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How to perform mirror EDM?

Not all machining types can be mirrored EDM

All of EDM can achieve the mirror effect, which is directly related to the carbon black layer produced in EDM. If the processed part can easily form a uniform carbon black layer, it means that it is easy to process the mirror effect.

  • 1) Simple shapes are easier to obtain mirrors than complex shapes by EDM. The easiest shape to obtain a mirror effect is a circle. The mirror effect is slightly worse at the corners of complex shapes.
  • 2) The bottom surface is easier to obtain a good mirror effect than the side surface; the deeper the cavity, the more difficult it is to obtain the mirror surface machining effect, especially the side surface.
  • 3) The larger the machining area, the more difficult it is to obtain a good mirror effect.
  • 4) The mirror effect of the opening part and the hollow part of the cavity is not good.

Not all mold steel can be mirrored EDM

The EDM of some mold steels can easily achieve the mirror effect, while some mold steels cannot achieve the mirror effect anyway. At the same time, the hardness of mold steel is higher, and the effect of EDM mirror is better. Please refer to the table below for various materials and mirror machining properties.

  • category
  • Material name
  • Mirror performance
  • A
  • S136
  • SKD61
  • PD555
  • NAK80
  • 718H
  • very good
  • very good
  • very good 
  • very good
  • very good
  • B
  • SKS3
  • SKH9
  • HPM38
  • S55C
  • H13
  • XW10
  • it is good 
  • it is good
  • it is good
  • it is good
  • it is good
  • Slightly worse
  • C
  • SKD11
  • NAK55
  • HPM1
  • DH2F
  • difference
  • difference
  • difference
  • difference
  • D
  • Bs
  • Al
  • Super hard alloy
  • Pear skin noodles
  • Pear skin noodles
  • Pear skin noodles

Some requirements for electrodes in mirror EDM

The electrode material used for mirror EDM is generally copper or chromium copper, and the electrode loss of chromium copper is smaller. The selected copper materials must have uniform texture and low impurities. Poor copper materials are prone to abnormalities such as large electrode loss and surface wrinkles during mirror machining. This should be paid attention to. Although copper-tungsten alloy electrodes can achieve extremely low electrode loss, the mirror effect is not as good as copper.

Electrodes used for mirror EDM must be refined and polished, and there must be no knife marks and defects, otherwise these defects will be copied to the processed surface of the workpiece, but the surface of the electrode is not required to meet the mirror requirements.

Generally, the size of the mirror machining electrode is 0.20.05 mm on one side, and the finishing electrode is generally 0.1mm on one side. It can be smaller when the machining area is relatively small, and it can be smaller when the profiling accuracy is high. It is appropriate for powder mixing Take the bigger one. If the electrode size scaling is too small, the machining speed will be greatly reduced.

Precision mirror EDM uses multiple electrodes. This requires good consistency of multiple electrodes, high manufacturing accuracy, repeated clamping and positioning accuracy of replacement electrodes. Generally, high-speed milling is used to manufacture electrodes, the positioning method using the reference ball measurement, and the 3R quick clamping positioning system for repeated positioning are used to meet the requirements.

Control the machining allowance of mirror EDM

The process of EDM is a process from rough machining to finishing. First, use the rough machining electrode, and under the condition of ensuring certain machining quality (dimensional accuracy, shape accuracy, position accuracy, surface roughness), a large amount of metal is eroded by larger discharge energy to shorten the machining time and improve the machining efficiency; Then change the finishing electrode and use a smaller discharge energy to complete the finishing. Regardless of whether it is roughing or finishing, multiple discharge conditions are used, the current is also from large to small, the bottom is smoothed by deep feed, and the side is smoothed by translation.

For each discharge condition, a machining allowance must be reserved for the following conditions. Reasonable machining allowance is the key to ensuring machining quality and machining efficiency. A larger machining allowance can ensure the surface quality, but will reduce the machining speed. Small machining allowance can achieve high efficiency, but the final processed mirror will have many pinholes or unevenness.

The most ideal machining condition is that after the first condition is processed, the subsequent machining is only to smooth the surface unevenness formed by the first machining condition, without destroying the new material, that is, the material allowance for each condition Treat as zero. However, in the case of mirror EDM, considering the ever-changing constraints on the discharge conditions, it is necessary to leave a little more machining allowance for safety.

Reasonable use of electric discharge parameters and machining control of mirror machining

Mirror EDM adopts negative polarity machining. The selection of some non-main electrical parameters is also very important. There are also some differences with the selection of conventional machining. For example, the discharge time should be set longer, the tool lift height should be shorter, and the tool lift speed should not be too fast. , The purpose of this setting is to maintain a stable low-energy electro-erosion process, because in mirror machining, it will not produce a lot of electro-erosion products itself, and the excessive knife lifting action will interfere with the continuous and stable discharge of the discharge.

The electro-erosion capability of the mirror machining electrical standard is very weak, and the machining process takes a long time. Since the size change during mirror machining is already very small, it only plays a role of polishing. In fact, the machining can be finished as long as the required surface roughness is processed. Therefore, the timing machining function of the CNC EDM machine can be used to determine based on experience How long to process.

The machining method of the working fluid during mirror machining is also very important. Generally, oil immersion machining is sufficient, so that the working fluid of the machining part is in a slight circulation state. Never use strong flushing fluid. Strong scouring will interfere with the normal micro-discharge process. form.

According to the machining experience, as long as the machining steps are handled properly, carbon deposits are generally not generated in mirror machining, so the machine should be stopped as little as possible during the machining, especially not to clean the ink layer left on the surface of the workpiece.

Large area machining uses mixed powder technology to achieve mirror machining effect

Powder-mixed EDM is a process method to improve the surface roughness of EDM. The so-called powder-mixed EDM refers to the addition of micro-powders, such as silicon powder, aluminum powder, chromium powder and related additives, to the working fluid, in order to make The machined surface obtains a mirror effect.

Using mixed powder machining technology, under the same electrical parameters, it can obtain faster machining speed (reducing the finishing time by 20%-30%) and a more uniform surface effect than without using powder mixing machining technology. For larger area EDM, it has a significant improvement effect.

Powder mixing EDM requires the selection of suitable powder additives and powder mixing working fluid circulation devices, the concentration management of powder additives, and the use of diffusion devices to eliminate concentration errors.

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