What Is Electrical Discharge Machining

Electrical Discharge Machining

（Belong to mechanical engineering）

Electric discharge machining refers to a method of machining a workpiece in a certain medium through the electric erosion effect of the pulse discharge between the tool electrode and the workpiece electrode. EDM is a method of processing using electricity and heat that began to be studied in the 1940s and gradually applied to production.

Origin time:1940s
Advantages:Can process any conductive material, etc.
Application:EDM, etc.
Subject:Mechanical engineering

Brief Introduction Of EDM

The electric corrosion phenomenon in EDM was discovered as early as the end of the 19th century. For example, the electric spark generated when plugs and switches are opened and closed will damage the contact surface. In the early 20th century, when Lazarinco of the Soviet Union was studying the phenomenon and causes of spark discharge corrosion damage to switch contacts, he found that the instantaneous high temperature of electric sparks would melt and vaporize local metals and be eroded away, thus pioneering and inventing Electric discharge machining method, and in 1943, the world's first practical electric discharge machining device was developed by using the principle of electric erosion, which really applied the phenomenon of electric erosion to actual production and processing. China began to study EDM equipment in the early 1950s and developed the first imitated wire-cut EDM machine in the early 1960s.

EDM is a new process that uses electrical energy and heat to process, commonly known as electrical discharge machining. The difference between EDM and general cutting is that the tool and the workpiece are not in contact during EDM. Instead, the pulse discharge between the tool and the workpiece is continuously generated, and the local and instantaneous high temperature generated during the discharge is used to gradually reduce the metal material. Eclipse down. Because there are visible sparks during the discharge process, it is called EDM.

Working principle of EDM

When performing EDM, the tool electrode and the workpiece are respectively connected to the two poles of the pulse power supply and immersed in the working fluid, or the working fluid is charged into the discharge gap. The tool electrode is controlled 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 will break down the working fluid and generate spark discharge.

A large amount of heat energy is instantaneously concentrated in the tiny discharge channel, the temperature can be as high as 10,000 degrees Celsius, and the pressure also changes sharply, so that the small amount of metal material on the working surface at this point is immediately melted, vaporized, and exploded into the working fluid In the process, it quickly condenses to form solid metal particles, which are taken away by the working fluid. At this time, a tiny pit mark is left on the surface of the workpiece, the discharge is temporarily suspended, and the working fluid between the two electrodes restores the insulation state.

Immediately afterwards, the next pulse voltage breaks down at another point relatively close to the two electrodes, generating spark discharge, and repeating the above process. In this way, although the amount of metal eroded by each pulse discharge is very small, because there are thousands of pulse discharges per second, more metal can be eroded, which has a certain productivity.　Under the condition of maintaining a constant discharge gap between the tool electrode and the workpiece, while eroding the workpiece metal, the tool electrode is continuously fed to the workpiece, and finally a 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 complex profiles can be processed.

Tool electrodes are commonly used materials with good conductivity, high melting point and easy processing, such as copper, graphite, copper-tungsten alloy and molybdenum. In the process of processing, the tool electrode is also lost, but it is less than the amount of metal erosion of the workpiece, and even close to no loss.

As the discharge medium, the working fluid also plays a role in cooling and chip removal during processing. Commonly used working fluids are media with low viscosity, high flash point and stable performance, such as kerosene, deionized water and emulsion.

Essentials of EDM

1. A certain discharge gap must always be maintained between the tool electrode and the processed surface of the workpiece, and this gap depends on the processing conditions. If the gap is too large, the voltage between the electrodes cannot break down the inter-electrode medium, so no spark discharge will occur; if the gap is too small, it is easy to form a short-circuit contact, and no spark discharge will also occur. Generally, the discharge gap should be controlled within the range of 1-100μm, which is related to the pulse size of the discharge current.
2. Pulse power must be used. The pulse power supply can make the heat generated by the discharge too late to conduct and diffuse to the rest, and limit each discharge point to a small range, otherwise it will burn the surface like continuous arc discharge and cannot be used for mold electrode processing.
3. Spark discharge must be carried out in an insulating liquid medium. The liquid medium must have a high dielectric strength, which is conducive to the generation of pulsed spark discharge. At the same time, the liquid medium can also suspend and remove the metal chips, carbon black and other electrical corrosion products generated during the EDM process from the discharge gap, and has a better cooling effect on the electrode and the surface of the workpiece. Kerosene is usually used as the discharge medium.
4. The power density of the discharge point is sufficiently high. Only in this way, the heat generated during discharge is enough to melt or vaporize the metal on the surface of the workpiece electrode instantaneously.

Features Of EDM

1. Any conductive material can be processed. The material removal in EDM is realized by the electric heating effect during discharge, and the machinability of the material mainly depends on the electrical conductivity and thermal characteristics of the material.
2. Suitable for processing low-rigidity workpieces and micro processing. Because the shape of the tool electrode can be copied to the workpiece, it is particularly suitable for processing workpieces with complex surfaces.
3. The surface of EDM is composed of numerous small pits and hard flanges. Its hardness is higher than that of machined surface, and it is beneficial to protect lubricating oil. Under the same surface roughness, its surface lubricity and wear resistance are also better than mechanical The machined surface is good, especially suitable for mold manufacturing.
4. However, the speed of EDM is generally slow.

EDM Applications

(1) EDM perforation

Perforation processing is the most widely used type of EDM. It is often used to process shaped holes (round holes, square holes, polygonal holes, special-shaped holes), curved holes, 4qL, micro holes, etc., such as cold punching dies, drawing dies, extrusion Various holes and small holes on die, nozzle, spinneret.

The dimensional accuracy of the perforation is mainly ensured by the size of the tool electrode and the gap of the spark discharge. The cross-sectional profile size of the electrode should be evenly smaller than the size of the hole to be processed by a processing gap, and its dimensional accuracy should be one level higher than that of the workpiece. At IT7 level, the surface roughness value is smaller than that of the workpiece, and the straightness, flatness and parallelism are not more than 0.01mm in the length of 100 mm.

(2) EDM cavity machining

EDM cavity processing includes forging die, die casting die, extrusion die, bakelite die, plastic die, etc. Cavity machining is more difficult, mainly because it is not through-hole machining, the amount of metal erosion is large, the working fluid circulation and the removal of electro-erosion products are poor, and the tool electrode can not be compensated by the feed after the tool electrode is worn. The cavity is complex and the electrode loss is uneven, which greatly affects the machining accuracy. Therefore, the cavity processing productivity is low and the quality is difficult to guarantee. In order to improve the machining accuracy of the cavity, in the electrode, pure copper and graphite with high corrosion resistance should be used as the electrode. In addition, some small plastic molds are frosted using EDM.