There are many reasons for the deformation of aluminum parts, which are related to the material, shape of the part, and production conditions. Machining deformation of aluminum parts mainly includes the following aspects: deformation caused by internal stress of blank, deformation caused by cutting force and cutting heat, and deformation caused by clamping force.
Process Measures to Reduce Machining Deformation of Aluminum Parts
1. Reduce The Internal Stress of Aluminum Blank
Natural or artificial aging and vibration treatment can partially eliminate the internal stress of the blank. Pre-processing is also an effective process method. For coarse blank, due to the large margin, the deformation after processing is also large. If the redundant part of the blank is processed in advance, the margin of each part is reduced, which can not only reduce the processing deformation of the subsequent process but also place for a period of time after pre-processing that can release a part of the internal stress.
2. Improve The Cutting Capability of The Tool
The material and geometrical parameters of the cutting tool have an important influence on cutting force and cutting heat. The correct choice of the cutting tool is very important to reduce the machining deformation of aluminum parts.
1)Reasonable selection of tool geometry parameters
①Rake angle: Under the condition of maintaining the strength of the tool, the rake angle should be larger. The larger Rake Angle, on the one hand, creates a sharp edge, and on the other hand, reduces cutting deformation and chip removal, thus reducing cutting force and cutting temperature. Never use negative rake cutting tools.
②Clearance Angle: The size of the clearance Angle has a direct effect on the wear of the back tool surface and the quality of the machined surface. The cutting thickness is an important condition for selecting the clearance Angle. In rough milling, due to the large feed, heavy cutting load, high heat, and the requirements of the tool heat dissipation conditions, therefore, the clearance Angle should be selected to be smaller. When finishing milling, the cutting edge is required to be sharp, to reduce the friction between the cutter surface and the processing surface, and to reduce the elastic deformation. Therefore, the rear Angle should be selected larger.
③Helix Angle: In order to make the milling smooth, and reduce the milling force, the helix Angle should be selected as large as possible.
④Plan Angle: Properly reducing the main deflection Angle can improve the heat dissipation conditions, so that the average temperature of the processing zone decreased.
2)Improve Tool Structure
①Reduce the number of cutter teeth, increasing the chip space. Due to the large plastic aluminum material, large cutting deformation in processing, and the need for large chip tolerance space, so the bottom radius of the chip tolerance groove should be large, and the number of milling cutter teeth is less.
②Fine grinding teeth. The roughness value of the cutting edge is less than Ra=0.4um. Before using a new knife, we should gently grind the teeth with a fine whetstone to eliminate burrs and slight serrations. In this way, not only cutting heat can be reduced but also the cutting deformation is relatively small.
③Strictly control tool wear standard. After tool wear, the workpiece surface roughness increases, the cutting temperature rises, and the workpiece deformation increases. Therefore, in addition to the use of tool materials with good wear resistance, the tool wear standard should not be greater than 0.2mm, otherwise it is easy to produce accumulated edges. When cutting, the temperature of the workpiece should generally not exceed 100 ° C to prevent deformation.
3. Improve The Clamping Method of Workpiece
For thin-walled aluminum workpieces with poor rigidity, the following clamping method can be adopted to reduce the deformation of aluminum parts:
①For thin-wall bushing parts, if the use of a three-jaw self-centering chuck or spring chuck from the radial clamping, once loosened after processing, the workpiece is bound to deformation. At this time, the method of axial end face pressing with good rigidity should be used. In order to locate the inner hole of the parts, a self-made threaded mandrel is set into the inner hole of the parts, and a cover plate is used to press the end face and then the nut is tightened back. The clamping deformation can be avoided when machining the outer circle, thus obtaining satisfactory machining accuracy.
②When machining a thin-wall thin sheet workpiece, it is best to choose a vacuum sucker to obtain evenly distributed clamping force, and then to process with small cutting consumption, which can well prevent workpiece deformation.
In addition, a tamping method can be used. In order to increase the processing rigidity of thin-walled workpieces, the workpiece can be filled with a medium to reduce the workpiece deformation during clamping and cutting. For example, to the workpiece filled with 3% to 6% potassium nitrate urea melt, after processing, the workpiece immersed in water or alcohol, you can dissolve the filling out.
4. Reasonable Arrangement of Aluminum Parts Machining Process
In high-speed cutting, vibration often occurs in the milling process due to large machining allowance and discontinuous cutting, which affects machining accuracy and surface roughness. Therefore, CNC high-speed cutting process can be generally divided into rough machining – half finishing – clear Angle machining – finishing, and other processes. For parts with high precision requirements, it is sometimes necessary to perform secondary semi-finishing and then finishing. After rough machining, the parts can be cooled naturally to eliminate the internal stress caused by rough machining and reduce deformation. The margin-left after rough machining should be greater than the deformation, generally 1 ~ 2mm. When finishing, the finishing surface of the parts should maintain a uniform machining margin, generally 0.2 ~ 0.5mm is appropriate so that the tool is in a stable state during the processing process, which can greatly reduce the cutting deformation, obtain good surface processing quality, and ensure the accuracy of the product.
Operation Techniques for Reducing Machining Deformation of Aluminum Parts
Parts of aluminum materials deform during machining. In addition to the above reasons, the operation method is also very important in practical operation.
- For the parts with a large processing allowance, in order to make them have better heat dissipation conditions in the process of machining, and to avoid heat concentration, when processing, it is appropriate to use symmetrical processing. If a 90mm thick sheet needs to be processed to 60mm, if one side is milled immediately after the other side is milled, and the final size is processed at one time, the flatness reaches 5mm; If repeated feeding symmetrical processing, each side is processed twice to the final size, the flatness can be guaranteed to reach 0.3mm.
- If there are multiple cavities on the plate parts, when processing, it is not appropriate to use the one cavity and one cavity order processing method, which is easy to cause uneven force on the parts and produce Machining Deformation of Aluminum Parts. The use of Layered multiple machining, each layer as far as possible to process to all the cavity, and then process the next level, so that the parts are evenly forced, reduces deformation of aluminum parts.
- Reduce cutting force and cutting heat by changing the cutting dosage. Among the three factors of cutting amount, the cutting force is greatly affected by the back cutting amount. If the machining allowance is too large, the cutting force of a pass is too large, which will not only deform the aluminum parts but also affect the rigidity of the machine tool spindle and reduce the durability of the tool. If the number of knives to be eaten by the back is reduced, the production efficiency will be greatly reduced. However, CNC machining is high – a speed milling, which will overcome this problem. The cutting force can be reduced and the machining efficiency can be guaranteed as long as the feed is increased correspondingly and the machine speed is increased.
- The order of cutting feed should also be paid attention to. Rough machining emphasizes improving processing efficiency and pursuing the cutting rate per unit time. Generally, reverse milling can be used. That is to remove the excess material on the surface of aluminum parts blank with the fastest speed and the shortest time to form the geometric contour required by finishing. The finishing emphasizes high precision and high quality, and it is advisable to use smooth milling. Because the cutting thickness of the tool tooth decreases gradually from the maximum to zero, the degree of work hardening is greatly reduced, and the degree of deformation of parts is reduced.
- Thin-walled workpieces are deformed during machining due to clamping, which is difficult to avoid even in finishing. In order to reduce the deformation of the workpiece to a minimum, before finishing to reach the final size, the compression parts lose, so that the workpiece is free to restore to its original state, and then slightly compressed, to just clamp the workpiece (completely by hand), so that the ideal processing effect can be obtained. In short, the point of action of the clamping force is best on the support surface, the clamping force should act in the direction of good stiffness of the workpiece, under the premise of ensuring that the workpiece does not loosen, the smaller the clamping force, the better.
- In the processing of cavity parts, processing cavity as far as possible not to let the milling cutter like drill directly down into the parts, resulting in milling cutter chip space is not enough, chip removal is not smooth, causing parts overheating, expansion and collapse knife, broken knife and other adverse phenomena. The hole is drilled with a drill bit of the same size or one size as the milling cutter and then milled with a milling cutter. Alternatively, CAM software can be used to produce spiral-cutting programs.
The main factor affecting the machining accuracy and surface quality of aluminum parts is that the parts are prone to deformation in the machining process, which requires operators to have certain operating experience and skills.