There are many reasons for the deformation of aluminum parts, which are related to the material, shape and production conditions. There are mainly the following aspects: deformation caused by internal stress of blank, deformation caused by cutting force and cutting heat, and deformation caused by clamping force.
1、 Technological measures to reduce processing deformation
1 reduce the internal stress of blank
The internal stress of the blank can be partially eliminated by natural or artificial aging and vibration treatment. Pre processing is also an effective process. For the blank of fat head and big ear, because of the large margin, the deformation after processing is also large. If the surplus part of the blank is pre processed and the surplus of each part is reduced, not only the processing deformation of the subsequent process can be reduced, but also the internal stress can be released after pre processing for a period of time.
2. Improve the cutting ability of tools
The material and geometric parameters of cutting tools have an important influence on cutting force and cutting heat. The correct selection of cutting tools is very important to reduce the machining deformation of parts.
(1) Reasonable selection of tool geometry parameters.
① Front angle: under the condition of maintaining the strength of the cutting edge, the front angle should be larger. On the one hand, it can grind out a sharp edge, on the other hand, it can reduce cutting deformation, make chip removal smooth, and then reduce cutting force and cutting temperature. Never use a negative rake tool.
② Back angle: the size of the back angle has a direct impact on the wear of the back cutter face and the quality of the machined surface. The cutting thickness is an important condition to select the back angle. In rough milling, due to the large feed, heavy cutting load and large heat output, it is required that the heat dissipation condition of the cutter is good, so the back angle should be smaller. When finishing milling, it is required to have sharp edge, reduce the friction between the back face and the machining surface, and reduce the elastic deformation. Therefore, the back angle should be larger.
③ Helix angle: in order to make milling stable and reduce milling force, the helix angle should be selected as large as possible.
④ Main deflection angle: properly reducing the main deflection angle can improve the heat dissipation conditions and reduce the average temperature of the processing area.
(2) Improve the tool structure.
① Reduce the number of milling cutter teeth, increase chip space. Because of the large plasticity of aluminum material, the large cutting deformation and the need for large chip space, the bottom radius of chip holding groove should be larger and the number of milling cutter teeth is less.
② Sharpen the teeth. The roughness value of cutting edge should be less than RA = 0.4um. Before using a new knife, it is necessary to use a fine oilstone to slightly grind the front and back of the knife teeth, so as to eliminate the remaining burrs and slight serrations when grinding the knife teeth. In this way, not only cutting heat can be reduced, but also cutting deformation is relatively small.
③ Strictly control the tool wear standard. After tool wear, the surface roughness of workpiece increases, the cutting temperature increases, and the deformation of workpiece increases. Therefore, in addition to the selection of good wear-resistant tool materials, the tool wear standard should not be greater than 0.2mm, otherwise it is easy to generate chip accretion. When cutting, the temperature of workpiece should not exceed 100 ℃ to prevent deformation.
3. Improve the clamping method of workpieces
For thin-walled aluminum parts with poor rigidity, the following clamping methods can be used to reduce deformation:
① For thin-walled bushing parts, if the three jaw self centering chuck or spring chuck is used to clamp from the radial direction, the workpiece will be deformed once it is loosened after machining. At this time, the method of axial end face compression with better rigidity should be used. Locate the inner hole of the part, make a self-made threaded through mandrel, insert it into the inner hole of the part, press the end face with a cover plate and back it with a nut. When machining the outer circle, the clamping deformation can be avoided and the satisfactory machining accuracy can be obtained.
② When machining thin-walled sheet metal, it is better to select vacuum sucker to obtain uniform clamping force, and then to process with a small cutting amount, which can prevent the deformation of the workpiece well.
In addition, the filling method can be used. In order to increase the technological rigidity of thin-walled workpiece, the medium can be filled in the workpiece to reduce the deformation of workpiece during clamping and cutting. For example, fill the workpiece with urea melt containing 3% – 6% potassium nitrate, after processing, immerse the workpiece in water or alcohol, then the filler can be dissolved and poured out.
4 reasonable arrangement of process
In high-speed cutting, because of large machining allowance and intermittent cutting, the milling process often produces vibration, which affects the machining accuracy and surface roughness. Therefore, NC High-speed machining process can be generally divided into: rough machining – semi finish machining – angle cleaning machining – finish machining and other processes. For parts with high precision requirements, sometimes it is necessary to carry out secondary semi finishing, and then finish machining. After rough machining, the parts can be naturally cooled to eliminate the internal stress caused by rough machining and reduce the deformation. The allowance left after rough machining shall be larger than the deformation, generally 1-2mm. During finishing, the machining allowance of the finished surface of the part shall be kept even, generally 0.2-0.5mm is the best, so that the cutter is in a stable state during the machining process, which can greatly reduce the cutting deformation, obtain good surface machining quality, and ensure the accuracy of the product.
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