Electrical discharge machining is a special processing method for etching conductive materials by electro-erosion caused by pulse discharge between two electrodes immersed in working fluid.
Also known as electrical discharge machining or electrolytic etching, referred to as EDM.
In 1943, the Soviet scholar Lazarenko discovered and invented EDM, and then developed rapidly with the improvement of pulse power and control systems.
The pulse power supply originally used was a simple resistor-capacitor loop.
Working principle
When performing EDM, the tool electrode and the workpiece are respectively connected to the two poles of the pulse power source, and immersed in the working fluid, or the working fluid is charged into the discharge gap.
Control the tool electrode to feed the workpiece through the gap automatic control system.
When the gap between the two electrodes reaches a certain distance, the pulse voltage applied on the two electrodes breaks down the working fluid to generate a spark discharge.
A large amount of thermal energy is instantaneously concentrated in the fine passage of the discharge, the temperature can be as high as 10,000 degrees Celsius or more, and the pressure also changes drastically.
Therefore, a small amount of metal material on the working surface is melted, vaporized, and splashed into the working fluid explosively, and rapidly condenses to form solid metal particles, which are carried away by the working fluid.
At this time, a slight pit mark is left on the surface of the workpiece, the discharge is short, and the working fluid between the two electrodes is restored to the insulation state.
Immediately thereafter, the next pulse voltage is broken down at another point where the two electrodes are relatively close, generating a spark discharge, and the above process is repeated.
Thus, although the amount of metal etched by each pulse discharge is extremely small, since there are tens of thousands of pulse discharges per second, more metal can be etched, which has a certain productivity.
Under the condition of maintaining a constant discharge gap between the tool electrode and the workpiece, the tool electrode is continuously fed to the workpiece while the workpiece metal is being etched, and finally the shape corresponding to the shape of the tool electrode is processed.
Therefore, as long as the shape of the tool electrode and the relative movement between the tool electrode and the workpiece are changed, various complicated profiles can be processed.
Tool electrodes are commonly used to resist electro-erosion materials such as copper, graphite, copper-tungsten alloys and molybdenum, which have good electrical conductivity, high melting point and easy processing.
During the processing, the tool electrode also has losses, but less than the amount of metal removed from the workpiece, even close to no loss.
The working fluid acts as a discharge medium and also functions as cooling and chip evacuation during the processing.
Commonly used working fluids are mediums with low viscosity, high flash point and stable performance, such as kerosene, deionized water and emulsion.
Development
The 1950s
Improved to circuits such as resistor-inductor-capacitor.
At the same time, a so-called long pulse power source such as a pulse generator is also used to improve the etching efficiency and reduce the relative loss of the tool electrode.
Subsequently, high-frequency pulse power sources such as high-power tubes and thyristors appeared, which increased productivity under the same surface roughness conditions.
The 1960’s
Transistors and thyristor pulsed power supplies have emerged, increasing energy efficiency and reducing tool electrode losses, and expanding the adjustable range of rough finishing.
The 1970’s
High-low voltage composite pulse, multi-loop pulse, equal-amplitude pulse and adjustable waveform pulse appear.
New progress has been made in machining surface roughness, machining accuracy and reducing tool electrode losses.
In terms of the control system, the discharge gap is simply maintained from the beginning, and the advance and retreat of the control tool electrode is gradually developed to use a microcomputer to timely control various factors such as electrical parameters and non-electrical parameters.
Main features of EDM
① It can process materials and complex shape workpieces that are difficult to cut by ordinary cutting methods;
② There is no cutting force during processing;
③ Does not produce defects such as burrs and knife marks;
④ Tool electrode material does not have to be harder than the workpiece material;
⑤ Directly use electric energy processing to facilitate automation;
⑥ After the processing, the surface has a metamorphic layer, which needs to be further removed in some applications;
⑦ The treatment of smoke pollution caused by the purification and processing of working fluid is troublesome.
Main application of EDM
① Processing molds and parts with complex shapes of holes and cavities;
② Processing a variety of hard and brittle materials such as hard alloys and hardened steel;
③ Processing deep pores, shaped holes, deep grooves, slits and cut sheets;
④ Processing tools and measuring tools such as various forming tools, templates and thread ring gauges.
EDM processing characteristics
- EDM speed and surface quality
Molds in the EDM machining generally use coarse, medium and fine grade processing.
Roughing is achieved with high power and low loss.
The middle and finishing electrodes have relatively large losses.
However, in general, the middle and finishing allowances are small, so the electrode loss is also extremely small.
It can be compensated by machining size control or ignored without affecting the accuracy requirements.
- Electric spark carbon residue and slagging
EDM machining can be carried out smoothly under the conditions of carbon residue generation and carbon residue removal.
In practice, carbon slag is often sacrificed at the expense of processing speed, such as high voltage, large pause pulse wave, etc. in middle and finishing.
Another reason for the exclusion of carbon residue is that the shape of the machined surface is complicated and the path of the chip removal is not smooth.
Only actively create conditions for good exclusion, and take some measures to deal with it.
- EDM workpiece and electrode mutual loss
The EDM machine has a long pulse wave discharge time, which is beneficial to reduce electrode loss.
EDM rough machining generally uses long discharge pulse wave and large current discharge, and the processing speed is fast and the electrode loss is small.
In the case of finishing, small current discharge must reduce the discharge pulse time.
This not only increases the electrode loss, but also greatly reduces the processing speed.
EDM is a new process that is completely different from machining.
With the development of industrial production and the advancement of science and technology, new materials with high melting point, high hardness, high strength, high brittleness, high viscosity and high purity have emerged.
More and more workpieces with various complex structures and special process requirements make traditional machining methods unworkable or difficult to machine.
Therefore, in addition to further developing and perfecting the machining method, people are also striving to find new processing methods.
EDM can adapt to the needs of production development and show a lot of excellent performance in the application.
Therefore, it has been rapidly developed and widely used.
EDM can process various materials with high melting point, high hardness, high strength, high purity and high toughness.
The processing mechanism is completely different from the cutting method.
It has the following characteristics:
- The energy density of pulse discharge is high, which is convenient for processing special materials and complex shapes of workpieces that are difficult to machine or cannot be processed by ordinary machining methods.
Not affected by the hardness of the material, not affected by heat treatment conditions.
- The pulse discharge duration is extremely short, the heat conduction diffusion range generated during discharge is small, and the material is affected by heat.
- When machining, the tool electrode is not in contact with the workpiece material, and the macro force between the two is extremely small.
The tool electrode material does not need to be harder than the workpiece material, so the tool electrode is easy to manufacture.
- Can reform the structure of the workpiece, simplify the processing technology, improve the service life of the workpiece, reduce the labor intensity of workers.
