The deformation of workpieces in mechanical processing is a difficult problem to solve. It is necessary to analyze the causes of deformation before taking corresponding measures.

The material and structure of the workpiece will affect its deformation

The magnitude of deformation is directly proportional to the complexity of the shape, aspect ratio, and wall thickness, as well as the rigidity and stability of the material. So, when designing parts, try to minimize the impact of these factors on workpiece deformation as much as possible.

Especially in the structure of large components, it is necessary to achieve a reasonable structure. Before processing, it is also necessary to strictly control the hardness, looseness and other defects of the blank to ensure the quality of the blank and reduce the deformation of the workpiece caused by it.

Deformation caused by workpiece clamping

When clamping workpieces, the first step is to select the correct clamping point, and then choose the appropriate clamping force based on the position of the clamping point. Therefore, it is advisable to align the clamping point with the support point as much as possible, so that the clamping force acts on the support. The clamping point should be as close as possible to the machining surface, and a position that is less likely to cause clamping deformation under stress should be selected. When there are clamping forces acting in several directions on the workpiece, the order of clamping forces should be considered. For the clamping force that brings the workpiece into contact with the support, it should be applied first,

And it should not be too large, as the main clamping force for balancing cutting force should act at the end. Secondly, it is necessary to increase the contact area between the workpiece and the fixture or use axial clamping force. Increasing the rigidity of parts is an effective way to solve clamping deformation, but due to the shape and structural characteristics of thin-walled parts, they have lower rigidity. Under the action of clamping force, deformation will occur. Increasing the contact area between the workpiece and the fixture can effectively reduce deformation during workpiece clamping.

When milling thin-walled parts, a large number of elastic pressure plates are used to increase the stress area of the contact parts; When turning the inner and outer diameters of thin-walled sleeves, whether using simple open transition rings or elastic core shafts, arc clamps, etc., the approach is to increase the contact area during workpiece clamping. This method is beneficial for bearing clamping force, thereby avoiding deformation of the parts. The use of axial clamping force is also widely used in production. Designing and manufacturing specialized fixtures can apply clamping force to the end face, which can solve the bending deformation of workpieces caused by thin walls and poor rigidity.

Deformation caused during workpiece processing

During the cutting process, the workpiece undergoes elastic deformation in the direction of the cutting force, which is commonly known as the tool yielding phenomenon. Corresponding measures should be taken on the cutting tool to deal with such deformation. During precision machining, the tool should be sharp, which can reduce the resistance caused by friction between the tool and the workpiece, and improve the heat dissipation ability of the tool when cutting the workpiece, thereby reducing residual internal stress on the workpiece.

For example, when milling large flat surfaces of thin-walled parts, the single edge milling method is used, and the tool parameters are selected to have a larger lead angle and a larger rake angle, in order to reduce cutting resistance. Due to its light cutting speed and reduced deformation of thin-walled parts, this type of tool has been widely used in production.

In the turning of thin-walled parts, a reasonable tool angle is crucial for the magnitude of cutting force, thermal deformation generated during turning, and the microscopic quality of the workpiece surface. The size of the tool rake angle determines the cutting deformation and the sharpness of the tool rake angle. A large rake angle reduces cutting deformation and friction, but if the rake angle is too large, the wedge angle of the tool will decrease, the strength of the tool will weaken, the heat dissipation of the tool will be poor, and wear will accelerate. Therefore, when turning thin-walled steel parts, high-speed cutting tools are generally used with a rake angle of 6 ° to 30 °; Use hard alloy cutting tools with a front angle of 5 ° to 20 °. The large back angle of the tool results in low friction and a corresponding decrease in cutting force, but excessive back angle can also weaken the strength of the tool.

When turning thin-walled parts, use high-speed steel cutting tools with a back angle of 6 ° to 12 °, use hard alloy cutting tools with a back angle of 4 ° to 12 °, and use larger back angles for precision turning and smaller back angles for rough turning. When determining the inner and outer circles of thin-walled parts in a car, take the larger main deviation angle. The correct selection of cutting tools is a necessary condition for dealing with workpiece deformation. The heat generated by the friction between the cutting tool and the workpiece during processing can also cause deformation of the workpiece, so high-speed cutting is often chosen for machining. In high-speed cutting, due to the cutting of chips in a relatively short period of time, the majority of cutting heat is carried away by the chips, reducing the thermal deformation of the workpiece; Secondly, in high-speed machining, the reduction of softened parts in the cutting layer material can also reduce the deformation of the parts during machining, which is beneficial for ensuring the dimensional and shape accuracy of the parts.

In addition, cutting fluid is mainly used to reduce friction and lower cutting temperature during the cutting process. Reasonable use of cutting fluid plays an important role in improving the durability of cutting tools, machining surface quality, and machining accuracy. Therefore, in order to prevent part deformation during processing, it is necessary to use sufficient cutting fluid reasonably. The use of reasonable cutting parameters in machining is a key factor in ensuring the accuracy of parts. When processing thin-walled parts with high precision requirements, symmetrical processing is generally adopted to balance the stress generated on the opposite sides, achieve a stable state, and make the workpiece flat after processing. But when a certain process adopts a large cutting amount, the workpiece will deform due to the imbalance of tensile and compressive stresses.

The deformation of thin-walled parts during turning is multifaceted, including clamping force when clamping the workpiece, cutting force when cutting the workpiece, elastic deformation and plastic deformation caused by the workpiece obstructing the cutting tool, which increase the temperature in the cutting zone and result in thermal deformation. So, when rough machining, we can take a larger amount of back cutting and feed rate; When precision machining, the tool amount is generally between 0.2-0.5mm, the feed rate is generally between 0.1-0.2mm/r, or even smaller, and the cutting speed is between 6-120mm/min. When precision machining, use the highest cutting speed possible, but not too high. Reasonably choose the cutting amount to achieve the goal of reducing part deformation.

Stress deformation after processing

After processing, the parts themselves have internal stresses, and the distribution of these internal stresses is a relatively balanced state. The appearance of the parts is relatively stable, but after removing some materials and heat treatment, the internal stresses change. At this time, the workpiece needs to reach force balance again, so the appearance changes. To solve this type of deformation, heat treatment can be used to stack the workpieces that need to be straightened to a certain height, use a certain fixture to press them into a straight state, and then place the fixture and workpiece together in a heating furnace. Depending on the material of the parts, different heating temperatures and heating times can be selected. After hot straightening, the internal structure of the workpiece is stable. At this point, the workpiece not only achieves high straightness, but also eliminates the phenomenon of work hardening, making it easier for further precision machining of the part.

Castings should undergo aging treatment to eliminate residual stresses as much as possible, using a method of rough machining aging reprocessing after deformation. For large parts, contour machining should be used, which means that the expected deformation of the workpiece after assembly should be reserved in the opposite direction during machining, which can effectively prevent deformation of the part after assembly.

In summary, for easily deformed workpieces, corresponding measures should be taken in both the blank and processing technology, and analysis should be conducted according to different situations to find a suitable process route. Of course, the above methods only further reduce the deformation of the workpiece. If you want to obtain higher precision workpieces, continuous learning, exploration, and research are needed.

Source: Lanxing Electric Power