Graphite has many advantages that have made it the material most widely used for EDM electrodes.
- It is easy to machine.
- It is very resistant to thermal shock.
- It has a low coefficient of thermal expansion (3 times lower than copper) which guarantees stability of electrode geometry during electro discharge machining.
- It is available in large blocks.
- It does not melt, but goes directly from the solid state to the gaseous at 3,400°C,
- which reduces wear.
- Its density is 5 times lower than that of copper, which results in lighter electrodes.
- It provides a higher metal removal rate than copper with less wear.
- It has the unique characteristic that the wear ratio tends to decrease as the peak current increases.
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.
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.
- apparent density
- average grain size
- electrical resistivity
- flexural strength
As a general rule, the wear rate decreases and the surface finish improves as the graphite density increases. Dense graphites are used to machine parts where geometry is critical. However, material removal is often better with less dense graphite.
With very fine grain grapites, the wear decreases and the surface finish improves. On the other hand, material removal is less, block sizes decrease, and prices increase.
The lower the electrical resistivity, the higher the thermal conductivity. Therefore, the graphite has the ability to dissipate the energy accumulated during each discharge. Thus, the wear rate decreases with electrical resistivity.
The higher the flexural strength, the easier the fine detailed machining and the lower the wear.
The higher the graphite hardness, the more difficult it is to machine the electrode.
Graphite used in negative polarity gives a faster machining speed than positive polarity with a wear rate of about 20% regardless of the intensity level. However, machining in positive polarity leads to wear rates approximately 10% to 50% in finish mode (with a current intensity less than 5A) and about 0% in roughing mode (current intensity greater than 25A), while diminishing the risk of abnormal discharges, as previously mentioned.
In practice, the operator will normally use graphite in positive polarity when in roughing mode, except for straight through holes where the wear is not a critical factor. Machining in finish mode will be accomplished in negative or positive polarity dependent upon the stability of the process. To obtain process stability, it is advisable to work with smaller current densities in negative polarity (45 A/in2) than in positive polarity (60 A/in2).