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Talking about the Broken Wire Condition and Measures of Slow-Walking Wire Cutting

Posted by: steel world 2021-10-18 Comments Off on Talking about the Broken Wire Condition and Measures of Slow-Walking Wire Cutting

Nowadays, as the advantages of slow-moving wire over fast-moving wire are gradually discovered and recognized, slow-moving wire cutting machines are widely used when processing high-precision parts.
Wire EDM can be divided into fast wire and slow wire according to the cutting speed. Because the surface roughness of the workpiece processed by fast wire is generally in the range of Ra=1.25~2.5 microns, while the slow wire can reach Ra=0.16 microns , And the fixed error, linear error and dimensional error of the slow-moving wire cutting machine are better than the fast wire-moving, so the slow-moving wire cutting machine has been widely used when processing high-precision parts.
Since the slow wire is the method of continuous wire feeding by the wire electrode, that is, the wire electrode is processed during the movement, so even if the wire electrode is worn out, it can be continuously supplemented, so the machining accuracy of the parts can be improved, but the wire EDM The broken wire has become an obstacle to these advantages, and it is necessary to solve this problem.
Broken wire mechanism: It is
generally believed that wire breakage is mainly due to the high temperature of the electrode wire caused by concentrated electric spark discharge, which is consistent with the detected wire breakage precursor. Therefore, studying the temperature distribution of the electrode wire from the theory of heat conduction has become the main way to study the mechanism of wire breakage. The research results show that: the thermal load before the wire breakage exceeds the average value; the pulse width and wire diameter have a great influence on the wire temperature; the thermal convection coefficient has a great influence on the wire temperature, and the state of flushing liquid is very important to avoid wire breakage; Joule heat The effect of and silk vibration can be relatively ignored.
For the equal energy pulse current power supply, research shows that there are two important precursors to wire breakage: 1. The spark discharge frequency suddenly rises in a short period of time. Because the discharge frequency is too high, the local temperature of the electrode wire is too high, which leads to wire breakage; 2. Normal spark The decrease in probability and the gradual increase in the probability of abnormal sparks are also a precursor to broken wires. As the wire loss rises, the electrode wire becomes thinner and eventually pulled off. KPRajurkar et al. pointed out that the sudden change in the thickness of the workpiece during the machining process is one of the main reasons for the concentration of discharge. Therefore, it is necessary to detect the change of the thickness of the workpiece online, adjust the corresponding process parameters, control the feed speed and discharge frequency of the electrode wire, and obtain a better cutting speed under the condition of continuous wire.
Due to the short duration of the broken wire precursor and the high real-time control of the broken wire, the selection of control parameters is very important. In the on-line cutting process, the pulse interval enlargement will increase the time for removing the corrosion products from the discharge gap, which can effectively improve the discharge concentration phenomenon and greatly reduce the chance of wire breakage. Therefore, the pulse interval has become the preferred control parameter for anti-breakage control.
First, factors associated with breakage of the wire electrode
1, the material properties of the wire electrode
wire requires a good discharge characteristics and high tensile strength, so we choose to lower the amount of zinc (10%) of the core for brass, zinc The high-volume brass coated electrode wire just meets the requirements of wire cutting. The electrode wire is made of famous brand molybdenum wire, which has high precision, high tensile strength and good quality. Low-temperature treatment of the electrode wire is also one of the measures to reduce the probability of wire breakage. An Ohio company in the United States compared the electrode wire cooled for 24H at a low temperature of -2000C with the electrode wire without low temperature treatment. As a result, the probability of wire breakage in the former was 30% lower than that in the latter.
During the processing, the load-bearing discharge capacity of the electrode wire is determined by the diameter of the electrode wire, so the wire diameter directly affects the wire breakage rate. Therefore, in the processing, you should choose the appropriate diameter, smooth coating surface, and no oxidation according to actual needs. Spots or high-speed cutting wire electrode after low temperature treatment, thereby reducing wire breakage.
2, yarn tension and yarn vibration
low-speed wire cut electric discharge machining, and is maintained as high as possible under stable tension strength limit of the wire electrode, the wire can roughing maintain a minimum lag in a discharge bend without explosive Broken wire. Proper tension can effectively reduce the vibration amplitude of the wire and keep the wire stable during processing.
3. Moving speed of electrode wire
Due to the small diameter of the wire electrode in the wire cutting process (usually 0.1~0.3mm), if the wire electrode moves too slowly, a certain point on the wire electrode may produce multiple discharges, which makes the amount of erosion at this point too large. The wire is easily broken under the action of wire tension and the explosive force of spark discharge. Therefore, under the condition that the wire allows a little continuous discharge times, the wire speed should be adjusted according to the discharge frequency according to the thickness of the workpiece. The discharge frequency of rough machining and finishing machining is different, and the wire speed is also different. If the diameter of the electrode wire is small, the workpiece is thick, the workpiece is rough, and the discharge frequency is required to be high, the wire speed should be relatively fast. In actual processing, you can refer to the electrode wire moving speed given in the process database of the low-speed wire EDM cutting machine.
4, the conductive block
conductive block made multi-silver-tungsten alloy, wear resistance and good conductivity. During the processing, the conductive block and the moving electrode wire keep in contact, which causes the conductive block to wear. Generally, the conductive block used in the low-speed wire EDM should be checked in time, removed, and cleaned with cleaning fluid to remove the dirt stuck on it. If the wear is serious, change the position or update the conductive block.
5, silk waste processing
low-speed wire EDM cutting wire walking is unidirectional, the processing will produce a large waste silk. If the waste wire is not taken out in time, it is easy to generate additional capacitance between the electrodes, and it may be directly connected to the electrode wire in the processing area, resulting in a concentrated release of energy, causing wire breakage or even short circuit, so that normal processing cannot be performed, so when the waste wire falls Take it out in time. At present, high-end machine tools have automatic waste processing devices. There are two treatment methods: one is to put the wire cutting device in the waste wire removal port; the other is to install the broken wire device on the processing head, and the cut waste wire is discharged through the flushing pipe.
 Second, broken wires factors related to the working fluid of
the current low-speed wire EDM cutting mostly with purified water and distilled water, cheap non-polluting. The use of working fluid has two main functions: insulation and cooling. Therefore, the working fluid should have good heat absorption, heat transfer and heat dissipation functions.
When the performance of the working fluid becomes poor, it means that the impurity ions in the working fluid are greatly increased, and the dielectric performance of the working fluid is significantly reduced. On the one hand, it will increase the probability of contact discharge caused by the conductive bridge composed of dielectric particles; on the other hand, due to the increase in the electrical conductivity of the working fluid, the processing gap increases. At this time, the processing waveform is characterized by a series of almost no open circuits and breakdowns. Delayed discharge pulse. At this time, the energy density input to the machining gap is very large, which can easily cause wire breakage. The working fluid must be replaced at this time.
The machining chips generated during the discharge process are also one of the factors that cause wire breakage. It is formed by bridging by machining chips, or when the relatively protruding sharp points on the two poles occasionally meet to form a micro-short-circuit state, which has a relatively large contact resistance. Due to the movement of the electrode wire, this micro-short circuit is easily pulled apart to form a spark discharge. Therefore, the energy density of the pulse power input to the machining gap at this time is much greater than during normal processing, which causes the concentrated release of pulse energy at the adhesion part of the electrode wire, causing the electrode wire to crack, which may cause wire breakage. Therefore, these particles must be washed away during the processing. . In order to effectively wash away the solid particles, when there is no restriction on the geometry of the workpiece, close processing should be selected as much as possible, so that water rushes into the kerf to better improve the scouring condition. If the direction of spraying water is inaccurate, the wire will break easily. Because the direction of spraying water is not correct, the working fluid cannot be sprayed into the cutting hole, and the electrode wire cannot be cooled, causing the local temperature of the electrode wire to rise suddenly, causing wire burning. The spraying position should be such that the electrode wire is covered by a water column, and the spraying water pressure should be equal up and down.
Third, the servo control associated with the breakage factor
servo feed speed of the workpiece in addition to erosion should be kept constant speed relationship, i.e., maintaining the machining gap constant. Because when the servo speed exceeds the erosion speed, frequent short circuits will occur, and the possibility of wire breakage will increase at the same time; on the contrary, if the servo speed is too slow, the poles will be biased towards the open circuit, and the short circuit will be caused by the open circuit during the processing. The cutting speed decreases and the surface roughness value increases. Therefore, the servo should feed uniformly and stably, avoid crawling, overshoot should be small, transmission rigidity should be high, there should be no obvious gap in the transmission chain, and the anti-interference ability should be strong. The frequency of electrical discharge machining is very high, and the state of the discharge gap changes rapidly. The feed adjustment system is required to adjust quickly according to the weak signal of the gap state. Therefore, the inertial area, time constant, and inertia of the movable part in the whole process should be small, the magnification should be sufficient, and the transition process should be short.
4. Broken wire factors related to pulse power supply
1, the main electrical parameters affect
(1) the pulse width of
the pulse width is one of the determinants of a single pulse energy. It can be seen from the experiment that the pulse width increases, the discharge energy of a single pulse increases, and the cutting speed gradually increases. However, when the pulse width exceeds a certain range, the discharge pulse energy lost between the workpiece and the electrode increases, which weakens the erosion effect. At the same time, since the throwing effect of the electro-erosion products will 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 single pulse energy, the vibration of the electric wire will be strengthened, thereby reducing the surface quality of the workpiece, increasing the surface roughness value, and wire breakage often occurs. Therefore, nano-scale Ruo pulse width is the main research direction of current low-speed wire EDM cutting machine manufacturers.
(2) pulse interval
experiments indicate that pulse interval a great influence on the cutting speed. Under the condition that the working fluid restores the insulation between the electrodes, the pulse interval can be appropriately reduced to increase the number of discharges per unit time, thereby increasing the cutting speed. Since the pulse width and the energy of a single pulse remain unchanged, the effect on the surface roughness of the workpiece is not obvious. However, if the pulse interval is too small, the medium in the work area cannot be restored to an insulating state, and the discharge products cannot be eliminated in time, which will make the processing state unstable, increase the probability of short circuit in the work area, and easily cause wire breakage.
2. Impact of pulse power control strategy
Because the low-speed wire EDM cutting is high-frequency electric discharge cutting (pulse width 50ns~2 microseconds, pulse interval 1~15 microseconds), it is difficult to control the energy of a single pulse online, but it has been developed by modern high technology. This kind of power supply increases the processing efficiency rapidly, and effectively prevents wire breakage and arc burning. For example, the “CLEAN CUT” (CC) new pulse power supply developed by the Swiss Charmilles company on the basis of the ROBOFIL240/440 machine tool has a maximum peak current of 1200A and a maximum breakdown voltage of 25%. In rough machining, the maximum The cutting speed reached 400 mm2/min. With such a high peak current, it can cut stably and efficiently without wire breakage because it uses a single discharge pulse energy control technology with precise pulse width. In addition to the effective pulse-by-pulse detection, the power supply also uses the PILOT-EXPERT expert system to control the energy of the concentrated discharge, so as to prevent wire breakage and solve the problem of local overheating.
Another anti-broken wire technology in the pulse power supply is the gas phase throwing during rough machining. The so-called gas-phase throwing process, that is, in electric discharge machining, uses thermal expansion and local micro-explosion to throw the molten and vaporized workpiece material in the gas phase. The gas phase throws away a large amount of energy at the same time, which not only greatly reduces the heating effect of the workpiece, but also greatly reduces the heating of the electrode wire. It will not cause micro-cracks due to instantaneous overheating and sudden cooling, thereby effectively preventing wire breakage.
Fifth, other factors that result in broken wire
broken wire there are other factors, such as workpiece material. In order to reduce broken wires and different materials, corresponding processing parameters should be selected during wire cutting.
1. Wire walking speed
The speed of the wire feeding directly affects the stay time of the wire electrode in the processing area and the number of discharges, thereby affecting the loss of the wire electrode. Due to the small diameter of the electrode wire in the wire cutting process, if the electrode wire moves too slowly, a certain point on the electrode wire may produce multiple discharges, which makes the erosion at this point too large, which will affect the wire tension and the blasting force of the spark discharge. The thread is easily broken underneath. In addition, by increasing the wire speed of the heating wire, the working fluid can be easily brought to the processing gap of the narrow slab, and the cooling of the electrode wire can be strengthened to bring the electrolytic corrosion products out of the gap. However, too high a wire speed will increase the vibration of the electrode wire and easily break the wire, reduce the precision of the processed surface and increase the surface roughness. Therefore, under the condition that the electrode wire allows a small number of continuous discharges, it is necessary to adjust the wire speed according to the discharge frequency according to the thickness of the workpiece.
2, the feed rate
over the cutting wire feed speed should be tracking its linear processing speed, fast feed, than the erosion rate of the workpiece may be likely to cause

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