Deformation control experience of thin-walled workpieces

The deformation of the workpiece in machining is a relatively difficult problem to solve. The reason for the deformation must be analyzed first, and then countermeasures can be taken.

1. The material and structure of the workpiece will affect the deformation of the workpiece

The amount of deformation is closely related to the complexity of the shape, the aspect ratio and the wall thickness, and the rigidity and stability of the material. Therefore, when designing parts, optimize these parameters as much as possible to reduce the influence of these factors on the deformation of the workpiece.

Especially in the structure of large parts, the structure should be reasonable. Before processing, it is necessary to strictly control the defects such as the hardness and porosity of the blank to ensure the quality of the blank and reduce the deformation of the workpiece.

2. Deformation caused by workpiece clamping

When the workpiece is clamped, the correct clamping point must be selected first, and then the appropriate clamping force should be selected according to the position of the clamping point. Therefore, make the clamping point and the supporting point as consistent as possible, so that the clamping force acts on the support, the clamping point should be as close as possible to the processing surface, and the selected position is not easy to cause clamping deformation.

When there are clamping forces in several directions on the workpiece, the order of the clamping forces should be considered. The clamping force should be applied first to make the workpiece contact the support, and it should not be too large. The main clamping force for balancing the cutting force , Should act at the end.

Secondly, it is necessary to increase the contact area between the workpiece and the fixture or use the axial clamping force. Increasing the rigidity of the parts is an effective way to solve the clamping deformation, but due to the characteristics of the shape and structure of the thin-walled parts, it has a lower rigidity. In this way, under the action of clamping force, deformation will occur.

Enlarging the contact area between the workpiece and the fixture can effectively reduce the deformation of the workpiece during clamping. For example, when milling thin-walled parts, a large number of elastic pressing plates are used to increase the force-bearing area of the contact parts. When turning the inner diameter and outer circle of the thin-walled sleeve, whether it is using a simple split transition ring, or using an elastic mandrel, full-arc jaws, etc., it is used to increase the contact area when the workpiece is clamped. This method is conducive to bearing the clamping force, thereby avoiding the deformation of the parts.

The axial clamping force is also widely used in production. The design and manufacture of special fixtures can make the clamping force act on the end surface, which can solve the bending deformation of the workpiece due to the thin wall of the workpiece and the poor rigidity.

3. Deformation caused by workpiece processing

Due to the cutting force during the cutting process, the workpiece produces elastic deformation in the direction of the force, which is what we often call the phenomenon of giving up the knife. Corresponding measures should be taken on the tool to deal with this kind of deformation. The tool is required to be sharp during finishing. On the one hand, it can reduce the resistance caused by the friction between the tool and the workpiece. On the other hand, it can improve the heat dissipation capacity of the tool when cutting the workpiece, thereby reducing the workpiece Residual internal stress.

For example, when milling large planes of thin-walled parts, single-edge milling is used, and the tool parameters select a larger entering angle and a larger rake angle in order to reduce cutting resistance. Because this kind of tool cuts lightly and reduces the deformation of thin-walled parts, it is widely used in production.

The deformation of thin-walled parts during turning is multifaceted. The clamping force when clamping the workpiece, the cutting force when cutting the workpiece, and the elastic deformation and plastic deformation generated by the workpiece hindering the cutting of the tool will increase the temperature of the cutting zone and cause thermal deformation. Therefore, in rough machining, the amount of back-grabbing and feed can be larger; in finishing, the amount of tool is generally 0.2～0.5mm, and the feed is generally 0.1～0.2mm/r, or even smaller. The speed is 6～120m/min, and the cutting speed should be as high as possible when finishing turning, but it should not be too high. Choose a reasonable amount of cutting, so as to achieve the purpose of reducing part deformation.

4. Stress and deformation after processing

After rough machining, the part itself has internal stress. The distribution of these internal stresses is a relatively balanced state, and the shape of the part is relatively stable in the short term. However, the internal stress changes after heat treatment and finishing removing some materials. At this time, the workpiece needs to reach the balance of force again, so the shape is prone to change to a certain extent within a certain period of time. This is also the typical reason why some workpieces deform and crack during the storage period and service period after delivery.

In the field of aerospace, "thin-walled parts" are often "parts that are easily deformed after processing and can be restored to geometric shapes after assembly". Generally, the ratio of the main dimension to the wall thickness is not less than 50. "Thin-walled workpieces" or "thin-walled" workpieces that meet the above conditions can be included in the modal broadband aging treatment range.

Compared with traditional thermal aging, modal broadband aging integrates residual stress simulation, modal dynamic stress simulation, residual stress detection, online modal analysis, tooling and fixture design as a whole, which is effective in solving problems such as delayed deformation and fatigue cracks after product delivery. Obviously, it can effectively improve the stability and reliability of the product after delivery.

Modal broadband aging requires short time and energy saving. The excitation frequency range can reach 0~3000Hz. It is suitable for small and medium-sized plates, disks, rings, frames and other workpieces. It is also good for parts with high rigidity and high modal frequency. Application effect. Because of its high frequency and low dynamic stress, it basically has no direct impact on the part itself. It can be used to solve the stress relief problem of small, light and thin-walled workpieces, filling the gap in the stress relief method of thin-walled parts after semi-precision addition.

In summary, for easily deformable light-weight thin-walled workpieces, corresponding countermeasures should be adopted in the blank and processing technology, combined with appropriate stress relief aging methods, to minimize the internal stress concentration after the finished product, and prevent the finished product from entering Delayed deformation and cracking during storage and service phases.