Everyone knows that in the aviation field, in order to reduce the weight of the aviation parts themselves, aluminum alloy materials will be used in large quantities. However, in terms of aluminum alloy precision machining, due to the relatively large expansion coefficient of the material, deformation is prone to occur during thin-wall machining, especially when free forging blanks are used, the machining allowance is large, and the deformation problem becomes more prominent. NS.
One, the cause of processing deformation
There are actually many reasons for the deformation of aluminum alloy parts, which are related to the material, the shape of the part, and the various conditions of production, such as the performance of the cutting fluid. To sum up, it roughly includes the following points, that is, the internal stress deformation of the blank, the cutting force, the cutting heat and the deformation caused by the clamping.
2. Process measures that should be formulated to reduce processing distortion
1, to reduce the internal stress of the blank
We can use natural or artificial aging and vibration treatment, which can partially eliminate the internal stress of the blank. Pre-processing is also an effective process method. Compared with the larger blank, the deformation after processing is also large due to the large margin. If we pre-process the excess part of the blank and reduce the margin of each part, not only can the processing deformation of the subsequent process be reduced, but also a part of the internal stress can be released after pre-processing for a period of time.
2, can improve the cutting ability of the tool
The material and geometric parameters of the tool have an important influence on the cutting force and cutting heat. The correct selection of the tool is essential to reduce the deformation of the part.
①Reasonably choose tool geometry parameters
Rake angle: Under the condition of maintaining the strength of the cutting edge, choose a larger rake angle. On the one hand, it can grind a sharp edge, and it can also reduce cutting deformation and make chip removal more smooth, thereby reducing cutting force and cutting heat. Never use negative rake angle tools.
Relief angle: The size of the relief angle has a direct effect on the wear of the flank surface 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 rate, heavy cutting load, and large heat generation, good heat dissipation conditions of the tool are required. Therefore, the relief angle should be selected smaller. When finishing milling, the cutting edge is required to be sharp, reduce the friction between the flank face and the machined surface, and reduce the elastic deformation. Therefore, the relief angle should be selected 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.
Entering angle: Appropriately reducing the entering angle can effectively improve the heat dissipation conditions and reduce the average temperature of the processing area.
②Improve the tool structure
Reduce the number of teeth of the milling cutter and increase the chip space. As aluminum alloy material has greater plasticity, larger cutting deformation during machining, and larger chip holding space, so the bottom radius of the chip pocket should be larger and the number of milling cutter teeth should be smaller. For example, milling cutters below φ20mm use two teeth; milling cutters with φ30-φ60mm are better to use three teeth to avoid deformation of thin-walled aluminum alloy parts caused by chip clogging.
Fine grinding teeth: The roughness value of the cutting edge of the teeth should be less than Ra=0.4um. Before using a new knife, you should lightly grind the front and back of the teeth with a fine oil stone to eliminate the residual burrs and slight serrations when sharpening the teeth. In this way, not only the cutting heat can be reduced, but also the cutting deformation is relatively small.
Strictly control the wear standard of the tool: After the tool wears, the surface roughness value of the workpiece increases, the cutting temperature rises, and the deformation of the workpiece increases. Therefore, in addition to the selection of tool materials with good wear resistance, the tool wear standard should not be greater than 0.2mm, otherwise it is easy to produce built-up edge. When cutting, the temperature of the workpiece should generally not exceed 100°C to prevent deformation.
③Improve the clamping method of the workpiece
For thin-walled aluminum alloy workpieces with poor rigidity, the following clamping methods can be used to reduce deformation:
For thin-walled bushing parts, if a three-jaw self-centering chuck or spring chuck is used to clamp from the radial direction, once it is loosened after processing, the workpiece will inevitably be deformed. At this time, the method of pressing the axial end face with better rigidity should be used. Use the inner hole of the part to locate, make a self-made threaded mandrel, sleeve it into the inner hole of the part, and use a cover plate to press the end face on it, and then tighten it with a nut. When machining the outer circle, clamping deformation can be avoided, so that satisfactory machining accuracy can be obtained.
When processing thin-walled and thin-plate workpieces, vacuum suction cups are used to obtain evenly distributed clamping force, and then processed with a smaller cutting amount, which can well prevent the deformation of the workpiece.
In addition, a stuffing method can also be used. In order to increase the process rigidity of thin-walled workpieces, medium can be filled inside the workpiece to reduce the deformation of the workpiece during clamping and cutting. For example, pour a urea melt containing 3%-6% potassium nitrate into the workpiece, and after processing, immerse the workpiece in water or alcohol to dissolve the filler and pour it out.
④Reasonable arrangement of procedures
During high-speed cutting, due to the large machining allowance and intermittent cutting, the milling process often produces vibration, which affects the machining accuracy and surface roughness. Therefore, the CNC high-speed cutting process can generally be divided into: rough machining-semi-finish machining-clear corner machining-finishing and other processes. For parts with high precision requirements, it is sometimes necessary to perform secondary semi-finishing and then finishing. After rough machining, the parts can be cooled naturally to eliminate internal stress caused by rough machining and reduce deformation. The margin left after rough machining should be greater than the amount of deformation, generally 1-2mm. During finishing, the finishing surface of the part must maintain a uniform machining allowance, generally 0.2-0.5mm is appropriate, so that the tool is in a stable state during the machining process, which can greatly reduce cutting deformation, obtain good surface machining quality, and ensure Product accuracy