Effective Strategies to Prevent Distortion in Aluminum Alloy Machining

Aluminum alloy is an important industrial raw material. Due to its relatively low hardness and large thermal expansion coefficient, it is prone to deformation during the machining of thin-walled and thin-plate parts. To minimize processing deformation, measures can be taken not only to improve tool performance and conduct pre-aging treatments to relieve the internal stresses of the material but also to adopt specific processing techniques.

For aluminum alloy parts that require significant material removal, it’s essential to enhance heat dissipation and reduce thermal deformation by avoiding excessive heat concentration. One effective method is symmetrical processing. For instance, consider a 90 mm thick aluminum alloy plate that needs to be milled down to 60 mm. If one side is milled first and then the plate is flipped to mill the other side, the continuous processing removal can lead to heat concentration. As a result, the flatness of the milled aluminum alloy plate may only reach 5 mm.

In contrast, using a symmetrical processing method that involves repeated feeding on both sides allows for each side to be processed multiple times until the final size is achieved. This approach promotes better heat dissipation and results in improved flatness, which can be controlled to within 0.3 mm.

▌ Layered multiple processing method

When processing multiple cavities in aluminum alloy plate parts, using a method that focuses on one cavity at a time can lead to deformation of the cavity walls due to uneven force distribution. A more effective approach is to use layered processing. This involves addressing all the cavities simultaneously, but instead of completing the machining in one go, the process is divided into several layers. Each layer is processed sequentially to reach the desired dimensions. This method ensures a more uniform distribution of force across the parts, thereby reducing the likelihood of deformation.

 

▌ Proper selection of cutting amount

Choosing the appropriate cutting amount can effectively reduce both cutting force and cutting heat during the machining process. When a large cutting amount is used, it results in excessive cutting force in a single pass, which can easily lead to deformation of the parts. This, in turn, can affect the rigidity of the machine tool spindle and the durability of the cutting tool.

Among the various elements of cutting amount, the back cutting amount has the most significant impact on cutting force. Reducing the back cutting amount can help prevent part deformation; however, this may also decrease processing efficiency. High-speed milling in CNC machining can address this issue. By reducing the back cutting amount while simultaneously increasing the feed rate and the machine tool’s speed, it is possible to lower the cutting force while maintaining machining efficiency.

 

▌ Improve the cutting ability of the tool

The material and geometric parameters of the tool significantly influence the cutting force and heat generated during machining. Selecting the appropriate tool is essential to minimize the deformation of parts in the machining process.

① Reasonable selection of tool geometric parameters.
Rake Angle: It is important to choose a larger rake angle while still maintaining the strength of the blade. A larger rake angle facilitates the creation of a sharp edge and helps reduce cutting deformation. This results in smoother chip removal, which in turn decreases cutting force and temperature. Therefore, tools with a negative rake angle should be avoided.

Back Angle: The size of the back angle directly affects both the wear of the back tool face and the quality of the machined surface. Cutting thickness plays a significant role in selecting the appropriate back angle. During rough milling, where there is a high feed rate, substantial cutting loads, and significant heat generation, good heat dissipation is essential. Thus, a smaller back angle is recommended. Conversely, during fine milling, a sharper edge is required to minimize friction between the back tool face and the machined surface, as well as to reduce elastic deformation. In this case, a larger back angle should be chosen.

Helix Angle: To achieve smoother milling and reduce the milling force, it is advisable to select the largest possible helix angle.

Main Deflection Angle: Reducing the main deflection angle appropriately can enhance heat dissipation and lower the average temperature in the processing area.

② Improve the tool structure.
To optimize the machining of aluminum alloy materials, it is important to reduce the number of teeth on milling cutters and increase the chip space. Due to the high plasticity of aluminum alloys, the cutting deformation during processing is substantial, necessitating a larger chip space. Therefore, the radius of the chip groove bottom should be larger, and the number of milling cutter teeth should be minimized. For instance, milling cutters with a diameter of less than 20 mm should have two teeth, while those with diameters between 30 mm and 60 mm should have three teeth. This approach helps to prevent deformation of thin-walled aluminum machined parts caused by chip blockage.

Additionally, the cutting edges of the milling cutter teeth should undergo fine grinding to achieve a roughness value of less than Ra = 0.4 μm. Before using a new cutter, it is advisable to gently grind the front and back of the cutter teeth with a fine oil stone a few times to eliminate any burrs and minor serrations left over from the sharpening process. This not only reduces cutting heat but also minimizes cutting deformation.

Moreover, it is crucial to strictly control the wear standards of the tool. When a tool becomes worn, the surface roughness of the workpiece increases, the cutting temperature rises, and the workpiece experiences greater deformation. Thus, it is essential to select tool materials with good wear resistance and ensure that tool wear does not exceed 0.2 mm to avoid the likelihood of built-up edge formation. During cutting operations, the workpiece temperature should generally be kept below 100°C to prevent deformation.

 

▌ The cutting sequence is particular

Roughing and finishing processes should utilize different cutting sequences. During the roughing phase, the goal is to quickly remove excess material from the surface of the blank in order to shape it to meet the geometric profile needed for finishing. As such, the focus is on processing efficiency and maximizing the material removal rate per unit of time, which is why reverse milling is recommended.

In contrast, the finishing phase has higher requirements for accuracy and surface quality, emphasizing the importance of processing quality; therefore, down milling is preferred. This method allows the cutting thickness of the cutter teeth to gradually decrease from the maximum to zero, significantly reducing the work hardening phenomenon. This also helps to minimize deformation of the parts to some extent.

 

▌ Secondary clamping of thin-walled parts

When processing thin-walled parts made from aluminum alloy, the clamping force applied during the clamping process can significantly contribute to deformation. This issue is often unavoidable, even with improvements in processing accuracy.

To minimize the deformation of the workpiece caused by clamping, it’s beneficial to loosen the clamped parts before reaching the final dimensions during the finishing process. By releasing the clamping force, the parts can return to their original shape, after which they can be gently re-clamped. It is advisable to position the secondary clamping points on the support surface, ensuring that the clamping force is directed along the line of greatest rigidity of the workpiece.

The clamping force needs to be sufficient to hold the workpiece securely without causing any loosening. This requirement places a high demand on the operator’s skill and sensitivity. By utilizing this method, processed parts exhibit reduced compression deformation.

▌ Drilling first and then milling method

When processing parts with cavities, inserting the milling cutter directly into the part often leads to poor chip removal due to the insufficient chip space of the cutter. This can result in the accumulation of cutting heat, causing the part to expand and deform. In severe cases, it may even lead to accidents such as chip collapse or cutter breakage.

The most effective method to address these issues is to drill first and then mill. Begin by drilling a hole with a drill bit that is at least the same diameter as the milling cutter. Once the hole is created, you can then insert the milling cutter into the drilled hole to start the milling process. This approach significantly improves chip removal and minimizes the associated risks.

 

 

 

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