The problem of workpiece deformation when machining aluminum alloy parts is a difficult problem to solve. First, you must analyze the cause of the deformation before you can take action.
1. The material and structure of the workpiece affect the deformation of the workpiece.
The amount of deformation is directly proportional to the complexity of the shape, the aspect ratio, the wall thickness, and the stiffness and stability of the material. Therefore, when designing parts, it is necessary to minimize the influence of these factors on the deformation of the workpiece.
The structure must be reasonable, especially the large-scale component structure. Before machining, it is necessary to strictly control the defects such as the hardness and pores of the blank to ensure the quality of the blank and reduce the deformation of the workpiece.
2. When clamping the workpiece to deform and clamp the workpiece, first select the correct clamping point, and then select the appropriate clamping force according to the position of the clamping point.
Whenever possible, the clamping point and the support point match, so the clamping force acts on the support. The clamping point should be as close to the work surface as possible, and the force is not easy to cause clamping deformation. If the workpiece has clamping forces in multiple directions, the order of clamping forces must be considered. First, the clamping force must be applied so that the workpiece is in contact with the support, and it should not be too large. The main clamping force that balances the cutting force needs to be applied last.
Second, you need to increase the contact area between the workpiece and the fixture or use axial clamping force. Increasing the rigidity of the parts is an effective way to solve the deformation of the clamp, but the shape and structure of the thin-walled parts reduce the rigidity. In this way, the application of clamping force causes deformation.
Increasing the contact area between the workpiece and the fixture can effectively reduce the deformation of the workpiece when the workpiece is clamped. For example, when milling thin-walled parts, a large number of elastic pressing plates are used to increase the receiving area of the contacting parts. It is used to increase the contact area of the working clamp when the inner and outer circles of the thin sleeve rotate. Whether to use a simple split transition ring, elastic mandrel, arc clamp, etc. This method helps to maintain the clamping force, thereby avoiding part deformation. Axial clamping force is also widely used in production. The special fixture designed and manufactured can apply clamping force on the end face to solve the bending deformation of the workpiece due to the thin wall and insufficient rigidity.
3. Deformation caused by workpiece machining
The workpiece is elastically deformed in the direction of the force due to the cutting force during the cutting process, which is usually called the phenomenon of tool release.
For this kind of deformation, it is necessary to take corresponding measures with the tool, and the sharpness of the tool is required for finishing. On the one hand, it can reduce the frictional resistance between the tool and the workpiece, and on the other hand, it can improve the heat dissipation capacity of the tool when cutting the workpiece, thereby reducing the residual internal stress of the workpiece. For example, when single-edge milling is used to mill large planes of thin-walled parts, the tool parameters select large entering angle and large rake angle to reduce cutting resistance. The cutting speed of the tool is light, the deformation of thin-walled parts is reduced, and it is widely used in production.
When turning thin-walled parts, the appropriate tool angle is very important for the cutting force during turning, the thermal deformation during turning, and the microscopic quality of the workpiece surface. The size of the rake angle of the tool determines the degree of cutting deformation and the sharpness of the rake angle of the tool. A large rake angle will reduce cutting deformation and friction, while a too large rake angle will reduce the tool wedge angle, reduce tool strength, reduce tool heat dissipation and accelerate wear. When using high-speed tools to turn thin steel parts, when using cemented carbide tools, the rake angle is 6° to 30°, so the rake angle is 5° to 20°.
The tool clearance angle is large, the friction is small, and the cutting force is correspondingly reduced. However, if the relief angle is too large, the strength of the tool will decrease. When turning thin-walled parts, use high-speed steel turning tools. The tool clearance angle is 6° to 12°. Carbide tools with an angle of 4° to 12°. Use a large relief angle for precision turning and a small relief angle for rough machining. When turning the inner and outer circles of thin-walled parts, take a large lead angle. The correct selection of the tool is a necessary condition for the deformation of the workpiece.
High-speed machining is usually selected because the heat generated by the friction between the tool and the workpiece during the machining process will also deform the workpiece. High-speed machining cuts chips in a short time, so most of the cutting heat is taken away by the chips, reducing the thermal deformation of the workpiece. Secondly, in high-speed machining, the reduction of the softened part of the cutting layer material also reduces the deformation of the part machining, which is beneficial to ensure the accuracy of the size and shape of the part. In addition, the cutting fluid is mainly used to reduce friction during the cutting process and lower the cutting temperature. The rational use of cutting fluid plays an important role in improving tool durability, machining surface quality and machining accuracy. Therefore, sufficient cutting fluid must be used to prevent parts from deforming during machining.
Using the right amount of cutting in machining is an important factor to ensure the accuracy of parts. For thin-walled parts that require high machining accuracy, symmetrical machining is generally used to balance the stress generated on both sides, stabilize the workpiece, and smooth the processed workpiece. However, when more cutting tools are used in a particular process, the workpiece will be deformed due to the imbalance between tensile and compressive stresses.
The deformation of thin-walled parts during turning is manifold. The clamping force when clamping the workpiece, the cutting force when cutting the workpiece, the workpiece prevents the elastic deformation and plastic deformation of the tool during cutting, and the temperature in the cutting zone increases, causing thermal deformation. Therefore, if rough machining is required, the depth of cut and feed can be increased. During finishing, the cutter volume is usually 0.2-0.5mm, the feed volume is usually 0.1-0.2mm/r or less, and the cutting speed is 6-120m/min. Use the best cutting speed when finishing turning, but it is not easy to be too high. Choose the right amount of cutting to achieve the purpose of reducing component deformation.
4. Stress after machining After deformation, internal stress acts on the part itself.
The distribution of these internal stresses is relatively balanced, and the shape of the part is relatively stable. However, after removing some materials and changing the internal stress after heat treatment, the workpiece needs to reach the force balance again, which causes the shape to change. In order to eliminate this deformation, the straightened workpieces can be stacked to a certain height by heat treatment and pressed into a flat state with a certain mold. Then put the mold and the workpiece into the heating furnace together, and choose different heating temperature and time according to the material of the part. After the heat is corrected, the internal structure of the workpiece is stable. At this time, not only the height of the workpiece is straight, but also the work hardening phenomenon is eliminated, making it easier to further finish the parts. Carry out the casting aging process, try to remove the internal residual stress, and then use the rough machining-aging-reworking machining method.
For large parts, contour analysis is required. In other words, the amount of deformation of the workpiece after assembly is predictable, and the amount of deformation in the opposite direction is guaranteed during the machining process, so it can effectively prevent the deformation of the parts after assembly.
In summary, for deformable workpieces, measures such as blanks and machining techniques should be taken. You need to analyze it for different situations and find the correct process route.
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