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The process of electroerosive metals treatment 21.03.2018

The basis of the process of electro-erosive cutting of metals is the effect on the detail of spark discharges formed as a result of the flow of a pulsed current at a frequency of about 240 kHz between the electrode-wire and the component, which are in close proximity to each other in the medium of a liquid dielectric. As a result of these discharges from the material of the part, the microparticles, which are carried out from the inter-electrode gap (GAP), are cut off by the jet of the dielectric. In addition, the dielectric acts as a catalyst for the decay process, since at high temperature of the discharge, the insulator in the erosion zone is converted into pairs. There is an additional microabsorption of steam, which can not immediately leave the interelectrode gap. In recent years, electro-erosion treatment not only fully consolidated its position in modern tooling production, but also continues to grow rapidly in the direction of improving the quality indicators and the proposal of new constructive solutions. The discovery of the formative effect of electric erosion on conducting materials took place in 1943 and belongs to our compatriots - the spouses B. and M. Lazarenko. When the best scientists in the world in the 1950s were serious about this technology, some Soviet leadership would have turned promising developments and led us to lag for many decades in the field of electro-erosion treatment. Dielectric in electro erosion treatment Soviet researchers, spouses Lazarenko, who for the first time discovered the possibility of using the phenomenon of electrical erosion for the processing of materials that were conductive, as the dielectric, used the ambient air at first. However, it soon became clear that derivatives of mineral oils have incomparable advantages in this regard: the discharge strength is greater, it is possible to work with smaller spark gaps, which improves the accuracy of the operation. The new material of the dielectric also allowed to increase the discharge frequency and it is better to wash particles of eroded metal. Since 1960, chemical companies that offer special compounds for use in electro-erosion plants began to enter the market. Sequence of process: Electrodes - tool and workpiece - fixed by the machine tool and do not come into contact with each other. The generator of electric pulses sets the frequency and intensity of the electric field around the electrodes. During the process of electroscope exposure a series of periodic wandering electrical discharges removes the finest layer of material from the workpiece. At the point where the electric field strength reaches the maximum, there is an electrical discharge - lightning. Under the action of an electric field, electrons and free positive ions are accelerated to high velocities and instantly form an ionization tunnel with electrical conductivity. There is an electric current, and a spark discharge is formed between the electrode and the workpiece, which leads to collisions of elementary particles. During this process, a gas bubble forms, the pressure of which constantly increases until the formation of the plasma zone. The plasma zone quickly reaches superheat temperatures - from 8000 to 12000 ° С - due to the increasing number of collisions of elementary particles. This process leads to the instant melting of the micro-layers of the substance in the electrode. In the absence of an electric field, a sudden decrease in temperature leads to an explosion of a plasma bubble, which is accompanied by the separation of part of the material from the workpiece, and the formation of a microscopic crater on this site. The eroded material is then re-formed in the form of small spheres that are washed out by a liquid-dielectric. With a very short discharge pulse in motion, more negatively charged particles are generated than positively charged. The more particles of a certain charge move to the electrode, the more heat is produced on its surface. Because of the larger size, positive particles contribute to the production of more heat at the same bombardment speeds of the target electrode. To minimize the removal of the workpiece material or the wear of the tool, the polarity is selected in such a way that as much heat is freed from the workpiece until the discharge is complete. For this purpose, at short discharges, the electrode-tool is connected to a negative terminal and, thus, has a negative polarity. At long discharges, on the contrary, the electrode-tool connects to the positive pole. The duration of the pulse, at which the polarity in the workpiece and the electrode tool should change, is affected by a number of factors that are more dependent on the physical parameters of the instrument and the properties of the electrode material. When the copper electrode is processed, the duration of the periodically generated pulse is about 8 microseconds. Electro-erosive processing technology continues to evolve in two main directions: wire electrical discharge machining and die-slicing electro-discharge machining (ram (die sinking)). Wire electro erosion cutting Wire cutting started its development from a clean sheet in the early 70's. The global improvement of the process took place between the mid-80's and the mid-1990s. Progress has been made in six key areas of the known price-quality ratio: • processing speed; • the size of the workpiece; • angle of conical cutting; • price; • accuracy; • Continued work without human intervention. Functions of the dielectric: Electrical insulating. The insulator must divide the workpiece and the electrode. Spark discharge should occur in the narrowest possible gap between the workpiece and the electrode, which allows the process to be productive and accurate. Ionization In a very short time it is necessary to create all the conditions for the formation of an electric field. After the momentum, the discharge zone should be very deionized to allow re-discharge. The insulator should contribute to the maximum narrowing of the spark zone in order to achieve high energy density in this zone. Cooling The spark discharge has an ultrahigh temperature, so the dielectric should cool both the electrode and the workpiece. It is also important to remove the metallic gases that are formed during the process of electroerosion. Washable In order to avoid electrical breakdown, the dielectric should effectively remove the eroded grains of the metal. Speed. The nominal maximum cutting speed to date exceeds 300 square meters. mm / min, (in laboratory conditions the cutting speed is 600 sq. mm / min.). Of course, the average speed of cutting in real production conditions is lower than the nominal values, but it is striking compared with the indicators of quarter-on-quarter (25-30 sq. M / min.). The emergence of new alloys for the wire can even push away these limits. The size of the workpiece. The maximum sizes of blanks suitable for machining on cutters, on axes X and V reach a meter and even more (1300-1380 mm - along the X axis). However, the size of the workpiece along the axis 2, which now reaches 400 mm, has increased significantly over the past decade. To this same mark went up and coordinate movements on the axis 2. All this was achieved thanks to new constructive solutions that united the principles of the integrity of the workpiece and more accurate and rigorous implementation of axial movements. Angle of conical cutting. In modern installations, angles of conical holes up to 30 degrees for blanks up to 400 mm are achieved. Moreover, some firms offer the option of an option with the ability to achieve taper to 45 degrees. Such opportunities have allowed since the beginning of 90 years to significantly expand the scope of electro-erosion cutting. So, if earlier the heads for extrusion units were assembled from a set of separately processed thin plates, then the new possibilities of the machines allowed to make extrusion heads from one workpiece and one operation. Due to this, it was possible to significantly reduce the amount of scrap and increase the productivity of the extrusion process. Price. For more than a quarter century of continuous work, the characteristics of cut-off machines have improved on an orderly basis, while during the same period equipment fell by about 75% (taking into account the inflation factor). A number of circumstances contributed to lower prices. Electronic filling of the equipment has absorbed many of the solutions that came from the computer industry, which at that time were excellent in terms of price-quality - increased computing power for less money. The volume of production of electroerosive equipment has greatly increased, which has increased the efficiency of such industries. In addition, the latest technical solutions have also contributed to a steady decline in installation prices. Precision. Back in the 70's and early 80's, the operator of the machine had to make a trial sample first and then adjust the initial settings before reaching the exact geometric dimensions of the product spark gaps. The spark gap separates the workpiece and the electrode. Even with a small depth of processing distinguish two types of gap: the front and side. The front is set by the control system, while the lateral spark gap depends on the duration and height of the discharge pulse, the combination of materials, idle voltage and other specified parameters. The power supply is the most important element of the electro-erosion plant. It converts alternating current from the power network into rectangular pulses of a certain polarity. The size of the pulses and the interval between them is set by the control system in accordance with the processing mode executed. The current strength of the discharge is proportional to the pulse height. The interval between pulses corresponds to the moment of absence of a spark discharge, and the length of the pulse corresponds to the duration of the spark discharge. Both of these quantities make up thousands or even millions of seconds. The interval between individual impulses (the duration of the absence of electric current) is also set by the control system. The interval is measured as a percentage of its own duration to the duration of the pulse. Today's precision of machine tools reaches 0.001 mm. The involvement of modern information technologies (CAD / CAM systems) in the technological process has also increased the accuracy of processing - new installations with the whole complex of modern equipment guarantee the repetition of the geometry of the program with a high degree of accuracy. As already noted, modern machine models are 75% cheaper than their early predecessors, at the same time they are 3-5 times more accurate. Continued work without human intervention. In the 70s, electro-erosion plants could work for hours without operator intervention - then the cutting speeds were too small. In modern conditions, when processing speeds have significantly increased, practically completely deserted production is ensured by the use of advanced automation systems: automatic welding of the wire, search of the hole, preventing the breakage of the wire, the system of automated loading of the workpieces, removing the jumpers, etc. D. Electrodes wearing Erosion with A small current removes a little material from the workpiece, while a large current allows for high material removal rates. However, the wear of the tool increases, especially when processing steel with copper electrodes. Graphite electrodes behave differently - wear to a certain point increases, then more or less stabilizes. Short pulses also lead to accelerated wear of the electrode. Conversely, wear is much slower with long pulses. In practice, when roughing steel with a copper and graphite tool, the optimum value of the pulse duration lies in the segment, at one end of which the pulse duration with maximum removal, and on the other - the duration of the pulse with the minimum intensity of the tool wear. Copy-flashing processing For the first time about electro-erosion machines became known with the advent in 1955 of the first duplicator-flashing device with manual control. The capabilities of the machine tools were significantly improved by the end of the 1970s, when the use of satellites allowed to significantly reduce the cycle time and the number of electrodes used. In about 1980, the first model of CNC-controlled machine was presented. By this time, the process of copying firmware had already undergone all possible improvements, and the characteristics of the firmware CNS-machines improved at a slower pace than the "junior" wire-cut machine tools. The most important improvement in comparison with early machines with manual control was not so much the cycle time, but the number of working hours required to form a given cavity in the material. Previously, it was observed that in the case of wire cutting, increased productivity and reduced production costs were achieved to a greater extent due to the cutting speed. With regard to electro-erosive firmware, the corresponding improvements were achieved due to a significant increase in the number of hours that the CNS machine can operate during the day. Both technologies have won a lot from lowering normohodyn on every detail. The transition from the manual control to the numerical software allowed the machine to work almost the entire first change, intensively in the second and third in desolate mode. If for a machine with a manual operation the most typical is work for 6 hours, then in the case of a machine with a CNS-control real task can be 16 hours. Although many industries achieve higher rates. Price. The leading manufacturers of embossing machines today offer CNS machines (without a system of automatic tool change and the C axis) at about the same price as in the mid-70s, machine tools with manual control (inflation is taken into account) were sold. Even if you add the cost of the necessary options, the adjusted price of such a new machine will be approximately equal to the price of a 70-s machine with manual control, which is equipped with a satellite. Any increase in prices will be substantially outweighed by a decrease in the cycle time, normohodin and an increase in the time that equipment actually works during the day. Accuracy of processing. Given the nature of the shaping process in electro-erosive stitching, the accuracy of the treatment will depend to a greater extent on the quality and accuracy of the electrode fabrication. Graphite - a cheaper and more handled material - in combination with modern technologies of high-speed milling allows you to receive electrodes of great complexity and high accuracy. Certain successes have been achieved with respect to the plane of the treated surfaces. For example, the total deviation for a rectangular cavity of 150x200 mm, obtained on a modern machine, is only 0008 mm.

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