Basic knowledge of thermal power spark

Basic knowledge of electric spark

first, what is electric spark machining

electric spark is a kind of self-excited discharge. Its characteristics are as follows: there is a high voltage between the two electrodes of spark discharge before discharge. When the two electrodes are close, the dielectric between them is broken down, and then spark discharge occurs. With the breakdown process, the resistance between the two electrodes decreases sharply, and the voltage between the two poles also decreases sharply. The spark channel must be extinguished in time after a short period of time (usually s) to maintain the "cold pole" characteristics of spark discharge (that is, the heat energy converted by the channel energy has no time to transmit to the depth of the electrode), so that the channel energy acts on a very small range. The effect of channel energy can make the electrode locally corroded. The method of dimensional machining of materials by using the corrosion phenomenon produced by spark discharge is called EDM. EDM is a kind of spark discharge in liquid medium in the lower voltage range

Second, the characteristics of EDM

EDM is a new process completely different from machining. 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 and high purity continue to appear. There are more and more workpieces with various complex structures and special process requirements, which makes the traditional machining methods unable to process or difficult 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. Therefore, it has been rapidly developed and increasingly widely used

the characteristics of EDM are as follows:

1 The energy density of pulse discharge is high, which is convenient for processing special materials and workpieces with complex shapes that are difficult or impossible to be processed by ordinary machining methods. It is not affected by material hardness and heat treatment

2. The duration of pulse discharge is very short, the range of heat conduction and diffusion generated during discharge is small, and the range of material heating influence is small

3. During machining, the tool electrode is not in contact with the workpiece material, and the macro force between them is very small. The tool electrode material does not need to be harder than the workpiece material, so the tool electrode is easy to manufacture

4. It can reform the workpiece structure, simplify the processing technology, improve the service life of the workpiece and reduce the labor intensity of workers

based on the above characteristics, the main uses of EDM are as follows:

1) manufacturing stamping dies, plastic dies, forging dies and die-casting dies

2) machining small holes, abnormal holes and threaded screw holes on cemented carbide

3) cut the parts on the metal plate

4) process narrow seams

5) grind flat and round surfaces

6) others (such as strengthening metal surface, taking out broken tools, punching on quenched parts, directly machining parts with complex profile, etc.)

III. composition and function of EDM machine tool

it can be seen from the above situation that in order to realize the EDM process, the machine tool must have three elements, namely, pulse power supply, mechanical part and automatic control system, working fluid filtration and circulation system. The functions of these three elements are briefly discussed one by one below

1. The voltage applied by the pulse power supply

to the discharge gap must be pulsed, otherwise, the discharge will become a continuous arc. The so-called pulse power supply is actually an electrical circuit or device, which can send out pulse voltage with sufficient energy

2. The function of the mechanical part and the automatic control system is to maintain an appropriate discharge gap between the tool electrode and the workpiece and adjust it

3. Working fluid purification and circulation system

the function of working fluid is to concentrate energy, strengthen the processing process, and take away the heat and corrosion products generated during discharge. The working fluid system includes the storage, cooling, circulation, regulation and protection of working fluid, filtration and forced circulation system using working fluid

the above three elements are sometimes referred to as the three major parts of EDM machine tools, which form a unified whole of EDM machine tools to meet the requirements of processing technology

IV. conditions for realizing EDM

to realize EDM, the following conditions should be met:

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 dielectric breakdown by pulse voltage and spark discharge, but also meet the requirements of dielectric 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. In EDM dimension machining of materials, liquid medium (special working fluid or industrial kerosene) is used 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, so 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 a/cm2 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. 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. There are two meanings here: first, two adjacent pulses do not form channels at the same point in time; Second, if the pulse discharge is concentrated in one area within a certain time range, the pulse discharge should be transferred to another area within another period of time. Only in this way can carbon deposition be avoided, and then electric arc and local burns be avoided

6. The electric erosion products after pulse discharge can be discharged out of the discharge gap in time, so that the repetitive discharge can be carried out smoothly

in the production practice of EDM, the above process is completed in two ways. On the one hand, spark discharge and electric corrosion process itself have the inherent characteristics of discharging corrosion products; The rest of the discharge products (such as the vaporization of the medium) other than the etch can also promote the above process; On the other hand, some artificial auxiliary process measures must be used, such as circulating filtration of working fluid, flushing and pumping measures used in processing, etc

v. polarity effect

during EDM, the corrosion amount of two electrodes of the same material is different. One electrode has more erosion than the other, which is called polarity effect. If the materials of the two electrodes are different, the polarity effect is more obvious

VI. coverage effect

EDM in oil medium will decompose negative free carbon particles. Under appropriate pulse width and pulse to pulse conditions, carbon particles will be covered on the positive electrode of the discharge, which is called coverage effect. The electrode loss can be reduced by using the coverage effect. Pay attention to the negative polarity processing, which is conducive to the coverage effect

VII. Machining speed

for the EDM machine, the machining speed refers to the volume or weight of the workpiece that is etched in unit time. Generally expressed in volume. If the volume of the workpiece being etched is v within time t, the processing speed VW is: vw=v/t (mm3/min). For wire cutting machines, the processing speed refers to the area where the workpiece is cut in unit time ③ the localization competition of multinational enterprises intensifies. It is expressed in mm2/min. At the specified surface roughness (such as ra=2.5 μ m) , the maximum processing speed when the relative electrode loss (such as 1%) is an important index to measure the process performance of electric machining machine tools. Generally, when the new standard puts forward higher safety requirements for plastic runway materials, the manufacturer gives the maximum processing current and the maximum processing speed that can be achieved under the best processing state. Therefore, in actual processing, due to the ever-changing size and shape of the workpiece to be processed, the processing conditions and chip removal conditions are far from the ideal state, even in rough processing, the processing speed is often much lower than the maximum processing speed index of the machine tool

VIII. Tool electrode loss

in EDM, tool electrode loss directly affects the profiling accuracy, especially for cavity machining, the process index of electrode loss is more important than the machining speed

electrode loss is divided into absolute loss and relative loss

absolute loss is most commonly used in two ways: volume loss ve and length loss veh, which respectively represent the volume and length of tool electrode eroded in unit time. That is,

ve=v/t (mm3/min)

veh=h/t (6. Relationship between gear amplification and range classification mm/min)

relative loss - the percentage of absolute loss of tool electrode and workpiece processing speed. Generally, the relative loss of length is more intuitive and convenient to measure. In cutting, the loss of electrode wire has little effect on the quality of workpiece, so it is generally not discussed. However, the use of molybdenum as electrode wire in fast wire walking machine tool is repeated discharge, so the loss of wire affects the service life of electrode wire, which should be properly considered in actual processing

in EDM, the loss rate of tool electrode is different in different parts

in finishing machining, generally, the selection of electrical standards is small, the discharge gap is too small, the channel is too narrow, and the eroded substances constantly impact the electrode surface under the action of explosion and working fluid, which accelerates the electrode loss. Therefore, if the electrical gap can be appropriately increased and the channel condition can be improved, the electrode loss can be reduced

IX. surface roughness

surface roughness refers to the micro geometric error on the machined surface. For the electric machining surface, it is the accumulation of discharge marks - pits on the machining surface. Because the pit surface will form a work hardening layer and can store lubricating oil, its wear resistance is better than the machined surface with the same roughness, so the machined surface is allowed to be larger than the required roughness. Moreover, under the same roughness, the brightness of EDM surface is lower than that of machined surface

the national standard stipulates that the machined surface roughness is evaluated by one of RA (average arithmetic deviation of contour) and RZ (average height of roughness)

the surface roughness of EDM workpiece directly affects its service performance, such as wear resistance, matching property, contact stiffness, fatigue strength and corrosion resistance. Especially for molds and parts working under high-speed, clean and high-pressure conditions, their surface roughness is often the key to determine their performance and service life

X. discharge gap

discharge gap, also known as overcut, refers to the distance between the two poles of pulse discharge in machining. The actual effect is reflected in the unilateral expansion of workpiece size after machining. The quantitative understanding of the discharge gap in EDM is the basis of determining the machining scheme. Including tool electrode