Stated electorde material kinds.Introduced characteristics and kinds of graphite electorde,choice method and application example of graphite electorde in EDM machining.
Graphite is a nonmetal, but it is usually classified as a metal-like material because it exhibits both metallic and nonmetallic physical properties.
Graphite can be changed from solid state to gaseous state at high temperature, and its sublimation temperature is very high. Its conductive and thermal conductivity is good, and processing is convenient, so it is an excellent electrode material.
There are many kinds and grades of graphite for EDM users. The properties of graphite are determined by its particle size, microstructure uniformity and other inherent physical properties. In order to distinguish effectively different graphite, classification is made according to particle size. In the same category of graphite, there are many brands and different levels to choose from. There are so many levels of EDM special graphite, is to adapt to different processing requirements, to achieve the corresponding expected results.
Graphs of various grades of graphite (micrograph 100 times magnification)
Although graphite is an excellent electrode material, not all graphite properties are the same because graphite used for EDM must first be machined by itself, and the raw materials, processing techniques and process control methods used by manufacturers are different, so although the basic procedures are similar, the final product The quality is different, and all manufacturers make different grades of graphite for different purposes.
The invention of high-temperature electric furnaces in the late 19th century made it possible to produce graphite. Since E, G, Acheson patented it in 1986, the basic method of graphite processing has changed little, and most of the raw materials for graphite processing are petroleum coke (amorphous carbon). The calcined coke is first milled into different sizes of particles, and then the desired coke particles are mixed with the coal char particles and extruded into the billet. The volatile substances in the coal tar are removed by vacuum roasting, and the mixture is combined into amorphous carbon. Finally, after high temperature treatment and graphitization treatment, the carbon is converted into graphite.
Since there is not much difference in the appearance of all graphite, it is necessary for all EDM users to understand the importance of graphite attributes and the impact of their attributes on EDM.
Like wood, many graphite products have fixed particle orientation, and note that it is a layered crystal structure. This causes the crystals to form microcrystals, and the microcrystals to form graphite particles. Graphite like wood, the strength in the same direction as the crystal structure is greater than its transverse strength. Similarly, the conductivity and other properties of graphite along crystals are often better than lateral ones.
Graphite particles are arranged in a certain direction during baking, and the microcrystals in each particle have a preferred orientation. Therefore, the final graphite particles may be larger, irregular in shape and size, and uneven in structure.
Unfortunately, the human eye can not see the internal grain structure of graphite, as can not see the internal structure of wood, therefore, anisotropic ordinary graphite, can not use its differences, along the composition of the grain processing.
EDM Graphite classification
EDM graphite is classified according to its particle size. This classification criterion is adopted because particle size is directly or indirectly related to other properties and properties of graphite. EDM graphite can be divided into five types: ultrafine graphite with particle size less than 1 micron, ultrafine stone black with particle size of 1-5 micron, ultrafine graphite with particle size of 6-10 micron, fine graphite with particle size of 11-20 micron and medium stone black with particle size of 21-100 micron. High grade graphite with particle size less than 5 m is difficult to work out.
(1) particle size.
Particle size is one of the most important properties of graphite. The diameter of the micro graphite particle is less than 1 m. In most cases, the size of EDM graphite is much larger.
The significance of particle size to EDM graphite is as follows:
EDM will consume electrodes. The graphite loss with large particle size is much larger than that with small particle size, which not only increases the electrode loss, but also causes DC arcing. Large particles may obstruct the discharge gap, resulting in unstable discharge, thereby reducing the processing speed. This effect is more obvious when the oil-flushing conditions are poor. Even if the use of advanced adaptive machine tools, due to the clearance of impurities, the speed will be significantly reduced.
Particle size and EDM
The particle size is closely related to the minimum value of surface finish. In the finishing process with low energy and high frequency, the electrode tends to duplicate its own structure on the surface of the workpiece. It is impossible to get a better surface finish using a graphite electrode with large particles, large pores and rough surface.
The strength is also related to particle size and pore size. The bonding force between particulates of ultrafine graphite and very fine graphite is much larger than that of other coarser graphite. Graphite with grain orientation is much stronger along the crystal direction than the transverse one.
The significance of strength for EDM graphite is as follows:
The flexural strength and compressive strength of electrodes are very important whether they are in processing or not. Many electrodes are damaged or destroyed before they are processed due to improper treatment or accidents. Electrodes made from high-strength ultra-micro graphite and ultra-fine graphite can withstand repeated use without cracking. High strength means that it can be used to process small ribs and fine electrodes. For precision machining, be sure to use materials with flexural strength greater than 10,000 PSI. Low strength materials often crack when machining thin walls and precision parts.
In machining, if the deflection strength of the thin rib electrode is not big enough, the pressure of the oil will be bent.
High strength also means low wear and tear. This is because stronger materials are better able to resist the impact of EDM.
Density is the ratio of graphite mass to volume. The density of most EDM graphite ranges from 1.55 g/cm3 to 1.85 g/cm3. When comparing different materials and densities, we should observe their grain structure diagram. The density of graphite with larger particles may be higher than that of the best small particles and porous graphite.
Usually, the higher the density of graphite given by a manufacturer, the better the physical properties (such as strength, etc.).
The significance of density to EDM graphite is as follows:
The same is high-density material. The particles are closely arranged, the graphite with small pores is larger than particle size, the graphite with loose arrangement is less loss, and the surface finish achieved is good.
For graphite, particle size, pore size and density all affect its hardness. Similarly, the temperature at high temperature (graphitization) will affect the temperature.
The hardness of most EDM graphite is between 45~85HS, while the hardness of 55~75HS is the most convenient material.
The significance of hardness to EDM graphite is as follows:
The hardness of graphite is very important to the machining of its electrode. The harder the material, the easier it is to break, the more difficult it is to process. The hardness will not affect the performance of the electrode in EDM.
Resistivity (ER) determines the resistance of materials to current resistance. The lower the resistivity, the better the conductivity. The density of material will affect its resistivity. In the same grade graphite, the greater the density, the lower the resistivity.
The significance of electrical resistivity to EDM is as follows:
Resistivity generally does not affect EDM, except when machining thin-walled ribs and thin rods, high resistivity will waste the energy in the electrode, make it hot, resulting in overheating ribs.
The resistance coefficients of the electrodes must be kept consistent when drilling with a small pole cutter, because the electrode with the slowest processing speed will affect the processing speed.
Five key elements of graphite electrode selection
The electrode performance factors concerned by EDM operators include processing speed, electrode loss rate, surface finish, machinability and raw material cost. If the machining accuracy of large cavity is not high, large size electrode material is needed. And the requirement electrode loss is low, processing speed is fast, the price is reasonable. So fine graphite is a qualified electrode material, because it is a big particle, affordable; If you want to process a small, fine cavity, you need to use micrographite. Because of this kind of processing on the electrode wear rate, machinability and surface finish strict requirements. This level of graphite particles is small, strong and capable of maintaining complex processing details, and the problem of raw material costs becomes secondary.
When choosing the electrode, we must take into account the different requirements of different processing conditions on the electrode performance. At the same time, we should also consider the work items, including: workpiece materials, electrode shape and size, cavity number and the number of electrodes needed for rough and finish machining.
The performance of the electrode is the key to success or failure. These elements, combined with the charts provided by the objective comparison test project, can be used to accurately evaluate the performance of a given material.
1 processing speed (MRR)
The rate of processing is usually expressed in terms of an hourly etched metal, with a cubic inch / hour (in3/h). In fact, it can be expressed directly in yuan / hour.
Processing speed (in3/h) = (electrode end area (in2) *depth (in) / processing time (micron in) *60
It is not enough to obtain high processing speed only by correct operating parameters. It also involves the energy loss in EDM process. The energy may be consumed in the following three places:
(1) inside the workpiece ：the physical properties of the workpiece will affect the machining speed. Its melting point and thermal conductivity are also important. For example, because copper is a good heat transfer conductor, its melting point is low, but the processing speed is still slow. The reason is that the higher thermal conductivity leads to the rapid emission of energy, thus reducing the processing speed. Although the melting point of tool copper is high, its thermal conductivity is lower than that of copper, so its processing speed is fast.
(2) In the gap：the particles of electrode loss remain in the gap, which will lead to machining instability and energy waste, which will also affect the machining speed.
(3) Inside the electrode：even if the electrode loss is as large as the workpiece loss, the machine tool can still run smoothly. We can not completely avoid electrode wear, but by choosing the appropriate electrode materials and workpiece combinations, using the most appropriate operating parameters to minimize electrode wear.
Obviously, all the energy lost in the workpiece is wasted except for the energy consumed in the workpiece. The maximum energy consumption occurs in the electrode, but this is also the easiest to avoid.
Usually, the larger the particle size of graphite electrode, the faster the processing speed is.
Of course, machining cavity velocity is not the only factor to be considered. With the increasing demand for accuracy, we must use graphite electrodes with small particle size.
2 Loss resistance
2.1 electrode loss
There are 4 kinds of attrition in machining: overall loss, angular loss, end face loss and side wear. Because the angle loss determines the accuracy of the final machining, its loss rate is the most important. If the most vulnerable part of the electrode can successfully resist the loss, then the overall loss can be minimized, and the life of the electrode can be maximized. The ability of the electrode itself to process to a certain accuracy and maintain accuracy is directly related to its loss resistance and machinability.
When the electrode is re repaired, the loss of the angle of view should be emphasized. The original damaged part must be taken into account when repairing.
(1) End-face loss-in EDM process, the electrode becomes shorter and shorter, is caused by end-face loss, through the difference between the original initial length of the electrode and the final length after processing can be obtained end-face loss:
End loss = initial length – final length
(2) End-face wastage rate-the percentage of end-face wastage to the initial effective length of the electrode can be expressed by the percentage of the final length, the depth of processing can be divided by the measured end-face wastage, and the percentage of end-face wastage can be obtained by dividing 100 by the end-face wastage rate:
End loss ratio = processing depth / end loss
Percentage of end face loss =100/ end loss rate
(3) Angular loss – Electromagnetic field tends to concentrate at the bend angle of the electrode, which makes angular loss more harmful than other losses. The sharper the angle is, the more sparks are generated and the more heat is accumulated, which aggravates the loss of the angle. The loss of the obtuse angle is smaller than that of the sharp angle. High strength, high density and small particle electrodes can be selected to minimize angular loss.
To determine the angular loss, the optical comparator can be used to determine the surface angular loss, that is, the part of the arc lost at the right angle. Then the real angular loss is obtained by adding the end loss and the surface angular loss.
Angular loss = surface angular loss + end loss
(4) the angular loss rate – the angular loss rate can be divided by the depth of processing divided by the angular loss.
Angular wastage = processing depth / corner loss
(5) Overall wear – refers to the ratio of the wear of the whole electrode surface to the amount of metal etched, which can be calculated by the weight of both or other general units of measurement. Measurements were made before and after processing. The volume loss rate can be drawn from the following formula:
Volume loss rate = metal removal / electrode wastage
(6) side loss – indicating the loss of the side of the electrode.
2.2 no translational electrodes
The lateral loss is caused by the particles in the discharge gap. When the impurities get out of the gap with the dielectric oil, the side of the electrode will be rubbed. If there are too many impurities, the spark will take place in the side gap, where the workpiece and the side of the electrode will be melted. In the case of poor oil flushing conditions, large particle size and many impurities, the electrode will produce side loss. Side wear leads to the tapered processing of the workpiece by the electrode. Its value can be determined by the difference between the bottom of the mold cavity and its top after processing.
2.3 translational electrode
The translation of the translational electrode is the surface of the entire electrode, not just its end face. When the electrode moves laterally, it erodes the side of the workpiece. Due to the large area of the electrode used in the process, it can be Increasing the machining current and increasing the machining speed, the translation of the electrode greatly reduces the taper of the workpiece. The end face loss and the angular loss of the flat-free electrode are larger than those of the translational electrode, and the side surface of the processed cavity is inclined, and the loss of the end face and the side surface of the translational electrode is uniform, and the side surface of the processed cavity remains vertical.
Generally, small particles, high-strength graphite have less graphite loss than large particles. If the correct operating parameters are used, all graphite electrodes can achieve a lossless state (ie, end face loss is less than 1%).
2.4 no loss state
Lossless machining, ie loss rate is no more than 1%. To achieve this, the electrode positive electrode and long pulse width must be used for processing, and the interval should be set to a minimum value that can maintain stable processing.
After the electrode enters a lossless state, the surface is covered with a silver coating, which is the result of the metal attached to the workpiece. If the coverage is too thick, the electrode is made larger, a bump is formed on the surface thereof, and its shape is changed.
The lossless state does not mean that the machining speed is the fastest. Although all graphite electrodes can reach a lossless state, not all combinations of electrodes and workpieces can achieve this state.
3 surface finish
Surface finish is determined by the pulse width, peak current, electrode material, and workpiece material. As the electrode continues to erode the workpiece. It will gradually form a mirror image of the electrode surface on the workpiece, and the defects of the electrode surface will also be reflected on the surface of the workpiece, including defects or high temperature ablation. If the surface of the electrode is pitted, there is a bump on the surface of the workpiece. The finished surface finish of the workpiece will reflect the surface finish of the electrode.
To get the best surface finish, use a short pulse width and low energy because the etch pits formed on the surface of the workpiece are relatively small. If such conditions are not met, even the best electrode materials will not produce a good surface finish. Ultra-fine graphite and ultra-fine graphite have high strength and small particles, which is the first choice for finishing electrodes.
(1) CNC machining effect.
A flat surface electrode can be used to obtain a better surface finish. When the electrode is translated outward according to the translational trajectory, the computer will continuously reduce the current, so that the formed oxide layer is thinner, and thus the surface finish is better.
(2) Measuring surface finish.
The surface finish measurement standard used in the United States is the arithmetic mean, in microinches or micrometers. The root mean square (VDI.RMS) used in Europe is outdated but still appears in some publications.
Most graphites are easy to process, and materials with too high hardness are difficult to process. Processing time is determined by the particle size, strength and processing accuracy of the material. The precision that graphite processing can achieve is limited by its strength, particle size and pore size. If the material itself does not achieve the desired accuracy, then processing it is a waste of time. Fragmentation of the material tends to result in higher electrode rejection rates.
High-strength, small-particle materials are suitable for processing into small rounded, high-precision electrodes, and are good for maintaining precision after processing. The ultra-fine graphite and ultra-fine graphite can be processed into 0.5mm thick and 25mm long tubes. The diameter of the oil hole in the middle is only φ0.1mm, while the ultra-fine and fine graphite is suitable for processing into coarse electrodes with low precision.
5 material cost
The cost of electrode processing, loss and trimming must be considered when selecting the best electrode materials and operating parameters. The material cost is only a small part of the overall EDM cost.
When roughing is performed and the accuracy and surface finish are not critical, ordinary graphite is used as the electrode material, and the cost is very low. But if they are used in other processes that are not suitable, then the cost is even higher. The work items to be considered are: workpiece material, cavity shape, size and number, and the number of electrodes required for roughing and finishing.
The selected electrode material will affect the processing time of the electrode and workpiece, labor and electrode loss. This is why we must understand the properties and properties of graphite and understand the performance of different graphites under different processing conditions. They are the key to success or failure.
Graphite electrode selection application examples
In general, we can get information about various material properties and characteristics. Any prestigious electrode material manufacturer will be happy to provide a product specification sheet. Those who are also involved in the EDM market can also provide performance parameters for EDM in different situations. Obtain the performance parameters of the electrode material when using different workpiece materials in different situations from the dealer. If you don’t get the relevant information, it doesn’t mean that the material you are buying is not suitable. Just as a user, you may have to do some trial processing to determine the performance of the material or to try your luck when predicting the processing result.
In addition to raw material costs, processability is the only element that can be determined without testing. As stated earlier, the precision that any material can achieve cannot be less than its largest pore size. So reliable data on particle size, pore size and strength are a good indication of the processability of the material. Of course, if you know that the material is isotropic, you can be sure that the properties in each direction are the same.
To determine the processing speed, loss resistance and surface finish, it must not be accurately and thoroughly tested repeatedly. Obviously, testing is the only way to determine material performance data, but one machine is only used for testing electrode materials, and few manufacturers can afford it.
In addition to narrow grooves and thin ribs, an excellent mechanic can use any grade of graphite to make a micron-sized electrode. The problem is that the processed cavity cannot achieve or maintain the same accuracy.
Generally, the electrode with a large particle size (fine graphite) has a high processing speed, but when the sharp angle is processed, the electrode with a small particle size is processed at a high speed, and since the energy is concentrated at the sharp corner of the electrode, the electrode with a large particle is excessively depleted, and the impurity may block the discharge. Clearance: Side flushing does not remove impurities, processing is not stable, and these factors reduce processing speed. Moreover, the tip of the electrode is rounded, and the bottom of the processing cavity is wider than that of the electrode with a small particle size, which is why it is important to use an appropriate level of graphite for microfabrication.
Author: Ma Guoting