Process Introduction
Electrical discharge machining (EDM), also known as electrical discharge machining ( EDM ) or electrical erosion machining, is one of the most popular four processing methods, and milling, turning and grinding.
Different from the principle of metal cutting, EDM is a process method for machining through the electrical corrosion effect of pulse discharge between tool electrode and workpiece electrode. Since sparks can be seen in the discharge process, it is called EDM.
According to the different EDM processes, EDM can be further divided into WEDM, EDM piercing forming, EDM grinding and boring, EDM synchronous conjugate rotary machining, EDM high-speed keyhole machining, EDM surface strengthening and wording.
At present, EDM technology has been widely used to process various high melting point, high strength and high toughness materials, such as quenching steel, stainless steel, die steel, cemented carbide, etc., as well as parts with complex surface and special requirements for processing molds.
Classification of EDM
EDM is the most widely used machining method in the electrical processing industry, accounting for about 90 % of the industry. According to the relative movement of tool electrode and workpiece, it can be roughly divided into six categories : EDM molding, wire cutting, EDM grinding, EDM synchronous conjugate rotary machining, EDM high-speed hole machining, EDM surface strengthening and engraving. Wire cutting accounted for60 % of EDM, EDM forming accounted for30 %. With the vigorous development of electrical processing technology, wire cutting has become a symbol of advanced technology.
The basic principle of EDM
The tool electrode is connected to one pole of the pulse power supply, and the workpiece electrode is connected to the other pole. Both poles are immersed in liquid medium ( kerosene or mineral oil or deionized water ) with certain insulation. The tool electrode is controlled by an automatic feed regulator to ensure that a small discharge gap ( 0 ) is maintained between the tool and the workpiece during normal machining. 01 – 0. 05 mm ). When the pulse voltage is added between the two poles, the liquid medium at the nearest point between the poles under the conditions at that time is penetrated to form a discharge channel. Due to the small cross-sectional area of the channel and the short discharge time, the energy is highly concentrated ( 10 ~ 107 W / mm ). The instantaneous high temperature generated in the discharge area is enough to make the material melt or even evaporate, resulting in a small pit. After the first pulse discharge, after a short interval of time, the second pulse is most recently clicked through the discharge between the other poles. The tool electrode continuously feeds into the workpiece, and its shape eventually replicates on the workpiece to form the required machined surface. At the same time, a small part of the total energy is also released to the tool electrode, resulting in tool loss.
Advantages of EDM
- Electrical discharge machining does not use mechanical energy, not rely on cutting force to remove metal, but directly use electrical energy and thermal energy to remove metal. Compared with mechanical cutting, EDM has the following characteristics :
- It is suitable for the processing of materials that are difficult to be processed by traditional mechanical processing methods, showing the characteristics of “ soft and rigid ”. Since the material removal is achieved by the thermal erosion of discharge, the machinability of the material mainly depends on the thermal properties of the material, such as melting point, specific heat capacity and thermal conductivity ( thermal conductivity ), which are almost independent of the mechanical properties such as hardness and toughness. Tool electrode materials need not be harder than the workpiece, so electrode manufacturing is relatively easy.
- Parts with special and complex shapes can be machined. Because there is no relative cutting motion between electrode and workpiece and there is no cutting force during machining, it is suitable for low stiffness workpiece and fine machining. Due to the short pulse discharge time, the heat affected range of the material processing surface is small, so it is suitable for the processing of thermal sensitive materials. In addition, since the shape of the tool electrode can be simply copied to the workpiece, it is particularly suitable for the machining of thin wall, low rigidity, elasticity, micro and complex shape surfaces, such as the machining of complex cavity molds.
- Process automation can be realized. The electrical parameters in the machining process are easier to realize digital control, adaptive control and intelligent control than mechanical quantities, which can facilitate the rough, semi-finishing and finishing processes and simplify the process. After the processing parameters are set, there is no need for manual interference in the processing.
- The structural design can be improved and the process-ability of the structure can be improved. After EDM, the mosaic and welding structure can be changed into the whole structure, which not only greatly improves the reliability of the workpiece, but also greatly reduces the volume and quality of the workpiece, and can shorten the die processing cycle.
- The process route of the parts can be changed. Since EDM is not affected by material hardness, it can be processed after quenching, which can avoid heat treatment deformation during quenching. If in die casting or forging die manufacturing, the die can be quenched to a hardness greater than 56 HRC.
Limitations of EDM
EDM has its unique advantages, but at the same time EDM also has some limitations, specific performance in the following aspects :
It is mainly used for the processing of metal materials. Can not process plastics, ceramics and other insulation non-conductive materials. However, recent studies have shown that semiconductors and polycrystalline diamonds can also be processed under certain conditions.
- Low processing efficiency. Generally, the processing speed per unit processing current is not more than 20 mm3 / ( A · min ). Compared with machining, EDM material removal rate is relatively low. Therefore, machining cutting is often used to remove most of the surplus, and then EDM is carried out. In addition, there is a prominent contradiction between processing speed and surface quality, that is, the processing speed is very low in finish machining, and rough machining is often limited by surface quality.
- The machining accuracy is limited. Electrode loss exists in EDM. Since EDM relies on electricity and heat to remove metal, the electrode will also suffer loss, and the electrode loss is mostly concentrated in the sharp angle or bottom surface, which affects the forming accuracy. Although the recent machine tool products can reduce the relative loss ratio of electrodes to less than 1 % in rough machining and to 0.1 % or even smaller in finish machining, the problem of low electrode loss in finish machining still needs further study.
- There are metamorphic layers or even microcracks on the machined surface. Due to the instantaneous high heat generated on the machined surface during EDM, thermal stress deformation will occur, resulting in a metamorphic layer on the machined surface.
- Limitation of minimum angular radius. In general, the minimum angular radius obtained by EDM is slightly larger than that obtained by machining the discharge gap, which is generally 0.02 – 0.03 mm. If the electrode is lossy or the flat head is used for machining, the angular radius will also increase, and it is impossible to achieve a true full right angle.
- Limitation of external processing conditions. The discharge part must be in the working fluid during EDM, otherwise it will cause abnormal discharge, which will bring trouble to observe the machining state and affect the size of the workpiece.
- Processing technology problems. Electrical discharge machining ( EDM ) is a highly technical work. Mastering the quality is the key to the success of machining, especially the equipment with low degree of automation. The selection of process methods, the selection of electrical standards, the clamping and positioning of electrodes, the monitoring of machining state and the determination of machining allowance are closely related to the technical level of operators. Therefore, the accumulation of experience in EDM is crucial.
