Aluminum parts can deform during processing due to various factors, including material properties, part shape, and production conditions. There are primarily three aspects to consider: deformation from internal stress of the blank, deformation from cutting forces and heat, and deformation due to clamping force.
1) Process measures to reduce processing deformation
1. Reduce the internal stress of the blank
Natural or artificial aging, along with vibration treatment, can help reduce the internal stress of the blank. Pre-processing is also an effective method for improvement. For blanks with a large head and significant protrusions, the deformation during processing can be substantial due to the oversized features. By pre-processing the excess material and reducing the margin in each area, we can lessen the deformation that occurs in subsequent processing steps and also alleviate some of the internal stress from the pre-processing.
2. Improve the cutting ability of the tool
The tool’s material and geometric parameters significantly affect cutting force and heat. Selecting the right tool is crucial for minimizing part deformation during processing.
1) Reasonably select the tool geometric parameters.
① Rake angle: To maintain blade strength, it is important to select a larger rake angle. This has two main benefits: first, it allows for the grinding of a sharper edge, and second, it reduces cutting deformation, which facilitates smooth chip removal. As a result, this approach helps lower both cutting force and cutting temperature. Additionally, it is advisable to avoid using tools with a negative rake angle.
② Back angle: The size of the back angle significantly affects both the wear of the back tool face and the quality of the machined surface. When selecting the back angle, it is crucial to consider the cutting thickness.
During rough milling, the high feed rate, heavy cutting load, and substantial heat generation necessitate effective heat dissipation. As a result, a smaller back angle should be chosen.
In contrast, during fine milling, a sharp edge is essential to reduce friction between the back tool face and the machined surface, as well as to minimize elastic deformation. Therefore, a larger back angle is recommended for fine milling.
③ Helix angle: In order to make milling smooth and reduce milling force, the helix angle should be selected as large as possible.
④ Main deflection angle: Reducing the main deflection angle can enhance heat dissipation and lower the average temperature in the processing area.
2) Improve tool structure.
- To optimize the CNC milling process for aluminum materials, it is important to reduce the number of teeth on the milling cutter and increase the chip space. Aluminum has a high plasticity, which leads to significant cutting deformation during processing. As a result, a larger chip space is necessary. Therefore, the radius at the bottom of the chip groove should be increased, and the milling cutter should have fewer teeth.
- Fine grinding of cutter teeth. The cutting edge of the cutter teeth should have a roughness value of less than Ra = 0.4 µm. Before using a new cutter, take a fine oil stone and gently grind the front and back of the cutter teeth a few times. This process helps eliminate burrs and minor sawtooth patterns that may remain after sharpening. By doing so, you can reduce cutting heat and minimize cutting deformation.
- It is essential to strictly monitor the wear standards of the cutting tool. As the tool wears down, the surface roughness of the workpiece increases, resulting in higher cutting temperatures and greater workpiece deformation. Therefore, in addition to selecting tool materials with excellent wear resistance, it is crucial to ensure that the tool wear does not exceed 0.2 mm. Exceeding this limit can lead to the formation of chip edges. Additionally, the temperature of the workpiece during cutting should typically not exceed 100°C to prevent deformation.
3. Improve the clamping method of the workpiece.
For thin-walled aluminum workpieces with poor rigidity, the following clamping methods can be used to reduce deformation:
① For thin-walled bushing components, using a three-jaw self-centering chuck or a spring collet for radial clamping can lead to deformation of the workpiece once it is loosened after processing. To address this issue, a more rigid clamping method should be employed, specifically the axial end face clamping method.
First, position the inner hole of the component and create a threaded through-mandrel that inserts into the inner hole. Then, use a cover plate to clamp the end face and secure it with a nut. This approach helps to prevent clamping deformation when processing the outer circle, ensuring satisfactory processing accuracy.
② When processing thin-walled sheet workpieces, using a vacuum suction cup is recommended to achieve a uniformly distributed clamping force. It is also advisable to work with a smaller cutting amount to effectively prevent deformation of the workpiece.
In addition, the packing method can be employed to enhance the processing rigidity of thin-walled workpieces. By filling the interior of the workpiece with a medium, we can minimize deformation during clamping and cutting. For example, a urea melt containing 3% to 6% potassium nitrate can be poured into the workpiece. After the machining process, the workpiece can be immersed in water or alcohol to dissolve the filler, allowing it to be easily poured out.
4. Reasonable arrangement of processes
During high-speed cutting, the milling process often generates vibrations due to large machining allowances and intermittent cutting, which can negatively affect machining accuracy and surface roughness. As a result, the CNC metal cutting service is typically divided into several stages: roughing, semi-finishing, angle cleaning, and finishing.
For parts that require high precision, a secondary semi-finishing step may be necessary before proceeding to the finishing stage. After the roughing process, it is advisable to allow the parts to cool naturally. This helps eliminate the internal stress generated during roughing and reduces deformation. The machining allowance left after roughing should exceed the expected deformation, typically ranging from 1 to 2 mm.
During the finishing stage, it is important for the finishing surface of the parts to maintain a uniform machining allowance, generally between 0.2 and 0.5 mm. This uniformity helps keep the tool in a stable position throughout the machining process, significantly reducing cutting deformation, improving surface quality, and ensuring product accuracy.
2. Operational skills to reduce processing deformation
In addition to the above reasons, the operation method is also very important in actual operation.
1. For parts with large processing allowances, symmetrical machining is recommended to improve heat dissipation and prevent heat concentration during the processing. For example, when processing a 90mm thick sheet down to 60mm, if one side is milled followed by immediate milling of the other side, and then the final size is processed in one go, the resulting flatness may only reach 5mm. However, if a repeated symmetrical machining approach is used, where each side is processed to the final size twice, the flatness can be improved to 0.3mm.
2. When dealing with multiple cavities on sheet parts, it is not advisable to use a sequential processing method that tackles one cavity at a time. This approach can easily lead to uneven stress on the parts and potential deformation. Instead, opt for layered multiple processing. Aim to process all cavities simultaneously on each layer before moving on to the next layer. This method helps ensure even stress distribution, thereby reducing the risk of deformation.
3. To reduce cutting force and cutting heat, it’s important to adjust the cutting amount. Among the three elements of the cutting amount, the back-cutting amount has the most significant impact on cutting force. If the machining allowance is too large, the cutting force during a single pass will be excessively high, which can deform the parts and compromise the rigidity of the machine tool spindle, ultimately reducing tool durability.
Reducing the back-cutting amount can decrease production efficiency; however, in CNC machining, high-speed milling can help mitigate this issue. By decreasing the back-cutting amount and simultaneously increasing the feed rate and machine tool speed, it is possible to lower the cutting force while maintaining machining efficiency.
4. The order of cutting should be important to consider. Rough machining focuses on improving machining efficiency and maximizing the material removal rate per unit of time. Typically, reverse milling is used during this phase. This method allows the excess material on the surface of the blank to be removed quickly, effectively shaping the geometric contour needed for finishing.
In contrast, finishing operations prioritize high precision and quality, and down milling is recommended for this stage. In down milling, the thickness of the cutting tool’s teeth decreases from a maximum value to zero. This gradual reduction significantly lowers the risk of work hardening and minimizes the deformation of the parts.
5. Thin-walled workpieces often deform due to clamping during machining, a situation that can be challenging to avoid, even during the finishing process. To minimize deformation, it is recommended to loosen the clamping piece before the final size is achieved. This allows the workpiece to return to its original shape freely. After this, the workpiece can be slightly clamped—just enough to hold it in place, relying on a tactile approach—to achieve the best processing results.
In summary, the clamping force should ideally be applied at the supporting surface, and directed in a manner that enhances the workpiece’s rigidity. While ensuring the workpiece remains secure, the clamping force should be kept to a minimum.
6. When processing parts with cavities, it is essential to avoid having the milling cutter penetrate the part directly, similar to a drill bit. This can lead to insufficient chip space for the milling cutter, hindered chip removal, overheating, part expansion, and other unfavorable issues such as chip collapse and tool breakage.
To prevent these problems, start by drilling a hole using a drill bit that is the same size or larger than the milling cutter. After creating the hole, utilize the milling cutter for the milling process. Alternatively, you can use CAM software to generate a spiral toolpath.
It’s important to note that machining accuracy and surface quality of aluminum parts can be affected by their tendency to deform during processing. Therefore, operators should possess a certain level of experience and skill to achieve optimal results.
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