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Precision molds how to mirror the EDM processing
Release date: 2017-09-19    Source: Qingdao Root Industrial and Trading Co., Ltd.    Browse: 480

EDM is widely used in precision cavity mold manufacturing. The so-called "mirror EDM" generally refers to the processing of surface roughness value of Ra <0.2 um, the processing surface with a mirror reflective effect of EDM processing. Mirror EDM for some precision machining can replace the manual polishing process to improve the quality of parts, with practical significance.


Not all machining types can be mirrored


All of the discharge machining can achieve the mirror effect, and there is a direct relationship with the carbon black layer produced in the discharge process. If the processing site can easily form a uniform carbon black layer, which means that easy to produce a mirror effect.


•Simple shape is easier to obtain than the complex shape of the EDM mirror. The shape of the mirror is most easy to obtain a circular shape. Complex shape of the corner, the arc surface effect is slightly worse.

•The bottom surface is easier to obtain a good mirror effect than the side. The greater the depth of the cavity, the more difficult to get the mirror processing effect, especially the side.

•The larger the processing area, the harder it is to achieve a good mirror effect.

•The open position, the cavity of the hollow part of the mirror effect is poor.

• Some of the mold steel EDM can easily achieve the mirror effect, and some mold steel in any case can not reach the mirror effect. At the same time, the hardness of the mold steel is higher, the effect of EDM mirror better. Please refer to the following table for various materials and mirror processing performance.


Item

Material

Mirror performance

A

SKD61

STAVAX (S136)

PD555

NAK80

718H

Very Good

Very Good

Very Good

Very Good

Very Good

B

SKS3

SKH9

HPM38

S55C

H13

XW10

Good

Good

Good

Good

Not Good Enough

C

SKD11

NAK55

HPM1

DH2F

Not Good

Not Good

Not Good

Not Good

D

Bs

Al

Hard alloy

Pear skin

Pear skin

Pear skin


Some requirements for electroless EDM electrodes


Electrode materials for mirror EDM are generally copper or copper, copper copper electrode wear less. Selection of copper must be uniform texture, with less impurities, poor copper in the mirror processing prone to electrode wear and tear, the surface wrinkles and other abnormalities, this should attach great importance. Copper copper alloy electrode can achieve very low electrode wear, but the mirror effect is poor.


The electrodes used for mirror EDM must be finely polished and can not be marked and defected. Otherwise, these defects will be copied to the machined surface of the workpiece, but the surface of the electrode is not required to meet the mirror requirements.


The general size of the mirror processing electrode size to take a single side 0.2 ~ 0.05 mm, finishing electrode generally take a single 0.1mm, in the processing area is relatively small when the smaller, high precision when the contours of the required small, mixed powder processing appropriate Take some bigger. If the electrode size is too small, the processing speed is greatly reduced.


Precise mirror EDM use multiple electrodes. This requires a greater degree of consistency of the electrode, the manufacturing accuracy is high, replace the electrode re-clamping, positioning accuracy is high. Generally use high-speed milling manufacturing electrodes, the use of reference ball measurement positioning method, the use of rapid clamping positioning system for repeated positioning and other advanced technology to meet.



Control the processing allowance of the mirror EDM


The process of EDM is a process from rough to finishing. (Size accuracy, shape accuracy, position accuracy, surface roughness), the use of large discharge energy to remove large amounts of metal, to shorten the processing time, improve the processing efficiency; and then use the rough processing electrode, to ensure a certain processing quality conditions (size accuracy, shape accuracy, Replace the finishing electrode, use a smaller discharge energy to complete the finishing. Whether it is roughing or finishing, will use multiple discharge conditions, the current is from large to small, through the depth of the bottom of the meridian, through the smooth side of the grinding.



Each discharge condition shall be reserved for the following conditions. Reasonable processing allowance is the key to ensure the quality of processing and processing efficiency. A larger machining allowance ensures surface quality, but reduces processing speed. The smaller machining allowance achieves high efficiency, but the final machined mirror will have a lot of pinholes or unevenness.



The most ideal processing condition is the first condition after processing, the subsequent processing is only the first processing conditions formed by the surface roughness, and not to destroy the new material, that is, the conditions of each material margin According to zero treatment. However, when the mirror EDM processing, taking into account the discharge conditions are subject to changes in the ever-changing, in order to safety or to stay a little more processing margin.


Large area processing using powder technology to achieve the effect of mirror processing


Mixing EDM is a process to improve the surface roughness of EDM. The so-called mixed-powder EDM is the addition of fine powder, such as silica fume, aluminum powder, chrome powder and related additives, to the working fluid. Processing the surface to obtain a mirror effect.



The use of blending processing technology, in the same electrical parameters under the conditions, than the use of powder processing technology to obtain faster processing speed (to reduce the finishing time of 20% -30%) and better surface roughness. For a larger area of EDM, it has a significant improvement.


Mixed powder EDM requires the selection of appropriate powder additives and mixed powder working fluid circulation unit, the concentration of powder additives management, the use of diffusion devices to eliminate the concentration of the error.

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