EDM refers to the method of processing the workpiece through the electric erosion effect of the pulse discharge between the tool electrode and the workpiece electrode in a certain medium. EDM is a method of processing using electricity and heat energy that was studied in the 1940s and gradually applied to production. Today we will learn about the principle of EDM.
The basic principle of EDM: The principle of EDM is based on the electrocorrosion phenomenon during the pulsed spark discharge between the tool and the workpiece (positive and negative electrodes) to remove excess metal, so as to achieve the size, shape and surface of the workpiece. Quality scheduled processing requirements.
The workpiece and tool electrodes are respectively connected to two electrodes with different polarities of the pulse power supply. Tool electrodes are usually made of electro-corrosion-resistant materials with good conductivity, high melting point and easy processing, such as copper, graphite, copper-tungsten alloy and molybdenum. During the machining process, the tool electrode also has loss, but it is less than the erosion amount of the workpiece metal, and even close to no loss.
As the discharge medium, the working fluid also plays the role of cooling and chip removal during the machining process. Commonly used working fluids are media with low viscosity, high flash point and stable performance, such as kerosene, deionized water and emulsion.
When the pulse voltage is applied between the two electrodes, when a proper gap is maintained between the workpiece and the electrodes, the working fluid medium between the workpiece and the tool electrodes will be broken down to form a discharge channel.
Instantaneous high temperature is generated in the discharge channel, which melts or even vaporizes the material on the surface of the workpiece. At the same time, it also vaporizes the working fluid medium, thermally expands rapidly in the discharge gap and explodes, and a small part of the material on the surface of the workpiece is eroded and thrown out, forming tiny Electric pits.
After the pulse discharge ends, after a period of time, the working fluid is restored to insulation. The pulse voltage is repeatedly applied to the workpiece and the tool electrode, and the above process is repeated continuously, and the workpiece material is gradually etched away. The servo system constantly adjusts the relative position of the tool electrode and the workpiece, and automatically feeds to ensure the normal progress of the pulse discharge until the required parts are processed.
1. EDM
The tool electrode is usually a copper or graphite shaped electrode, which can be in any shape that can be manufactured, and the processed shape is the corresponding cavity.
2. Wire EDM
WEDM is divided into slow wire cutting and fast wire cutting. Generally, wire electrodes with a diameter of 0.1~0.3mm are used to process through ruled surface parts, which can be punch parts or die holes.
What is changed during EDM is not only the surface of the workpiece, but also its subsurface. The surface structure of the processed workpiece is divided into three layers (Figure 1-3). The impact layer on the surface of EDM is formed by the impact of the thrown molten metal and a small amount of electrode particles. This layer is easily removed.
The next layer is the hard layer (oxide layer). EDM substantially changes the metallurgical structure and properties of the hard layer. Under the action of the medium oil, the molten metal is cooled rapidly, and the molten metal that has not been thrown out solidifies in the cavity to form a hard layer. This hard and brittle oxide layer develops microscopic cracks. If this layer is too thick, or cannot be thinned or removed by polishing, the piece may fail prematurely under some conditions of use.
The last layer is the heated or annealed layer. It just heats up, it doesn't melt. The thickness of the hard layer and the heated layer is determined by the heat dissipation capability of the workpiece material and the processing energy. In any case, the changed metal layer will affect the original properties of the workpiece surface. The automatic finishing circuit on the CNC EDM machine can effectively reduce the formation of the hard layer, but it still cannot eliminate the annealed layer.
Compared with traditional processing methods, EDM has many advantages, such as it can process any conductive material, including those metal materials with higher hardness that cannot be processed by traditional processes.
Using EDM, you can reach depths that are impossible to achieve with cutting tools, which is an ideal processing method for demanding deep processing.
EDM does not apply additional mechanical force to the workpiece during processing, which ensures the mechanical properties of the workpiece. In addition, the surface finish after EDM is usually better than that of traditional processes.
However, compared with traditional machining techniques, EDM is slower and consumes a lot of power, which increases manufacturing costs.
