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 '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.
Graphite electrode is nothing more than a sharp graphite rod, taken from a pile, subject to a support conductor of the current, copper tube, and at a reasonable distance a plastic handle, PVC tube. The handle should be far enough away so that it does not reach so much heat to soften it. Anyway it is clear that PVC is the worst plastic that can be used for this but it was there.
This is a sheet metal welder for almost any metal. It does not require input of material and practically does not admit it unless they are very thin rods. It is not about electric arc welding. What melts the metal is the graphite tip that shortly turns to bright white. What is heated by the current is graphite, not metal, Graphite electrode because the first one has a much greater resistance and dissipates most of the power. It is important that the graphite tip is sharp for two reasons:
The finer the point of contact between the material and the tip more resistance to current and more temperature reach. If it is too thick, heat is transmitted easily from the tip to the soldering iron and a large part of it dissipates without reaching the necessary temperature. The soldering iron only works if it concentrates most of the power at the point to be soldered. Everything has to be thought for that.
The truth is that to have been done in 10 minutes, nothing else came up with the idea, it was pretty good. The way to hold the Graphite electrode was the idea of a friend and is interesting for its simplicity. It involves making two cuts on the edge of the tube longitudinally dividing it into 4 more or less equal parts. Two of them are eliminated and the tube is left with two tabs. Each one is tightened with pliers to give it a round shape and it adapts to the bar and then approaches each other. You are looking for a large brass nut that snaps into place and, without the rod; you turn the nut with force, making a thread in the copper. Then open, put the bar and do the same again and is perfectly subject. This system allows changing the bar quickly, adjusting its position, and providing a good electrical contact.
Graphite electrode is mainly used in electric arc furnace. They are presently the only products available that have the high levels of electrical conductivity and the capability of sustaining the extremely high levels of heat generated in EAF. Graphite electrodes are also used to refine steel in ladle furnaces and in other smelting processes. Graphite electrodes are divided into 4 Types: RP Graphite electrodes, HP Graphite electrodes, SHP Graphite electrodes, UHP Graphite electrodes.
Diameter and length for all grades:
Diameter mm
Length mm
Nominal Diameter
Actual Diameter
Nominal Length
Allowance
mm
inch
Max
Min
mm
Standard
Short
200
8
205
200
± 100
-275
250
10
256
251
300
12
307
302
/
350
14
357
352
/
400
16
409
403
/
450
18
460
454
/
500
20
511
505
/
550
22
562
556
/
600
24
613
607
/
650
26
663
657
/
700
28
714
708
/
Machining Dimension of Electrode and Nipple&#;
Applicable
Dia.
mm
IEC code
Nipple
Socket
Large Dia.
mm
Length
mm
Small Dia.
mm
Socket Depth
mm
UHP
SHP
HP
RP
250
155T3N
155.57
220.00
147.14
116.00
300
177T3N
177.16
270.90
168.73
141.50
350
215T3N
215.90
304.80
207.47
158.40
400
215T3N
215.90
304.80
207.47
158.40
400
241T3N
241.30
338.70
232.87
175.30
450
241T3N
241.30
338.70
232.87
175.30
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450
273T3N
273.05
355.60
264.62
183.80
500
273T3N
273.05
355.60
264.62
183.80
500
298T3N
298.45
372.60
290.02
192.20
550
298T3N
298.45
372.60
290.02
192.20
UHP
SHP
HP
RP
200
122T4N
122.24
177.80
115.92
94.90
250
152T4N
152.40
190.50
146.08
101.30
300
177T4N
177.80
215.90
171.48
114.00
350
203T4N
203.20
254.00
196.88
133.00
400
222T4N
222.25
304.80
215.93
158.40
400
222T4L
222.25
355.60
215.93
183.80
450
241T4N
241.30
304.80
234.98
158.40
450
241T4L
241.30
355.60
234.98
183.80
500
269T4N
269.88
355.60
263.56
183.80
500
269T4L
269.88
457.20
263.56
234.60
550
298T4N
298.45
355.60
292.13
183.80
550
298T4L
298.45
457.20
292.13
234.60
600
317T4N
317.50
355.60
311.18
183.80
600
317T4L
317.50
457.20
311.18
234.60
650
355T4N
355.60
457.20
349.28
234.60
650
355T4L
355.60
558.8
349.28
285.40
700
374T4N
374.65
457.20
368.33
234.60
700
374T4L
374.65
558.80
368.33
285.40
Graphite electrodes uses
Graphite electrodes are used primarily in electric arc furnace steel manufacturing. Graphite electrodes can provide high levels of electrical conductivity and capability of sustaining the extremely high levels of generated heat. Graphite electrodes are also used in the refinement of steel and similar smelting processes.
1. The electrode holder should be held in the place beyond the security line of the top electrode; otherwise the electrode would be easily broken. The contact surface between the holder and the electrode should be cleaned up regularly to maintain a good contacting. The cooling jacket of the holder shall be avoided from water leakage.
2. Identify the reasons if there is gap in the electrode junction, do not use the muntil the gap is eliminated.
3. If there is falling off of nipple bolt when connecting electrodes, it is necessary to complete the nipple bolt.
4. The application of electrode should avoid of tilting operation, particularly, the group of connected electrodes should not be put horizontally so as to prevent from breaking.
5. When charging materials to the furnace, the bulk materials should be charged to the place of the furnace bottom, so as to minimize the impact of the large furnace materials on the electrodes.
6. The large pieces of insulation materials should be avoided of stacking on the bottom of the electrodes when smelting, so as to prevent from affecting the electrode use, or even broken.
7. Avoid of collapsing the furnace lid when rising or dropping the electrodes, which may result in electrode damage.
8. It is necessary to prevent the steel slag from splashing to the threads of the electrodes or nipple stored in the smelting site, which my damage the precision of the threads.
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