Common process measures and six operation methods to solve the processing deformation of aluminum parts
In other words, fixed point machining requires three points and one fixed point. There are many reasons for the deformation of common aluminum parts, which are related to the material, the shape of parts and the production conditions. There are several aspects: deformation caused by internal stress of blank, deformation caused by cutting force and cutting heat, deformation caused by clamping force.
Technological measures to reduce deformation in aluminum processing
Table of Contents
- 1 Technological measures to reduce deformation in aluminum processing
- 2 Six operation methods to avoid aluminum deformation
Reduce the internal stress of blank
Natural or artificial aging and vibration treatment can partly eliminate the internal stress of the blank. Pre processing is also an effective process. For larger blanks, the deformation is also large after machining because of the large allowance. If the surplus part of the blank is pre processed and the allowance of each part is reduced, not only the machining deformation of the later process can be reduced, but also a part of the internal stress can be released after a period of time after pre processing.
For example, figure 1 shows the beam part. The shape of the blank is shown by the double dot dash. The weight of the blank is 60kg, while the part only weighs 3kg. The flatness error can be as high as 14mm if it is processed in one time as shown by the dotted line in the figure. If it is pre processed according to the solid line in the figure and processed to the required parts after natural aging for a period of time, the flatness error can be reduced to 3mm.
Figure.2 shows the part of a certain type of cover piercing device, the local minimum thickness is only 3 mm, and the blank thickness before processing is 20 mm. Parts can be directly processed to the size by pressing plates in the machining center. However, when the parts are removed from the worktable, the two ends at the bottom of the parts will be upwarped, resulting in serious out of tolerance and even scrapping.
Therefore, before processing, first open a stress relief groove on the blank, as shown in the solid line position in Fig. 3, and then remove it from the worktable, natural aging for 1-2 h, so that the deformation can occur as much as possible at this time. After that, a fitter leveling process is added to level the parts, and the deformation of the parts in the subsequent processing will be greatly reduced.
Improve the cutting ability of cutting tools
The material and geometric parameters of cutting tools have important influence on cutting force and cutting heat. It is very important to select the cutting tools correctly to reduce the machining deformation of parts.
The geometric parameters of the tool are selected reasonably.
- ① Rake angle: under the condition of maintaining the strength of the cutting edge, the front angle should be properly selected to be larger. On the one hand, it can grind the sharp edge, on the other hand, it can reduce the cutting deformation, make the chip removal smoothly, and then reduce the cutting force and cutting temperature. Do not use negative rake tool.
- ② Back angle: the size of the back angle has a direct impact on the wear of the flank and the quality of the machined surface. The cutting thickness is an important condition for selecting the back angle. In rough milling, due to the large feed rate, heavy cutting load and high heat output, the tool heat dissipation condition is required to be good, so the back angle should be selected smaller. In finish milling, the cutting edge is required to be sharp to reduce the friction between the flank and the machined surface and reduce the elastic deformation. Therefore, the back angle should be larger.
- ③ Helix angle: in order to make milling smooth and reduce milling force, spiral angle should be selected as large as possible.
- ④ The main processing temperature can be improved by reducing the main deflection angle.
Improve tool structure.
- ① Reduce the number of milling cutter teeth, increase chip space. Due to the high plasticity of aluminum parts and the large cutting deformation in machining, large chip holding space is required. Therefore, it is better to have a larger chip holding groove bottom radius and a smaller number of milling cutter teeth.
- ② Fine grinding of cutter teeth. The roughness value of cutting edge of cutter tooth should be less than RA = 0.4um. Before using the new cutter, it is necessary to use a fine oilstone to grind the front and back of the cutter teeth gently to eliminate the residual burr and slight serration when grinding the cutter teeth. In this way, not only can the cutting heat be reduced, but also the cutting deformation is relatively small.
- ③ Strictly control tool wear standard. After tool wear, workpiece surface roughness increases, cutting temperature rises, and workpiece deformation increases. Therefore, the tool wear standard should not be greater than 0.2mm except for the selection of tool materials with good wear resistance, otherwise it is easy to produce chip accretion. When cutting, the temperature of the workpiece should not exceed 100 ℃ to prevent deformation.
Improve the clamping method of workpiece
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 three claw self centering chuck or spring chuck is used to clamp from radial direction, once the workpiece is loosened after machining, the workpiece will inevitably deform. At this time, the method of axial end face compression with better rigidity should be used. According to the location of the inner hole of the part, a self-made piercing mandrel with thread is inserted into the inner hole of the part. A cover plate is used to compress the end face, and the nut is used to back up. The clamping deformation can be avoided when machining the outer circle, so the satisfactory machining accuracy can be obtained.
- (2) When machining thin-walled workpieces, it is better to select vacuum chuck to obtain uniform clamping force, and then to process with small cutting parameters can prevent the deformation of the workpiece.
- (3) The packing method was used. In order to increase the process rigidity of thin-walled workpiece, the medium can be filled in the workpiece to reduce the workpiece deformation during clamping and cutting. For example, the molten urea containing 3% – 6% potassium nitrate is poured into the workpiece. After processing, the workpiece is immersed in water or alcohol, and the filler can be dissolved and poured out.
Arrange the working procedure reasonably
In high-speed cutting, due to large machining allowance and intermittent cutting, vibration often occurs in milling process, which affects machining accuracy and surface roughness. Therefore, the NC high-speed cutting process can be generally divided into: rough machining, semi finishing, angle clearing and finishing. For parts with high precision requirements, sometimes it is necessary to carry out secondary semi finishing and then finish machining. After rough machining, parts can be naturally cooled to eliminate internal stress and reduce deformation. The allowance left after rough machining should be greater than the deformation, generally 1-2mm. When finishing, the surface of parts should be kept uniform machining allowance, generally 0.2 ~ 0.5mm is appropriate, so that the tool can be in a stable state in the process of machining, which can greatly reduce the cutting deformation, obtain good surface processing quality and ensure the accuracy of products.
Six operation methods to avoid aluminum deformation
In addition to improving the tool performance and eliminating the internal stress of the material by aging treatment in advance, the proper operation method can effectively avoid the material deformation in actual operation.
Symmetrical processing method
For parts with large machining allowance, symmetrical machining should be adopted in order to provide better heat dissipation conditions and avoid heat concentration. If a piece of 90mm thick sheet needs to be processed to 60mm, if one side is milled and the other side is immediately milled to the final size, the flatness can reach 5mm; if repeated feed symmetrical machining is adopted, each side is processed twice to the final size, which can ensure the flatness of 0.3mm.
Layered multiple processing method
If there are multiple cavities on the sheet part, as shown in the following figure. When a cavity is machined in such a way, it is not easy to produce a uniform cavity. Each layer is processed to all cavities at the same time as far as possible, and then the next layer is processed to make the parts evenly stressed and reduce deformation.
Select cutting parameters properly
Cutting force and cutting heat can be reduced by changing cutting parameters. Among the three factors of cutting parameters, the back cut has a great influence on the cutting force. If the machining allowance is too large and the cutting force is too large, it will not only deform the parts, but also affect the rigidity of the machine tool spindle and reduce the tool life. If we reduce the amount of back knife, the production efficiency will be greatly reduced. However, high speed milling can overcome this problem. The cutting force can be reduced and the machining efficiency can be ensured at the same time, as long as the feed is increased correspondingly and the rotation speed of the machine tool is increased.
Pay attention to the cutting sequence
Roughing and finishing should adopt different tool feed sequence. Rough machining emphasizes the improvement of machining efficiency and the pursuit of cutting rate per unit time. Generally, reverse milling can be used. That is to cut off the surplus material on the surface of the blank with the fastest speed and the shortest time, so as to form the geometric contour required by the finish machining. The emphasis of finish machining is high precision and high quality, so it is suitable to use down milling. Because the cutting thickness of the cutter teeth gradually decreases from the maximum to zero during the down milling, the work hardening degree is greatly reduced, and the deformation degree of the parts is reduced at the same time.
Secondary compression of thin wall parts
The deformation of thin-walled workpieces due to clamping is difficult to avoid even after finishing. In order to reduce the deformation of the workpiece to a minimum, before the final dimension of the finishing machining is reached, the pressing part can be loosened so that the workpiece can freely return to its original state, and then slightly press down, subject to the rigid clamping of the workpiece, so as to obtain the ideal processing effect. In a word, the best point of clamping force is on the bearing surface, and the clamping force should act on the direction of good rigidity of the workpiece. On the premise of ensuring the workpiece is not loose, the smaller the clamping force is, the better.
Drilling before milling
When machining the parts with cavity, try not to let the milling cutter directly plunge into the part like a drill bit when machining the cavity, which will result in insufficient chip holding space and unsmooth chip removal of the milling cutter, resulting in overheating, expansion, breakage and other adverse phenomena. First, drill the hole with the same size as the milling cutter or a larger drill bit, and then mill it with a milling cutter. Or you can use CAM software to produce programs.
Source: China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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