In the process of forming cnc machining parts，we most often use cutting technology. When cutting the workpiece, due to cutting heat, friction heat between machine parts, internal stress of the workpiece, and clamping force, the workpiece will be deformed, resulting in poor accuracy, and resulting in waste. Therefore, when machining, it is necessary to have a certain understanding of the cause of deformation of the workpiece, and to prevent deformation.
A. Thermal deformation
1. Thermal deformation of the tool
Due to the cutting heat, the cutting edge and the tool body will become hot, causing the tool head to deform and elong and change the size of the workpiece. The extension of the cutter head has a certain relationship with the depth of the cutter head, the size of the section, the thickness of the blade, and the sharpness of the blade. The greater the depth of the cutter head, the greater the elongation; the cross section of the shank is inversely proportional to the elongation; the thicker the blade, the smaller the elongation.
2. Thermal deformation of the machine tool
The heat generated by the friction between the cutting heat and the cnc parts of the machine tool will cause some parts of the machine tool to heat up and deform. For example, the deformation of the spindle box of a lathe will increase the height of the spindle center and move in the horizontal direction.
3. Thermal deformation of the workpiece
Due to the cutting heat, the workpiece becomes hot and the temperature rises. There are two types of workpiece heating: balanced heating and unbalanced heating. Even heating will change the size of the workpiece, but the shape will remain unchanged; when uneven heating, not only the size of the workpiece, but also the shape will also change.
B. 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 proportional to the rigidity and stability of the material. Therefore, the influence of these factors on the deformation of the workpiece is reduced as much as possible when designing parts.
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.
C. Deformation caused by cnc manufacturing.
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 must be sharp during finishing. On the one hand, it can reduce the resistance caused by the friction 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 workpiece the residual internal stress.
For example, in cnc machine shop, 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 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.
In the turning of thin-walled parts, a reasonable tool angle is very important to the cutting force during turning, the thermal deformation generated during turning, and the microscopic quality of the workpiece surface. The size of the rake angle of the tool determines the cutting deformation and the sharpness of the rake angle of the tool. Large rake angles reduce cutting deformation and friction, but too large rake angles will reduce the wedge angle of the tool, weaken the tool strength, poor heat dissipation of the tool, and accelerate wear. Therefore, generally when turning thin-walled parts of steel materials, use high-speed tools with a rake angle of 6°-30°, and a cemented carbide tool with a rake angle of 5°-20°.
The clearance angle of the tool is large, the friction force is small, and the cutting force is correspondingly reduced, but too large clearance angle will also weaken the strength of the tool. When turning thin-walled parts, use high-speed steel turning tools, the tool clearance angle is 6°-12°, with cemented carbide tools, the clearance angle is 4°-12°, and the larger clearance angle is used for fine turning. When taking the smaller back angle. When turning the inner and outer circles of thin-walled parts, take a large entering angle. The correct choice of tools is a necessary condition to deal with the deformation of the workpiece.
The heat generated by the friction between the tool and the workpiece will also deform the workpiece during processing, so high-speed cutting is often selected. In precision machining, since the chips are removed in a short time, most of the cutting heat is taken away by the chips, reducing the thermal deformation of the workpiece.
Secondly, in high-speed machining, due to the reduction of the softened part of the cutting layer material, the deformation of the parts can also be reduced, which is beneficial to ensure the size and shape accuracy of the parts. 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 the durability of the tool, the quality of the processed surface, and the accuracy of the processing. Therefore, in order to prevent parts from deforming, sufficient cutting fluid must be used reasonably.
The use of reasonable cutting amount in processing is a key factor to ensure the accuracy of parts. When processing thin-walled parts with high precision requirements, symmetrical processing is generally adopted to balance the stresses generated on the two opposite sides to a stable state, and the workpiece is smooth after processing. However, when a large amount of knife is taken in a certain process, the workpiece will be deformed due to the loss of balance between the tensile stress and the compressive stress.
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