Common problems and solutions when machining aluminum parts by CNC wire cutting

Wed Jun 08 16:03:10 CST 2022

Common problems and solutions when machining aluminum parts by CNC wire cutting

1. The problem of wire-cutting aluminum parts easy to break

1. Reasons for wire-cut aluminum parts easy to break

When wire cutting aluminum parts, because the material is relatively soft, chip removal is difficult, and aluminum is very easy to form a hard oxide film at high temperatures. A large amount of aluminum oxide or aluminum chips are easily adhered to the molybdenum wire, making the molybdenum wire and the feed block Deep grooves will soon be ground at the contact area. Soft aluminum and hard abrasive grains are mixed together and stuffed in the groove. Once brought in, the groove will be squeezed to death, and the wire will be jammed.

2. Solution

In view of the problem that the molybdenum wire is stuck in the feed block and the molybdenum wire is clipped, adjust the position of the feed block of the upper and lower wire racks to prevent the worn groove of the feed block from causing the clamping wire to break, resulting in low efficiency and multiple processing surfaces Quality degradation and material waste caused by sub-cutting.

The thickness of the processed material exceeds 40mm. Generally, after processing for 3 to 4 hours, rotate the feed block slightly by an angle, as shown in Figure 1b, first rotate to the direction shown in Figure 1a, and then rotate the direction shown in Figure 1b after 8 hours of processing. Rotate and adjust the position of the feed block after every 8 hours of processing to reduce the probability of wire breakage. The actual processing shows that this method greatly reduces the processing cost and improves the economic benefits. Note that the overvoltage of the feed block and the electrode wire is generally 0.5 to 1 mm. When there are cutting grooves at three points on one surface, rotate the feed block by 90°, and so on until there are cutting grooves on all four surfaces. Then put a shim of 1~2mm on the side close to the sampling line to adjust the position of the feed block to the left, which is equivalent to replacing the feed block with a new one, saving production costs.
For example: the thickness of each piece of aluminum is 2mm, and a total of 8 pieces are stacked together to be processed at the same time. The contact surface of the feed block is adjusted every 8h, and the wire is continuously processed for 8h every day. The wire breaks on the sixth day. Then use a micrometer to measure the diameter of the molybdenum wire is only 0.125mm (the diameter of the original molybdenum wire is 0.18mm). By adjusting the position of the feed block, the problem of wire breakage in the feed block is greatly avoided.

Of course, it must be noted that when the dimensional accuracy requirements of the workpiece are relatively high, the diameter of the molybdenum wire must be measured, and the compensation amount must be modified in time to ensure the dimensional accuracy requirements. Metal processing WeChat, the content is good and worthy of attention.

Second, the problem of deformation of the workpiece

1. The cause of deformation of the workpiece

Due to the uneven heating, internal structure phase transformation, force deformation and other effects of the workpiece blank during manufacturing, cutting and heat treatment, residual stresses are generated inside the workpiece. The stress distribution is relatively balanced within a period of time without external influence, but During the on-line cutting process, because the workpiece material is cut and cut in large quantities, its stress distribution will be changed, and it will gradually balance with the passage of time, which will cause the workpiece to deform. This deformation phenomenon is more obvious for aluminum alloy parts.

2. Solution

(1) Stress relief before cutting processing The parts are first heat treated to eliminate the internal stress of the material before cutting processing, so that there will be no large stress deformation during cutting to stabilize the size. Of course, different materials have different processing methods.

(2) Outer contour processing method In outer contour processing, it is usually possible to cut in from the outside of the blank without punching holes. As shown in Figure 2a, this method is very easy to cause large deformation due to the release of the internal stress of the material after the blank break , Resulting in reduced parts processing accuracy. In order to avoid and reduce the occurrence of this kind of deformation, the method of punching wire holes can be used to maintain the closedness of the blank contour, as shown in Figure 2b, which can minimize the stress and deformation during the processing.

(3) Adopt secondary cutting method. For parts with higher processing accuracy requirements, it is best to use secondary cutting method.

For example, when processing the tail wing of a bullet as shown in Figure 3, a V-shaped iron is used to clamp the φ8mm end, while the other end is suspended. After finishing the processing according to the normal processing technology, measuring with a micrometer, the processing size near the V-shaped iron is 2.00mm, and the processing size at the other end is only 1.86mm. This is during the on-line cutting process, because the workpiece material is cut to change its stress field distribution, so that the workpiece is deformed.
The improved method of processing is to increase the thickness left during the first processing from 2mm to 2.4mm, and leave 0.2 allowance on one side during the first cutting, (when using φ0.18mm molybdenum wire) roughing can be carried out quickly. After the first cutting, the blank is damaged from the original internal stress balance state, and then reaches a new balance, and then performs a second finishing process. The measured size at the end of the process is 1.99mm, which reaches the size required by the test.

Three, aluminum parts are easy to short-circuit when cutting and the problem of cutting track distortion

1. Analysis of the cause of the problem

In daily processing, it is difficult to cut large-thickness workpieces. Due to the restriction of the erosion conditions of electrical discharge machining, the workpiece is thick to a certain extent, and there is not enough coolant to enter the workpiece. The electrical corrosion products in the gap cannot be removed normally, and the processing is very unstable. , Until there is a current without discharge short circuit occurs, this phenomenon is more likely to occur when cutting aluminum parts.

Under normal circumstances, the WEDM machine tool has a short-circuit protection function. Once a short-circuit occurs, the machine tool table will immediately stop moving and remain in place for processing. However, when processing aluminum parts, due to the light weight, softness and difficulty of chip removal, sometimes the discharge point is transferred from the processing area to the feed block after a short circuit, causing spark discharge between the feed block and the molybdenum wire. The electrode wires are in a short-circuit state, but the discharge state of the machine tool is normal. Therefore, 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 will still cut and process according to the normal procedure. In normal shape and size, only molybdenum wire pulls grooves on the softer aluminum workpiece, which causes the cutting track to be distorted and the workpiece is scrapped. When processing gears as shown in Figure 4, the first two pieces were processed with distortion of the cutting trajectory and the workpiece was scrapped.

2. Solution

(1) The first is to optimize the process parameters and set different wire cutting parameters according to different thicknesses. Through many experiments, it is found that processing aluminum alloy materials can appropriately reduce the pulse width. This is because the pulse width is reduced, which reduces the discharge energy of a single pulse, and the discharge trace is also small, and the size and number of alumina particles can also be reduced. , Effectively reduce the wear of the feed block. Relatively increasing the pulse gap is conducive to chip removal, reducing wire sticking, improving cutting stability, and improving the surface roughness of the workpiece. If the pulse gap is too small, the discharge products are too late to be eliminated, and the discharge gap is too late to deionize, which will make the processing unstable and easily cause wire breakage or distortion of the cutting track.

The set feed rate is lower than the actual possible erosion speed of the workpiece, the processing state will be open, and the cutting speed will be slow. And because of the low melting point and vaporization point of aluminum alloy, the machining erosion amount of the same discharge energy is increased, so that the discharge gap is large. Due to the large discharge gap, the pulse voltage cannot break down the liquid medium between the electrodes in time, which greatly reduces the utilization of the pulse. At the same time, too large a distance between the electrodes will increase the electrode wire amplitude, make the processing unstable, and even cause wire breakage. . When the set feed speed is greater than the actual possible erosion 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 will decrease instead, and it does not use the electro-erosion material to exclude aluminum. , Causing broken wires and short-circuit suffocation. Therefore, reasonably adjust the frequency conversion feed to achieve a better processing state. The entire variable frequency feed control circuit has multiple adjustment links, most of which are installed inside the machine control cabinet and generally should not be changed. There is also an adjustment knob installed on the console operation panel. This knob can be set in a suitable position according to the specific conditions during processing to ensure stable readings of the ammeter and voltmeter, the molybdenum wire shakes little, and the processing is in the best tracking state.

For example, when processing thick workpieces, generally at the beginning of (5mm) processing, because the electrode wire is easy to shake and the coolant concentration is high, the energy of a single pulse should be increased, and the pulse interval should be increased. At least the ratio of pulse width to pulse interval is (1 : 8) This is mainly to ensure that there is enough single pulse energy and enough interval time to remove the electro-erosion products, while reducing the processing current, generally below 2A. When the processing is stable, the pulse interval is reduced accordingly, and the processing current is increased by about 3A and the voltage is about 75V. The purpose is that the average value of the current carrying capacity of the molybdenum wire can form a spark discharge without increasing, and the explosive power of the spark can be enhanced. Therefore, the electrical parameters should be appropriately larger, otherwise the processing will be unstable and the quality of the processed parts will decrease.

The requirements of cutting speed and surface roughness are two contradictory process indicators. Therefore, high cutting speed must be pursued under the premise of satisfying surface roughness.

(2) Choose the appropriate discharge gap according to the thickness of the workpiece. The discharge gap should not be too small, otherwise it will easily cause a short circuit, which is not conducive to cooling and the discharge of electrical corrosion. If the discharge gap is too large, it will affect the surface roughness and processing speed. When cutting workpieces with larger 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) Maintain the cleanliness of the coolant and replace it with new coolant in time, and use a filter at the return port of the workbench to filter the impurities of the coolant, and then put a sponge at the outlet of the coolant to absorb the coolant. The effect is very good. In order to improve the chip removal ability and prevent the processing path from being deformed, detergent and soap bars can also be added to the coolant, so that the washing performance becomes better, the chip removal ability is increased, and the chip removal state is improved. At the same time, it is necessary to rotate and adjust the position of the power supply block in time or replace it with a new one. Frequently clean the chip particles at the power supply block, maintain a good contact state between the molybdenum wire and the power supply block, and avoid discharge phenomenon there. , It can effectively avoid the distortion of the cutting track and cause the workpiece to be scrapped.

Fourth, the conclusion

Starting from the actual operation of wire cutting, it is analyzed that aluminum oxide particles are easily generated when processing aluminum parts, which causes the conductivity to decrease, the feed block wears, and the broken wires and short circuits occur, which seriously affect the processing quality and efficiency. Summarizes the problems and solutions that often occur in practice. These methods and measures are practical and have good effects on improving the quality of wire cutting, reducing wire breakage, short-circuiting, and cutting track distortion of processed aluminum parts.