Process measures and operating techniques to reduce deformation of aluminum parts!

There are many reasons for aluminum part deformation during machining, all of which are related to the material, part shape, and production conditions. These factors primarily include deformation caused by internal stress in the blank, deformation due to cutting forces and heat, and deformation due to clamping forces.

A. Process Measures to Reduce Machining Deformation

1. Reducing Internal Stress in the Blank

Natural or artificial aging, as well as vibration treatment, can partially eliminate internal stress in the blank. Pre-machining is also an effective process. For thick, oversized blanks, large stock removal leads to significant post-machining deformation. Pre-machining the blank to remove excess material and reduce stock removal not only reduces deformation in subsequent steps but also relieves some internal stress by allowing the blank to rest for a period of time after pre-machining.

2. Improving Tool Cutting Capability

The tool material and geometry significantly influence cutting forces and heat. Proper tool selection is crucial for reducing part deformation during machining.

1) Reasonable Selection of Tool Geometry

① Rake Angle: While maintaining cutting edge strength, a larger rake angle can produce a sharp cutting edge and reduce cutting deformation, facilitating chip evacuation and, consequently, lowering cutting forces and temperatures. Avoid using tools with negative rake angles.

② Clearance Angle: The clearance angle has a direct impact on flank wear and surface quality. Cutting thickness is a key factor in selecting the clearance angle. During roughing, high feed rates, heavy cutting loads, and high heat generation require good heat dissipation from the tool, so a smaller clearance angle should be used. During fine milling, a sharp cutting edge is required to reduce friction between the flank and the machined surface and minimize elastic deformation. Therefore, a larger clearance angle should be used.

③ Helix Angle: To ensure smooth milling and reduce milling forces, the helix angle should be as large as possible.

④ Lead Angle: Appropriately reducing the lead angle can improve heat dissipation and lower the average temperature of the machining area.

2) Improve Tool Structure.

① Reduce the number of cutter teeth to increase chip clearance. Because aluminum is highly plastic, significant deformation occurs during machining, requiring a larger chip clearance. Therefore, a larger chip flute bottom radius and fewer teeth are recommended.

② Fine grinding of the teeth. The roughness of the cutting edge should be less than Ra = 0.4 μm. Before using a new cutter, lightly grind the front and back of the teeth with a fine oilstone to remove burrs and slight serrations left over from sharpening. This not only reduces cutting heat but also minimizes deformation.

③ Strictly control tool wear. As tools wear, the workpiece surface roughness increases, cutting temperatures rise, and workpiece deformation increases. Therefore, in addition to selecting tool materials with good wear resistance, tool wear should not exceed 0.2 mm, otherwise built-up edge is likely to occur. During cutting, the workpiece temperature should generally not exceed 100°C to prevent deformation.

3. Improve Workpiece Clamping Methods

For thin-walled aluminum workpieces with poor rigidity, the following clamping methods can be used to reduce deformation:

① For thin-walled bushings, if a three-jaw self-centering chuck or collet is used for radial clamping, the workpiece will inevitably deform once released after machining. In this case, a more rigid axial end-face clamping method should be used. Position the workpiece using the inner hole of the part, create a threaded through-shaft, insert it into the inner hole of the part, and press the end face with a cover plate, then tighten it with a nut. This can avoid clamping deformation when machining external diameters, achieving satisfactory machining accuracy.

② When machining thin-walled plate workpieces, it is best to use a vacuum suction cup to achieve evenly distributed clamping force. This allows for minimal cutting effort and effectively prevents workpiece deformation.

Another alternative is the tamping method. To increase the process rigidity of thin-walled workpieces, a medium can be filled inside the workpiece to reduce deformation during clamping and cutting. For example, a molten urea solution containing 3% to 6% potassium nitrate can be poured into a workpiece. After processing, the filler can be dissolved and removed by immersing the workpiece in water or alcohol.

4. Proper Process Arrangement

During high-speed cutting, large machining allowances and intermittent cutting often cause vibration during the milling process, affecting machining accuracy and surface finish. Therefore, the CNC high-speed cutting process can generally be divided into the following steps: roughing, semi-finishing, corner cleaning, and finishing. For parts requiring high precision, a second semi-finishing step is sometimes required before finishing. After roughing, the part is allowed to cool naturally to eliminate internal stresses generated by roughing and minimize deformation. The remaining allowance after roughing should be greater than the deformation, generally 1 to 2 mm. During finishing, a uniform machining allowance should be maintained on the finished surface of the part, generally 0.2 to 0.5 mm. This ensures that the tool remains stable during processing, significantly reducing cutting deformation, achieving good surface quality, and ensuring product accuracy.

B. Techniques for Reducing Machining Deformation

In addition to the aforementioned reasons, aluminum parts can deform during machining. In practice, the correct machining method is also crucial.

1. For parts with large machining allowances, symmetrical machining is recommended to ensure optimal heat dissipation during machining and avoid heat concentration. For example, if a 90mm thick sheet needs to be machined to 60mm, milling one side immediately before milling the other side, achieving the final dimension in one pass, will result in a flatness of 5mm. Using symmetrical machining with repeated feeds, machining each side twice to the final dimension, can guarantee a flatness of 0.3mm.

2. If a sheet metal part has multiple cavities, machining them sequentially, cavity by cavity, is not recommended. This can easily result in uneven stress and deformation. Instead, use layered, multi-processing techniques, machining all cavities simultaneously on each layer before moving on to the next layer. This ensures even stress distribution and minimizes deformation.

3. Reduce cutting forces and heat by varying cutting parameters. Among the three key elements of cutting parameters, back-cutting has a significant impact on cutting force. Excessive machining stock and excessive cutting force in a single pass can not only cause part deformation but also affect the rigidity of the machine tool spindle and reduce tool durability. Reducing back-cutting significantly reduces production efficiency. However, high-speed milling, commonly used in CNC machining, can overcome this challenge. By reducing back-cutting and increasing the feed and machine speed accordingly, cutting forces can be reduced while maintaining machining efficiency.

4. The order of cuts must also be considered. Roughing emphasizes improving machining efficiency and achieving the highest removal rate per unit time. Therefore, up-cut milling is generally used. This involves removing excess material from the blank surface at the fastest speed and in the shortest possible time, essentially creating the geometric profile required for finishing. Finishing, on the other hand, emphasizes high precision and quality, so down-cut milling is preferred. Down-cut milling significantly reduces the cutting thickness of the cutter teeth from maximum to zero during down-cut milling, significantly reducing work hardening and part deformation.

5. Thin-walled workpieces can deform during machining due to clamping, which is unavoidable even during fine machining. To minimize workpiece deformation, loosen the clamping element just before reaching the final dimension, allowing the workpiece to return to its original shape. Then, lightly tighten the clamp, ensuring it just holds the workpiece securely (based entirely on feel). This will achieve optimal machining results. In short, the clamping force should ideally be applied on the support surface and in the direction of maximum workpiece rigidity. As long as the workpiece does not loosen, the lower the clamping force, the better.

6. When machining parts with cavities, avoid inserting the milling cutter directly into the part like a drill. This will result in insufficient chip clearance, disrupted chip evacuation, and potentially overheating, expansion, and chipping and breakage. First, drill the cutter hole with a drill bit of the same size or larger than the cutter before milling. Alternatively, use CAM software to generate a spiral cutter program.