3、 Problems of short circuit and distortion of cutting path in aluminum cutting
one Cause analysis of the problem
In the daily processing of Guangdong mechanical processing plant, it is difficult to cut large thickness workpiece, because the workpiece is thick to a certain extent, there is not enough coolant into the workpiece, the electrical corrosion products in the gap can not be removed normally, and the processing is very unstable, until there is a short circuit of current and no discharge, which is more likely to occur in cutting aluminum parts. In general, the offline cutting machine tool has the function of short circuit protection. Once there is a short circuit, the worktable of the machine tool immediately stops moving and remains in the original position for processing. However, when machining aluminum parts, due to the light, soft and difficult chip removal, sometimes the discharge point is transferred from the processing area to the feed block after the short circuit, resulting in spark discharge between the feed block and the molybdenum wire. Therefore, although the workpiece and the electrode wire are in the short circuit state, the discharge state of the machine tool is normal, so the machine tool will not realize short circuit protection due to the short circuit between the workpiece and the molybdenum wire, At this time, the machine tool is still cutting according to the normal procedure, and the workpiece can not be machined with normal shape and size due to no discharge at the processing place. Only the molybdenum wire is pulled out of the groove on the soft aluminum workpiece, resulting in the distortion of the cutting track and the scrapping of the workpiece. As shown in Figure 4, when machining the gear, the first two parts have the cutting trajectory distortion and the workpiece is scrapped

                                              Figure 4 Aluminum gear processing

2. solutions
(1) Firstly, the optimization of process parameters is to set different cutting parameters according to different thickness. Through many experiments, Guangdong Machinery Factory found that the pulse width can be reduced properly by processing aluminum alloy materials. This is because the pulse width decreases, and the energy of single pulse discharge is reduced, the discharge mark is also small, and the size and quantity of alumina particles can be reduced and the wear of the feed block can be effectively reduced. The increase of pulse gap is helpful to chip removal, reduce wire sticking, improve cutting stability and improve the surface roughness of workpiece. If the pulse gap is too small, the discharge product can not be eliminated, and the discharge gap is not able to eliminate ionization, which will make the processing unstable and easily cause wire breakage or cutting track distortion. If the feed speed is less than the actual possible etching speed of the workpiece, the machining state will open and the cutting speed is slow. Because of the low melting point and vaporization point of aluminum alloy, the machining erosion of the same discharge energy increases, which makes the discharge gap large. Because of the large discharge gap, the pulse voltage can not break through the liquid medium between the electrodes in time, which greatly reduces the utilization rate of the pulse. At the same time, the electrode wire amplitude increases due to the too large distance between the electrodes, which makes the processing unstable and even leads to wire breakage.
When the setting feed speed is greater than the actual possible etching speed of the workpiece (called over tracking or over feed), it is easy to short circuit during processing, and the actual feed and cutting speed also decrease. Moreover, the electrical corrosion materials excluding aluminum materials are not used, resulting in wire breaking and short circuit suffocation. Therefore, the feed of frequency conversion is adjusted reasonably to achieve a better processing state. The whole frequency conversion feed control circuit has several adjustment links, most of which are installed in the machine control cabinet, and should not be changed generally. Another adjusting knob is installed on the console operation panel. During processing, the knob can be set in the proper position according to the specific conditions to ensure the reading of ammeter and voltmeter is stable, the molybdenum wire jitter is small and the processing is in the best tracking state. If the thick workpiece is processed, when the thick workpiece is processed (5mm), because the electrode wire is easy to shake and the concentration of coolant is high, the energy of a single pulse should be added to increase the pulse interval. At least the pulse width to pulse interval ratio is (1:8), which mainly ensures that there is enough single pulse energy and enough time to eliminate the electrical corrosion products, and reduce the processing current, Generally, it is less than 2A. When the processing is stable, the pulse interval is reduced accordingly, which will increase the processing current by about 3a and the voltage of 75V. The aim is to increase the average value of the current carrying capacity of molybdenum wire, and to form the spark discharge ability without increasing the explosive force of the spark. Therefore, the electrical parameters should be taken larger properly, otherwise the machining will be unstable, the quality of the processed parts will be reduced, the requirements of cutting speed and surface roughness are contradictory two technological indicators. Therefore, the high cutting speed must be pursued on the premise of meeting the surface roughness.
(2) The discharge gap of the discharge gap should not be too small according to the thickness of the workpiece, otherwise it is easy to produce short circuit and is not conducive to the cooling and discharge of the electrical corrosion; The excessive discharge gap will affect the surface roughness and processing speed. When cutting the workpiece with a large thickness, molybdenum wire with larger diameter and large pulse width current should be selected as far as possible to increase the discharge gap, enhance the chip removal effect and improve the stability of cutting.
(3) Keep the cleanness of the coolant and replace the new coolant in time. Filter the impurities of the coolant with a filter screen at the return port of the working table coolant, and then place a layer of sponge at the outlet of the coolant to absorb the impurities of the coolant. The effect is very good. In order to improve the chip discharge capacity and prevent the abnormal processing track, detergent and soap block can be added to the coolant, so that the washing performance is better and the chip discharge capacity is increased, and the chip discharge state is improved. At the same time, it is necessary to rotate and adjust the position of feed block timely or replace a new feeder block, clean up the chip particles at the feed block frequently, keep the good contact state between molybdenum wire and feed block, avoid discharge phenomenon, and effectively avoid cutting track distortion and cause scrap of work piece.
4、 Conclusion
Based on the practical operation of wire cutting, Guangdong machining factory analyzes the phenomena of aluminum oxide particles, which lead to the decrease of conductivity, abrasion of feed block, wire breaking and short circuit, etc. in the process of aluminum parts, which seriously affect the quality and efficiency of processing, and summarizes the problems and solutions that often appear in practice. These methods and measures are feasible, which have good effect on improving the quality of wire cutting, reducing wire breaking, short circuit and distortion of cutting track of aluminum parts.