The problem of workpiece deformation in machining is more difficult to solve. First of all, we must analyze the cause of the deformation before we can take measures to deal with it.
01 The material and structure of the workpiece will affect the deformation of the workpiece
The amount of deformation is proportional to the complexity of the shape, aspect ratio and wall thickness, and is proportional to the rigidity and stability of the material. Therefore, when designing parts, reduce the influence of these factors on the deformation of the workpiece as much as possible.
Especially in the structure of large parts should be more reasonable structure. Before processing, it is necessary to strictly control the blank hardness, looseness and other defects to ensure the quality of the blank and reduce the deformation of the workpiece.
02 Deformation caused by workpiece clamping
When the workpiece is clamped, first select the correct clamping point, and then select the appropriate clamping force according to the position of the clamping point. Therefore, as far as possible, the clamping point and the supporting point should be the same, so that the clamping force acts on the support. The clamping point should be as close to the processing surface as possible, and the position where the clamping force is not easily caused by the clamping force should be selected.
When there are several directions of clamping force on the workpiece, the order of the clamping force must be taken into consideration. The clamping force of the workpiece and the support should be applied first, and not too large, the main clamping force for balancing the cutting force. It should act at the end.
Second, increase the contact area between the workpiece and the fixture or use axial clamping force. Increasing the rigidity of the part is an effective way to solve the problem of clamping deformation, but due to the shape and structure characteristics of thin-walled parts, it has a lower rigidity. In this way, under the action of clamping force, deformation will occur.
Increasing the contact area between the workpiece and the fixture can effectively reduce the deformation of the workpiece when it is clamped. For example, when milling thin-walled parts, a large number of elastic press plates are used to increase the contact area of ​​the contact parts. When turning the thin-walled sleeve inner diameter and outer circle, either simple opening transition rings or elastic mandrels are used. , The entire arc claws, etc., are used to increase the contact area when the workpiece is clamped. This method is conducive to carrying the clamping force, thereby avoiding the deformation of the parts. The use of axial clamping force is also widely used in production. The design and production of a special fixture allows the clamping force to act on the end face, which can solve the bending deformation of the workpiece due to the thin wall and poor rigidity of the workpiece.
03 Deformation caused by workpiece processing
During the cutting process, due to the action of the cutting force, the workpiece undergoes elastic deformation toward the direction of force, which is what we often call the knife phenomenon. Corresponding measures should be taken to deal with such deformation on the tool. The sharpness of the tool is required for precision machining. On the one hand, the resistance caused by the friction between the tool and the workpiece can be reduced. On the other hand, the heat-dissipating ability of the tool when cutting the workpiece can be improved, thereby reducing the number of workpieces. Residual internal stress.
For example, when milling a thin-walled part on a large plane, a single-edged milling method is used, and the tool parameters select a larger main declination angle and a larger rake angle in order to reduce the cutting resistance. Due to the quick cutting of this type of tool, the deformation of thin-walled parts is reduced and it is widely used in production.
In the turning of thin-walled parts, a reasonable tool angle is critical for the size of the cutting force during turning, the thermal deformation during turning, and the microscopic quality of the workpiece surface. The size of the tool rake angle determines the sharpness of the cutting deformation and the tool rake angle. The rake angle is large, the cutting deformation and friction force are reduced, but the rake angle is too large, the wedge angle of the tool is reduced, the tool strength is weakened, the tool heat dissipation is poor, and the wear is accelerated. Therefore, when turning thin-walled parts of steel materials in general, use a high-speed tool with a rake angle of 6° to 30° and a carbide tool with a rake angle of 5° to 20°.
The back angle of the tool is large, the friction force is small, and the cutting force is also reduced. However, if the back angle is too large, the tool strength will be weakened. When turning thin-walled parts, use a high-speed steel lathe tool, the tool back angle is 6 ° -12 °, with a carbide tool, the rear angle to take 4 ° -12 °, when the fine car to take a larger angle, Take a smaller rear corner when roughing. When the thin-walled parts inside and outside the car, take a large main angle. The correct choice of tool is a necessary condition to deal with the deformation of the workpiece.
The heat generated by the friction between the tool and the workpiece during machining also deforms the workpiece, so high-speed cutting is often selected. In high-speed cutting, because the chips are removed in a short time, most of the cutting heat is taken away by the chips, which reduces the thermal deformation of the workpiece. Second, in the high-speed machining, due to the reduction of the softened part of the cutting layer material, Can reduce the deformation of parts processing, help to ensure the part's size, shape accuracy. In addition, the cutting fluid is mainly used to reduce the friction during cutting and reduce the cutting temperature. The rational use of cutting fluid plays an important role in improving the tool's durability, machining surface quality and machining accuracy. Therefore, it is necessary to use a sufficient amount of cutting fluid in order to prevent the deformation of parts during processing.
The use of a reasonable amount of cutting during processing is a key factor in ensuring the accuracy of the part. When processing high-precision thin-walled parts, symmetry processing is generally adopted to balance the stresses on the opposite sides to a stable state, and the workpieces are smooth after processing. However, when a certain process takes a large amount of knife, the workpiece will be deformed due to loss of balance between tensile stress and compressive stress.
Deformation of thin-walled parts when turning is multi-faceted, the clamping force when clamping the workpiece, the cutting force when cutting the workpiece, and the elastic deformation and plastic deformation caused by the workpiece when the workpiece is blocked from cutting, so that the temperature in the cutting zone increases and thermal deformation occurs. Therefore, we need to take a larger amount of backing and feed in roughing; when finishing, the cutting amount is generally 0.2-0.5mm, and the feed is generally 0.1-0.2mm/r, even Smaller, cutting speeds of 6-120 m/min. When turning a car, the cutting speed is as high as possible, but not too high. A reasonable choice of cutting amount, so as to achieve the purpose of reducing the deformation of the parts.
04 Stress deformation after processing
After processing, the part itself has internal stress. The internal stress distribution is a relatively balanced state. The part shape is relatively stable, but some materials are removed and the internal stress changes after heat treatment. At this time, the workpiece needs to reach the balance of force, so the shape is It has changed. To solve this kind of deformation, the workpieces that need to be straightened can be stacked to a certain height by means of heat treatment. The workpieces are pressed into a flat state by a certain tooling. The tooling and the workpieces are then put together in a heating furnace. According to the different materials of the parts, Different heating temperatures and heating times. After the heat is straightened, the internal structure of the workpiece is stable. At this point, the workpiece not only has a high degree of straightness, but also the work hardening phenomenon is eliminated, which is more convenient for further finishing of the parts. Castings should be treated with ageing, and internal residual stress should be eliminated as much as possible, and the method of deformation after reprocessing should be adopted, that is, roughing-aging-reworking.
For large parts, profiling is used, that is, the deformation of the workpiece is expected to be assembled, and the deformation is reserved in the opposite direction during machining, which can effectively prevent the deformation of the parts after assembly.
In summary, for the easily deformable parts, corresponding countermeasures must be taken for the blanks and the processing technology. It is necessary to analyze according to different situations and find a suitable process route. Of course, the above method only further reduces the deformation of the workpiece. If you want to get a higher-precision workpiece, you need to continue to learn, explore and study.
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