Top Considerations When Choosing Graphite Used for EDM

EDM (Electrical Discharge Machining) graphite should actually be called Graphite used for EDM. While there are many companies, Entegris l Poco, Toyo Tanso, as examples, that manufacture graphite used in EDM, all the materials produced have additional uses outside of EDM. The reason these materials are referred to as EDM graphite is the fact that they all work well in the EDM process.

To answer this, first we need to identify what the EDM process is asking of the graphite during machining operation.

A graphite electrode attached to an EDM machine approaches a conductive workpiece (metal) which is going to be shaped through spark erosion.  High electrical voltage is sent through an electrode, causing a spark which removes a small amount of material at a time in the workpiece.  Dielectric fluid is sent through this distance between the graphite electrode for cleaning (flushing) the space and helping achieve temperature stability of the workpiece. Through continual sparking from the electrode and erosion of material, the desired shape is created in the workpiece.

The material used for EDM Graphite must be of a premium quality and consistency to perform the job it needs to perform.

Now that we briefly covered the process in which the graphite is used, we need to take a deeper look at what makes these materials able to perform under such intense conditions.

What does this mean?  Look at a 200x photomicrograph (below) for a material produced by Entegris l Poco which is their EDM 3 material.  

By looking at the material you can see the conistency throughout the material is very good.  Consistency in the graphite material provides stabilization of the electrode and workpiece during the EDM process.  If the graphite materials are not consistent in structure, the results during the EDM machining process will be inconsistent as well.  The consistency of the material is also critical during the machining of the graphite electrode.  This consistency is one of the key factors which give the material and the machinist the ability to hold extremely tight tolerances with premium surface finishes.

Particle size selection of graphite is determined by the specific application and the priorities in which have been deemed most important.  As an example, if priorities are the speed to finish the process and, as an example, finish is not as important as speed, a slightly larger (10-12 micron) particle size would be a better choice than a five-micron material.  If the reverse were the case and surface finish was the priority, a five (5) micron material would be the better choice.  

The reliability of supply for graphite is as important if not more important than many other products.  Purchasing material from a reliable source is paramount to having consistent EDM machining performance out of the graphite material.   Many suppliers in the industry private label different graphite grades and offer their specific brand and grade designation to their customers.  This is more acceptable if a) the material supplier allows private labeling and b) your supplier will not substitute materials under the same grade number.  Unfortunately substitutions under the same grade number happen all the time. 

Substitution can cause so much turmoil and confusion for the machinist as they could get different results which is especially important in the aerospace/ jet engine market, and in tight tolerance tool & die work.  At Graphel, we private label several different grade of graphite but never substitute one material for another at any time.  When prints have a call out of a specific graphite material, the call out needs to be honored and never substituted.  

There are many articles on the topic of Graphites used for EDM and the material selection process, however the one that stands out the most would be one from Entegris l Poco authored by Mr. Rick Slimp titled "EDM Graphite Buyers Guide". This was written years ago but has stood the test of time as a go-to-guide for new and experienced EDM machinists alike:

https://graphel.com/week-2-edm-graphite-buyers-guide/