As we all know, aluminum alloy is an important industrial raw material. In aircraft structures, in order to reduce weight, a large number of thin-walled parts made of aluminum alloy are used. Due to their relatively low hardness and large thermal expansion coefficient, they are prone to deformation during machining of thin-walled and thin-plate parts. From the perspective of machining technology, some measures can also be taken to minimize the machining deformation of the material. There are many reasons for the deformation of aluminum alloy parts. In addition to improving the performance of the tool and using the aging treatment to eliminate the internal stress of the material, it is related to the choice of material, part shape, production conditions, cutting fluid performance, and machining method.

It can be seen that the choice of materials to eliminate internal stress is the first factor to reduce machining deformation, but it is not the determining factor. Because the deformation of aluminum alloy machining can be divided into many factors, not all machining deformation is caused by aluminum alloy raw materials. There are other factors, such as deformation caused by cutting force, deformation caused by cutting heat, deformation caused by clamping force, and so on.

    Therefore, during the machining, measures to reduce machining distortion are particularly important. Such as improving the cutting ability of the tool, rationally selecting the geometric parameters of the tool, improving the tool structure, improving the clamping method of the workpiece, and using the machining method reasonably. Today, I will focus on exploring the methods of reasonable use of machining.

    1.the symmetrical machining method

    For aluminum alloy parts with large machining allowances, in order to create better heat dissipation conditions and reduce thermal deformation, it is necessary to avoid excessive concentration of heat as much as possible. The method that can be adopted is symmetrical machining. For example, there is a 90 mm thick aluminum alloy plate that needs to be milled to 60 mm thick. If one side is milled and the other side is milled immediately after milling, because each side is processed once to the final size, it is continuously processed. A large margin will cause the problem of heat concentration. In this way, the flatness of the milled aluminum alloy plate can only reach 5 mm. If a symmetrical machining method with repeated feeds on both sides is used, each side is processed at least twice until the final size is reached, which is conducive to heat dissipation and the flatness can be controlled at 0.3 mm.

    2.layered multiple machining method

    When multiple cavities need to be processed on aluminum alloy plate parts, if one cavity and one cavity are sequentially processed, it is easy to cause the cavity wall to be entangled and deformed due to uneven force. The best solution is to adopt the multi-layer machining method, that is, all the cavities are processed at the same time, but not in one process, but in several layers, and processed to the required size one by one. In this way, the force of the part will be more uniform, and the chance of deformation is smaller.

    3.drill first and then milling

    When machining a part with a cavity, if a milling cutter is used to directly pierce the part down, the chip will not be discharged smoothly due to insufficient chip space, which will cause the part to accumulate a large amount of cutting heat and expand and deform. There may be accidents such as chipping or breaking the knife. The best method is to drill first and then mill, that is, first use a drill with a size not smaller than the milling cutter to drill the hole, and then use the milling cutter to extend into the hole to start milling. This can effectively solve the problems mentioned above.

    4.Improve the cutting ability of the tool

    The material and geometric parameters of the tool have an important impact on the cutting force and cutting heat. Correct selection of the tool plays a vital role in reducing machining distortion of the part.

    [1] Reasonable selection of tool geometry

    Front angle: Under the condition of maintaining the strength of the cutting edge, choose a larger front angle. On the one hand, it can sharpen the sharp edge, and on the other hand, it can reduce cutting deformation and smooth chip removal, thereby reducing cutting force and temperature. Never use a negative rake tool.

    Rake angle: The size of the rake angle has a direct effect on the flank wear and the quality of the machined surface. Cutting thickness is an important condition for selecting the relief angle. During rough milling, due to the large feed amount, heavy cutting load and large heat generation, good heat dissipation conditions of the tool are required, so the back angle should be selected smaller. During fine milling, sharp edges are required to reduce the friction between the flank and the machined surface and reduce elastic deformation. Therefore, the rake angle should be larger.

    Helix angle: In order to make the milling smooth and reduce the milling force, the helix angle should be selected as large as possible.

    Main declination: Properly reducing the main declination can improve the heat dissipation conditions and reduce the average temperature in the machining area.

    [2] Improved tool structure

    Reduce the number of cutter teeth and increase the chip space. Due to the greater plasticity of the aluminum material, the cutting deformation during machining is large, and a large chip space is required. Therefore, the bottom radius of the chip groove should be larger, and the number of milling cutter teeth is better.

    Fine grinding knife teeth. The roughness value of the cutting edge of the tooth is smaller than Ra = 0.4um. Before using a new knife, you should use a fine whetstone to grind it a few times in front of and behind the teeth to eliminate burrs and slight jagged marks remaining when sharpening the teeth. In this way, not only the cutting heat can be reduced, but also the cutting deformation is relatively small.

    Strict control of tool wear standards. After the tool is worn, the workpiece surface roughness value increases, the cutting temperature increases, and the workpiece deformation increases. Therefore, in addition to the selection of tool materials with good abrasion resistance, tool wear standards should not be greater than 0.2mm, otherwise it is easy to produce chippings. When cutting, the temperature of the workpiece should generally not exceed 100 ° C to prevent deformation.

    5.Improve the clamping method of the workpiece.

    For thin-walled aluminum workpieces with poor rigidity, the following clamping methods can be used to reduce deformation:

    [1] For thin-walled bushings, if a three-jaw self-centering chuck or collet is used to clamp it from the radial direction, the workpiece will be deformed once it is loosened after machining. At this time, the method of pressing the axial end face with good rigidity should be used. Based on the positioning of the inner hole of the part, a self-made threaded mandrel is made, and the inner hole of the part is sleeved. A cover plate is used to press the end surface and then a nut is used to back it. When machining the outer circle, clamping deformation can be avoided, so as to obtain satisfactory machining accuracy.

    [2] When machining thin-walled and thin-plate workpieces, it is best to use a vacuum chuck to obtain a uniform clamping force, and then process it with a small amount of cutting, which can prevent the workpiece from deforming well.

    Alternatively, a padding method can be used. In order to increase the rigidity of the thin-walled workpiece, a medium can be filled inside the workpiece to reduce the deformation of the workpiece during clamping and cutting. For example, a urea melt containing 3% to 6% potassium nitrate is poured into a workpiece, and after machining, the workpiece is immersed in water or alcohol to dissolve and pour the filler.

     6.You can pay attention to the operation skills

    In addition to the reasons mentioned above, parts of aluminum parts are deformed during machining. In actual operation, the operation method is also very important.

    [1] For parts with large machining allowances: in order to have better heat dissipation conditions during machining and avoid heat concentration, symmetrical machining should be used during machining. If a 90mm thick sheet needs to be machined to 60mm, if one side is milled, the other side is milled immediately, and the flatness is 5mm once; if repeated feeding is used for symmetrical machining, each side is processed twice. The final size can guarantee flatness of 0.3mm.

    [2] There are multiple cavities on plate parts. When machining, it is not appropriate to use the sequential machining method of one cavity and one cavity. This will easily cause uneven parts to be deformed. Multi-layer machining is used, and each layer is processed to all the cavities at the same time as far as possible, and then the next layer is processed to uniformly force the parts and reduce deformation.

    [3] Reduce cutting force and cutting heat by changing the amount of cutting. Among the three factors of cutting consumption, the amount of cutting back has a great effect on cutting force. If the machining allowance is too large, the cutting force in one pass will not only deform the part, but also affect the rigidity of the machine tool spindle and reduce the durability of the tool. If you reduce the number of back-knives, production efficiency will be greatly reduced. However, high-speed milling is used in NC machining to overcome this problem. While reducing the amount of back knife, as long as the feed is increased accordingly and the speed of the machine tool is increased, the cutting force can be reduced and the machining efficiency can be guaranteed.

    [4] The order of cutting should also pay attention to rough machining. The emphasis is on improving machining efficiency and pursuing the resection rate per unit time. Generally, reverse milling can be used. That is to remove the excess material on the surface of the blank at the fastest speed and the shortest time, basically forming the geometric contour required for finishing. The emphasis on precision machining is high precision and high quality, and down milling should be used. Because the cutting thickness of the cutter teeth gradually decreases from maximum to zero during down milling, the degree of work hardening is greatly reduced, and the degree of deformation of the part is also reduced.

    [5] The problem of pressing parts: The thin-walled workpiece is deformed due to the clamping during machining. In order to reduce the deformation of the workpiece to a minimum, you can loosen the pressing part just before the final machining reaches the final size, so that the workpiece can be freely restored to its original state, and then slightly compressed, which can only be grasped by the workpiece (completely By feel), this can get the ideal machining effect. In short, the action point of the clamping force is best on the support surface. The clamping force should be applied in the direction of the workpiece with good rigidity. On the premise of ensuring that the workpiece is not loose, the smaller the clamping force, the better.

    [6] When machining a cavity part, try not to let the milling cutter directly drill down into the part like a drill, resulting in insufficient milling chip space, chip removal, and overheating, expansion and chipping Knife, broken knife and other adverse phenomena. You must first use a drill bit of the same size or larger as the milling cutter to drill the hole, and then use the milling cutter to mill.