The causes of unstable discharge of precision EDM and improvement measures

2021/09/14



In the process of precision EDM, there is often a discharge instability phenomenon, making it difficult for the processing to proceed smoothly. Therefore, we need to operate according to the causes of the discharge instability phenomenon, respectively, to take the corresponding measures can improve the stability of processing. Next, let's take a look at the following article.
 
The causes of unstable discharge of precision EDM and improvement measures
 
(1) EDM machine tool with poor discharge performance often occurs in the processing of discharge instability phenomenon. EDM processing is mainly dependent on the good performance of the machine tool to complete the processing. High-grade EDM machine tools are equipped with a variety of pulse output circuit, spindle with high speed and high response servo characteristics, these characteristics can meet the high stability in the processing to achieve high quality processing. If the machine does not perform well, it is often difficult to control the discharge instability in the process, which seriously affects the quality of processing. The machine pulse power supply performance, the machine servo feed system is the main reason for the EDM machine itself in the processing of often occurring discharge instability phenomenon.
 
The most common problem is that the waveform of the machine's pulse power supply is out of whack, making the process unstable. To achieve stable discharge processing, the machine must require the pulse power supply can output a series of benign pulses, stable and reliable work, free from external interference. The machine's servo feed system should be highly sensitive, able to accurately detect spark gaps in the discharge state, automatically adjust to abnormal discharges, adjust and hysteresis as small as possible, and be highly resistant to interference. Because of the machine performance abnormalities in the problem is relatively complex, the user after confirming the machine tool problems, should promptly contact the machine maintenance service department, ask professionals to repair and troubleshoot.
 
(2) incorrect adjustment of electrical parameters have a negative impact on the discharge stability. Incorrect use of electrical parameters is the main cause of the discharge instability. Electrical regulation is mainly composed of current, pulse width, pulse gap three major electrical parameters. Using too much current, too much pulse width, too little pulse gap is the main reason for the discharge instability caused by the unreasonable adjustment of electrical parameters. These three parameters should be set and selected according to the stability of the process and the process index requirements.
 
In the case of unstable discharge, the first consideration is to increase the pulse gap, which can ensure the process to eliminate ionization and improve the chip discharge condition, and has little impact on the process index. The second consideration is to reduce the pulse width, too large a pulse width makes the processing in a short period of time too much discharge, processing in time to eliminate ionization, easy to produce burning arc. Other parameters in processing are also important. Such parameters as tool lifting speed, discharge time, tool lifting height, etc., which directly affect the effect of chip removal.
 
In case of unstable discharge, the tool lifting speed should be accelerated, the discharge time should be reduced and the tool lifting height should be increased. When dealing with electrical parameters, special attention should be paid to the difference in discharge stability between rough machining and finishing machining. In rough machining, due to the large discharge energy, large spark gap and good chip removal, more stable machining is often achieved, while in finish machining, the opposite is true and the discharge instability is easy to occur. Therefore, finish machining should be especially monitored. The polarity of machining should be correct. If negative polarity (electrode is negative) is mistakenly used in normal machining, the phenomenon of unstable discharge will also occur and it will be impossible to machine down, so the polarity of machining should be changed.
 
(3) Improper liquid handling and the quality of the processing liquid cause unstable discharge. EDM processing is carried out in the working fluid medium. The insulating properties of the working fluid play a role in eliminating ionization in the process of pulse discharge, accelerating the cooling of the electrode and the workpiece in the process, and making the electrolytic products suspended and excreted from the discharge gap. In EDM, flushing and pumping methods are often used for chip removal to avoid arc discharge and to stabilize the process. However, improper fluid handling can also affect the stability of the discharge. Excessive punching and pumping pressures can make it difficult to form discharge channels and produce unstable partial discharges, especially in finishing operations. It is possible to control the punching and pumping pressures within the critical pressure range for stable machining.
 
The incorrect direction of oil punching will cause the accumulation of chips and lead to unstable discharge, and carbon will be easily formed. Generally, the oil is flushed towards the open part and the blind bottom part is dripped downwards. Although the flushing and oiling process can discharge the machining chips well and improve the stability of the discharge, the uneven flow field will cause concentrated discharge and secondary discharge, which will have a great impact on the flatness and roughness of the workpiece and produce the phenomenon of uneven discharge gap. And the strong scouring will lead to abnormal loss of electrode edges. So in the precision machining usually use no punching liquid, oil immersion processing of liquid processing, relying on the action of lifting the tool to achieve stable processing chip. This places higher demands on the machine configuration.
 
When the machine spindle is fed with a high acceleration, the suction effect is generated, so that the machining chips, tar and exhaust gas that exist between the electrode and the workpiece can be discharged effectively. The oil temperature will be too high during the long processing time, and the surface of the processing part will be easy to produce unstable discharge if it cannot be cooled well. The oil temperature should not be higher than 35°C. If necessary, a cooling device should be installed in the upper part of the machining fluid circulation system to control the oil. The quality of the working fluid during machining is also important for the stability of the machining discharge. Dirty working fluids containing too much machining chips cannot relieve the contamination in the discharge gap in time, resulting in the formation of harmful arc discharge due to the non-dispersion of the discharge point.
 
Poor-quality fluids also cause unstable discharge during machining due to their poor performance. The EDM fluid to be used must have low viscosity, high insulation, ability to unblock the discharge channel, and good fluidity and permeability. At present, there are many types of EDM fluids, and some results of adding additives to the fluids can improve the performance of the discharge and enhance the stability of the discharge, which can be adopted.
 
(4) The type of electrode material selected, the quality of the material, and the influence of different electrode material processing electrical parameters on the discharge stability. Electrode materials must have good electrical conductivity and discharge stability. Pure copper electrode has good processing performance, especially processing stability, not easy to arc discharge or transition arc discharge, in most of the processing can be stable discharge, is widely used. The most outstanding feature is that it can maintain stable discharge in rough machining with high current and ensure low loss of electrode, but in finishing machining, it is easy to have unstable discharge and arcing burns.
 
Copper-tungsten alloy and silver-tungsten alloy are rarely used as electrode materials because of the high price of the materials. They can be used in precision machining because of their high price. They can still discharge very stably in the difficult parts such as micro parts and deep grooves, and the electrode loss is very small. The electrode material chosen must be of high quality to ensure stable discharge during machining. Pure copper must be electrolytic copper without impurities and preferably wrought. There are several classifications of graphite electrode materials, such as emi, extra-fine, ultra-fine, fine, etc., which can be selected according to the precision and efficiency requirements of the process.
 
The quality of graphite material should be uniform and strong, and not easy to produce spalling during processing. The use of different electrode materials for processing should be flexible to deal with the electrical parameters of the pairing, in order to achieve stable discharge in the processing, good processing results, to play the value of the selected material purposes. Now many electric processing machine tools are able to automatically pair electrical parameters according to different combinations of processing materials. Electrical parameter matching is mainly dealing with the current, pulse width, pulse gap size.
 
Depending on the properties of the electrode material, the appropriate electrical parameters are chosen to take advantage of the material and to deal with the defects in the machining. Depending on the machine tool used, a set of data sheets on the relationship between the main electrical parameters is provided here for reference, as shown in the following table (parameter values are specific).
 
Table of the main electrical parameters of different electrode materials pair relationship
 
(5) The selected process method is not reasonable, which makes the discharge unstable during processing. Whether the EDM process is reasonable and feasible is also the key to achieve stable processing. Most of the processing is: using roughing electrodes to etch out a large amount of metal to complete the rough machining, and then change the semi-finishing or finishing electrodes to complete the transition machining or finishing machining. The key to this process is to use electrode shaking to improve chip removal and achieve stable machining. Multi-stage machining conditions are achieved by using a free-floating method in which the other two axes are simultaneously expanded as the machining depth is fed.
 
The shaking method is widely used to make the discharge more stable, reduce the secondary discharge phenomenon, and achieve more uniform surface roughness on the sides and bottom surfaces. The size of the shaking amount depends on the shape of the machining part and the accuracy requirements, and is generally about 0.03mm in finishing. Since the amount of shaking is small, it has no effect on the accuracy of profiling. If the shaking method is not used in the machining, it is difficult to achieve stable machining under small gap discharge conditions. Unstable machining due to this reason can be easily found in the finishing process. The secondary discharge caused by the unstable discharge may result in large machining dimensions, and the actual spark gap may be smaller than the electrode scaling under good chip removal conditions, resulting in small machining dimensions that cannot be accurately controlled. These problems can be solved by using the shaking method to achieve stable machining.
 
(6) Difficult machining parts are not conducive to stable machining. Some processing parts are prone to unstable discharge during processing due to their processing limitations. For example, at the beginning of the processing of surface parts, the processing of clear corner parts, processing of sharp and thin parts, due to the actual discharge area is small, the current density is large, the concentration of local electrolysis products is too high, the discharge point can not be dispersed and transferred, the residual heat after the discharge is too late to spread and accumulate, resulting in overheating, which destroys the stability of processing. The current must be temporarily reduced, and then gradually increase the electric gauge after the actual processing area is gradually expanded and the processing is gradually stabilized. When machining deep holes, sloping parts and deep cavities, the discharge instability will also occur during machining due to the difficulty of chip removal.
 
It is necessary to take appropriate measures to improve the chip removal performance. The sloped electrode must have a high chip discharge height. For deep holes and deep cavities, a larger size reduction can be chosen for the electrode to improve chip removal by leveling. Deep round holes can be machined very stably by rotating the C-axis with the C-axis and using the EROWA fixture in a concentric manner. The 3-axis linkage method is used for clear corners where possible, i.e. oblique machining to avoid unstable discharge due to the small area of the machined part. In addition, the use of horizontal servo machining in some difficult parts can also improve the discharge stability. The use of the electrode to avoid empty, open the exhaust chip hole, etc. can also improve the discharge instability under some difficult machining conditions.
 
(7) The discharge instability caused by the processing problems in the processing operation. The unstable discharge phenomenon can also occur when some small parts of the machining operation are not handled properly. Small pieces or workpieces that are difficult to clamp or electrodes do not take reliable clamping methods, resulting in loosening during processing and unstable discharge. If the workpiece is not demagnetized and used directly for EDM, the chips will be adsorbed and difficult to discharge, resulting in unstable discharge during processing. The presence of debris, rust, and burrs on the machined part can make the discharge very unstable at the beginning of machining.
 
The machining of through-hole part is not considered to use the hole to discharge the chips in the clamping, so it becomes blind bottom machining, which greatly reduces the machining stability. The "discharge" phenomenon during machining may loosen the workpiece, and the process of gas ignition also affects the stability of machining. Misalignment of the electrode and tilting of the workpiece also affects the stability of the discharge. In the processing of deep cavity parts, the brush is not used to remove the chip accumulation in time.
 
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