There are many reasons for the deformation of aluminum parts during CNC aluminum processing, which are related to materials, part shapes, production conditions, etc. These are mainly the following aspects: deformation caused by the internal tension of the blank, the cutting force, the cutting heat and also the clamping force. aluminum CNC Processing has a great impact on the machining of aluminum CNC parts, especially for customized aluminum parts, greater attention needs to be paid to the main process.
Process measures to reduce processing deformations
1. Reducing the internal stress of the blank
The natural stress of the blank can be partially eliminated by natural or artificial aging and vibration treatment. Pre-processing is also an effective process. For the large head of the thick head, the deformation after processing is also large due to the large rim. If the excess part of the blank is processed in advance and the remaining quantity of each part is reduced, not only can the processing deformation of the subsequent process be reduced, but also some of the internal stress can be released for a certain period of time after pre-processing and placement.
2. Improve tool cutting ability
Tool material and geometry parameters have a great influence on cutting force and cutting heat. Proper tool selection is essential to reduce part deformation.
2.1 Sensible selection of tool geometry parameters
Front angle: Under the condition of maintaining the strength of the cutting edge, the front angle is selected accordingly. On the one hand, the sharp cutting edge can be ground and the cutting deformation can be reduced to make chip removal smooth, thereby reducing the cutting force and cutting temperature. Never use a cutter with a negative rake angle.
Back angle: The size of the back angle has a direct influence on flank wear and the quality of the machined surface. The thickness of the cut is an important requirement for the selection of the back angle. When grinding, the back angle should be smaller due to the high feed speed, high cutting load, large heat generation and good heat dissipation conditions of the tool. When finishing milling, the edge must be sharpened, the friction between the flank and the machined surface is reduced, and the elastic deformation is reduced. Therefore, the angle of incidence should be chosen larger.
Spiral angle: To allow uniform milling and reduce milling force, the spiral angle should be as large as possible.
Tilt angle: Reducing the tilt angle appropriately can improve heat dissipation and reduce the average temperature in the processing area.
2.2 Improvement of tool structure
- Reduce the number of cutter teeth and increase chip space. Due to the high plasticity of aluminum material, the cutting deformation during processing is large and a lot of space is required for chipping. Therefore, the bottom radius of the chip groove should be large and the number of cutter teeth should be small.
- Use fine teeth. The roughness value of the cutter cutting edge is less than Ra = 0.4 µm. Before using a new knife, you should carefully sand the front and back of the teeth with a fine stone to remove burrs and small zigzags left from sharpening the teeth. In this way, not only can the cutting heat be reduced, but also the cutting deformation is relatively small.
- Strictly control the tool wear pattern. As the tool wears, the surface roughness of the workpiece increases, the cutting temperature increases, and the deformation of the workpiece also increases. Therefore, in addition to selecting highly abrasion-resistant tool materials, the wear pattern of the tool should not be more than 0.2mm, otherwise it is easy to build up edges. When cutting, the temperature of the workpiece should not exceed 100°C to avoid deformation.
2.3 Improvement of the part clamping method
For aluminum parts with thin walls and low rigidity, the following fastening methods can be used to reduce distortion:
- For thin-walled bushing parts, when the three-jaw self-centering chuck or chuck is used for clamping in the radial direction, the workpiece will inevitably be deformed when it is released after machining. At this time, an axial end surface pressing method with good rigidity should be used. To position the inner hole of the part, a threaded mandrel is made and inserted into the inner hole of the part, and a cover plate is pressed against the end surface and then tightened with a nut. When the outer circle is machined, clamping deformation can be avoided and satisfactory machining accuracy can be achieved.
- When processing thin-walled and thin-walled parts, it is best to use vacuum suction cups to achieve even distribution of clamping force, and then process with a small amount of cutting, which can avoid deformation of the part.
- Additionally, a stuffing method can also be used. In order to increase the process rigidity of the thin-walled part, the medium can be filled into the part to reduce the deformation of the part during the clamping and cutting process. For example, a urea melt containing 3 to 6% potassium nitrate is poured onto the workpiece. After processing, the part is dipped in water or alcohol and the filler can be dissolved and poured out.
2.4 Appropriate procedural rules
In high-speed cutting, due to large machining tolerances and interrupted cuts during milling, vibrations are often generated, which affect machining accuracy and surface roughness. Therefore, the high-speed CNC machining process can generally be divided into roughing, semi-finishing, cleaning, finishing and finishing. For parts with high precision requirements, it is sometimes necessary to perform secondary prefinishing before finishing.
After grinding, parts can be cooled naturally, eliminating internal stresses caused by grinding and reducing deformation. The margin remaining after grinding should be greater than the deformation, generally 1 to 2 mm. When finishing, the finished surface of the workpiece must maintain a uniform machining margin, generally 0.2-0.5mm, so that the tool is in a stable state during the machining process, significantly reducing cutting deformation and achieving good surface machining quality and product precision can.
Reduction in processing capabilities when processing deformation
Parts of the aluminum material are deformed during processing. In addition to the above reasons, the operation method in actual operation is also very important.
- To ensure better heat dissipation conditions during the machining of parts with a large machining margin, heat concentration must be avoided and symmetrical machining must be used during machining. If a piece of material 90mm thick needs to be machined to 60mm, the other side will be milled and the flatness will be 5mm once. If processed by repeated delivery, each page will be processed twice. The final size guarantees a flatness of 0.3 mm.
- If the plate parts have multiple cavities, it is not advisable to use the cavities and cavities arrangement method during processing, as it is easy to cause deformation of the parts due to uneven force. Multilayer processing is used, each layer is processed in all cavities at the same time, and then the next layer is processed to load the parts evenly and then reduce deformation.
- Reduce cutting force and cutting heat by changing the cutting amount. Of the three cutting quantity factors, the counter knife quantity has a great influence on the cutting force. If the machining tolerance is too large, the cutting force of one pass will be too large, which not only deforms the parts, but also affects the rigidity of the machine spindle and reduces the durability of the tool. Reducing the number of consecutive knives will significantly reduce production efficiency. However, high-speed milling in CNC machining can solve this problem. By reducing the amount of counter feed, as long as the feed is increased accordingly and the speed of the machine tool is increased, the cutting force can be reduced and the machining efficiency can be guaranteed.
- The order of the knives must also be taken into consideration. Thinning focuses on improving processing efficiency and aiming for a resection rate per unit time. In general, up milling can be used. This removes excess material from the surface of the blank at the highest speed and in the shortest time, essentially creating the geometric contour necessary for finishing. The finishing focuses on high precision and high quality, and the use of milling is recommended. As the cutting thickness of cutters gradually decreases from maximum to zero during milling, the degree of work hardening is greatly reduced and the degree of deformation of parts is alleviated.
- Thin-walled parts deform due to clamping during machining, even if final machining can hardly be avoided. To minimize deformation of the part, the pressure part can be released before the finish reaches the final size, so that the part can freely return to its original shape, and then lightly pressed just to secure the part.
Depending on how you feel, the desired processing results may be achieved. In short, the application point of the clamping force is preferably on the supporting surface, and the clamping force should be applied in the direction of the rigidity of the part. As long as the workpiece is not loose, the clamping force is as small as possible.
- When machining cavities, try not to let the cutter penetrate directly into the parts like a drill when machining the cavity. This leads to insufficient space for the cutter and chip flow is not smooth, resulting in overheating, expansion and collapse of parts, as well as adverse phenomena such as broken knives and knives. First drill the hole with a drill bit the same size or larger than the router bit and then mill it with a milling cutter. Alternatively, CAM software can be used to create a spiral undercut program.
The main factor that affects the processing accuracy and surface quality of aluminum parts is that they are prone to deformation during the processing of these parts, which requires the operator to have certain operating experience and skills.
