Aluminum Parts Machining: Common Methods to Solve the Machining Deformation of Aluminum Parts

 

 

 

 

In layman's terms, the fixture is six points (3+2+1: three-point fixed surface, two-point fixed line, one-point fixed), and machining needs to solve the deformation. There are many reasons for the deformation of common aluminum parts machining, which are related to the material, shape of the part, and production conditions. There are several main 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 the deformation of aluminum parts

 

 

1. Reduce the internal stress of the 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 larger blanks, due to the large allowance, the deformation after processing is also large. If the excess parts of the blank are machined in advance and the allowance of each part is reduced, it can not only reduce the deformation of the CNC machining in the subsequent process, but also release a part of the internal stress after being placed for a period of time after the CNC machining in advance.

 

 

 

 

For example, Figure 1 shows a beam part, the shape of the blank as shown by the double-dotted line in the figure weighs 60kg, while the part weighs only 3kg. If it is processed and formed at one time according to the dotted line in the figure, the flatness error can be as high as 14mm. If the pre-processing is carried out according to the solid line in the figure, and then processed into the required aluminum parts after natural aging for a period of time, the flatness error can be reduced. as small as 3mm.

 

Figure 2 shows a part of a certain type of cap piercer, the local minimum thickness is only 3mm, and the thickness of the blank before CNC machining is 20mm. The aluminum parts can be directly processed to the size by changing the pressure plate on the CNC machining center, but when they are removed from the worktable, the bottom ends of the aluminum parts will be lifted up, resulting in serious oversize or even scrapped.

 

 

 

 

Therefore, before CNC machining, a stress relief groove is first opened on the blank, as shown by the solid line in Figure 3, and then removed from the worktable, and the natural aging is 1 to 2 hours, so that the deformation occurs as much as possible at this time. After that, a fitter leveling process is added to level the part, and the deformation of the part in the subsequent processing will be greatly reduced.

 

 

 

 

2. Improve the cutting ability of the tool

 

 

The material and geometric parameters of the tool have an important influence on the cutting force and cutting heat. The correct selection of the tool is very important to reduce the deformation of aluminum parts.

 

(1) Reasonable selection of tool geometric parameters.

 

①Rake angle: Under the condition of maintaining the strength of the blade, the rake angle is appropriately selected to be larger, on the one hand, it can grind a sharp edge, and on the other hand, it can reduce the cutting deformation, make the chip removal smooth, and then reduce the cutting force and cutting temperature. Never use tools with negative rake angle.

 

②Relief angle: The size of the relief angle has a direct impact on the wear of the flank and the surface quality of CNC machining. The cutting thickness is an important condition for selecting the clearance angle. During rough milling, due to the large feed rate, heavy cutting load, and large heat generation, the tool requires good heat dissipation conditions. Therefore, the clearance angle should be selected to be smaller. When fine milling, the cutting edge is required to be sharp, the friction between the flank and the CNC machined surface is reduced, and the elastic deformation is reduced. Therefore, the clearance angle should be larger.

 

③ Helix angle: In order to make the milling smooth and reduce the milling force, the helix angle should be as large as possible.

 

④Main declination angle: Properly reducing the main declination angle can improve the heat dissipation conditions and reduce the average temperature of the CNC machining area.

 

 

(2) Improve the tool structure.

 

 

①Reduce the number of teeth of the milling cutter and increase the chip space. Due to the large plasticity of the aluminum material and the large cutting deformation during processing, a large chip space is required, so the bottom radius of the chip groove should be large and the number of milling cutter teeth should be small.

 

② Finely grind the teeth. The roughness value of the cutting edge of the cutter teeth should be less than Ra=0.4um. Before using a new knife, you should use a fine oil stone to lightly sharpen the front and back of the knife teeth a few times to eliminate the burrs and slight serrations left when sharpening the teeth. In this way, not only can the cutting heat be reduced, but also the cutting deformation is 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 rises, and the workpiece deformation increases. Therefore, in addition to the selection of tool materials with good wear resistance, the tool wear standard should not be greater than 0.2mm, otherwise it is easy to produce built-up edge. When cutting, the temperature of the workpiece should generally not exceed 100 ℃ to prevent deformation.

 

 

 

3. Improve the clamping method of Aluminum parts machining

 

 

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 a three-jaw self-centering chuck or a spring collet is used for radial clamping, once it is released after CNC machining, the workpiece will inevitably deform. At this time, the method of pressing the axial end face with better rigidity should be used. 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 on it, and then tighten it with a nut. When CNC machining the outer circle, clamping deformation can be avoided, so as to obtain satisfactory machining accuracy.

 

(2) When performing CNC machining on thin-walled and thin-plate workpieces, it is best to use vacuum suction cups to obtain evenly distributed clamping force, and then perform CNC machining with a small amount of cutting, which can well prevent deformation of the parts.

 

(3) Use the packing 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, urea melt containing 3% to 6% potassium nitrate is poured into the workpiece. After CNC machining, the workpiece can be immersed in water or alcohol, and the filler can be dissolved and poured out.

 

 

4. Reasonable arrangement of processes

 

During high-speed cutting, due to the large CNC machining allowance and intermittent cutting, the milling process often generates vibration, which affects the CNC machining accuracy and surface roughness. Therefore, the CNC high-speed cutting process can generally be divided into: roughing-semi-finishing-corner-clearing-finishing and other processes. For the processing of aluminum parts with high precision requirements, it is sometimes necessary to perform secondary semi-finishing and then finishing. After rough machining, aluminum parts can be cooled naturally, eliminating the internal stress caused by rough machining and reducing deformation. The allowance left after rough machining should be greater than the deformation, generally 1 to 2 mm. When finishing, the finishing surface of the parts should maintain a uniform machining allowance, generally 0.2 ~ 0.5mm, so that the tool is in a stable state during the CNC machining process, which can greatly reduce cutting deformation and obtain good surface CNC machining quality , to ensure the accuracy of Aluminum parts machining.

 

 

 

 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 by aging treatment in advance, 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 allowance, in order to make them have better heat dissipation conditions during the CNC machining process and avoid heat concentration, symmetrical machining should be adopted during CNC machining. If a 90mm thick sheet needs to be processed to 60mm, if one side is milled and the other side is milled immediately, and the final size is processed at one time, the flatness will reach 5mm; if it is processed symmetrically with repeated feeding, each side is processed twice to The final dimension can guarantee a flatness of 0.3mm.

 

 

2. Layered multiple processing method

 

If there are multiple cavities on the sheet part, as shown in the picture below. During CNC machining, it is not appropriate to use the sequential CNC machining method of one cavity and one cavity, which will easily cause uneven processing of aluminum parts and cause deformation. Multi-layer processing is adopted, and each layer is processed to all the cavities at the same time as possible, and then the next layer is processed to make the aluminum parts process evenly stressed and reduce deformation.

 

 

 

 

3. Proper selection of cutting amount

 

The cutting force and cutting heat can be reduced by changing the cutting amount. Among the three elements of cutting amount, the amount of back-engagement has a great influence on the cutting force. If the CNC machining allowance is too large, the cutting force of one 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 amount of knives to be eaten by the back is reduced, the production efficiency will be greatly reduced. However, high-speed milling is used in CNC machining, which can overcome this problem. While reducing the amount of back-engagement, 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 processing efficiency of aluminum parts can be guaranteed at the same time.

 

 

4. Pay attention to the order of the knives

 

Roughing and finishing should use different pass sequences. Rough machining emphasizes improving the processing efficiency of aluminum parts and pursuing the removal rate per unit time. Generally, up-cut milling can be used. That is, the excess material on the surface of the blank is removed at the fastest speed and the shortest time, and the geometric contour required for finishing is basically formed. While finishing emphasizes high precision and high quality, it is advisable to use down milling. Because the cutting thickness of the cutter teeth gradually decreases from the maximum to zero during down milling, the degree of CNC machining hardening is greatly reduced, and the deformation degree of Aluminum parts machining is also reduced.

 

 

5. Secondary compression of thin-walled parts

 

Thin-walled workpieces are deformed due to clamping during CNC machining, and even finishing is unavoidable. In order to reduce the deformation of the workpiece to a minimum, you can loosen the pressing part before finishing the final size, so that the workpiece can freely return to its original state, and then slightly press it, as long as the workpiece can just be clamped, so that The ideal CNC machining effect can be obtained. In short, the action point of the clamping force is preferably on the supporting surface, and the clamping force should act in the direction of the rigidity of the aluminum parts. On the premise of ensuring that the aluminum parts are not loosened, the smaller the clamping force, the better.

 

 

6. Drilling before milling

 

When CNC machining aluminum parts with a cavity, try not to let the milling cutter plunge directly into the part like a drill when machining the cavity, resulting in insufficient space for the milling cutter to hold the chips, and the chip removal is not smooth, resulting in overheating and expansion of the aluminum parts. And the unfavorable phenomena such as broken knife and broken knife. First drill the hole with a drill of the same size as the milling cutter or one size larger, and then mill it with the milling cutter. Alternatively, CAM software can be used to produce helical rundown programs.