In layman's terms, a fixture is six points (3+2+1: three fixed points, two fixed points, and one fixed point), and machining needs to address the deformation. There are many common causes of deformation in machining aluminum parts, which are related to the material, part shape, and production conditions. There are several main aspects: deformation caused by internal stress in the blank, deformation caused by cutting force and cutting heat, and deformation caused by clamping force.
Process measures to reduce distortion in aluminum processing
1. Reduce internal stress of the blank
Internal stresses on the blank can be partially relieved by natural or artificial aging and vibration treatment. Pre-machining is also a proven process method. For large blanks, the deformation after machining is large due to the large allowance. Pre-machining the excess parts of the blank to reduce the allowance of each part can not only reduce the deformation in the subsequent process, but also release a part of the internal stress after a certain period of pre-processing.
A stress relief groove is first made in the blank, as shown in Figure 3, at the solid line position, and then removed from the workbench with a natural aging of 1 to 2h to allow as much deformation as possible to occur at this time. After that, add a clamper leveling process to level the part, the deformation of the part in subsequent machining will be greatly reduced.
2. Improve the cutting ability of the tool
Tool material, geometric parameters of the cutting force, cutting heat has an important impact on the correct selection of the tool, to reduce the part machining deformation is critical.
(1) a reasonable choice of tool geometry parameters.
① Front angle: in the condition of maintaining the strength of the cutting edge, the front angle properly selected larger, on the one hand, you can grind out a sharp edge, the other can reduce the cutting deformation, so that chip removal smoothly, and thus reduce the cutting force and cutting temperature. Do not use negative front angle tools.
Rear angle: the size of the rear angle has a direct impact on the cutter surface wear and machining surface quality. Cutting thickness is an important condition for the selection of the back angle. Rough milling, due to the large feed, heavy cutting load, heat generation, requiring good tool heat dissipation conditions, therefore, the back angle should be selected smaller. Precision milling requires a sharp cutting edge to reduce the friction between the cutter surface and the machined surface and to reduce the elastic deformation, therefore, the back angle should be selected larger.
Helix angle: In order to make the milling steady and reduce the milling force, the helix angle should be as large as possible.
(4) Main offset angle: Properly reduce the main offset angle can improve heat dissipation conditions, so that the average temperature of the machining area is reduced.
(2) Improve the tool structure.
(1) Reduce the number of milling cutter teeth and increase the chip space. Due to the plasticity of the aluminum material, the cutting deformation in machining is large, requiring a larger chip space, so the chip capacity of the bottom radius of the groove should be larger, the number of milling cutter teeth is less good.
Precision grinding cutter teeth. Before using the new cutter, you should use a fine oil stone to gently grind the teeth before and after a few times to eliminate the residual burrs and slight serration pattern when sharpening the cutter teeth. This will not only reduce the cutting heat but also reduce the cutting deformation.
③Strict control of tool wear standards. When the tool wears, the surface roughness value of the workpiece increases, the cutting temperature rises, and the deformation of the workpiece increases. Therefore, in addition to the selection of good wear resistance of the tool material, tool wear standard should not be greater than 0.2mm, otherwise it is easy to produce chip tumor. When cutting, the temperature of the workpiece generally should not exceed 100 ℃, in order to prevent deformation.
3. Improve the clamping method of workpieces
For thin-walled aluminum parts that are less rigid, the following clamping methods can be used to reduce distortion.
(1) For thin-walled bushing parts, if the three-jaw self-centering chuck or spring chuck is clamped radially, the workpiece is bound to be deformed once it is released after machining. In this case, the more rigid method of axial face pressing should be used. To position the bore of the part, make your own threaded mandrel, which is inserted into the bore of the part, with a cover plate on which the end face is pressed and then tightened with a nut. Clamping deformation can be avoided when machining the outer circle, so as to obtain satisfactory machining accuracy.
(2) When machining thin-walled sheet workpieces, it is best to use a vacuum chuck to obtain a uniform distribution of clamping force, and then with a small amount of cutting to machine, can prevent the workpiece deformation.
(3) Use the plugging method. To increase the process rigidity of thin-walled workpieces, fill the media inside the workpiece to reduce the deformation of the workpiece during clamping and cutting. For example, the workpiece can be filled with 3% to 6% potassium nitrate urea melt, and after machining, the filler can be dissolved and poured out by immersing the workpiece in water or alcohol.
4. Rational arrangement of the process
In high-speed cutting, due to large machining allowances and intermittent cutting, the milling process often produces vibration, which affects machining accuracy and surface roughness. Therefore, the CNC high-speed cutting process can be generally divided into: roughing - semi-finishing - corner clearing - finishing and other processes. For parts with high precision requirements, sometimes it is necessary to carry out semi-finishing machining twice, and then finish machining. After roughing, the part can be cooled naturally to eliminate internal stresses and reduce deformation. After roughing, the residual allowance should be larger than the deformation, generally 1~2mm, and when finishing, the surface of the part should maintain an even machining residual allowance, generally 0.2~0.5mm, so that the tool in the process of processing in a stable state, can greatly reduce the cutting deformation, to obtain good surface processing quality, to ensure the accuracy of the product.
6 Ways to Avoid Aluminum Distortion
In addition to improving the performance of the tool and eliminating internal stresses in the material by pre-ageing, the use of appropriate methods can effectively avoid deformation of the material in practice.
1. Symmetric Machining
For the processing of large spare parts, in order to have better heat dissipation conditions in the process, to avoid heat concentration, processing, it is appropriate to use symmetric processing. If there is a 90mm thick plate needs to be processed to 60mm, if the milling side immediately after milling the other side, a processing to the final size, the flatness of 5mm; If repeatedly into the tool symmetrical processing, each side in two processing to the final size, to ensure that the flatness of 0.3mm.
2. Layered multiple processing method
If there are multiple cavities on a sheet part, as shown in the figure below. It is not advisable to process one cavity after another, as this will easily cause uneven force and deformation of the part. It is better to use layered multiple processing, each layer try to process all the cavities at the same time, and then process the next layer, so that the parts are evenly stressed and deformation is reduced.
3. Proper selection of cutting amount
Reduce cutting force and heat by changing the amount of cutting. Of the three elements of cutting capacity, the amount of back feed has a great impact on the cutting force. If the machining allowance is too large, the cutting force of a single tool walk is too great, which will not only deform the part, but also affect the rigidity of the machine tool spindle and reduce the durability of the tool. If you reduce the amount of back-eating tooling, but also make the production efficiency is greatly reduced. However, in CNC machining are high-speed milling, can overcome this problem. In reducing the amount of back tool eating at the same time, as long as the corresponding increase in feed, increase the speed of the machine, you can reduce the cutting force, while ensuring processing efficiency.
4. Go tooling order to pay attention to
Roughing and finishing machining should use different tooling sequences. Roughing emphasizes the improvement of machining efficiency and the pursuit of the removal rate per unit of time, which is generally achieved by reverse milling. This means that the excess material on the surface of the blank is removed at the fastest speed and in the shortest time, and the geometric profile required for finishing is basically formed. The finishing emphasis is on high precision and quality, it is appropriate to use cis-milling. Because of the progressive milling, the cutting thickness of the teeth gradually decreases from maximum to zero, the degree of hardening is greatly reduced, while reducing the degree of deformation of the parts.
5. Second pressing of thin-walled parts
Deformation of thin-walled workpieces during machining due to clamping is unavoidable, even in finishing. In order to minimize the deformation of the workpiece, it is possible to loosen the clamping part before the final size is reached in the finishing process, so that the workpiece is free to return to its original state, and then press the clamping part again slightly, in order to just be able to clamp the workpiece, so as to obtain the desired machining results. In short, the point of action of the clamping force is best on the supporting surface, the clamping force should act in the direction of the workpiece rigidity, under the premise of ensuring that the workpiece does not loosen, the smaller the clamping force the better.
6. Milling method after drilling
When machining parts with cavities, try not to let the milling cutter like a drill bit directly down into the part, resulting in insufficient chip space for the milling cutter, chip removal is not smooth, resulting in overheating, expansion of the part, as well as the unfavorable phenomenon of collapse, broken tool. It is necessary to use a drill of the same size as the milling cutter or a larger drill bit to drill the lower cutter hole, and then use the milling cutter to mill the part. Alternatively, CAM software can be used to produce a helical cutter program.
Contact: Jacky Wang
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