Graphite has many advantages that have made it the material most widely used for EDM electrodes.
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More About Graphite
Graphite used for EDM machining is an isotropic material with a grain size ranging from a few microns to about 20 microns. In the 1970's, improvements made by graphite manufacturers (isotropic properties, consistent quality, large size billets) combined with the emergence of EDM machines equipped with iso-plus generators, allowed graphite to become the most commonly used material for EDM machining electrodes.
Three separate groups of graphite can be defined:
1. Large grain graphite (about 20 µm) with low densities (1.76 g/cm3)
2. Fine grain graphite (~10 µm) of high density (1.82 g/cm3)
3. Very fine grain graphite (~4 µm) with densities greater than 1.86 g/dm3
Larger grained graphite is used for machining in roughing modes while fine grain graphites produce the best surface finishes for finishing operations. As graphite has become more affordable, EDM machining shops will often inventory two or even three types or grades of graphite. A less expensive large grained graphite for the roughing operation; followed by a finer grained graphite for finishing or a combination of both roughing and finishing performance; and possibly an expensive very fine grained graphite for fine finishing and precision operations.
Why Graphite?
Graphite has several advantages over other materials. It is resistant to thermal shock. It is the only material in which mechanical properties increase with temperature. It has a low CTE for geometrical stability. It is easily machined. It does not melt but sublimes at very high temperature (3,400ºC), and finally, its density is lower (five times less than copper) which means lighter electrodes. 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. Therefore, graphite is suited for the 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, as a result of this tradeoff, machining tungsten carbides is more difficult than with copper-tungsten electrodes. Also, since graphite is a ceramic, it is sensitive to mechanical shock, and consequently must be handled and machined with care.
Comparing Graphite Grades
It is not advisable to compare a grade of graphite to another just by looking at physical properties without also performance testing the graphite in actual EDM operations. However, the following is a list of physical properties of graphite that exhibit some effect on performance in EDM operations.
To understand the myriad applications and benefits of working with graphite, it's important to have a clear understanding of the material we are working with. So, what is graphite?
When we talk about graphite in most industrial applications we are discussing synthetic graphite. Synthetic graphite is the crystalline form of carbon. Synthetic graphite is a man made material that is extremely resistant to high temperatures and acidic or basic solutions. Graphite can be engineered to obtain specific properties such asdensity, electrical resistance, hardness, porosity, compressive strength, flexural strength, coefficient of thermal expansion and thermal conductivity.
Typically graphite is produced from petroleum coke which is heated to incandescence, which drives off many volatiles. The coke is then crushed and ground to specific particle sizes dictated by the final grade. The coke powder is mixed with coal tar pitch and other additives which act as a binder. This mixture can be extruded or molded into desired blocks and rounds. There are various methods of producing synthetic graphite shapes. The most commonly used methods are extrusion, compression molding and isostatic molding.
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Once the "Green" or raw carbon blocks are molded they undergo an extended baking cycle to convert the pitch into solid carbon. This process may take up to 60 days and is carefully controlled to prevent the material from fracturing.
Once the baking cycle is complete the "Baked Carbon" is ready for the final process of graphitization. The conversion to graphite from baked carbon takes extremely high temperature. The temperature normally required for complete graphitization is 5,000 degrees Fahrenheit or higher. This temperature is typically reached in a controlled atmosphere induction furnace. An added benefit of the extremely high temperature is the expulsion of most of the impurities.
The result of the final graphitization process is a solid graphite block or round made up of graphite particles held together by the converted binder. The characteristics of the graphite are dictated by the recipe used which will specify the particle size, type of coke, final porosity, additives, and method of molding.
Typically extrusion is used to produce larger particle size (0.030-0.060") material which is used as a general purpose material. While isostatically molded graphite is composed of extremely small particles (4-10 micron). The isostatically molded graphite takes much longer to manufacture due to the fact that it takes extra steps to mill the raw materials to a very fine and consistent powder prior to molding.
Typically extruded graphite is less than half the cost of the higher cost isostatically molded graphite, but the price difference reflects the characteristics obtained as the isostatically molded graphite has very low porosity, high density, consistent resist and high strength.
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