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cnc problems machining

2019-04-04

When CNC wire cutting aluminum parts are used, there are often problems such as serious wear of the feed block, short circuit, broken wire, distortion of the machining track and deformation of the workpiece, and some directly cause the scraped products to be scrapped. Based on the experience accumulated over many years of processing, the author summarizes the key technical issues in the processing practice.

First, the problem of wire-cut aluminum pieces easily broken

1. Reasons for wire-cut aluminum pieces that are easily broken

When the wire is cut aluminum, the material is relatively soft, the chip removal is difficult, and the aluminum is easy to form a hard oxide film at a high temperature, and a large amount of alumina or aluminum chips are easily processed to adhere to the molybdenum wire, so that the molybdenum wire and the feed block are formed. The contact area will soon be ground out of the deep groove. The soft aluminum and the hard abrasive grains are mixed together and stuffed in the grooves. Once brought in, the grooves are squeezed and the wires are broken.

2. Solution

In order to solve the problem that the molybdenum wire card pinches the molybdenum wire in the feeding block, adjust the position of the feeding block of the upper and lower wire frame to prevent the groove of the feeding block from being broken, resulting in low efficiency and high processing surface. Sub-cutting caused by quality degradation and material waste.

The thickness of the processed material exceeds 40mm. After the normal processing for 3~4h, the feed block is slightly rotated by an angle. As shown in Fig. 1b, first rotate to the direction shown in Fig. 1a, and then rotate the direction shown in Fig. 1b after 8h. After 8 hours of processing, the position of the feed block is adjusted by rotating 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 wire is generally 0.5 to 1 mm. When the three points of a face have cutting grooves, the feed block 90o is rotated, and so on until the four faces have cutting grooves. Then, by placing a 1~2mm spacer on the side close to the sampling line, the position of the feeding block can be adjusted to the left, which is equivalent to replacing a new feeding block, which saves production cost.



Figure 1 Schematic diagram of the installation orientation of the conductive block

For example, the thickness of each aluminum piece is 2mm, and a total of 8 pieces are stacked and processed at the same time. The contact surface of the feed block is adjusted every 8h, and it is processed continuously for 8 hours every day, and the broken wire appears on the sixth day. The diameter of the molybdenum wire was then measured using a micrometer to be only 0.125 mm (the original molybdenum wire diameter was 0.18 mm). By adjusting the position of the feed block, the problem of broken wire of the feed block is largely avoided.

Of course, it must also 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 should be modified in time to ensure the dimensional accuracy requirement.

Second, the problem of deformation of the workpiece

1. The cause of deformation of the workpiece

Due to the uneven heating, internal phase transformation, and deformation of the workpiece during the manufacturing, cutting and heat treatment of the workpiece blank, residual stress is generated inside the workpiece, and the stress distribution is relatively balanced under the influence of the outside world for a period of time, but During the online cutting process, since the workpiece material is cut and cut in a large amount, the stress distribution is changed, and gradually becomes balanced as time passes, thereby deforming the workpiece. This deformation is more pronounced for aluminum alloy parts.

2. Solution

(1) Before the cutting process, the stress-relieving parts are first heat-treated before the cutting process to eliminate the internal stress of the material, so that no large stress deformation occurs during cutting to stabilize the size. Of course, different materials are handled differently.

(2) Outer contour processing method Generally, the outer contour processing can be cut from the outside of the blank without piercing the wire hole. As shown in Fig. 2a, this method is very easy to cause large deformation due to the release of stress in the material after the blank is broken. , resulting in reduced machining accuracy of parts. In order to avoid and reduce the occurrence of such deformation, the method of piercing the wire hole can be used to maintain the sealing property of the blank contour, as shown in Fig. 2b, to minimize the stress deformation during the processing.

(a) outside cutting (b) piercing wire processing


Figure 2 Schematic diagram of the outer contour processing method

(3) It is better to use the secondary cutting method for the parts with higher processing precision requirements.

For example, when processing a bullet tail as shown in Fig. 3, a V-shaped iron is used to clamp the end of φ8 mm, and the other end is suspended. After the normal machining process is finished, it is measured with a micrometer, and the processing size is 2.00 mm near the V-shaped iron, and the processing size at the other end is only 1.86 mm. This is the process of the online cutting process, because the workpiece material is cut a lot to change its stress field distribution, so that the workpiece is deformed.


Figure 3 A bullet tail diagram

The improvement of the processing method is that the thickness left in the first processing is increased from 2 mm to 2.4 mm, and the first side is left with a residual amount of 0.2, and (when φ 0.18 mm molybdenum wire is used) roughing is performed rapidly. After the first cutting, the blank is destroyed by the internal stress balance state, and then a new balance is reached, and then a second finishing is performed. The measuring size is 1.99 mm at the end of the processing, and the size required for the test is achieved.

Third, aluminum parts are easy to short-circuit and the distortion of the cutting track

1. Analysis of the cause of the problem

In daily processing, the cutting of large thickness workpieces is difficult, because the workpiece is thick to a certain extent due to the erosion processing conditions, there is not enough coolant to enter the workpiece, the corrosion products in the gap can not be eliminated normally, and the processing is unstable. Until a short circuit occurs with no current discharge, this phenomenon is more likely to occur when cutting aluminum parts.

Normally, the wire cutting machine has a short-circuit protection function. Once a short circuit occurs, the machine table stops moving immediately and remains in place for waiting. However, in the processing of aluminum parts, due to the lightness, softness and difficulty of chip removal of aluminum, sometimes the discharge point is transferred from the processing area to the feed block after the short circuit, causing the feed block and the molybdenum wire to have a spark discharge, so despite the workpiece and The wire is short-circuited, but the machine discharge state is normal, so the machine will not be short-circuit protected by the short circuit between the workpiece and the molybdenum wire. At this time, the machine tool is still cut according to the normal procedure, and the workpiece cannot be processed because the machine is not discharged. The normal shape and size, only the molybdenum wire pulls out the groove on the soft aluminum workpiece, causing the cutting track to be distorted, resulting in the scrapping of the workpiece. When the gear is machined as shown in Fig. 4, the cutting of the cutting trajectory and the scrapping of the workpiece occur in the first two parts.

Img 7219

Figure 4 aluminum gear processing

2. The solution

(1) First, optimize the process parameters to set different wire cutting parameters according to different thicknesses. Through many experiments, it is found that the processing of the aluminum alloy material can appropriately reduce the pulse width, because the pulse width is reduced, the single pulse discharge energy is reduced, the discharge trace is also small, and the size and number of the alumina particles can be reduced. , effectively reducing the wear of the feed block. Relatively increasing the pulse gap facilitates chip removal, reduces sticking, improves cutting stability, and improves workpiece surface roughness. If the pulse gap is too small, the discharge product is too late to be eliminated, and the discharge gap is not enough to eliminate the ionization, which will make the processing unstable, and easily cause the broken wire or the cutting track distortion.

If the set feed speed is less than the actual possible erosion speed of the workpiece, the machining state will be open and the cutting speed will be slow. Moreover, since the melting point and the vaporization point of the aluminum alloy are low, the amount of etching erosion of the same discharge energy is increased, so that the discharge gap is large. Due to the large discharge gap, the pulse voltage can not penetrate the liquid medium between the poles in time, which greatly reduces the utilization of the pulse. At the same time, the distance between the poles is too large, which causes the amplitude of the wire to increase, which makes the processing unstable and even causes broken wires. . When the set feed speed is greater than the actual possible erosion speed of the workpiece (called over-tracking or over-feeding), the machining is easy to short-circuit, the actual feed and cutting speed are also reduced, and the electrolytic corrosion of the aluminum is not used. , causing broken wire and short circuit suffocation. Therefore, the frequency conversion feed is reasonably adjusted 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 should not be changed. Another adjustment knob is installed on the console operation panel. The knob can be set to the appropriate position according to the specific conditions during processing to ensure stable reading of the ammeter and voltmeter, and the molybdenum wire jitter is small, and the processing is in the best tracking state.

For example, when processing thick workpieces, the general condition (5mm) processing, because the electrode wire is easy to shake, the concentration of the coolant is high, the energy of a single pulse should be increased, and the interval of the pulse 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 time to eliminate the electrolytic corrosion products, while reducing the processing current, generally less than 2A. When the processing is stable, the pulse interval is further reduced, and the machining current is increased by about 3A and the voltage is about 75V. The purpose is to form the spark discharge capacity without increasing the average value of the molybdenum wire current carrying capacity, and the spark explosive force is enhanced. Therefore, the electrical parameters should be appropriately larger, otherwise the processing will be unstable and the quality of the machined parts will be degraded.

The requirements of cutting speed and surface roughness are two contradictory process indexes, so it is necessary to pursue a high cutting speed while satisfying the surface roughness.

(2) Select the appropriate discharge gap according to the thickness of the workpiece. The discharge gap should not be too small, otherwise it will easily cause short circuit, which is not conducive to cooling and discharge of electro-erosion. If the discharge gap is too large, it will affect the surface roughness and processing speed. When cutting a workpiece with a large thickness, the molybdenum wire with a large diameter and the large pulse width current should be selected as much as possible to increase the discharge gap, enhance the chip removal effect, and improve the stability of the cutting.

(3) Keep the coolant clean and replace the new coolant in time, and use the filter to filter the impurities in the coolant at the return port of the table coolant, and then put a sponge on the outlet of the coolant to absorb the coolant. The impurities are good. In order to improve the chip removal ability and prevent the processing track from being deformed, detergent and soap pieces can be added to the cooling liquid, so that the washing performance is improved, the chip discharging ability is increased, and the chip discharging state is improved. At the same time, it is necessary to rotate and adjust the position of the feed block in time or replace a new feed block, often clean the chip particles at the feed block, maintain a good contact state between the molybdenum wire and the feed block, and avoid discharge at that place. , 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 when processing aluminum parts, alumina particles are easily generated, resulting in decreased conductivity, wear of feed blocks, broken wires and short circuits, which seriously affect the quality and efficiency of processing. Summarize the problems and solutions that often occur in practice. These methods and measures are feasible and have good effects on improving the quality of wire cutting processing, reducing wire breakage, short circuit and processing distortion of aluminum parts.



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