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reduce deformation for aluminum alloy parts with CNC machining 2017-07-31

As the aluminum alloy parts make from aluminum casting or forging, of the thermal expansion coefficient of large, thin-walled process is easy to deformation. Especially in the use of free forging rough, while more machining allowance, deformation is more prominent.

1. aluminum alloy parts deformation reasons

Aluminum alloy parts for many reasons for deformation, and material, parts shape, production conditions, cutting fluid performance and so have a relationship. Mainly has the following aspects: rough deformation caused by roughness, cutting force, deformation caused by cutting heat, clamping force caused by deformation.

2 to reduce the processing of deformation of the process measures

(1) to reduce the internal stress of the blank

The use of natural or artificial aging and vibration treatment, can be part of the blank to eliminate the internal stress. Pre-processing is also an effective process. On the fat head of the rough, due to the margin, so after processing deformation is also large. If the excess part of the blank is machined in advance, the margin of each part is reduced, not only the processing deformation of the subsequent process can be reduced, but also a part of the internal stress can be released after being pre-processed for a period of time.

(2) to improve the cutting capacity of the tool

Tool material, geometric parameters on the cutting force, cutting heat has an important impact, the correct choice of tools, to reduce the deformation of parts is essential.

1) Reasonable selection of tool geometry.

Front angle: to maintain the strength of the blade under the conditions of the rake angle appropriate to choose a larger, on the one hand can grind sharp edge, the other can reduce the cutting deformation, so that chip removal, and thus reduce the cutting force and cutting temperature. Avoid using negative rake tool.

Rear angle: The horn angle has a direct effect on the flank wear and the machined surface quality. The cutting thickness is an important condition for selecting the rear corner. Roughing, due to the large feed, cutting load weight, heat a large amount of requirements, the tool cooling conditions are good, so the horn should be selected smaller. Fine milling, the requirements of sharp edges, reduce the flank and the surface of the friction, reduce the elastic deformation, therefore, the horn should be selected larger.

Helix angle: To make the milling smooth and reduce the milling force, the helix angle should be chosen as large as possible.

The main declination: the appropriate reduction of the main declination can improve the heat dissipation conditions, so that the average temperature of the processing area decreased.

2) to improve the tool structure.

Reduce the number of milling cutter teeth, increase the chip space. Due to the large plasticity of the aluminum alloy material, the machining deformation is larger and the larger chip space is needed. Therefore, the radius of the chip bottom should be larger and the number of milling cutter teeth is less. For example, φ20mm below the milling cutter with two knife teeth; φ30-φ60mm cutter with three knife teeth is better to avoid the chip plug caused by thin-walled aluminum alloy parts deformation. The roughness value of the cutting edge is less than Ra = 0.4um. Before using the new knife, you should use fine stone in the knife before the teeth, gently grinding a few, to eliminate the blade when the teeth of the remaining burrs and slightly sawtooth. In this way, not only can reduce the cutting heat and cutting deformation is relatively small.

3) strict control of the tool wear standards:

After the tool wear, the workpiece surface roughness value increases, the cutting temperature increases, the workpiece deformation increases. Therefore, in addition to the use of good wear resistance of the tool material, the tool wear standards should not be greater than 0.2mm, or prone to BUE. Cutting, the workpiece temperature is generally not more than 100 ℃, to prevent deformation.

4) to improve the workpiece clamping method.

For less rigid thin-walled aluminum alloy workpieces, the following clamping method can be used to reduce deformation:

For thin-walled bushing parts, if you use a three-jaw self-centering chuck or spring chuck to clamp from the radial, once the workpiece is released, the workpiece must be deformed. At this point, should be used to improve the rigidity of the axial end of the method of compression. To the parts of the hole positioning, homemade a threaded thread through the shaft, into the parts of the hole, with a cover on the end of the face and then tighten the nut. Machining the outer circumference can avoid clamping deformation, so as to obtain satisfactory machining accuracy.

For thin-walled sheet workpieces, it is best to use a vacuum chuck to obtain a uniform distribution of clamping force, and then a smaller amount of cutting to the processing, can be very good to prevent deformation of the workpiece.

In addition, you can also use stuffing method. In order to increase the rigidity of the thin-walled workpiece, the medium can be filled with the medium to reduce the workpiece during the clamping and cutting process. For example, a urea melt containing 3% to 6% potassium nitrate is poured into the workpiece, and after processing, the workpiece can be immersed in water or alcohol.

5) reasonable arrangements for the process.

High-speed cutting, due to large processing margin and intermittent cutting, so the milling process often produce vibration, affecting the processing accuracy and surface roughness. Therefore, the NC high-speed machining process can be divided into; rough processing - semi-finishing - Qing angle processing - finishing and other processes. For parts with high precision requirements, it is sometimes necessary to perform secondary semi-finishing and then finish the finishing. After roughing, the parts can be naturally cooled to eliminate the internal stress generated by roughing and reduce the deformation. Roughing after leaving the margin should be greater than the amount of deformation, usually 1-2mm. Finishing, the parts of the finishing surface to maintain a uniform processing margin, generally 0.2-0.5mm is appropriate, so that the tool in the processing process in a stable state, can greatly reduce the cutting deformation, access to good surface processing quality, to ensure that Product accuracy.

3 operating skills

Aluminum alloy parts in the process of deformation, in addition to the above reasons, in practice, the operation method is also very important.

1) for the processing of large spare parts, in order to make it in the processing process have better heat dissipation conditions, to avoid heat concentration, processing, should adopt symmetrical processing. If a piece of 90mm thick sheet needs to be processed to 60mm, if the milling side immediately after milling the other side, a processing to the final size, the flatness of 5mm; if repeated use of symmetry processing, each side of the two processing The final size, can guarantee the flatness of 0.3mm.

2) If there are multiple cavities on the plate part, as shown in Figure 4. Processing, it is not appropriate to use a cavity of a cavity in the order of processing methods, which is likely to cause uneven parts of the workpiece deformation. Using layers of multiple processing, each layer as much as possible at the same time processing to all the cavity, and then processing the next level, so that parts even force, reduce deformation.

3) By cutting the amount of cutting to reduce the cutting force, cutting heat. In the three elements of the amount of cutting, the amount of knife back on the cutting force of a great impact. If the processing margin is too large, a knife cutting force is too large, not only will deformation of parts, but also affect the machine tool spindle stiffness, reduce tool durability. If you reduce the amount of back to eat knife, but also make the production efficiency greatly reduced. However, in CNC machining are high-speed milling, can overcome this problem. In reducing the amount of kneading knife at the same time, as long as the corresponding increase in feed, improve the speed of the machine, you can reduce the cutting force, while ensuring processing efficiency.

4) walking order should pay attention. Roughing emphasizes the improvement of processing efficiency, the pursuit of unit time within the removal rate, generally can be used reverse milling. That is, the fastest speed, the shortest time to cut off the rough surface of the excess material, the basic formation of the required geometric contour finishing. The finishing is emphasized by the high-precision high-quality, should adopt smooth milling. Because the cutting thickness of the knife teeth is gradually decreasing from the maximum to the zero, the degree of hardening of the work is greatly reduced, and the deformation degree of the parts is reduced.

5) Thin-walled workpieces are deformed during machining due to clamping, even if finishing is difficult to avoid. In order to minimize the deformation of the workpiece, it is possible to loosen the pressing member before the finish is about to reach the final dimension, so that the workpiece is free to return to its original state and then slightly pressed to allow the workpiece to be clamped With the feel), so you can get the desired processing effect. In short, the role of clamping force on the support surface of the best, the clamping force should be applied in the direction of good rigidity of the workpiece, to ensure that the workpiece is not loose under the premise of the smaller the better clamping force.

6) in the processing of cavity parts with the cavity, the processing cavity as much as possible not to let the cutter like a drill bit like a straight down into the parts, resulting in milling cutter chip space is not enough, chip is not smooth, resulting in parts overheating, expansion and collapse Knives, broken knives and other adverse phenomena. First with the cutter with the same size or large drill bit drill hole, and then milling cutter. Alternatively, you can use the CAM software to produce a spiral cutter program.

4 workpiece surface black

Aluminum oxide processing Aluminum alloy casting is generally made of metal type, metal aluminum and aluminum alloy has a good mobility and plasticity, but in the course of easy to black, because:

1) process design unreasonable. Aluminum alloy die castings in the cleaning or pressure inspection after improper handling, for the aluminum alloy die-casting mold to create the conditions of black, accelerated the formation of mildew.

2) Warehouse management is not in place. Aluminum alloy die castings stored in the warehouse at different heights, the moldy situation is also different.

3) internal factors of aluminum alloy. A lot of aluminum die casting manufacturers in the die-casting, machining processes, do not do any cleaning, or simply flush with water, can not be completely cleaned, die-casting aluminum surface residual mold release agent, cutting fluid, saponification and other corrosive Substances and other stains, these stains to speed up the aluminum alloy die casting slow mold point black speed.

4) External environmental factors of aluminum alloy. Aluminum is an active metal, in a certain temperature and humidity conditions easily oxidized black or moldy, which is determined by the characteristics of aluminum itself.

5) use cleaning agent should not be. Selection of cleaning agents with strong corrosive, resulting in corrosion corrosion of die-casting aluminum.

 

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