In the aircraft structure, in order to reduce the weight, a large number of thin-walled parts of aluminum alloy material are used, and since the aluminum alloy part material has a large thermal expansion coefficient, it is easily deformed during the thin-wall processing. Especially when using free forging blanks, the machining allowance is large and the deformation problem is more prominent.
There are many reasons for the deformation of aluminum alloy parts, which are related to the material, the shape of the part, the production conditions, and the performance of the cutting fluid.
The main causes of deformation are as follows:
变形 Deformation caused by internal stress of the blank
变形 Deformation caused by cutting force
变形 Deformation caused by cutting heat
变形 Deformation caused by clamping force
Therefore, measures to reduce processing distortion during processing are particularly important.
Measures to reduce processing distortion mainly include the following aspects:
降低 Reduce the internal stress of the blank
改善 Improve the cutting ability of the tool
合理 Reasonable selection of tool geometry parameters
改善 Improve tool structure
改善 Improve the clamping method of the workpiece
合理 Arrange the process reasonably
Reducing the internal stress of the blank The internal stress of the blank can be partially eliminated by natural or artificial aging and vibration treatment. Pre-processing is also an effective process. For the blank of the big head of the fat head, due to the large margin, the deformation after processing is also large. If the excess part of the blank is processed in advance and the remaining amount 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 and placed for a certain period of time.
Improve the cutting ability of the tool The material and geometric parameters of the tool have an important influence on the cutting force and the cutting heat. The correct selection of the tool is essential to reduce the deformation of the part.
(1) Reasonable selection of tool geometry parameters.
1 Front angle: Under the condition of maintaining the strength of the cutting edge, the front angle is appropriately selected. On the one hand, the sharp cutting edge can be ground, and the cutting deformation can be reduced to make the chip removal smooth, thereby reducing the cutting force and the cutting temperature. Never use a negative rake cutter.
2 Back angle: The size of the back angle has a direct influence on the flank wear and the quality of the machined surface. Cutting thickness is an important condition for selecting the back angle. In roughing, due to the large feed rate, heavy cutting load, large heat generation, and good heat dissipation conditions of the tool, the back angle should be smaller. When finishing milling, the edge is required to be sharp, the friction between the flank and the machined surface is reduced, and the elastic deformation is reduced. Therefore, the relief angle should be selected to be larger.
3 helix angle: In order to make the milling smooth and reduce the milling force, the helix angle should be as large as possible.
4 lead angle: Properly reducing the lead angle can improve the heat dissipation condition and reduce the average temperature of the processing area.
(2) Improve the tool structure.
1 Reduce the number of milling cutter teeth and increase the chip space. Due to the large plasticity of the aluminum material, the cutting deformation during processing is large, and a large space for chipping is required. Therefore, the bottom radius of the chip groove should be large, and the number of teeth of the milling cutter is small.
2 fine grinding teeth. The roughness value of the cutting edge of the cutter is less than Ra = 0.4 um. Before using a new knife, you should use a fine stone to grind gently in front of and behind the teeth to eliminate burrs and slight zigzag remaining when sharpening the teeth. In this way, not only the cutting heat can be reduced but also the cutting deformation is relatively small.
3 Strictly control the wear standard of the tool. After the tool wears, the surface roughness of the workpiece increases, the cutting temperature increases, and the workpiece deformation increases. Therefore, in addition to the selection of high-abrasion tool materials, the tool wear standard should not be greater than 0.2mm, otherwise it will easily lead to built-up edge. When cutting, the temperature of the workpiece should not exceed 100 °C to prevent deformation.
Improved method of clamping workpieces For thin-walled aluminum workpieces with poor rigidity, the following clamping methods can be used to reduce distortion:
1 For thin-walled bushing parts, if the three-claw self-centering chuck or the collet chuck is used to clamp from the radial direction, once the workpiece is loosened after machining, the workpiece is inevitably deformed. At this time, a method of pressing the axial end face with good rigidity should be utilized. To position the inner hole of the part, a threaded threading mandrel is made and inserted into the inner hole of the part, and a cover plate is pressed against the end surface and then tightened with a nut. When the outer circle is machined, the clamping deformation can be avoided, and satisfactory machining accuracy can be obtained.
2 When processing thin-walled thin-plate workpieces, it is best to use vacuum suction cups to obtain a uniform distribution of clamping force, and then process with a small amount of cutting, which can prevent deformation of the workpiece well.
In addition, a packing method can also be used. In order to increase the process rigidity of the thin-walled workpiece, the medium can be filled inside the workpiece to reduce the deformation of the workpiece during the clamping and cutting process. For example, a urea melt containing 3% to 6% of potassium nitrate is poured into the workpiece, and after the processing, the workpiece is immersed in water or alcohol, and the filler can be dissolved and poured out.
Reasonable Arrangement Process During high-speed cutting, due to large machining allowance and intermittent cutting, the milling process often produces vibration, which affects machining accuracy and surface roughness. Therefore, the numerical control high-speed machining process can be generally divided into: roughing-semi-finishing-clearing-finishing-finishing. For parts with high precision requirements, it is sometimes necessary to perform secondary semi-finishing before finishing. After roughing, the parts can be naturally cooled, eliminating internal stresses caused by roughing and reducing distortion. The margin left after roughing should be greater than the amount of deformation, typically 1 to 2 mm. When finishing, the finished surface of the part should maintain a uniform machining allowance, generally 0.2~0.5mm, so that the tool is in a stable state during the machining process, which can greatly reduce the cutting deformation and obtain good surface processing quality. Product accuracy.
Parts of the aluminum material are deformed during processing. In addition to the above reasons, the operation method is also very important in actual operation.
For parts with large machining allowance, in order to make them have better heat dissipation conditions during processing, and avoid heat concentration, symmetrical machining should be used during processing. If a piece of 90mm thick material needs to be machined to 60mm, if the other side is milled, the other side will be milled, and the flatness will be 5mm once. If it is processed by repeated infeed, each side will be processed twice. The final size guarantees a flatness of 0.3mm.
There are multiple cavities on the plate parts. When processing, it is not advisable to use a cavity and a cavity ordering method, which is easy to cause uneven deformation of the parts. Multi-layer processing is used, each layer is processed into all the cavities at the same time, and then the next layer is processed to make the parts evenly stressed and reduce the deformation.
Reduce cutting force and cutting heat by changing the amount of cutting. Among the three factors of cutting amount, the amount of backing knife has a great influence on the cutting force. If the machining allowance is too large, the cutting force of one pass will be too large, which will not only deform the parts, but also affect the rigidity of the machine spindle and reduce the durability of the tool. If you reduce the amount of back-to-back knives, it will greatly reduce production efficiency. However, high-speed milling in CNC machining can overcome this problem. While reducing the amount of back-feeding, 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 ensured.
The order of the cutting should also pay attention to the rough processing emphasizes the improvement of processing efficiency, the pursuit of resection rate per unit time, generally can use up-cut milling. 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 finishing work emphasizes high precision and high quality, and it is recommended to use down milling. Because the cutting thickness of the cutters gradually decreases from the maximum to zero during the milling, the degree of work hardening is greatly reduced, and the degree of deformation of the parts is alleviated.
Problems with the pressing parts Thin-walled workpieces are deformed by the clamping during processing, even if finishing is difficult to avoid. In order to minimize the deformation of the workpiece, the pressing piece can be loosened before the finishing is reached to the final size, so that the workpiece can be freely restored to its original shape, and then slightly pressed, just to clamp the workpiece. According to the feel, this can achieve the desired processing results. In short, the point of application of the clamping force is preferably on the bearing surface, and the clamping force should be applied to the direction of the rigidity of the workpiece. Under the premise of ensuring that the workpiece is not loose, the clamping force is as small as possible.
When machining a cavity with a cavity part, try not to let the milling cutter directly plung into the part like a drill bit, resulting in insufficient space for the milling cutter, and the chip removal is not smooth, causing overheating, expansion, and chipping and breaking of the part. Unfavorable phenomena such as knives. First drill the hole with a drill of the same size or larger size as the milling cutter, and then mill with a milling cutter. Alternatively, the CAM software can be used to produce a spiral undercut program.
Blackening of the workpiece
Aluminum is an active metal, which is easily oxidized and blackened or moldy under certain temperature and humidity conditions. This is determined by the characteristics of aluminum itself. The cleaning agent selected is highly corrosive, causing corrosion corrosion of die-cast aluminum.