Many reasons cause aluminium parts deformation, which are related to the material quality, the parts shape and the production conditions.
What cause aluminum deformetion?
There are 3mainly reasons:
- deformation caused by internal stress of blank.
- deformation caused by cutting force, cutting heat.
- deformation caused by clamping force.
How to avoid Aluminum deformation?
Technological Measures for Reducing Aluminum Deformation .
1.Reducing the Internal Stress of aluminum rough part
The internal stress of rought part can be partly eliminated by natural or artificial aging and vibration treatment. Pre-processing is also an effective process. For big rough aluminum part, the deformation after machining is also large. If cut the extra aluminum material from rought part, get precise tolerance the deformation can be improved. Also a part of internal stress can be released after stay for a period of time.
2.Improving Cutting Ability of Cutting Tools
Material and geometric parameters of cutting tools have an important influence on cutting force and heat. Correct selection of cutting tools is very important to reduce machining deformation of parts.
1.Reasonable selection of tool geometry parameters.
- Front angle: Under the condition of maintaining the edge strength, the front angle should be selected appropriately larger. On the one hand, it can grind a sharp edge, on the other hand, it can reduce cutting deformation, make chip removal smoothly, and then reduce cutting force and cutting temperature. Never use negative rake tool.
- Back angle: The size of back angle has a direct impact on the wear of the flank and the quality of the machined surface. Cutting thickness is an important condition for choosing rear angle. In rough milling, because of the large feed, heavy cutting load and high heat output, the cutter is required to have good heat dissipation conditions, so the back angle should be selected smaller. In finishing milling, sharp edges are required to reduce friction between the flank and the machined surface and reduce elastic deformation. Therefore, a larger rear angle should be chosen.
- Helix angle: In order to make milling smooth and reduce milling force, the helix angle should be selected as large as possible.
- Principal deviation angle: Appropriate reduction of the main deviation angle can improve the heat dissipation conditions and reduce the average temperature of the processing area.
2. Improve cutting tool structure.
- Reduce the number of milling cutter teeth and increase chip space. Because of the large plasticity of aluminium material and the large cutting deformation in processing, it needs a larger chip space, so the radius of the bottom of the chip groove should be larger and the number of milling cutter teeth should be less.
- Fine grinding of cutter teeth. The roughness of the cutting edge is less than Ra=0.4um. Before using a new cutting tool, a few gentle rubbings should be made on the front and back of the cutter teeth with fine asphalt to eliminate the burrs and slight serrated marks remaining when grinding the tool teeth. In this way, not only cutting heat can be reduced, but also cutting deformation is relatively small.
- Strict control of tool wear standards. After tool wear, the roughness of workpiece surface increases, cutting temperature increases, and workpiece deformation increases. Therefore, in addition to choosing tool materials with good wear resistance, the tool wear standard should not be greater than 0.2 mm, otherwise it is easy to produce chip tumors. When do cutting by CNC Milling or CNC Turning, the temperature of the machining workpiece should not exceed 100 ℃ in order to prevent deformation.
3.Improving the clamping method of workpiece
For Thin-walled Aluminium workpieces with poor rigidity, the following clamping methods can be used to reduce deformation:
- For thin-wall CNC Machining bushing parts, if three-jaw self-centering chuck or spring chuck is used to clamp from the radial direction. Once it 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. A threaded mandrel is made for positioning the inner hole of the part. It is inserted into the inner hole of the part. A cover plate is used to press the end face and a nut is used for back tightening. The clamping deformation can be avoided when the outer circle is machined, and the satisfactory machining accuracy can be obtained.
- When machining thin-wall sheet workpiece, it is better to choose vacuum sucker to obtain uniform clamping force. And then to process with smaller cutting parameters, which can prevent machined workpiece deformation.
- In addition, the filling method can also be used. In order to increase the technological rigidity of thin-walled workpiece, medium can be filled in the workpiece to reduce the deformation of workpiece during clamping and cutting. For example, urea melt containing 3% to 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.
4.Reasonable Arrangement of Processes
In high-speed cutting, because of large machining allowance and intermittent cutting, milling process often produces vibration, which affects the processing accuracy and surface roughness. Therefore, the NC high-speed cutting process can be generally divided into roughing, semi-finishing, angle clearing and finishing processes. For parts requiring high precision, it is sometimes necessary to finish them twice and half, and then finish them. After rough machining, the parts can be cooled naturally to eliminate the internal stress and reduce the deformation. The allowance left after roughing should be greater than the deformation, generally 1-2 mm. When finishing, the surface of parts should be kept uniform. Generally, 0.2-0.5mm is the best way to keep the tool in a stable state during the process of finishing, which can greatly reduce the cutting deformation, obtain good surface processing quality and ensure the accuracy of the product.
Operational Skills for Reducing Aluminum Machining Deformation
Except the above reasons, the operation method is also very important in the actual operation.
For parts with large machining allowance
In order to make them have better heat dissipation conditions and avoid heat concentration in machining, symmetrical processing should be adopted. If a 90 mm thick sheet needs to be processed to 60 mm, if the other side is milling immediately after milling one side, and the flatness reaches 5 mm once to the final dimension. If the repeated feed symmetrical processing is used, each side is processed to the final dimension twice, the flatness can be guaranteed to reach 0.3 mm.
If there are more than one cavity in the sheet metal parts
The sequential processing method of one cavity and one cavity should not be adopted in the processing, which is easy to cause uneven force and deformation of the parts. Layered multi-processing is adopted, each layer is machined to all the cavity at the same time, and then the next layer is machined, so that the parts are uniformly stressed and the deformation is reduced.
Reduce cutting force and heat by changing cutting parameters.
Among the three factors of cutting parameters, back feed has a great influence on cutting force. If the machining allowance is too large and the cutting force of the tool is too large. Not only will the parts be deformed, but also will affect the rigidity of the machine tool spindle and reduce the tool durability. If we reduce the amount of knives eaten by the back, the production efficiency will be greatly reduced. However, high-speed milling can overcome this problem in NC machining. At the same time, the cutting force can be reduced and the processing efficiency can be guaranteed as long as the feed is increased correspondingly and the speed of the machine tool is increased.
We should also pay attention to the sequence of cutting tool walking way.
Rough milling emphasizes the improvement of processing efficiency and the pursuit of cutting rate per unit time. Reverse milling can be generally used. That is to say, the superfluous material on the surface of the blank is removed at the fastest speed and in the shortest time, and the geometric contour required for finishing is basically formed. Precision machining emphasizes high precision and high quality, and it is advisable to use forward milling. Because the cutting thickness of the cutter teeth decreases gradually from the maximum to zero, the degree of work hardening is greatly reduced, and the degree of deformation of the parts is also reduced.
The deformation of thin-walled workpiece is inevitable even in finishing because of clamping.
In order to minimize the deformation of the workpiece, it is possible to loosen the pressing part before finishing is about to reach the final dimension, so that the workpiece can be freely restored to its original state, and then slightly compressed, whichever is rigid enough to hold the workpiece (completely by hand), so as to achieve the desired processing effect. In a word, the point of action of clamping force is best on the supporting surface. The clamping force should be applied in the direction of good rigidity of the workpiece. The smaller the clamping force, the better, on the premise of ensuring that the workpiece is not loose.
Don’t use milling cutter like drill
When machining the parts with cavity, try not to let the milling cutter penetrate directly into the parts like a drill bit, resulting in inadequate chip space and unsmooth chip removal of the milling cutter, resulting in overheating, expansion of the parts, tool breaking and other adverse phenomena. First drill the hole with a bit of the same size or size as the milling cutter, and then milling with the milling cutter. Alternatively, the spiral downcutter program can be produced with CAM software.
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