EDM, Graphite Electrodes, and Quality of Finish

Many expected the use of EDM or electrical discharge machining to dwindle at the turn of the century due to the development of new machining technology. More than a decade has passed since the prediction but EDM remains in use and preferred by many workshops. The reason for this staying power is the lower cost of operation, reliability of EDM machines that allowed them to stay in very good condition, and quality based on the experience of the operator. A newer technology might offer far better potential in terms of quality but until an operator is able to master its operation, that potential will remain as is.

Graphite electrodes have also made EDM machines for applications that are more than just rough cutting. The graphite used for electrodes in sinker EDM have varying grades. The type of grade determines the quality of finish.

Fine grade graphite electrodes are made of particles with sizes between 11 to 20 microns. These electrodes are primarily used for roughing in large cavities in work pieces. The finish is very crude and will require further polishing down the line. Polishing equipment like diamond tools can be used for improving the quality of the work piece right after the EDM process.

The superfine grade graphite electrodes feature particle sizes between 6 to 10 microns. These electrodes are used for creating cavities that larger details. The quality of finish is still very crude will require polishing.

The ultrafine and angstrofine grade graphite electrodes are the ones used for finishing applications. Ultrafine grade features particle sizes of less than 5 microns while angstrofine have submicron particles. These are used for creating intricate details with very high quality finish.

How Graphite is Made

Graphite is the most preferred electrode material for sinker EDM applications. It has been over half a century since graphite was introduced to the EDM industry. But even with the modern advancements in machining technology, graphite electrodes remain an integral part of the industry. It is cost-effective and provides excellent results. It is similar to diamond tools that have also withstand the test of time and the arrival of new technology by being easy, affordable to use and providing the needed results.

Graphite is made using carbon powder which is derived from petroleum. The carbon powder is mixed with a petroleum-based binder. The mixture is then compacted to create a block. The way the graphite is compacted greatly influences its properties. In the early days, graphite was made by compressing the mixture in only one direction. This resulted to the formation of grains. These grains affected the structural strength of the formed graphite and other properties like resistance to wear and machinability.

The needs of the aerospace industry resulted  a newer method of compacting the carbon-binder mixture. The isostatic pressing of the mixture created blocks with uniform properties all throughout.  This compacting process is used to produce the modern high-quality graphite which is used for sinker EDM electrodes. The block then undergoes several heat treatments that converts carbon to graphite. The fineness of the carbon powder determines that finishing quality of the EDM.