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Industry News

How to do a good job in high precision discharge machining

Author: ComeFrom: Date:2016-07-12 11:05:06 Hits:1441
     In recent years, the development of mold industry, the precision of the mold is also getting higher and higher. High added value such as micro mould connectors, IC plastic mould, LED mould, has become very popular on the market a variety of molds. Although the technology of high speed milling for mould micro machining is in progress, the fine parts of high value added mould parts are processed by EDM.
    The core of precision EDM is mainly reflected in the size precision, profiling precision, surface quality requirements.
     With the processing size should be strictly required up to ±2-3 μm;
     The profiling precision angular clear bottom corner, the value of R should be as small as possible (generally require a R value of less than 0.02mm);
     The depth error of the plane degree from the edge part to the central part is within 2 μm;
     Surface quality requirements of the surface roughness uniformity, Ra should be less than 0.4 μm (VDI12), or the requirements of mirror surface processing, the microscopic examination of the surface without abnormal;
     The edge integrity of the discharge inlet and the opening angle is not larger than that of the 0.015mm.
     Many factors affect the precision of EDM machining. In the mechanism of electrical discharge machining (EDM), the discharge process of discharge gap, inevitable existence of electrode loss, finishing in the poor removal effect is directly affects the problem of precision electrical discharge machining accuracy control.
In practical machining, the machining accuracy is affected by multiple links. Only grasp the essence of processing technology, in the processing of each link to be strictly controlled, and continuously accumulated processing experience, to ensure a high success rate.
1.The premise of precision EDM
     The use of machine tools for processing, processing accuracy can not go beyond the accuracy of machine tools. For the electrical discharge machining of high value-added mold parts, the general discharge machine is difficult to complete, the choice of high precision EDM machine tools is the premise.
       Precision EDM machine tools in the mechanical properties of the structure, spindle and the various geometric accuracy of the working table should meet the requirements of high precision. High precision grating ruler full closed loop control method, can get a high automatic positioning accuracy. Machine performance of pulse power supply decided to discharge machining performance, size precision, contour clear requires discharge gap is very small, in the small clearance processing conditions can realize stable machining and automatic monitoring the state of each discharge pulse and ensure that bad pulse does not have the destructive effect.
       High speed lift knife technology can make the processing of waste debris and tar effectively discharged, to obtain a stable discharge gap.
       High performance servo control system can make the machining depth error control to achieve high precision machining.
        The size of the compensated surface profile in the process of machining. Such as corner translation, translation is a very important feature of spherical.
        In order to use the waveform with the slope control to achieve low electrode loss, at the same time, it is required to be able to produce high quality surface.
        The high degree of automation and intelligence of this kind of high precision electric spark machining tools are prominent, which greatly reduce the dependence on artificial.
2. Precision electrical discharge machining process
     Multi electrode processing technology for precision electric spark machining. According to the different characteristics of the discharge gap in the rough, semi finish and finish machining, the rough, semi finish and finish machining of a type of cavity with the corresponding size scaling is used.
      First, a large amount of metal is removed by rough machining, and then the semi finishing electrode is processed to finish the transition process of rough machining. The final finishing process should be replaced with multiple electrodes to compensate for the loss of the electrode. From the general precision machining electrode size scale 0.03 - unilateral 0.1mm/, processing the multi segment processing conditions by shaking method of side and bottom surfaces of the piecewise Xiuguang.
       Because wave momentum is small, still have very high accuracy of profiling and joggling the discharge is more stable, get a side surface and a bottom surface more uniform surface roughness and is widely used. In precision machining, the need for multiple electrodes, as long as the repeated positioning accuracy, can achieve satisfactory results.
3. Method for precision machining and positioning accuracy
     Precision EDM machining to ensure the high precision of repeated positioning, do not reduce the processing efficiency, the use of fast fixture positioning system, such as EROWA fast clamping fixture. The use of this type of fixture, as long as in front of the supporting of precision finishes the debugging, in processing highest repeat positioning accuracy up to + 2 μm, electrode and the workpiece can rapid reloading, before processing using a reference ball tool, using CNC EDM machine tool automatic function, spindle chuck and workpiece zero Center positioning.
      Machining of the workpiece to be taken to measure the size and then continue to process is very convenient. The method can ensure high precision of repeated positioning by using the method of reference, and the contact area between the reference ball and the workpiece is a little at the time, and the positioning accuracy can be avoided due to the presence of fine debris.

      Precise positioning of the electrode and the workpiece must have a clear positioning reference, the electrode to take the center of the reference point, so that the size of the error distribution of the electrode size. According to the situation, the workpiece can be based on one side or the reference center, and the datum must be smooth and level, especially when the depth is located.

4. Requirements for precision machining of electrodes
     In the precision machining of the electrode, it is necessary to use precision manufacturing methods to ensure the quality of the electrode. Electrode manufacturing should meet the requirements of high accuracy, general precision electrode manufactured by high-speed machining center to complete. After the completion of the processing should be reached as far as possible without artificial processing requirements of the electrode, need to be perfected polishing. This is to avoid electrode process affects the accuracy of electrode, but also of electrode manufacturing process level proposed high demand.
       Electrode material is a key issue in precision machining. Copper in electrical machining field is the most used electrode material, can meet the processing requirements of high precision, but in micro machining, in order to reduce the electrode loss, reduce number of electrodes, improve the quality of machined surface, to achieve high and stable machining, should consider to choose more expensive copper tungsten alloy electrode material.
Electrode used for electric spark processing to deal with the size and shape of the detailed projection measurement, comprehensive testing before they are used to process.
        In the actual production and processing, due to the problem caused by the failure of the electrode is a lot of scrap. Therefore, it is very important to do a good job in the manufacture of electrode in precision machining.
5. Control of electric gauge in precision machining
       Precision machining of the use of high-grade machine tool electric control method is generally intelligent control. In the process of machining, the state of the gap of EDM is monitored and judged, and the machining efficiency can reach the highest processing condition in the range of stable arc.
Man can make a little change to the stop time and the condition of the knife, but try not to modify the main parameters of the electric pulse (such as current, pulse width, etc.), so as not to affect the machining conditions of the machining conditions of the spark gap.
      In the machine tool automatic selection of processing conditions on the required input conditions should be accurate input, such as processing area, processing shape, electrode taper, electrode size scaling, shaking, processing surface roughness value, etc.. Machine tool selection of processing conditions generally meet the processing requirements, the operation is simple, to avoid the human intervention in the processing process. With proper control of machining experience, the machine tool can be easily controlled.
6. Liquid treatment methods in precision machining
       Electrical discharge machining is carried out in the working fluid medium. The insulation performance of the working liquid is played in the process of pulse discharge, and the action of the electrode and the workpiece is accelerated to cool off, so that the electric corrosion product is suspended and discharged from the discharge gap.
      In EDM often use slag punching, drainage, can make the stable process, improve the efficiency of. But the uneven flow field, resulting in uneven processing debris accumulation, causing concentrated discharge and the two discharge, the workpiece flatness, roughness effect is very large, resulting in uneven discharge gap phenomenon. And the strong erosion can cause the abnormal loss of the edge of the electrode. So precision machining and precision machining is usually used without red liquid, liquid immersion processing approach, rely on lift the knife in the action to the chip.
       Like spindle with a very high acceleration feed processing, resulting in the suction effect, so that the existence of the electrode and the workpiece between the processing chips, tar and waste gas can be effectively discharged.
         The quality of working fluid is also very important in precision machining, there are many types of EDM special working fluid, the main point is the viscosity should be low, precision machining performance is better.
7.Processing of precision machining
       In precision machining, due to the existence of the error of each processing link, so before dying in general should be repeated measurement, processing to ensure the final dimensional accuracy. The size of the beginning of processing as much as possible to get the deviation, the size of the processing is better.
        After the measurement of the processing size, the size difference is modified, which is referred to as the cutting tool. Repair knife processing is in the process of cleaning the surface after cleaning without the condition of tar, so the discharge clearance is smaller than before, can be more accurate to complete the subtle processing. Fast loading and clamping workpiece repositioning system plays a strong supporting role in the processing of the repair tool. Machining accuracy can be further improved by means of the tool repair.
8. The requirements of the processing technology of precision machining
        High value added mold processing requirements of the entire process of standardization, with a mature technology level. All parts of the processing link should have a clear process and requirements of the process. Each process in strict accordance with the technical methods, requirements for processing, in order to ensure the success rate of high precision parts manufacturing. Particularly in processing of precision parts taboo welding repair, a processing to link any appear quality problems will lead to scrap the whole parts of. EDM is often the whole process of the post process, and strictly control the quality of processing is particularly important.
Some specifications for precision edm:
        Design department split electrode should meet the requirements of precision machining, should pay special attention to the thin, subtle parts of the processing.
For the processing of the content should be related to the production of processing maps, the use of accurate positioning, accurate processing of data.
        Because many parts processing parts, not easy to grasp the shape of the processing before processing, should provide a detailed basis for the 3D.
         For the processing of parts should be treated earlier, pre process force demagnetization, processing and so on the past.
         The positioning surface of workpiece with grinding machine grinding and chamfering should be treated.

         The electrodes shall be provided with a reference angle of the direction of discrimination, and the reference angle shall be corresponding to the reference angle of the workpiece.
          Electrodes should be marked on the number of electrodes, coarse fine labeling, processing map should be given the details of the processing, such as the number of electrodes, the size of the electrode size, processing tolerances, surface roughness requirements.
         The processing process specification, clear processing can make the staff follow the prescribed order to reduce the error rate, and improve the processing quality.
9. The requirements of precision machining for machining environment and the use of tools
         Precision machining of the processing environment is also required, high precision machining should be set up a special finishing room, so as to prevent the operation of other machine tools in the work of vibration, dust and affect the processing accuracy.
        Processing room should be the installation of air-conditioning, room temperature control at about 20 DEG C plus or minus 2 DEG C, thin small parts size will affect the temperature measurement error caused by, electric spark machine tool only under the condition of constant temperature can play a predetermined processing performance.
      The tools used in precision machining should be with high precision, such as clamp, sinusoidal disk, punch forming grinder, gauge, dial gauge, measurement instrument etc.. With these tools to complete the auxiliary operation, the accuracy of the tool also directly affects the machining accuracy.
10. Requirements of precision machining for technical personnel
        The final realization of the precision machining is the operation of the processing technical personnel, to control the completion of the. Processing technical personnel's work attitude, processing technology level of the ability to complete the task of precision machining is very important.
         Precision machining requires the operator to work seriously, responsible for the work attitude, the entire process is very skilled, each step of the operation can be carefully controlled, there is a wealth of work experience. In the precision machining, the machining size is small, the machining allowance is little, the main principle of processing is to guarantee the processing precision.
          Precision machining is also a pressure of work, human operation in a little careless will result in the occurrence of abnormal processing, so the enterprise to create a relaxed and harmonious working environment is also very important. Technical personnel is the precision machining task of the realization, in the processing plays a decisive role.
      Concluding remarks
        High value-added mold EDM process in every aspect of a processing are and machining accuracy has a close relationship, grasp the good processing in every detail, according to a set of precision machining of professional craft processing, in order to achieve the high precision requirement of the final.