1.EDM characteristics of graphite materials.
1.1.Discharge machining speed.
Graphite is a non-metallic material with a very high melting point of 3, 650 ° C, while copper has a melting point of 1, 083 ° C, so the graphite electrode can withstand greater current setting conditions.
When the discharge area and the scale of electrode size are larger, the advantages of high efficiency rough machining of graphite material are more obvious.
The thermal conductivity of graphite is 1/3 that of copper, and the heat generated during the discharge process can be used to remove metal materials more effectively. Therefore, the processing efficiency of graphite is higher than that of copper electrode in medium and fine processing.
According to the processing experience, the discharge processing speed of graphite electrode is 1.5~2 times faster than that of copper electrode under the correct use conditions.
1.2.Electrode consumption.
Graphite electrode has the character that can withstand the high current conditions, in addition, under the condition of appropriate roughing setting, including carbon steel workpieces produced during machining removal in content and working fluid at high temperature decomposition of carbon particles, the polarity effect, under the action of partial removal in content, carbon particles will adhere to the electrode surface to form a protective layer, ensure the graphite electrode in small loss in rough machining, or even “zero waste”.
The main electrode loss in EDM comes from rough machining. Although the loss rate is high in the setting conditions of finishing, the overall loss is also low due to the small machining allowance reserved for parts.
In general, the loss of graphite electrode is less than that of copper electrode in rough machining of large current and slightly more than that of copper electrode in finishing machining. The electrode loss of graphite electrode is similar.
1.3.The surface quality.
The particle diameter of graphite material directly affects the surface roughness of EDM. The smaller the diameter is, the lower the surface roughness can be obtained.
A few years ago using particle phi 5 microns in diameter graphite material, the best surface can only achieve VDI18 edm (Ra0.8 microns), nowadays the grain diameter of graphite materials have been able to achieve within 3 microns of phi, the best surface can achieve stable VDI12 edm (Ra0.4 mu m) or more sophisticated level, but the graphite electrode to mirror edm.
The copper material has low resistivity and compact structure, and can be processed stably under difficult conditions. The surface roughness can be less than Ra0.1 m, and it can be processed by mirror.
Thus, if the discharge machining pursues extremely fine surface, it is more suitable to use copper material as electrode, which is the main advantage of copper electrode over graphite electrode.
But copper electrode under the condition of large current setting, the electrode surface is easy to become rough, appear even crack, and graphite materials would not have this problem, the surface roughness requirement for VDI26 (Ra2.0 microns) about mold processing, using a graphite electrode can be done from coarse to fine processing, realizes the uniform surface effect, the surface defects.
In addition, due to the different structure of graphite and copper, the surface discharge corrosion point of graphite electrode is more regular than that of copper electrode. Therefore, when the same surface roughness of VDI20 or above is processed, the surface granularity of the workpiece processed by graphite electrode is more distinct, and this grain surface effect is better than the discharge surface effect of copper electrode.
1.4.The machining accuracy.
The coefficient of thermal expansion of graphite material is small, the coefficient of thermal expansion of copper material is 4 times that of graphite material, so in the discharge processing, graphite electrode is less prone to deformation than copper electrode, which can obtain more stable and reliable processing accuracy.
Especially when deep and narrow rib is processed, local high temperature makes copper electrode bend easily, but graphite electrode does not.
For copper electrode with a large depth-diameter ratio, a certain thermal expansion value should be compensated to correct the size during machining setting, while graphite electrode is not required.
1.5.Electrode weight.
The graphite material is less dense than copper, and the weight of the graphite electrode of the same volume is only 1/5 of that of the copper electrode.
It can be seen that the use of graphite is very suitable for the electrode with a large volume, which greatly reduces the load of the spindle of EDM machine tool. The electrode will not cause inconvenience in clamping due to its large weight, and it will produce deflection displacement in the processing, etc. It can be seen that it is of great significance to use graphite electrode in the large-scale mold processing.
1.6.Electrode manufacturing difficulty.
The machining performance of graphite material is good. The cutting resistance is only 1/4 that of copper. Under the correct processing conditions, the efficiency of milling graphite electrode is 2~3 times that of copper electrode.
Graphite electrode is easy to clear Angle, and it can be used to process the workpiece which should be finished by multiple electrodes into a single electrode.
The unique particle structure of graphite material prevents burrs from occurring after electrode milling and forming, which can directly meet the use requirements when the burrs are not easily removed in the complex modeling, thus eliminating the process of manual polishing of the electrode and avoiding the shape change and size error caused by polishing.
It should be noted that, because graphite is dust accumulation, milling graphite will produce a lot of dust, so the milling machine must have a seal and dust collection device.
If it is necessary to use edM to process graphite electrode, its processing performance is not as good as copper material, cutting speed is about 40% slower than copper.
1.7.Electrode installation and use.
Graphite material has good bonding property. It can be used to bond graphite with the fixture by milling the electrode and discharging, which can save the procedure of machining screw hole on the electrode material and save working time.
The graphite material is relatively brittle, especially the small, narrow and long electrode, which is easy to break when subjected to external force during use, but can immediately know that the electrode has been damaged.
If it is copper electrode, it will only bend and not break, which is very dangerous and difficult to find in the process of use, and it will easily lead to the scrap of the workpiece.
1.8.Price.
Copper material is a non-renewable resource, the price trend will become more and more expensive, while the price of graphite material tends to stabilize.
Copper material price rising in recent years, the major manufacturers of graphite improving the process in the production of graphite make its competitive advantage, now, under the same volume, generality of graphite electrode material price and the price of copper electrode materials is quite, but the graphite can achieve efficient processing, than the use of copper electrode to save a large number of working hours, equivalent to reduce the production cost directly.
To sum up, among the 8 edM characteristics of graphite electrode, its advantages are obvious: the efficiency of milling electrode and discharge processing is significantly better than that of copper electrode; large electrode has small weight, good dimensional stability, thin electrode is not easy to deform, and surface texture is better than copper electrode.
The disadvantage of graphite material is that it is not suitable for fine surface discharge processing under VDI12 (Ra0.4 m), and the efficiency of using edM to make electrode is low.
However, from a practical point of view, one of the important reasons affecting the effective promotion of graphite materials in China is that special graphite processing machine is needed for milling electrodes, which puts forward new requirements for processing equipment of mold enterprises, some small enterprises may not have this condition.
In general, the advantages of graphite electrodes cover the vast majority of edM processing occasions, and are worthy of popularization and application, with considerable long-term benefits. The deficiency of fine surface processing can be made up by the use of copper electrodes.
2.Selection of graphite electrode materials for EDM
For graphite materials, there are mainly the following four indicators that directly determine the performance of the materials:
1) Average particle diameter of the material
The average particle diameter of the material directly affects the discharge condition of the material.
The smaller the average particle of graphite material is, the more uniform the discharge is, the more stable the discharge condition is, the better the surface quality is, and the less the loss is.
The larger the average particle size is, the better removal rate can be obtained in rough machining, but the surface effect of finishing is poor and the electrode loss is large.
2) The bending strength of the material
The flexural strength of a material is a direct reflection of its strength, indicating the tightness of its internal structure.
The material with high strength has relatively good discharge resistance performance. For the electrode with high precision, the material with good strength should be selected as far as possible.
3) Shore hardness of the material
Graphite is harder than metal materials, and the loss of the cutting tool is greater than that of the cutting metal.
At the same time, the high hardness of graphite material in the discharge loss control is better.
4) The inherent resistivity of the material
The discharge rate of graphite material with high inherent resistivity will be slower than that with low resistivity.
The higher the inherent resistivity, the smaller the electrode loss, but the higher the inherent resistivity, the stability of the discharge will be affected.
At present, there are many different grades of graphite available from the world’s leading graphite suppliers.
Generally according to the average particle diameter of graphite materials to be classified, particle diameter ≤ 4 m is defined as fine graphite, particles in 5~ 10 m is defined as medium graphite, particles in 10 m above is defined as coarse graphite.
The smaller the particle diameter is, the more expensive the material is, the more suitable graphite material can be selected according to the requirements and cost of EDM.
3.Fabrication of graphite electrode
The graphite electrode is mainly made by milling.
From the point of view of processing technology, graphite and copper are two different materials, and their different cutting characteristics should be mastered.
If the graphite electrode is processed by the process of copper electrode, problems will inevitably occur, such as frequent fracture of the sheet, which requires the use of appropriate cutting tools and cutting parameters.
Machining graphite electrode than copper electrode tool wear, on the economic consideration, the choice of carbide tool is the most economical, choose diamond coating tool (called graphite knife) price is more expensive, but diamond coating tool long service life, high processing precision, the overall economic benefit is good.
The size of the front Angle of the tool also affects its service life, the 0° front Angle of the tool will be up to 50% higher than the 15° front Angle of the tool’s service life, cutting stability is also better, but the greater the Angle, the better the machining surface, the use of 15° Angle of the tool can achieve the best machining surface.
The cutting speed in machining can be adjusted according to the shape of the electrode, usually 10m/min, similar to the machining of aluminum or plastic, the cutting tool can be directly on and off the workpiece in rough machining, and the phenomenon of Angle collapse and fragmentation is easy to occur in finishing machining, and the way of light knife fast walking is often adopted.
Graphite electrode in the cutting process will produce a lot of dust, in order to avoid graphite particles inhaled machine spindle and screw, there are two main solutions at present, one is to use a special graphite processing machine, the other is the ordinary processing center refit, equipped with a special dust collection device.
The special graphite high speed milling machine on the market has high milling efficiency and can easily complete the manufacturing of complex electrodes with high precision and good surface quality.
If EDM is needed to make a graphite electrode, it is recommended to use a fine graphite material with a smaller particle diameter.
The machining performance of graphite is poor, the smaller the particle diameter is, the higher the cutting efficiency can be obtained, and the abnormal problems such as frequent wire breaking and surface fringe can be avoided.
4.EDM parameters of graphite electrode
The selection of EDM parameters of graphite and copper is quite different.
The parameters of EDM mainly include current, pulse width, pulse gap and polarity.
The following describes the basis for rational use of these major parameters.
The current density of graphite electrode is generally 10~12 A/cm2, much larger than that of copper electrode. Therefore, within the range of current allowed in the corresponding area, the larger the current is selected, the faster the graphite discharge processing speed will be, the smaller the electrode loss will be, but the surface roughness will be thicker.
The larger the pulse width is, the lower the electrode loss will be.
However, a larger pulse width will make the processing stability worse, and the processing speed slower and the surface rougher.
In order to ensure low electrode loss during rough machining, a relatively large pulse width is usually used, which can effectively realize low loss machining of graphite electrode when the value is between 100 and 300 US.
In order to obtain fine surface and stable discharge effect, a smaller pulse width should be chosen.
In general, the pulse width of graphite electrode is about 40% less than that of copper electrode
The pulse gap mainly affects the discharge machining speed and machining stability. The greater the value, the better the machining stability will be, which is helpful for obtaining better surface uniformity, but the machining speed will be reduced.
Under the condition of ensuring the processing stability, the higher processing efficiency can be obtained by choosing a smaller pulse gap, but when the discharge state is unstable, the higher processing efficiency can be obtained by choosing a larger pulse gap.
In graphite electrode discharge machining, pulse gap and pulse width are usually set at 1:1, while in copper electrode machining, pulse gap and pulse width are usually set at 1:3.
Under stable graphite processing, the matching ratio between pulse gap and pulse width can be adjusted to 2:3.
In the case of small pulse clearance, it is beneficial to form a covering layer on the electrode surface, which is helpful to reduce the electrode loss.
The polarity selection of graphite electrode in EDM is basically the same as that of copper electrode.
According to the polarity effect of EDM, positive polarity machining is usually used when machining die steel, that is, the electrode is connected to the positive pole of the power supply, and the workpiece is connected to the negative pole of the power supply.
Using large current and pulse width, selecting positive polarity machining can achieve extremely low electrode loss. If the polarity is wrong, the electrode loss will become very large.
Only when the surface is required to be fine processed less than VDI18 (Ra0.8 m) and the pulse width is very small, the negative polarity processing is used to obtain better surface quality, but the electrode loss is large.
Now CNC edM machine tools are equipped with graphite discharge machining parameters.
The use of electrical parameters is intelligent and can be generated automatically by the expert system of the machine tool.
Generally, the machine can configure the optimized processing parameters by selecting the material pair, application type, surface roughness value and inputting the processing area, processing depth, electrode size scaling, etc. During programming.
Set for graphite electrode of edm machine tool library rich processing parameters, the material type can choose in the coarse graphite, graphite, graphite corresponds to a variety of workpiece material, to subdivide the application type for the standard, deep groove, sharp point, large area, large cavity, such as fine, also provides low loss, standard, high efficiency and so on the many kinds of processing priority choice.
5.Conclusion
The new graphite electrode material is worth popularizing vigorously and its advantages will be gradually recognized and accepted by the domestic mould manufacturing industry.
The correct selection of graphite electrode materials and the improvement of related technological links will bring high efficiency, high quality and low cost benefit to mold manufacturing enterprises
Post time: Dec-04-2020