Discussion on Improving the Appearance Quality of EDM
Because EDM can process difficult-to-cut materials, and can process parts with special and complex shapes. Therefore, EDM skills are widely used in machinery, aerospace, electronics, instruments, light industry and other parts. However, because there are many technical parameters in the processing process, there is a certain influence between the various technical parameters, which often brings some technical questions, which directly affect the quality and power of the processing. Therefore, the analysis of the factors affecting the quality of the processing surface has important implications.
1 Analysis of the appearance quality of EDM
The appearance quality of EDM mainly includes appearance roughness, appearance transformation layer and appearance mechanics function. The appearance of EDM is mainly composed of countless non-directional small pits and hard convex edges. The calculation of the surface roughness is the same as that of mechanical processing. It is indicated by the uniform arithmetic deviation value Ra of the microscopic general flatness. Because EDM removes the data by the high temperature between the object and the workpiece due to the discharge, during the deionization period, because of the rapid cooling of the working fluid, the outer surface of the workpiece is greatly changed. The surface degeneration layer is further divided into three levels: melting and condensation layer, heat-affected layer, and micro-crack, as shown in Figure 1.
The surface mechanics functions are mainly the indicators of microhardness, residual stress, fatigue resistance and so on. A large number of experimental studies have confirmed that after EDM, the microhardness and wear resistance of the workpiece have improved, and the degree of improvement is also different for different materials. Because in the process of EDM, the surface of the workpiece experiences instantaneous expansion and then shortening, which makes the surface of the workpiece constitute residual stress, most of the residual stress is tensile stress, and there will be a small compressive stress in the outer surface. Due to the effect of residual stress and possible micro-cracks, the fatigue resistance of the workpiece after EDM will be greatly reduced, so it is necessary to adopt a certain heat treatment to eliminate this adverse effect.
2 The influence of electrical parameters on the surface roughness
Electrical discharge machining mainly relies on the instantaneous high temperature and high heat generated by the pulse discharge between the east and west electrodes and the workpiece to remove data. The discharge trace formed by removing the data is mainly determined by the pulse energy. The energy released by a single pulse discharge depends on the inter-electrode discharge voltage, discharge current, and discharge continuation time (ie pulse width). The calculation formula is:
Therefore, the electrical parameters that affect the surface roughness of EDM mainly include peak voltage, peak current and pulse width. Because under certain processing conditions, the peak voltage does not change much and generally takes a fixed value, so the surface roughness mainly depends on the peak current and pulse width.
According to experimental research, when copper is used as the east-west electrode and steel is used as the workpiece, the experience formula of surface roughness, peak current and pulse width can be obtained:
2.1 The influence of peak current on external surface roughness
When the pulse width must be observed, following the increase of the peak current, the energy of a single pulse is also added to increase the surface roughness value, as shown in Figure 2. Therefore, in order to obtain a smaller surface roughness value, the peak current value should be reduced as much as possible.
2.2 The effect of pulse width on external surface roughness
When the peak current must be observed, the pulse width is large, the energy of a single pulse is large, the discharge traces of discharge corrosion are large and deep, and the surface roughness becomes worse, as shown in Figure 3. Therefore, when the peak current must be observed, a smaller pulse width is generally selected to obtain a better surface roughness.
3 The influence of the east and west electrodes on the surface roughness
3.1 East and West electrode data and surface roughness
In the process of EDM, the materials of the east and west electrodes are different, and the functions of the electrodes are different. The wear of the electrodes during the machining process is different, and the effect on the surface roughness of the workpiece is also different. Poor data quality, uneven arrangement, and impurity-containing electrodes will cause the surface roughness of the processed workpiece to be large, which will not meet the processing requirements. Generally, in the rough machining standards, the pulse width is larger, and the copper electrode has better surface roughness than the graphite electrode at this moment. In the finishing standards, the pulse width is small, and the surface roughness of the graphite electrode is better. EDM is to copy the surface of the electrode to the surface of the workpiece. The surface roughness of the electrode will directly affect the roughness of the processed surface. Especially in the finishing process, the east and west electrodes generally need to be polished.
3.2 The influence of processing polarity
In the pulse discharge process, the surface of the positive and negative electrodes are separated by the bombardment of negative electrons and positive ions, so the energy distributed on the surfaces of the two electrodes is not the same. Because the mass and inertia of the electrons are small, during the initial period of discharge, a greater speed and initial velocity can be obtained, and the surface of the positive electrode can be bombarded, so that the electrode material quickly melts and vaporizes. However, the mass and inertia of the positive ions are large, and as the discharge continues, the energy they obtain is larger, and the shelling effect on the surface of the negative electrode is strong. Therefore, when the pulse width is small, positive polarity processing should be selected; when the pulse width is large, negative polarity processing should be selected. Generally speaking, the surface roughness of positive polarity processing is better than the surface roughness of negative polarity processing. In order to obtain better appearance quality, positive polarity processing is selected during finishing.
4 Influence of processing area
Experimental research shows that under the same other processing conditions, the processing area is different, and the surface roughness varies greatly. When the processing area is large, even with a small pulse energy, the surface roughness value of the processed workpiece is difficult to be less than 0.32 μm, and as the processing area increases, the surface roughness becomes worse. This is because in the process of processing, the east-west electrode and the workpiece are equivalent to the two poles of the capacitor. According to the calculation formula of parallel plate capacitance:
It can be seen that with the increase of the machining area, the parasitic capacitance in EDM increases, that is, the energy storage effect of the two electrode plates of EDM increases. In the case of small electrical regulations, electrical energy is “absorbed” by this capacitor. At this moment, it can only play a “charging” effect without causing spark discharge. Usually, only one discharge can be generated by stacking multiple pulses, which makes the processed discharge pits larger and the surface roughness value larger.
The emergence of “mixed powder processing” new technology has dealt with this question. The use of “mixed powder” processing technology can process a bright surface with a roughness value of 0.05 to 0.1 μm on a larger area. Powder mixing processing is to mix a certain amount of conductive powder such as silicon or aluminum into the working fluid, so that the resistivity of the working fluid is reduced, and the latent cloth capacitance is reduced. The discharge channel is divided into many small discharge channels together, so that the discharge energy is greatly refined, and a smaller surface roughness value can be obtained. Fig. 4 shows the electric field distribution diagram between ordinary processing and powder mixing processing.
5 The influence of electro-erosion products on the surface roughness
As the processing progresses, the number of electro-erosion products between the east and west electrodes and the workpiece increases. If too much electro-erosion products are swept and dispersed from the electrode in time, this will not only make the oil dirty, increase the viscosity, and not be conducive to taking away. The electro-erosion product between the electrode and the workpiece will change the composition of the gap medium, reduce its insulation strength, increase the number of secondary discharges between the workpiece and the workpiece, and affect the state of inter-electrode discharge. The discharge points are gathered in a certain part, which damages the deionization process and affects the stability of discharge. In this way, the pulsed spark discharge turns the vicious circle into a harmful and stable arc discharge condition. The high temperature of the working fluid may form the appearance of carbon deposits, and the coke particles will accumulate there, which will burn the east and west electrodes and the surface of the workpiece. The roughness value increases. In addition, inadequate discharge of electro-erosion products can simply cause short-circuit appearance, reduce processing speed, and long-term short-circuit effects increase the current in the circuit, which affects the life of the pulse power supply. In order to deal with this question, it is necessary to filter and clean the working fluid frequently, strengthen the filtration cycle, and manually sort the accumulated carbon black and metal particles in the working fluid. In the process of processing, in order to strengthen the discharge of electro-erosion products, proper working fluid circulation must be carried out. Working fluid circulation methods can be divided into two methods: flushing and pumping. The oil flushing or pumping method is shown in Figure 5.
The oil flushing method has a strong chip removal ability and can completely discharge the electro-erosion products from the electrode. However, in the process of chip removal, the electro-erosion products will pass through the processed area, which will cause “secondary discharge” between the tool and the workpiece, and then damage the processed surface and reduce the surface quality. Using the oil pumping method, the electro-erosion products are discharged from the surface to be processed, although the quality of the processed surface will not be affected. However, chip removal is not sufficient, and the combustible gas generated by the processing process is simply accumulated in the dead corner of the oil pumping circuit, resulting in the appearance of “fire shot”. This will not only lead to safety hazards, but also aggravate the wear of the electrode, make the electrode form a taper, and affect the accuracy of processing. Therefore, the oil flushing method is generally selected to enhance the discharge of electro-erosion products.
The choice of oil flushing method has led to another discussion on factors affecting appearance quality. How to avoid the secondary discharge caused by the discharge of electro-erosion products from the processed area has become a hot topic that everyone pays attention to. Especially in micro-hole EDM, the discharge gap between the east and west electrodes and the workpiece is very small, resulting in a very high probability of secondary discharge. Even if excellent appearance quality can be obtained during the machining process, it is in the process of discharging the electro-erosion product. It will also be damaged. In order to deal with this question, Japanese experts plated a thin insulating layer on the surface of the electrode. This medium vaporizes during the processing process and adheres to the surface of the processed workpiece, so that the east and west electrodes and the processing surface are covered by the insulating medium, so that the processed surface will not be processed again due to the “secondary discharge”, which will damage its appearance. . After the workpiece is processed, the workpiece is rinsed to remove the insulating medium attached to the processed surface. The current composition of this medium is not clear for the time being, and further in-depth study by experts in the field of data is required. The study of the plan to avoid damage to the processed surface by the secondary discharge is also an indispensable key element to ensure the quality of the processed surface, and its study has important research value.
In summary, there are many factors that affect the appearance quality of EDM, and the treatment plan for each factor is very simple. However, in the process of processing, it is necessary to summarize and think about the influence of these elements on the processing quality and the mutual influence between the elements, analyze the system and allocate them scientifically, and finally find the optimal parameter control and allocation, and then improve the appearance. Quality, reduce the surface roughness.