sharp edges or corners. Because of the molecular and bonding structure in this grade class, you will experience high removal rates at the expense of high electrode wear. Although this grade class machines somewhat easily—due to its structure—there is an issue with it chipping and breaking with small electrodes.

Once you enter the finishing/detail aspect of the grade selection process, you will find that the grades generally have particle sizes from 8 to 5 microns. The most common application for this grade class is forging dies and die cast molding dies. They also can be used in less complex powdered and sintered metal applications, and for larger plastic injection parts. The downfall to these grades is that they are limited on strength and you cannot produce thin fragile electrodes, or superior surface finishes either.

If the workpiece requires smaller and more intricate features, sharper corners, thin ribs or fine detailed smaller electrodes, another grade class usually is required. The fine/detail category will provide the necessary results. These grades usually have a particle size from 3 to 5 microns.

When the application calls for wire cutting, aerospace applications, blow molding, plastic injection, threading electrodes or medical applications, the fine/detail category will meet all of your needs. These applications require that the graphite provides sharp corners, superior surface finishes, and exhibits high strength for small and thin electrode applications.

There are applications that require ultra fine surface finishes, extremely small parts, intricate details on the electrode, best wear conditions, precision or thin wire cutting and superior electrode wear. If this is the prerequisite then the precision class will work best for them. The particle size for this grade class ranges from 3 to 1 micron. It works exceptionally when applied to exotic aerospace type metals (high nickel and copper alloys) and carbide applications.

Finally, there also are copper-impregnated grades that provide an advantage over non-impregnated grades. Typically companies impregnate two classes of graphites: (1) the finishing/detail grades and (2) the fine detail grades.

When graphite is impregnated with copper, micron size particles of copper fill in the holes (otherwise known as porosity) of the graphite structure. By impregnating the graphite with copper it increases the graphite’s electrical resistivity and strength.

Now the electrode manufacturer has the ability to make extremely thin and fragile electrodes, such as a very thin deep rib electrode. Copper-impregnated graphites also are the preferred grades for aerospace applications because the added copper helps to stabilize machining of exotic, copper and nickel alloys.

If you require the finest finish available, impregnated graphites provide the absolute finest finish. Copper-impregnated grades also allow for stable machining when nonfavorable conditions exist—such as poor flushing or when the operator is not very experienced with the application.