Low-speed wire-cut EDM wire cutting has an important position in China's manufacturing industry due to its wide application fields, high processing precision and high efficiency. However, interrupting the wire at low speed wire-cut wire EDM is a tricky problem. Based on a large number of domestic and foreign references, combined with practical work experience, this paper analyzes in detail the occurrence of wire breakage in low-speed wire-cut EDM wire cutting from electrode wire, working fluid, servo control and pulse power control strategy. The main reason, and based on this, put forward corresponding improvement measures.
1 Broken wire factor related to wire electrode 1.1 Wire material characteristics The common electrode wire used in low-speed wire-cut wire electric discharge machine is mostly single-structured brass. The high-speed cutting electrode wire uses a brass with a low zinc content (about 10%) as the inner core and is coated with brass with a high zinc content (35% to 40%). Because alloys with high zinc content have good discharge characteristics, and alloys with low zinc content have high tensile strength at high temperatures, these two characteristics are in line with the requirements for wire cutting for wire cutting. Using this kind of electrode wire to test on the low-speed wire-cut wire electric discharge machine produced by Mitsubishi Corporation of Japan, the breaking rate of the wire is nearly 20% lower than that of the ordinary brass wire. The low temperature treatment of the wire is also the probability of reducing the wire breakage. a measure. A company in Ohio, USA, used a wire electrode cooled at -200T for 24 hours and a wire electrode without low temperature treatment. The result is that the breaking probability of the former is reduced by 30% compared with the latter. During the processing, the wire carries the discharge capacity. The size of the wire is determined by the wire diameter of the electrode, so the wire diameter will directly affect the wire breaking rate. The wire diameter is small, and the load carrying capacity is small. When the electrode wire is not made, the teaching and research of the technology.
It can easily cause broken wires when it can carry peak current and average current. Therefore, in the processing, according to the actual needs, the electrode wire with appropriate diameter should be selected, and the high-speed cutting electrode wire with smooth surface, no oxidation spots or low temperature treatment should be selected. Thereby reducing wire breakage and improving processing efficiency.
1.2 Wire tension and wire vibration Low-speed wire-cut EDM wire cutting process, maintaining the highest and stable tension under the wire wire strength limit, can keep the wire under the discharge explosive force to maintain the minimum hysteresis bending during rough machining, and Will not break the wire. A suitable tension can effectively reduce the vibration amplitude of the wire and keep the wire stable during processing.
Since the wire cutting process itself requires that the wire tension be only in the cutting area, the tension in front of the wire drawing wheel tends to be higher than the required tension and is uneven. This is because the force is not only the loading device, the wire is pulled out from the wire shaft, and there is resistance on the steering wheel, the conductive block and the yarn guide, and finally added to the wire drawing wheel, if the value is close to the wire electrode. The strength limit is easy to break here, especially in the roughing process, the electrode wire before the wire drawing wheel has a certain loss and deterioration layer after discharge. Therefore, the more advanced machine tools use multi-stage tensioning mechanism, the tension is uniform and stable. In the past, many machine tools used magnetic powder clutches or magnetic powder brakes to control the tension of the wire. However, due to the disadvantage of uneven control torque, the magnetic powder clutch was gradually replaced by a DC motor.
For example, Japan's Sodick Company, the early low-speed wire EDM wire cutter uses a magnetic powder clutch to control the tension, and now the new models use DC motors to control the tension, and the recommended tension is 1200g (. 2A) UJ 1.3 wire movement The speed is small due to the diameter of the wire used in the wire cutting process (generally 0.1~0.3mm). If the wire moves too slowly, a certain discharge may occur at a certain point on the wire, making the amount of erosion too large. Under the action of the tension of the wire and the explosive force of the spark discharge, it is easy to break the wire. Therefore, under the condition that the wire allows a certain number of continuous discharges, the thickness of the n pieces should be combined, and the wire speed should be correctly adjusted according to the discharge frequency. The frequency of roughing and finishing T discharge is different, and the wire speed is different. If the wire diameter is small, the workpiece is thick, rough, and the discharge frequency is required to be high, the wire speed is relatively faster. In the actual processing, the moving speed of the electrode wire given by the process database of the low-speed wire-cut EDM wire cutting machine is 1.4. The conductive block of the conductive block is made of silver-tungsten alloy, which has good electrical conductivity and wear resistance.
In Gading. During the process, the conductive block keeps good contact with the moving electrode wire, which causes the conductive block to wear. The conductive block used in the general low-speed wire EDM machine is processed after 60~120h. Check once. In actual processing, such as breaking the wire near the upper and lower yarn guides, then usually caused by the conductive block, the conductive block should be inspected, the conductive block should be removed, and the dirt adhering to the surface should be washed away with cleaning liquid. Change position or update the conductive block 1.5 waste wire processing Low-speed wire-cut wire EDM is a one-way wire, which will produce a lot of waste wire during processing. If the waste wire is not taken out in time, it is easy to generate additional capacitance between the poles, and it may be directly connected to the electrode wire of the processing zone and the domain, thereby generating concentrated energy release, causing wire breakage or even short circuit, so that normal processing cannot be performed. Therefore, when the waste silk falls, it should be taken out in time. At present, high-end machine tools have automatic waste wire processing devices. There are two ways to deal with it: one is to put the wire cutting device at the waste wire discharge port. For example, Japan's Mitsubishi new wire cutting machine KEF-L waste wire cutting is a special method of stretching and breaking by means of a rotating blade. This can lead to the winding of the wire and avoid interruption of the wire cutter. The second is to install the broken wire device in the processing head, and the cut waste wire is discharged through the flushing pipe.
2 Broken wire factors related to working fluid At present, the working fluid for low-speed wire-cut EDM wire cutting processing is mostly pure water and distilled water, which is cheap and pollution-free. The main functions of using working fluid are: 1 insulation. The working fluid insulation performance should be proper, the resistance is generally around, the insulation performance is too low, and the electrolysis will not be formed into a breakdown spark discharge; if the insulation performance is too high, the discharge gap is small, the chip removal is difficult, and the flushing effect is poor. When the working fluid resistance can no longer return to normal, the ion exchange resin should be replaced in time to ensure the insulation performance of the working fluid. 2 cooling effect. During the discharge process, the local instantaneous temperature of the discharge point is extremely high, especially at high current processing, reaching 0000~12000*. In order to prevent the wire from being blown and the workpiece surface partially annealed, it must be sufficiently cooled. Therefore, the working fluid should have good heat absorption, heat transfer and heat dissipation properties.
When the performance of the working fluid is significantly deteriorated, it means that the amount of impurity particles present in the working fluid is greatly increased, and the dielectric properties of the working fluid are remarkably lowered. In this respect, the probability of contact discharge caused by the conductive bridge composed of the impurity particles is greatly increased; on the other hand, the processing gap is increased due to the increase in the conductivity of the working fluid, and the processing profile at this time is a series of almost no Open and break down the delayed discharge pulse. At this time, the energy density of the input to the machining gap is very large, and it is easy to cause broken wires. At this time, the working fluid must be replaced.
The machining waste generated during the discharge process is also one of the factors causing the broken wire. 3 is formed by the bridge of machining chips, or the micro-short state formed by the relatively protruding cusps on the two electrodes sometimes has a large contact resistance due to the electrode. Silk motion, this micro short circuit is easily pulled apart to form a spark discharge. Such a spark discharge has no breakdown delay and is in a partial short circuit state or a micro short circuit state. Therefore, at this time, the energy density of the pulse power input to the processing gap is much larger than that during the normal processing, so that the pulse energy is concentrated and released at the adhesion portion of the electrode wire, which causes cracks in the electrode wire, which may cause broken wires. Therefore, these particles must be washed away during processing. From the point of view of the rinsing ability, the processing of the material for processing low sublimation heat is good. These materials are easily converted into gases, leaving only a few particles. The material of the high-heating heat will produce a large amount of particles, and the flushing effect is poor. The amount of material cut is always the largest during the first processing, and the waste generated is also the most, so the amount of flushing during roughing is larger than the next few times. In order to effectively wash away the solid particles, if there is no limitation on the geometry of the workpiece, it should be as close as possible to make the water rush into the slit to better improve the flushing condition. If the direction of the water spray is not accurate, it is easy to break the wire. Because the water spray direction is not accurate, the working fluid can't be sprayed into the cutting seam, and the electrode wire can't be cooled, so that the local temperature of the wire suddenly rises and causes the wire to burn. The position of the water spray should be such that the water column encloses the electrode wire, and the pressure of the upper and lower spray water is equivalent.
3 Broken wire factors related to servo control EDM is different from cutting machining and belongs to “non-contact machiningâ€. During normal EDM, there is a discharge gap between the tool and the workpiece. Since the spark discharge gap is small, it is generally ol ~0.01mm, and it is related to the machining standard, the workpiece erosion speed, etc., so it is difficult to adjust manually, and it is not possible to use "maneuvering" constant speed feed like drilling. An automatic feed adjustment system must be used.
This non-equal speed discharge gap automatic feed adjustment system is also called a discharge gap servo control system, and its task is to maintain a certain servo feed speed must maintain a certain relationship with the workpiece erosion speed, that is, to maintain the machining gap Certain value. Because when the servo speed exceeds the erosion speed by 24 degrees, frequent short circuit will occur and the possibility of broken wire will be increased. On the contrary, the servo speed is too slow, lower than the etching speed, and the two poles are biased to open circuit. Short circuit due to open circuit, the cutting speed is reduced, and the surface roughness is deteriorated. Therefore, the servo should be fed evenly and stably, avoiding creeping, the overshoot should be small, the transmission stiffness should be high, there should be no obvious gap in the transmission chain, and the anti-interference ability should be strong.
The frequency of the electrical discharge machining is very high, and the state of the discharge gap is rapidly changing, and the feed adjustment system is required to quickly adjust according to the weak signal of the gap state. For this reason, the inertia of the entire system, the time constant, and the inertia of the movable part are required to be small, the magnification should be sufficient, and the transition process should be short.
4 Broken wire factor related to pulse power supply 4.1 Influence of main electrical parameters Pulse width is one of the determinants of single pulse energy, so it has an important impact on cutting speed, surface roughness and so on. It can be seen from the experiment that the pulse width is increased, and the single pulse discharge energy is also increased, so that the cutting speed is gradually increased. However, when the pulse width exceeds a certain range, the energy of the discharge pulse between the workpiece and the electrode is increased, and the etching effect is weakened. At the same time, since the throwing effect of the electrolytic corrosion product does not increase significantly with the increase of the pulse width, this will inevitably increase the probability of short circuit, reduce the cutting speed, and even break the wire. In addition, due to the increase of the single pulse energy, the electrode wire vibration is strengthened, thereby reducing the surface quality of the workpiece, increasing the surface roughness value, and the broken wire often occurs. Therefore, the narrow pulse width of the nanosecond is the main direction of the current low-speed wire-cut wire EDM machine manufacturers.
Through the experiment of changing the pulse width, the thickness of the workpiece and the number of power tubes, it was found that the pulse interval has a great influence on the cutting speed. Under the condition that the working fluid recovers the insulation between the poles, the pulse interval can be appropriately reduced, so that the number of discharges per unit time is increased, thereby increasing the cutting speed. Since the pulse width and the single pulse energy are constant, the influence on the surface roughness of the workpiece is not obvious. However, the pulse interval is too small, the medium in the working area can not be restored to the insulation state, and the discharge product can not be eliminated in time, which will make the processing state unstable, increase the probability of short circuit in the working area, and easily cause broken wire.
4.2 Impact of the swimming impulse power control strategy The most popular research at present is the precision control of a single discharge pulse energy source. Beijing: People's Publishing House, Research Office of the Central Committee of the Communist Party of China. Mao Zedong's Chronicle (middle volume). Beijing: People's Publishing House, Central Publishing House, 1993.401 Guo Dehong. The eternal spirit of Yan'an. Tianjin: Tianjin Ancient Books published the Research Office of the Central Committee of the Communist Party of China. Mao Zedong's Chronicle (middle volume). Beijing: People's Publishing House, Central Publishing House, 1993.341 Department of History, Nankai University. Proceedings of the International Academic Symposium on China's Anti-Japanese Base History. Tianjin: Archives Press, 1985.268. c on page 25) 6 Conclusions This paper focuses on the various causes of wire breakage caused by low-speed wire-cut EDM, and proposes some solutions. In practical applications, due to different design strategies and processing techniques of various machine tools, the method of breaking the wire is different, and the cause of the broken wire is multi-faceted, sometimes not only caused by one factor, but also may be two or more. The combination of factors, so it should be analyzed according to the specific circumstances.
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