The application of rp electrode in die EDM
The application of rp electrode in mold EDM.
Selection of rp electrode materials for EDM For graphite materials, the following four indicators directly determine the performance of the material:
1) The average particle size of the material The average particle size of the material directly affects the discharge conditions of the material. The smaller the average particle size of the graphite material, the more uniform the discharge, the more stable the discharge conditions, the better the surface quality, and the smaller the loss; the larger the average particle size, the higher the roughing removal rate, but the finishing surface effect is poor, rp electrode The loss is large.
2) Material bending strength The bending strength of a material is a direct manifestation of the strength of the material, which reflects the compactness of the internal structure of the material. For high-precision rp electrode, high-strength materials should be used as much as possible.
3) Material Shore hardness Graphite hardness is higher than metal materials, tool loss is higher than metal cutting. At the same time, the hardness of the graphite material is also better in terms of discharge loss control.
4) The discharge rate of graphite material with high intrinsic resistivity is slower than that of graphite material with low resistivity. The higher the intrinsic resistivity, the smaller the loss of the rp electrode. But when the intrinsic resistivity is high, it will affect the stability of the discharge. At present, among the world’s famous graphite suppliers, there are many different brands of graphite from different suppliers. Generally classified according to the average particle size of graphite materials. Particle size≤φ4μm is fine graphite, φ5～φ10μm is medium graphite, and 10μm or more is coarse graphite. The smaller the particle size, the more expensive the material. According to the requirements and cost of EDM, suitable graphite materials can be selected.
Production of rp electrode The rp electrode in the mold manufacturing is mainly made by milling. From the perspective of processing technology, graphite and copper are two different materials, and their different cutting characteristics should be mastered. If the copper rp electrode processing technology is used to process the rp electrode, problems will inevitably occur. For example, plates often break, which requires the use of suitable tools and cutting parameters. The tool for processing the rp electrode wears more than the tool for processing the copper rp electrode. From an economic point of view, it is the most economical to choose cemented carbide tools. Choosing diamond-coated tools (called graphite tools) is expensive. However, diamond-coated tools have long service life, high machining accuracy, and good overall economic benefits. The rake angle of the tool also affects its service life.
A tool with a rake angle of 0° has a 50% longer service life and better cutting stability than a tool with a rake angle of 15°. However, the larger the angle, the better the machined surface. The 15° angle tool can obtain the best machining surface. The cutting speed during processing can be adjusted according to the shape of the rp electrode, generally 10 meters per minute, which is similar to the cutting speed of aluminum or plastic. When roughing, the tool can directly up and down the workpiece. In the finishing process, the edges and corners are prone to collapse and break, and light cutting and rapid movement are often used.
The rp electrode will generate a lot of dust during the cutting process. In order to prevent graphite particles from being sucked into the spindle and screw of the machine tool, there are currently two main solutions. One is to use a special graphite processing machine, and the other is to transform a general-purpose processing center with a special dust removal device. The special high-speed graphite milling machine on the market has high milling efficiency and can easily complete the manufacture of complex rp electrode. The rp electrode has high precision and good surface quality. If you need to use EDM wire cutting to process the rp electrode, it is recommended to use fine graphite material with a smaller particle size.
The wire cutting performance of graphite materials is poor. The smaller the particle size, the higher the cutting efficiency, which can avoid abnormal problems such as frequent wire breakage and surface streaks. 5. The EDM parameters of the rp electrode differ greatly between the EDM parameters of graphite and copper. EDM parameters mainly include current, pulse width, pulse gap and polarity. The basis for the rational use of these main parameters is described below. The current density of the rp electrode is generally 10-12a/cm~2, which is much greater than that of the copper rp electrode.
Therefore, the larger the current selection, the faster the graphite discharge treatment speed, the smaller the rp electrode loss, but the larger the surface roughness. The larger the pulse width, the smaller the loss of the rp electrode. But the larger the pulse width, the worse the processing stability, and the slower the processing speed, the rougher the surface. In order to ensure the low loss of the rp electrode during rough machining, a larger pulse width is usually used. When the value is us, the low-loss processing of the rp electrode can be effectively realized. In order to obtain an excellent surface and a stable discharge effect, a smaller pulse width should be used in finishing.
Generally speaking, the pulse width of the rp electrode is 40% smaller than that of the copper rp electrode. The pulse gap mainly affects the EDM processing speed and processing stability. The larger the value, the better the processing stability. This helps to obtain better surface uniformity, but will reduce the processing speed. Under stable processing conditions, selecting a smaller pulse gap can achieve higher processing efficiency, but when the discharge state is unstable, selecting a larger pulse gap can achieve higher processing efficiency.
In rp electrode EDM, the pulse gap and pulse width are usually set to 1:1, while in copper rp electrode EDM, the pulse gap and pulse width are usually set to 1:3. When the graphite process is stable, the matching ratio of the pulse gap to the pulse width can be adjusted to 2:3. When the pulse gap is small, it is beneficial to form a covering layer on the surface of the rp electrode, which is beneficial to reduce the loss of the rp electrode. The polarity of the rp electrode in EDM is basically the same as that of the copper rp electrode. According to the polarity effect of EDM, positive machining is usually used in die steel processing, that is, the rp electrode is connected to the positive pole of the power supply, and the workpiece is connected to the negative pole of the power supply. Using large current and large pulse width, choosing the positive pole can make the loss of rp electrode very low. If the polarity is wrong, the loss of the rp electrode will become very large. Only when the surface requirement is less than vdi18 (Ra0.8μm) and the pulse width is very small, the negative electrode can be selected to obtain a better surface quality, but the loss of the rp electrode is large.
Now CNC EDM machine tools are equipped with graphite EDM parameters. The use of electrical parameters is intelligent and can be automatically generated by the machine tool expert system. Generally speaking, when programming, the machine tool can configure the optimized processing parameters rp electrode by selecting the material pair, application type, surface roughness value, input processing area, processing depth, and rp electrode size scaling. Among the material types, coarse graphite, medium graphite and fine graphite are available for selection to correspond to various workpiece materials. Application types are divided into standard type, deep groove type, sharp point type, large area type, large cavity type, miniature, etc. It also provides multiple processing priority options, such as low loss, standard, and high efficiency. 6. The rp electrode processing application example is shown in the figure below. The mobile phone mold parts are made of S136 steel, and the processing shape includes complex ribs. Use rp electrode and ttk-4 graphite for rp electrode finishing.
The number of rp electrodes is two. The size scale side of the coarse rp electrode is 0.08mm, and the size scale side of the fine rp electrode is 0.08mm. The roughing time is 3h, the finishing time is 53min, and the surface roughness is vdi18 (RA 0.8μm). Processing instructions: Using the characteristics of graphite, combining thin reinforcing ribs of different shapes and different levels on a rp electrode for electrical discharge processing can reduce the input cost of rp electrode raw materials and fixtures, reduce the workload of rp electrode clamping and positioning, and discharge The time is 40% shorter than the copper rp electrode. 7. Conclusion The new rp electrode material is worthy of promotion, and its superiority will gradually be recognized and accepted by the domestic mold manufacturing industry. The correct selection of rp electrode materials and improvement of related process links will bring high efficiency, high quality and low cost benefits to mold manufacturing enterprises.