Graphite and Electrodes

Electrical discharge machining or EDM is a process used for cutting intricate shapes, cavities and contours in pre-hardened steel. There is no heat treatment required. Compared to other machining methods, EDM involves less processes as there is no need to soften and then re-harden the steel. One of the key components for EDM are the electrodes. Rapid recurring current discharges between two electrodes remove the material from the work piece.

There are different types of electrodes used for EDM. Graphite electrodes is one of the most widely used. There are several reasons why graphite is preferred by many. It is fast and easy to produce. It is also resistant to thermal shock. Thermal shock happens when different parts of an object to expand by different amounts. This differential expansion can causes stress on the material. At some point, this stress can exceed the strength of the material, causing a crack to form.

Graphite electrode does not melt. It goes directly from solid to gaseous state. Graphite removes material better than copper or copper-tungsten while wearing slower. The wear rate tends to diminish as the discharge increases, unlike copper, whose wear increases at higher currents. This makes graphite more suited for machining of large electrodes since working with a high current intensity provides decreased roughing time.

Although graphite is prone to abnormal discharge, this can be eliminated through quality flushing, and lowering the intensity of discharge during negative polarity machining. However, since graphite is a ceramic, it is sensitive to mechanical shock. It must be handled and machined with care.

Keeping Your Edge: Diamond Tools

Diamond tools require proper care to ensure that it performs as needed. Unlike the graphite electrodes in EDM machines, diamond tools tend to be expensive.  Premature replacement of the tools can cost a company plenty of money. Knowing about the best practices with regards to caring for diamond tools is crucial to maximize the profitability of the operation.

Caution must be taken when handling the tools. Although diamond is high resistance to wear, placing the cutting tip of the tool on hard surfaces can have long-lasting effect on its sharpness and cutting ability.  This is especially true when dealing with tools that have been used very recently and are still very hot. The pressure from the weight of the tool can cause the binder that holds the diamond particles suffer deformation and effectively blunting the tool.

In cleaning the tools, make sure to use the right cleaner. Some cleaners are too acidic and can damage the tool. Check the pH and other properties of the cleaner to ensure that it is suited for cleaning diamond tools.

Another things to keep an eye on is overheating as extreme heat can damage both the binder and diamond particles. Lengthy cutting operation that generates extreme heat requires the use of coolant to keep temperature under control.

Using diamond tools on high-carbon steel and alloys can cause a reaction between the diamond particles and carbon from the work piece. This molecular structure of the diamond particles can get altered, turning it into soft graphite.

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.