Analysis and countermeasures on the causes of wire breakage in high-speed wire cutting machines
． 1 Broken wire related to electrode wire
(1) Wire tension and wire speed. For high-speed wire cutting processing, molybdenum wire of ?0.06~0.25mm is widely used because of its loss resistance, high tensile strength, not easy to become brittle and less wire breakage. Increasing the tension of the electrode wire can reduce the impact of wire vibration, thereby improving accuracy and cutting speed. Fluctuations in wire tension have a great impact on processing stability. The reason for the fluctuation is: the tension of the electrode wire on the wire storage drum is different when it moves forward and backward; after working for a period of time, the electrode wire will stretch again, causing the tension to drop (it is generally believed that the tension is 12~15N, which is more suitable for the consequences of the tension drop. The wire vibration is intensified and the wire is easily broken.
As the wire speed increases, the processing speed will also increase within a certain range. At the same time, the increase in wire speed will help the electrode wire bring the working fluid. In the discharge gap of a workpiece with a larger thickness, it is beneficial to the elimination of electrical corrosion products and the stability of discharge machining. However, haste makes waste, the wire running speed is too high, and the electrode wire shakes seriously, which destroys the stability of the machining. Not only will the processing speed be reduced, but the processing accuracy and surface roughness will also be deteriorated, and wire breakage will easily occur. However, the wire running speed cannot be too low, otherwise the wire will be easily broken due to large losses during processing. The general rule of thumb is less than 10m/ s is appropriate.
(2) The selection of electrode wire is nothing more than the type and diameter of the electrode wire. Usually the electrode wire material used in wire EDM processing should have good conductivity. The wire itself should have low electronic overflow work, high tensile strength, and good electrical corrosion resistance. The wire itself should not be bent or knotted. Its materials usually include molybdenum wire, tungsten wire, tungsten-molybdenum wire, brass wire, and copper-tungsten wire. Wires, etc. Among them, molybdenum wire and brass wire are used most. Tungsten wire processing can achieve higher processing speed, but the wire becomes brittle after discharge and is easy to break, so it is generally used at a lower wire speed. Slow and weak current gauges should be used on time. Molybdenum wire has low melting point and tensile strength, but has good toughness. It is not easy to become brittle and break during frequent rapid cooling and heating changes. Therefore, although some properties are not as good as tungsten wire, It is still the most widely used electrode wire. The processing effect of tungsten and molybdenum wire (50% each of tungsten and molybdenum) is worse than the previous two, so the service life and processing speed are higher than that of molybdenum wire, but it is more expensive. The processing of copper wire The speed is high and the processing process is stable, but the tensile strength is poor and the loss is large. It is generally used in low-speed wire cutting processing. In summary, the type of electrode wire should be determined according to the processing conditions, otherwise it will often cause breakage. Wire.
For high-speed wire cutting processing, the diameter of the electrode wire is generally between ?0.06~0.25mm, and the commonly used one is between ?0.12~0.18mm. When the fillet radius is very small, choose an electrode wire of ?0.04mm. Proper selection of electrode wire will greatly reduce the occurrence of wire breakage.
(3) New molybdenum wire and molybdenum wire breakage. There is a layer of black oxide on the surface of the new molybdenum wire. During processing, the cutting speed is fast and the surface of the workpiece is rough black. At this time, the power supply energy is too large and the wire is easy to break. Therefore, for the new molybdenum wire, the processing current needs to be appropriately reduced, and the electrode wire is basically equal. After turning white, the normal electrical parameters can be restored. When the machine tool has not been used for a long time, it is found that the molybdenum wire is broken. This is due to the temperature difference causing the material to expand and contract, plus the tension of the molybdenum wire itself. . If the machine tool is stopped, the wire storage drum should be shaken to the end and the molybdenum wire should be loosened.
1.2 Broken wire related to the workpiece
(1) When processing thin workpieces. of broken wire. Thin workpieces generally refer to thicknesses below 3mm. The main reason for the broken wire is that the opening distance between the upper and lower guide wheels of the wire frame is fixed, usually about 70mm. When cutting thin workpieces and running the wire at high speed, the electrode wire loses the damping effect of the coolant produced when processing thick workpieces, and the silver wire is prone to shaking due to the influence of spark discharge. The solution is to adjust the processing voltage to about 50V; adjust the processing current to about 0.3A; adjust the pulse width to less than 10?m; reduce the wire jitter. If the wire storage drum is driven by a DC motor, you can Change the armature voltage and reduce the speed; if it is driven by an AC motor, connect a 10-15Ω, 75W wire-wound resistor in series to any two of the three phases to reduce the phase voltage and increase the commutation transition time. Slightly longer to achieve soft commutation, which can effectively reduce vibration; use auxiliary material thickening between the upper and lower guide wheels to increase thickness and increase damping, which can also prevent molybdenum wire vibration.
This method is simpler and does not require adjustment of processing electrical parameters.
(2) Broken wire when processing thick workpieces. Thick workpieces generally refer to workpieces larger than 100mm. Wire breakage when cutting thick workpieces may occur when sparks are generated just after feeding, during the cutting process of the workpiece, or after the workpiece is cut. The main reasons for broken wires are:
① Broken wire at the beginning of cutting. When cutting from outside the workpiece, the wire breaks as soon as sparks are generated. This is because during initial cutting, the molybdenum wire is outside the workpiece, and the distance between the upper and lower godets is large. The molybdenum wire vibrates due to no damping, leaving the gap between the molybdenum wire and the workpiece in a poor state, or excessive emulsion. As a result, the insulation resistance is reduced, the arc extinguishing performance is poor, and arc discharge is included in the discharge gap, causing copper wire burns. In EDM, arc discharge is the main factor causing corrosion damage to the negative electrode. Coupled with poor clearance, arc discharge is easy to form. As long as the arc is concentrated on a certain section, it will cause wire breakage. Moreover, the greater the short-circuit current, the more serious the arc burns to the molybdenum wire, and the greater the possibility of wire breakage. ② Broken wire during cutting. When molybdenum wire is cut into a workpiece, due to the narrow slit, it is difficult for the emulsion to penetrate, and the electrical corrosion (carbon black and metal) in the slit will not come out, which will worsen the processing conditions and often cause secondary or tertiary discharges in the slit. Processing causes the slit to become wider, and like cutting thin workpieces, the gap is in poor condition, causing the pulse to form an arc discharge. If the arc discharge is concentrated in a certain section, the molybdenum wire will be burned out soon. ③ When the cutting is almost completed. Broken wire. In addition to the above reasons, the cause of wire breakage is the deformation caused by the self-weight of the workpiece and the internal stress of the workpiece material, which causes clamping. The wire is broken. The solution is to make a simple tooling fixture and perform necessary heat treatment on the material before processing.
(3) The broken wire appears to be caused by non-conductive substances in the workpiece. When the material is being cut normally, a "short circuit" phenomenon suddenly occurs, and no matter how it is eliminated, it cannot be solved. This situation is mostly caused by impurities contained in the forged or smelted material. These impurities do not have good conductivity, causing the problem during processing. Continuous short circuit will eventually break the steel wire. The solution is to program a program that moves back 0.5~1mm every time it advances 0.05~0.1mm, and uses it repeatedly during processing, and increases the coolant flow. Generally, this can be done. Wash away the impurities and resume normal cutting.
(4) Most of the workpieces processed by wire cutting should be demagnetized after flat grinding according to normal processes. The electrical corrosion particles generated in the cutting process are easy to be adsorbed in the cutting seams, especially when the workpiece is thick. Without demagnetization, it is easy to cause uneven cutting feed and increase the surface roughness value, causing short circuits and broken wires.
( 5) The wire breaks during automatic centering during wire cutting. This is because there are oil stains, burrs or some non-conductive substances on the wall of the process hole. When the electrode wire moves to the hole wall, there is no spark discharge, causing the machine tool to be unable to automatically reverse direction, and the workpiece is damaged. Bend the molybdenum wire and finally break the steel wire. Therefore, the process hole must be cleaned before processing
1.3 Broken wires related to pulse power supply
(1) Processing. The current is very large and the spark discharge is abnormal, resulting in broken wires. Most of this failure is caused by the output of the pulse power supply changing to DC output. Check the waveform from the output stage of the pulse power supply to the multivibrator step by step, and replace the damaged components. It can only be put into use when the output reaches the required pulse waveform.
(2) The output current exceeds the limit and the wire breaks. During the processing, the spark discharge suddenly turns into a blue arc discharge, and the current exceeds the limit. , burn out the molybdenum wire. Use an oscilloscope to measure the output of the input terminal and the oscillation part. It can be determined that the fault is in the oscillation part. It is found that there is an internal open circuit between the active poles and an internal breakdown between the middle poles. Replace this. tube, the high-frequency power supply returns to normal. Another situation is that the wire suddenly breaks during processing, and the current is above the limit. Use an oscilloscope to measure the output end of the high-frequency power supply. The amplitude of the waveform decreases, and there are negative waves, while the pulse The width meets the requirements, and the frequency, pulse width and amplitude of the driving stage waveform are measured to determine that the fault is in the power amplifier part. Checking the power tube, it was found that the internal breakdown between the ce poles of one of the tubes caused the final current to be directly added between the steel wire and the workpiece, causing arc burnout of the molybdenum wire. Replace the tube and return to normal.
(3) Burn points appear on the molybdenum wire and the wire breaks. Once a "pimple"-like burn point appears on the molybdenum wire, it is very easy for the wire to break.
It is generally believed that this is caused by machining chips (anode material) adhering to the electrode wire. This adhesion plays a role in concentrating discharge on the electrode wire. At this time, if the cooling and heat dissipation conditions are poor, it is likely to cause the discharge. The temperature at the point increases, so that during continuous discharge, other processing chips may continue to adhere to the vicinity of the point, thus creating a vicious cycle and eventually leading to burns at the point.
As for why machining chips adhere to the electrode wire, the main reason is related to the pulse parameters and the cooling status of the discharge gap. The solution is to increase the no-load voltage amplitude of the pulse power supply, or use a double pulse method (similar to what is commonly known as grouped pulse), which can reduce the possibility of machining chips adhering to the electrode wire; increase the coolant flow to improve cooling conditions.
(4) There are ablation points on the molybdenum wire and wire breakage. In the wire forehead, there is an ablation point every other section (about 10mm). A slight one looks like a mold spot, and a serious one can clearly see the ablation spots of the molybdenum wire. This is caused by the arc between the electrode wire and the workpiece. For some reason, point A on the workpiece and point B on the molybdenum wire draw an arc. The electrode wire is moving, and the arc between points A and B becomes longer and longer. , point A starts to arc with the closest point B', and so on, forming regularly spaced corrosion points, which greatly reduces the strength of the electrode wire. The main reason for this phenomenon is that the final output of the feed system is unbalanced. This phenomenon can be eliminated by adjusting the feed system.
1.4 Broken wires related to the wire-moving device and the working fluid
(1) The root cause of the broken wires related to the wire-moving device is the deterioration of the accuracy of the device. In particular, the wear of the different wheels will increase the vibration of the molybdenum wire, destroy the normal gap of spark discharge, and easily cause large current concentrated discharge, thereby increasing the chance of wire breakage. The accuracy of the guide wheel mechanism can be checked from three aspects: ① The V-shaped groove of the guide wheel becomes wider. This will cause the electrode wire to reciprocate in the Y-axis direction, which is manifested in the phenomenon of no feed or skipped feed when the wire storage drum reverses direction; ② The bottom diameter of the V-shaped groove of the guide wheel is not round. This is because the bearing supporting the guide wheel is damaged, the molybdenum wire does not enter the V-shaped groove of the guide wheel during processing, or dirt blocks the guide wheel, causing the molybdenum wire to pull out of the deep groove. When shaking the wire storage drum by hand, It will be found that the electrode wire shifts forward and backward in the X-axis direction. ③ The conductive shaft and the conductive wheel are not in good contact, causing wire breakage. During processing, it was found that the ammeter pointer swung widely from left to right, and the feed speed was uneven. Sometimes the ammeter pointer returned to zero, and the console feed speed was very fast. Because there was no discharge, the molybdenum wire was finally broken. At this time, new conductive wheels and conductive shafts need to be replaced.
(2) For workpieces that require high cutting speed or large thickness, the ratio of the working fluid can be appropriately lighter with a concentration of about 5% to 8%, so that the processing is more stable and the wire is not easy to break.
(3) The working fluid is dirty and the overall performance deteriorates after long use, which is an important reason for wire breakage. In practice, the deterioration of the working fluid can be measured in this way: when the processing current is about 2A, the cutting speed is about 40mm2/min, and the working fluid is 8 hours a day, the effect will be best after two days of use. If it is continued to be used for 8 to 10 days, it will be easy to break. The working fluid must be replaced with new one.
(4) Some studies believe that the working fluid prepared with high-purity water will work stably and have less broken wires during processing. The reason is that it is estimated that high-purity water is removed during the ion exchange purification process