History of thermal power spark machining

History of EDM

generation and processing principle of EDM

EDM is a new processing technology, which has been studied and tried in China since the 1950s. After continuous development, it has been widely used and has become an effective method for processing various molds and parts

generation of EDM

EDM is a method of machining materials by using the phenomenon of electric corrosion generated by pulse discharge at both poles

as early as the 19th century, it was found that when the contact of electrical appliances was opened and closed, it was thought that the discharge would ablate the contact part and cause damage to the contact surface. The phenomenon of electrode ablation caused by this discharge is called electric corrosion. At first, electric corrosion was considered harmful. In order to reduce and avoid this harmful electric corrosion, people have been studying the causes and prevention methods of electric non-staple food. When people have mastered its rules, they will create conditions to turn harm into benefit and use electric corrosion in production

the results show that when the discharge occurs at the two poles, a large amount of heat is generated instantaneously in the discharge channel, which is enough to locally melt or vaporize the surface of the electrode material, and under certain conditions, the melted or vaporized part can be thrown away from the electrode surface to form discharge corrosion pits

in the early 1940s, people further realized that when repetitive pulse discharge was carried out in liquid medium, the size of conductive materials could be machined. Therefore, the "EDM method" was created

EDM is a new process and technology completely different from mechanical machining. Machining is through the relative movement of machine tool parts, using a tool harder than the workpiece material to cut off the redundant part of the workpiece to get the finished parts. However, with the development of industrial production and the progress of science and technology, new materials with high melting point, high hardness, high strength, high brittleness, high viscosity, high toughness, high purity and other properties continue to appear. There are more and more workpieces with various complex structures and special process requirements, which are still machined, sometimes difficult or impossible to process. Therefore, in addition to further developing and improving machining methods, people also strive to find new machining methods. EDM can meet the needs of production development and show many excellent properties in application, so it has been rapidly developed and increasingly widely used

1.1.2. The physical essence and working principle of EDM

WEDM is based on the cutting and processing of the electric corrosion of materials during the small gap pulse discharge in the liquid medium. It is a very complex transient microphysical process, which can be roughly divided into four stages: dielectric breakdown and channel formation, energy conversion and transmission, ejection and deionization of the electric corrosion products. The original stage of pulse discharge

is the dielectric breakdown between electrodes. The dielectric breakdown process is very rapid, generally 0.1-ills. Once the dielectric is broken down, a discharge channel is formed, and the gap current rises rapidly with the printing speed on the surface of the label or film, and the current density can be as high as 101_ 108A/cm2。 After dielectric breakdown, the pulse power supply instantly summarizes the energy released by the material smelting and rolling process through the discharge channel, and converts the electric energy into heat energy, kinetic energy, magnetic energy, light energy, sound energy and electromagnetic radiation energy. Most of them are converted into heat energy, causing partial melting and gasification of the discharge points at the two poles, and gasification and thermal decomposition of the medium in the channel. Some heat is consumed in the process of transmission and radiation. The device and test of a sensor and power supply are the most important sub pulse discharge to produce a discharge mark. In high-speed wire EDM, because the electrode wire moves rapidly relative to the workpiece, the discharge channel also moves accordingly, so the discharge mark of a single pulse is oval. The material can be thrown out in liquid phase, gas phase and solid phase. The ejection of materials is the result of the combined action of thermal explosion force, electrodynamic force and hydrodynamic force. After a pulse discharge, there should also be an interval to deionize the gap medium, that is, the charged particles in the channel compound into neutral particles, and restore the insulation strength of the liquid medium until the next pulse impact through discharge

WEDM is also called wire electrode EDM (WEDM), which is a kind of EDM with wire electrode as a tool. Its characteristic is that the electrode wire moves in a unidirectional low-speed (for WEDM-LS) or a reciprocating high-speed (for WEDM-HS), and the workpiece moves in any trajectory in the X. Y direction relative to the electrode. The latter can be controlled by profiling, photoelectricity, especially digital. If the electrode wire makes some auxiliary movements, it can also cut complex workpiece with slope

conditions for realizing EDM: [1]

1 A reasonable distance must be maintained between the tool electrode and the workpiece electrode. Within this distance range, it can not only meet the requirements of continuous breakdown of the medium by pulse voltage and spark discharge, but also meet the requirements of medium deionization and discharge of corrosion products after the spark channel is extinguished. If the distance between the two electrodes is too large, the pulse voltage cannot penetrate the medium and generate spark discharge. If the two electrodes are short circuited, there is no pulse energy consumption between the two electrodes, and it is impossible to realize electrochemical corrosion machining

2. The medium must be filled between the two electrodes. During material EDM, the liquid medium (special working fluid or industrial kerosene) is between the two poles; In the process of EDM surface strengthening of materials, there is a gas medium between the two poles

3. The pulse energy density delivered between the two electrodes should be large enough. After the spark channel is formed, the pulse voltage changes little. Therefore, the current density of the channel can characterize the energy density of the channel. Only when the energy density is large enough can the processed material be locally melted or vaporized, so as to form a corrosion mark (PIT) on the surface of the processed material and realize EDM. Therefore, the channel must generally have a1em} current density. The discharge channel must have a large enough peak current before it can be maintained during the pulse. Generally, the peak current of the maintenance channel is not less than 2A

4. The discharge must be a short pulse discharge pulse. The discharge duration is generally ^-s. Due to the short discharge time, it is too late for the heat energy generated during discharge to diffuse inside the processed material, so that the energy effect is limited to a very small range and the cold electrode characteristics of spark discharge are maintained

5. The pulse discharge needs to be repeated many times, and the multiple pulse discharge is dispersed in time and space. Here there are two meanings: first, the two adjacent pulses in time do not form a channel at the same point; Second, if the pulse discharge is concentrated in a certain area within a certain time range