XavierOverview of methods to solve the deformation of aluminum parts processing
In layman’s terms, the fixture is six points (3+2+1: three-point fixed surface, two-point fixed line, one-point fixed), and processing has to face the problem of part deformation, but the deformation problem must also be solved to ensure the normal operation of the processing process. There are many common reasons for the deformation of aluminum parts, which are closely related to the material, shape, production conditions, etc. of the parts. It is mainly manifested in the following aspects: deformation caused by internal stress of the blank, deformation caused by cutting force, cutting heat, and deformation caused by clamping force.
1.Process measures to reduce the deformation of aluminum parts
1) Reduce the internal stress of the blank
Natural or artificial aging, vibration treatment, etc. can partially eliminate the internal stress of the blank. Pre-processing is also an effective process method. For larger blanks, due to the large machining allowance, the deformation after processing is also large. If the excess part of the blank is processed in advance and the machining allowance of each part is reduced, it can not only reduce the deformation of the subsequent CNC processing, but also release part of the internal stress by placing it in advance for a period of time after CNC processing.
As shown in the figure, a beam part, the weight of the blank in the shape shown by the double dashed line in the figure is 60kg, and the weight of the part is only 3kg. If the part is processed and formed in one go as shown by the dotted line in the figure, its flatness error can be as high as 14mm; if it is pre-processed as shown by the solid line in the figure, and then processed into the required aluminum part after a period of natural aging, its flatness error can be reduced to less than 3mm.
The figure below shows a part of a bottle cap perforator. The local minimum thickness is only 3mm, while the thickness of the blank before CNC processing is 20mm. The aluminum part can be directly processed to size by replacing the pressure plate on the CNC machining center, but when it is removed from the workbench, the bottom of the aluminum part will be lifted, causing serious deviation or even scrapping.
Therefore, before CNC machining, a stress relief groove is first opened on the blank, as shown by the solid line in the figure, and then it is removed from the workbench and naturally aged for 1 to 2 hours to allow deformation to occur as little as possible at this time. After that, a benchwork leveling process is added to level the part, and the deformation of the part will be greatly reduced in subsequent processing.
2) Improve the cutting ability of the tool
The tool material and geometric parameters have an important influence on the cutting force and cutting heat. The correct selection of the tool is crucial to reducing the deformation of aluminum parts.
The following are reasonable tool geometry parameters:
① Rake angle
Under the condition of maintaining blade strength, the rake angle should be appropriately larger. On the one hand, it can grind a sharp edge, and on the other hand, it can reduce cutting deformation, make chip removal smooth, and thus reduce cutting force and cutting temperature. At the same time, it is important to avoid using tools with negative rake angles.
② Back angle
The size of the back angle has a direct impact on the wear of the back tool face and the surface quality of CNC machining. Cutting thickness is an important condition for selecting the back angle. During rough milling, due to the large feed rate, heavy cutting load, and high heat generation, the tool requires good heat dissipation conditions, so a smaller back angle should be selected. During fine milling, the cutting edge is required to be sharp, the friction between the back tool face and the CNC machining surface is reduced, and the elastic deformation is reduced, so a larger back angle should be selected.
③ Helix angle
In order to make milling smooth and reduce milling force, the helix angle should be as large as possible.
④ Main deflection angle
Properly reducing the main deflection angle can improve heat dissipation conditions and reduce the average temperature of the CNC machining area.
The following are methods to improve tool structure:
① Reduce the number of milling cutter teeth and increase chip space
Due to the large plasticity of aluminum materials, the cutting deformation is large during processing, and a large chip space is required. Therefore, the radius of the chip groove bottom should be large and the number of milling cutter teeth should be small.
② Fine grinding of cutter teeth
The roughness value of the cutter tooth edge should be less than Ra=0.4um. Before using a new cutter, use a fine oil stone to lightly grind the front and back sides of the cutter teeth several times to eliminate the burrs and fine serrations left during grinding. In this way, not only can the cutting heat be reduced, but the cutting deformation is also relatively small.
③ Strictly control the wear standard of the tool
After the tool is worn, the surface roughness value of the workpiece increases, the cutting temperature increases, and the deformation of the workpiece increases. Therefore, in addition to selecting tool materials with good wear resistance, the tool wear standard should not be greater than 0.2mm, otherwise it is easy to produce chip edge. During cutting, the workpiece temperature should generally not exceed 100℃ to prevent deformation.
3) Improve the clamping method for aluminum processing
For thin-walled aluminum parts with poor rigidity, the following clamping methods can be used to reduce deformation:
① Three-jaw self-centering chuck
For thin-walled bushing parts, if a three-jaw self-centering chuck or spring collet is used for radial clamping, the workpiece will inevitably deform once it is loosened after processing. At this time, a method with better rigidity should be used for axial end face clamping. Position the inner hole of the part, make a threaded mandrel, insert it into the inner hole of the part, press the end face with a cover plate, and then tighten it with a nut. When processing the outer circle, clamping deformation can be avoided, thereby obtaining satisfactory processing accuracy.
② Vacuum suction cup
When CNC machining thin-walled and thin-plate workpieces, it is best to use a vacuum suction cup to obtain a uniformly distributed clamping force, and then perform CNC machining with a smaller cutting amount, which can well prevent the deformation of the parts.
③ Use filler method
In order to increase the process rigidity of thin-walled workpieces, a medium can be filled inside the workpiece to reduce the deformation of the workpiece during clamping and cutting. For example, pour urea melt containing 3% to 6% potassium nitrate into the workpiece. After CNC machining is completed, the workpiece can be immersed in water or alcohol, and then poured out after the filler is dissolved.
4) Reasonable arrangement of processes
During high-speed cutting, due to the large CNC machining allowance and intermittent cutting, the milling process often produces vibration, affecting the CNC machining accuracy and surface roughness. Therefore, the CNC high-speed cutting process can generally be divided into: rough machining-semi-finishing-angle clearing-finishing and other processes.
For aluminum parts with high precision requirements, sometimes secondary semi-finishing is required, and then finishing. After rough machining, the aluminum parts can be cooled naturally to eliminate the internal stress generated by rough machining and reduce deformation. The allowance left after rough machining should be greater than the deformation, generally 1 to 2mm. During finishing, the finishing surface of the part should maintain a uniform machining allowance, generally 0.2 to 0.5mm, so that the tool is in a stable state during CNC machining, which can greatly reduce cutting deformation, obtain good surface CNC machining quality, and ensure the accuracy of aluminum parts machining.
2.6 operating methods to avoid deformation of aluminum parts
In addition to improving the performance of the tool and eliminating the internal stress of the material through early aging treatment, in actual operation, the use of appropriate operating methods can effectively avoid the deformation of the material during CNC machining.
1) Symmetrical processing method
For aluminum parts with large CNC machining allowances, in order to have better heat dissipation conditions during CNC machining and avoid heat concentration, symmetrical processing should be used during CNC machining. If a 90mm thick sheet needs to be processed to 60mm, if the other side is milled immediately after one side is milled, the final size can be processed at one time, and its flatness can reach 5mm; if symmetrical processing with repeated feeding is used, each side is processed twice, and a flatness of 0.3mm can be guaranteed.
2) Layered multiple processing method
If there are multiple cavities on the sheet, as shown in the figure below. During CNC machining, it is not advisable to use a sequential CNC machining method of one cavity by one cavity, which is easy to cause uneven machining of aluminum parts and cause deformation. Use multi-layer processing, try to process each layer to all cavities at the same time, and then process the next layer, so that the aluminum parts are evenly stressed during machining and deformation is reduced.
3) Proper selection of cutting parameters
Cutting force and cutting heat can be reduced by changing the cutting parameters. Among the three elements of cutting parameters, the back cutting amount has a great influence on the cutting force. If the CNC machining allowance is too large, the cutting force of one pass will be too large, which will not only deform the aluminum parts, but also affect the rigidity of the machine tool spindle and reduce the tool durability. If the back cutting amount is reduced, the production efficiency will be greatly reduced. However, CNC machining uses high-speed milling to overcome this problem. While reducing the back cutting amount, as long as the feed rate is increased accordingly and the machine tool speed is increased, the machining efficiency of aluminum parts can be guaranteed while reducing the cutting force.
4) Pay attention to the order of cutting
Different cutting orders should be used for roughing and finishing. Roughing emphasizes improving the processing efficiency of aluminum parts and pursuing the removal rate per unit time. Generally, reverse milling can be used, that is, removing excess material on the surface of the blank at the fastest speed and in the shortest time, basically forming the geometric contour required for finishing. While finishing emphasizes high precision and high quality, it is appropriate to use down milling, because the cutting thickness of the cutter teeth gradually decreases from the maximum to zero during down milling, which greatly reduces the degree of hardening in CNC machining and also reduces the degree of deformation of aluminum parts.
5) Secondary compression of thin-walled parts
During CNC machining, thin-walled workpieces are deformed due to clamping and even cannot be finished. In order to minimize the deformation of the workpiece, the clamping part can be loosened before finishing to the final size, so that the workpiece can be freely restored to its original state, and then gently clamped. As long as the workpiece can just be clamped, the ideal CNC machining effect can be obtained. In short, the point of action of the clamping force is best on the support surface, and the clamping force should act in the rigid direction of the aluminum part. Under the premise of ensuring that the aluminum part does not loosen, the smaller the clamping force, the better.
6) Drilling first and then milling
When CNC machining aluminum parts with cavities, try not to let the milling cutter cut directly into the part like a drill bit when machining the cavity, resulting in insufficient space for the milling cutter to accommodate chips, poor chip removal, overheating and expansion of the aluminum part, and undesirable phenomena such as broken and broken knives. First, drill the hole with a drill bit of the same size or larger than the milling cutter, and then mill with the milling cutter. Alternatively, you can also use CAM software to create a spiral grooving program.