EDM is widely used in manufacture of precision cavity molds. So-called "mirror electric discharge machining" generally refers to an electric discharge machining in which surface roughness value of machined surface is Ra < 0.2 um and surface of machined surface has a specular reflection effect. Mirror EDM can replace manual polishing process for some precision machining and improve quality of parts. It has practical significance.

All of electrical discharge machining can achieve a mirror effect, which is directly related to carbon black layer generated during electrical discharge machining. If processed part can easily form a uniform carbon black layer, it means that mirror effect is easy to process.
Simple shapes are easier to obtain mirror effect than EDMs with complex shapes. Shape that is most easily obtained with a mirror effect is a circle. Corners of complex shapes and mirror surface at curved surface are slightly inferior.
Bottom surface is easier to get a good mirror effect than side. The larger depth of cavity, the harder it is to obtain mirror finish, especially on the side.
The larger processing area, the harder it is to obtain a good mirror effect.
Opening portion and hollow portion of cavity have a poor mirror effect.

Some EDMs of die steel can easily achieve mirror effect, and some mold steels can not achieve mirror effect anyway. At the same time, the hardness of mold steel is higher, the effect of EDM mirror is better. Please refer to table below for various materials and mirror finishes.

 

Some requirements for mirror EDM

Electrode material used for mirror EDM is generally red copper or Luoyang copper, and electrode loss of Luoyang copper is smaller. Selected copper material must have uniform texture and less impurities. Poor copper material is prone to abnormalities such as large electrode loss and surface wrinkling in mirror processing. This should be highly valued. Although copper alloy electrode can achieve extremely low electrode loss, mirror effect is poor.
Electrodes used for mirror EDM must be finished and polished without knife marks and defects. Otherwise, these defects will be copied to machined surface of workpiece, but surface of electrode is not required to meet mirror requirements.
Generally, size of mirror finished electrode is 0.2 to 0.05 mm on one side, and polished electrode is generally 0.1 mm on one side. When processing area is small, it can be smaller. When precision of profiling is high, it can be smaller. It can be larger when mixing powder processing. If electrode size scaling is too small, processing speed is greatly reduced.
Precision mirror EDM requires use of multiple electrodes. This requires that consistency of multiple electrodes is good, manufacturing precision is high, repeated clamping of replacement electrodes and positioning accuracy are high. It is generally satisfied by advanced processes such as high-speed milling of electrodes, positioning methods using reference spheres, and repeated positioning using a fast-clamp positioning system.

Process of EDM is a process from roughing to finishing. Firstly, rough machining electrodes are used to ensure a certain processing quality (size, shape, positional accuracy, surface roughness), a large amount of metal is used to etch a large amount of metal to shorten processing time and improve processing efficiency; Electrode is then reworked and finished with a smaller discharge energy. Whether it is roughing or finishing, multiple discharge conditions are used, current is also from large to small, bottom is polished by deep feed, and side is smoothed by translation.
For each discharge condition, machining allowance is reserved for subsequent conditions. Reasonable machining allowance is key to ensure processing quality and efficiency. A large machining allowance guarantees surface quality but reduces machining speed. A small machining allowance can achieve high efficiency, but finished mirror will have many pinholes or unevenness.
The most ideal processing condition is that after first condition is processed, subsequent processing is only to repair surface irregularity formed by first processing condition, without destroying new material, that is treating material balance of each condition as zero. However, in the case of mirror EDM, constraints caused by discharge condition are ever-changing, and it is necessary to leave a little more machining allowance for safety.

Mirror EDM is processed by negative polarity. Selection of some non-main electrical parameters is also very important. There are some differences with choice of conventional processing. For example, discharge time should be longer, lift height should be shorter, and lift speed should not be too fast. Purpose of this setting is to maintain a stable small energy erosion process, because in mirror processing, it will not produce a lot of electrolytic products, excessive knife lifting action of machine will interfere with continuous and stable discharge.
Electro-erosion ability of mirror-finished electrical gauge is very weak, and processing takes a long time. Since dimensional change during mirror processing is very small, it only plays role of light repair. In fact, as long as required surface roughness is processed, machining can be finished. Therefore, timing processing function of numerical control EDM machine can be used, and processing time is determined according to experience.
Processing method of working fluid during mirror processing is also very important. Generally, oil immersion processing can be performed, so that working fluid in processing part is in a slight circulation state. Never use strong flushing, which can interfere with normal formation of microdischarge process.

 

According to processing experience, as long as processing is properly processed, there is generally no carbon deposition in mirror processing. Therefore, machine should be stopped as little as possible during processing, especially powder layer left on workpiece surface.

Mixed powder EDM is a process to improve surface roughness of EDM. So-called EDM process refers to addition of micro-powder, such as silicon powder, aluminum powder, chromium powder and related additives in the working fluid to obtain a mirror effect on machined surface.
Using powder mixing technology, it is possible to obtain faster processing speeds (reduced finishing time by 20%-30%) and better surface roughness under same electrical parameters than without powder mixing technology. For a large area of EDM, there is a significant improvement.
Mixed powder EDM requires selection of suitable powder additives and mixed powder working fluid circulation devices for concentration management of powder additives, use of diffusion devices to eliminate concentration errors.