Exploration on improving the efficiency of EDM process

EDM is an important process method of mold manufacturing, especially in plastic mold manufacturing. The cavity of plastic mold, many groove corners of sliding block, insert, inclined pin and other parts need EDM. Because EDM is a process method with slow processing speed, EDM needs to spend more man hours in mold manufacturing. How to improve the machining speed of EDM machine tools on the premise of ensuring the machining quality through effective and feasible process measures, and how to shorten the mold manufacturing cycle and reduce the mold manufacturing cost by relying on high-efficiency EDM production has always been the concern of mold enterprises.

Starting from practical machining experience, the following summarizes three aspects directly related to EDM efficiency: EDM process, adjustment of EDM machine tool and EDM operation. According to these three aspects, the process measures to improve the efficiency of EDM are discussed in detail, hoping to help some enterprises solve practical problems.

I Key points related to EDM process

1. Most of the material is removed by machining

Before machining the EDM parts, the mechanical machining method should be used for rough machining, and only the parts that are difficult or unable to be finely milled by the tool should be left to EDM. In this way, the amount of materials in EDM can be greatly reduced and the efficiency of EDM can be greatly improved.

In order to avoid the trouble of work, some technicians directly carry out EDM without mechanical rough machining, resulting in the extremely low efficiency of EDM, which requires a correct understanding of the machining speed of EDM. EDM relies on micro thermal melting process to remove materials a little bit, which is much slower than mechanical cutting. For large cavities, even if mechanical rough machining is carried out, too much allowance for EDM will reduce the machining efficiency.

2. Determine the processing technology according to the processing conditions

EDM includes single electrode direct forming process, multi electrode replacement forming process and so on. When selecting the process, not only the machining speed, but also the requirements of machining accuracy and surface roughness should be considered in detail. The single electrode direct forming process uses only one electrode to process the required cavity. This process method is used to process the cavity with simple shape and low precision requirements. It does not need repeated clamping operation, and can improve the efficiency of EDM. For the occasions with high processing requirements, the process method of multi electrode processing is usually adopted. Firstly, a large number of materials are etched with rough machining electrodes, and then finish machining is carried out with finish machining electrodes. In finish machining, multiple electrodes can also be considered to compensate for the loss of electrodes.

If only one electrode is used for processing in the processing occasions with high precision requirements, due to insufficient understanding of the process, the original intention is to improve the processing efficiency by reducing the number of electrodes, but in the actual processing process, the processing efficiency is extremely low because large electrical parameters cannot be selected for rough processing; In another case, when the processing requirements are not high, using multiple electrodes for processing will inevitably reduce the processing efficiency. > Cutting edge CNC technology wechat deserves your attention.

3. Reasonable selection of electrode materials

The selection of electrode materials is directly related to the effect of discharge. To a large extent, whether the selection of materials is appropriate determines the final situation of discharge speed, machining accuracy and surface roughness. Electrode materials should be selected according to the actual needs of different types of mold processing. Red copper electrode and graphite electrode are usually used in EDM. Many die and mould enterprises seldom consider when selecting electrode materials. All large and small electrodes are used to selecting the same electrode material, which will affect the efficiency of EDM.

Red copper is easy to obtain a stable processing state, can obtain a cavity with clear contour, and has high surface finish. Generally, the processing conditions of low loss are adopted. Because the average current of low loss processing is small and is not suitable for high current processing, its productivity is not high; Graphite electrode can realize low loss and high-speed rough machining under the condition of large machining erosion, but the discharge stability is poor in finish machining, which is easy to transition to arc discharge, so it can only be processed under the machining conditions with large loss.

Most enterprises use red copper as electrode material. According to the processing experience, the processing speed of using graphite electrode to process the large cavity of forging die can save about 1 / 3 of the time compared with using red copper electrode; Some enterprises have recognized the advantages of graphite, so they are used to selecting graphite electrode, which leads to the requirement that the surface roughness of the mold is lower than Ra0 3um processing occasions are always difficult to stably meet the processing requirements, and the repeated repair processing greatly reduces the processing efficiency. If the red copper electrode is selected, the problem will be solved. It can be seen that when selecting electrode materials, reasonable selection should be made according to the processing requirements.

4. Determine the appropriate electrode scaling amount

The selection of electrode scaling should consider many factors. There are two ways of EDM: translational machining and non translational machining. CNC EDM machine tools can generally adopt translational machining, while traditional EDM machine tools cannot carry out translational machining if they are not equipped with translational head. The selection of electrode scaling is different between the two processing methods.

When translational machining is not used, the machining size mainly depends on the control of finish machining. When determining the scaling amount of finish machining electrode, the electrical parameter conditions to be selected to achieve the predetermined surface roughness shall be considered first, and the size of spark gap under this condition shall be determined, and then the scaling amount of electrode shall be determined, generally 0.02 ~ 0.06mm on one side; When determining the scaling amount of rough machining electrode, consider the machining speed and reserve appropriate allowance for finish machining as the standard, and generally take 0.15 ~ 0.25mm on one side.

When using electrode translation machining, the dimensional accuracy of machining depends on the control of discharge gap, electrode scaling and momentum. Since the translation amount is controllable, the translation amount can be adjusted according to the size of the discharge gap, the processing size can be easily controlled, and the electrode scaling amount can be relatively large, especially for finishing, and can be flexibly selected according to some specific conditions. Generally, translational processing is not used in rough machining, and the scaling amount of electrode is 0.15 ~ 0.3mm on one side. The finishing adopts multi-stage processing conditions, and the translational method is used to improve the chip removal condition, so as to achieve stable processing and obtain more uniform surface roughness on the side and bottom. The scaling amount of the electrode is generally 0.05 ~ 0.15 mm.

In die EDM, the machining efficiency is usually low because the electrode scaling is too small to select large discharge conditions. If the machining is allowed, the scaling amount of rough machining electrode is 0.3mm on one side, and the selected electrical parameter is more than 2 ~ 3 times faster than that of 0.15mm on one side. During translational machining, the precision machining electrode can appropriately increase the electrode scaling according to the machining situation, which is conducive to improve the machining speed. It must be noted that after increasing the electrode scaling amount, the energy of discharge parameters must be increased correspondingly, and the electrode scaling amount cannot be increased blindly, otherwise the machining efficiency will be reduced. For example, in rough machining, the electrode scaling is taken as 0.8mm on one side, while in actual machining, although larger machining conditions are selected, the actual discharge gap may be only about 0.3mm on one side, which will leave more material allowance for subsequent machining and will reduce the machining efficiency. If the NC EDM machine tool does not use the translation machining function according to the scaling method of the electrode of the traditional machine tool, it will reduce the machining efficiency, especially in the finishing machining. For example, if the electrode scaling value is 0.03mm on one side, only a small discharge parameter can be selected for machining. If there is a little more material allowance in rough machining, it may even cause immobility of machining; If the electrode scaling amount is increased to 0.08mm on one side, the discharge parameters of machining can be increased, the material allowance can be removed quickly, and the side can be polished gradually by using the translation function. This is especially true for machining electrodes that are round and square. Properly increasing the electrode scaling amount in ordinary machining will not have much impact on the shape, so it should be treated specially in precision machining.

5. Consider “area effect” in electrode design

“Area effect” is the effect of the change of machining speed and other process indexes caused by the change of machining area in EDM.

When the area of EDM is small to a critical area, the machining speed will be significantly reduced. Because of the small processing area and the excessive concentration of pulse discharge per unit area, the electro-corrosion products in the discharge gap are not discharged smoothly. At the same time, the phenomenon of gas excluding liquid will occur, resulting in the discharge processing being carried out in the gas medium, which greatly reduces the processing speed. Therefore, when designing the electrode, it should be considered not to split the electrode with too small processing area and use more integral electrodes. However, it should also be noted that when the area of the integral electrode is relatively large, the processing depth is deep and chip removal is difficult, it should be considered to divide the integral electrode into several electrodes for sub processing. The electrode can be divided into main cavity electrode and sub cavity electrode according to the geometry of the cavity. Otherwise, unstable discharge will occur in machining, resulting in poor machining efficiency and difficult to ensure accuracy.

2、 Key points of adjustment of EDM machine tool

1. Regulation of electrical parameters

The selection of electrical parameters directly affects the process indexes of processing. The ultimate purpose of electrical parameter adjustment is to achieve the predetermined machining size, surface roughness requirements and high machining efficiency. When adjusting electrical parameters, we should basically consider: the number of electrodes, electrode loss, machining surface roughness requirements, electrode scaling, machining area, machining depth and other factors.

At present, the intelligent performance of NC EDM machine tool has been greatly improved. The machine tool stores a large number of complete sets of electrical parameters for the combined processing of various materials. The electrical parameters can be automatically selected and configured only by inputting the process conditions according to the programming requirements in the programming process. During machining, the machine tool relies on Intelligent Control Technology (such as “fuzzy control” technology), monitors and judges the state of EDM gap by computer, automatically adjusts electrical parameters, maintains stable EDM and achieves high machining efficiency. The intelligent electrical parameters of advanced EDM machine tools can generally meet the machining requirements, and greatly reduce the skill requirements of machine tools for machine operators. The traditional EDM machine tool requires the operator to have rich working experience and be able to flexibly configure the electrical parameters according to the machining requirements.

The intelligent control technology of machine tool is not omnipotent, or the role of manual adjustment of electrical parameters can not be ignored. Especially in some special machining occasions such as deep hole machining, large taper machining and large area machining, it is necessary to manually adjust the electrical parameters. When adjusting electrical parameters, priority should be given to adjusting parameters other than the main specifications of electrical parameters, such as tool lifting height, discharge time, tool lifting speed, etc; Secondly, the pulse gap, pulse width and processing current can be adjusted in order. Negative polarity processing can be used for special material processing (the electrode is negative). On the premise of stable machining state, reducing the action and amplitude of tool lifting, reducing the pulse gap and increasing the machining current are conducive to improve the machining efficiency. However, in the case of unstable machining, we must keep lifting the knife frequently and select a large pulse gap appropriately, otherwise it will reduce the machining efficiency and even cause arc discharge, so that the machining process can not be carried out normally. According to the processing experience, properly and conservatively adjusting the electrical parameters can maintain the normal processing and obtain high processing efficiency. > Cutting edge CNC technology wechat deserves your attention.

Pulse width has a great influence on machining speed, but some technicians have misunderstandings in understanding the influence of pulse width on machining speed. Some believe that increasing the pulse width can improve the processing speed, while others believe that reducing the pulse width can improve the processing speed. In theory: when the pulse peak current is constant, the pulse width increases and the processing speed increases. When the pulse width increases to a certain value, the processing speed is the highest. Then continue to increase the pulse width, and the processing speed decreases. But in actual production, we must have a quantitative understanding of pulse width. According to a large number of machining examples, it must be pointed out here that the pulse width corresponding to the highest machining speed is often very small, so the electrode loss is large, which should not be used in many cases. However, in the actual machining, the electric standard selected by the machine tool generally takes into account the reduction of electrode loss. Then, in the low loss machining standard, if you increase the pulse width, the machining speed will inevitably decrease. If you reduce the pulse width, the machining speed will be improved to a certain extent.

2. Control of processing allowance

NC EDM is processed with multiple condition segments, and there must be a certain machining allowance between the condition segments. For example, the depth required for processing is 5mm, and the scaling amount of electrode is 0.15mm on one side. If the electric gauge used is divided into three sections from large to small, set the feed depth and translation radius according to the discharge gap of each electric gauge: the feed depth of the first gauge is 4.85 mm, the translation radius is 0.02 mm, the feed depth of the second gauge is 4.92 mm, the translation radius is 0.13 mm, the feed depth of the third gauge is 4.97 mm, and the translation radius is 0.17 mm. The control of machining allowance between condition segments has a great relationship with machining efficiency. Properly reducing the machining allowance can improve the efficiency of EDM, especially in large-area finishing workshops. If the feed depth of the first gauge in the above example is changed to 4.9mm, the translation radius is 0.1mm, the feed depth of the second gauge is 4.95mm, the translation radius is 0.15mm, and the third gauge remains unchanged, the machining efficiency can be improved to a certain extent.

The machining allowance given in the automatic programming of NC EDM machine tool is on the premise of ensuring the surface roughness, which is relatively conservative. In order to further improve the machining efficiency, the machining allowance of the automatic programming program can be manually modified according to the machining requirements. However, it must be noted that to reduce the processing allowance, it must be ensured that the next processing gauge can polish the previous processing gauge. In addition, reducing the processing allowance is of little significance to improve the processing efficiency of small area.

3. Selection of translational machining

Translation machining is an important process method of NC EDM. Different CNC EDM machine tools have different translational machining methods. Translational machining should be applied flexibly and reasonably according to the machine tools used. There are two translational machining methods of a NC EDM machine tool: free translational and servo translational. Free translation refers to that when the spindle is servo machined, the other two axes expand according to a certain track at the same time until the machining reaches the specified depth. Servo translation refers to the expanding movement of the other two axes according to a certain track after the spindle is processed to a specified depth. Free translation is generally used for shallow machining. During machining, translation while beating can improve chip removal performance, improve machining speed and reduce carbon deposition; However, for deep occasions, it will reduce the processing speed and increase the corner loss of the electrode. Servo translation is generally used in processing occasions with deep processing depth. The bottom surface is processed first and then the side surface is repaired. When the depth is shallow, the processing effect is not as good as automatic translation. It is also commonly used in processing grooves and rings on the side wall of the cavity, as well as other occasions of two axis translation, etc.

4. Timing processing

NC EDM machine tools generally have the function of timing machining, which can be used to control the machining time of the last few electrical parameters of electrode finishing with large area. The electric erosion ability of electric gauge in finish machining is very weak. Due to the influence of machining chips and other factors in the gap, it takes a long processing time, and even the processing is not completed for a period of time. As the size change of the last few conditions has been very small, in fact, we can finish the machining as long as we process to the required surface roughness. We can adopt the timed machining method according to experience, which can greatly improve the finishing efficiency.

5. Method of changing vertical servo machining

At present, most controlled EDM machine tools can realize horizontal machining and multi axis linkage machining. However, these functions have not been well applied in mold enterprises. If we can give full play to the function of the machine tool, the machining efficiency of the machine tool can be improved. For example, there are relatively thin and deep glue positions around the molding inserts of the injection mold. If these parts are processed by z-axis servo, the discharge instability will occur in the processing process and the processing speed will be slow due to the small partial discharge area and large processing depth. By improving the process method and using the transverse servo function of NC EDM machine tool, the machining speed can be several times higher than that of z-axis servo machining. The phenomenon of unstable discharge occurs in die “corner clearing processing” due to the small area of the processing part. The method of X, y and Z axis linkage, i.e. oblique processing, can stabilize the discharge processing and improve the processing efficiency.

3、 Key points of EDM operation

1. Improve the accuracy of repeated positioning

Die EDM often needs to use many electrodes for machining, which needs repeated positioning. In actual machining, repeated positioning accuracy is also an important factor affecting the efficiency of EDM, which mainly occurs in finishing machining. For example, if the first electrode is not aligned with the depth, resulting in the shallow depth of the first processing, the amount of materials to be processed will be too much if the processing depth of the second processing is to be guaranteed, and large electrical standards cannot be selected in finishing, which will inevitably lead to low processing efficiency. According to the processing experience, if the large-area processing is close to 750px2, it will take more than 1 hour to process the depth of 0.02mm under the finish machining standard; In addition, after the first electrode is processed, the processing position of the second electrode is quite different from that of the first electrode. One side cannot be repaired, and the processing volume of the other side increases, which will also reduce the processing efficiency.

The positioning method of contact perception between electrode and workpiece is widely used in mold enterprises. This positioning method is relatively simple and familiar to operators. However, the error of this positioning method is large, and it is difficult to ensure stable positioning accuracy. Abnormal processing problems caused by positioning out of tolerance occur from time to time, which reduces the processing efficiency. In order to improve the accuracy of repeated positioning, indirect positioning method can be used. If the reference ball is used for indirect positioning, this positioning method can eliminate the error in the maximum range because it adopts point contact. At present, many mold enterprises do not know the method of positioning with reference ball. Using the reference ball for positioning is not only easy to ensure the accuracy, but also easy to realize the centering in the case of complex workpiece shape; The centering process of the electrode to the reference ball avoids the movement of the electrode with a large stroke and saves the operation time; For the processing of multiple workpieces, there is no need to use the electrode for multiple centring, as long as the reference ball is centring once, which saves a lot of operation time, and the advantage is particularly obvious in multi electrode processing. Another best way to improve the accuracy of repeated positioning is to adopt the positioning system of rapid clamping, such as 3R and EROWA. The electrodes and workpieces can be changed quickly, and the ideal positioning effect can be achieved, which greatly improves the processing efficiency.

2. Method and size of flushing

In order to control the pressure and discharge the chips in the EDM process in a stable way, so as to improve the efficiency of metal punching and discharge the chips in time. It is generally necessary to flush or pump oil. Properly increasing the oil flushing pressure will increase the processing speed, but after the oil flushing pressure exceeds a certain value, it will continue to increase and the processing speed will decrease slightly.

4、 Others

1. Machining performance of machine tool

A good discharge machine tool is the prerequisite for realizing high-efficiency discharge machining. Pulse power supply plays an extremely important role in improving the machining speed. The performance of pulse power supply determines the micro process of EDM and whether the machining can be completed stably and efficiently under the condition of small discharge gap in finish machining. The adaptive adjustment of adaptive power supply in machining plays an important role in achieving high machining efficiency. The operation time of the machine tool and intelligent system is shortened. The full-automatic machine tool equipped with ATC device can operate continuously and automatically without manual intervention, which greatly improves the processing efficiency. The tool lifting technology of the machine tool also has a certain impact on the machining speed. For example, the high acceleration feed machining of the spindle plays an obvious role in improving the efficiency of machining deep, narrow and small seams.

2. Influence of working fluid

The processing speed is also different with different working fluid. At present, synthetic EDM working fluid is used in EDM. Different EDM working fluids have different characteristics, so they should be selected comprehensively. Generally, the mirror processing time is relatively long. The processing efficiency can be greatly improved by using powder mixing working fluid. It is to add a certain proportion of conductive powder (such as silicon powder, chromium powder, magnesium powder, etc.) to the working fluid. > Cutting edge CNC technology wechat deserves your attention.

3. Influence of workpiece material

Under the same processing conditions, different workpiece materials are selected, and the processing speed is also different. This mainly depends on the physical properties of the workpiece material (melting point, boiling point, thermal conductivity, specific heat capacity, melting heat, gasification heat, etc.). For example, the efficiency of EDM machining aluminum is very high, but the efficiency of machining cemented carbide is quite low.

5、 Concluding remarks

In practical machining, there are many factors affecting the efficiency of EDM. According to the specific requirements of the machining object, we should comprehensively consider and grasp all links of the process, so as to make the EDM machine tool achieve high machining efficiency.