Holistic Processing Strategy for Large-Cavity Thin-Walled Parts via 3-Axis Systems

This paper examines the machining challenges and structural characteristics of large-cavity, thin-walled parts, while proposing a strategy for process improvement. The strategy includes optimizing machining allowances, adding support pads at suspended cavities, and designing auxiliary fixtures. These measures effectively reduce part deformation during machining, enhance clamping efficiency and positioning accuracy, and ensure quality in the final product. Additionally, this approach serves as a valuable reference for three-axis CNC machining of similar components.

 

01 Introduction

The center cabin is a crucial structural component of a series of UAVs, as it connects the arms, landing gear, and core assemblies. These parts are characterized by large cavities, thin walls, and irregular shapes. In previous outsourced machining processes, they were primarily produced using five-axis or horizontal machining centers, which had limited clamping options. When the machining plan was revised to utilize the company’s sole three-axis machining center, a lack of experience led to poor clamping positioning accuracy and low efficiency. After conducting a thorough analysis and implementing optimizations, we successfully achieved integrated milling of these parts on the three-axis machining center.

 

02 Component Structural Characteristics

Figure 1 displays a 3D model of a component located in a central compartment. The external dimensions of the component are 443.4 mm × 443.4 mm × 73 mm, and the internal cavity has a wall thickness ranging from 1.5 mm to 2 mm. The material used for the component is 6061 aluminum plate. The component possesses the following characteristics:

 

1) The part is a typical large-cavity, thin-walled component, with a maximum dimension-to-thickness ratio of approximately 300:1.

2) The part features a complex structure, including characteristics on all six sides, as well as irregularly shaped protruding connecting structures at each of the four corners.

3) The irregularly shaped connecting structures at the four corners are designed with arm connections and mounting holes (see Figure 2), which are highly precise. The wall thickness of these connecting holes is just 3 mm.

 

4) The bottom of the part has numerous suspended thin-walled structures.

5) The part has a large number of holes; the two main surfaces alone have over 1800 mounting holes or heat dissipation holes.

 

03 First-Piece Machining Issues

First-Piece Trial Machining Process:

- Rough milling of the outer shape
- Milling of the inner cavity
- Milling of the backside
- Milling of the back hole system
- Milling of the outer shape and inner hole for the four legs
- Milling of the arm connection holes
- Milling of the four side heat dissipation grooves The main problems exposed during the first-piece trial machining are as follows:

1) During rough machining, the inner cavity was machined to its full extent, except for a 2mm allowance on the bottom plate. After machining, the part showed significant deformation, with the flatness of the opening surface measuring 3mm. The geometric tolerances did not meet the K-grade requirements outlined in GB/T 1184—2008. Additionally, during the subsequent machining of the four corner structures, the part’s clamping rigidity was insufficient, leading to further deformation.

2) During back milling, the inner cavity had a significant area of unsupported space, leading to noticeable vibration marks on the surface of the plate. As a result, it did not meet the required surface quality standards.

3) When machining the connecting holes of the four corner arms, the clamping was only positioned by the adjacent structural end faces and then aligned using a dial indicator. Repeated clamping led to large errors, resulting in a notch in one of the connecting holes, which ultimately caused the part to be scrapped.

 

04 Key Process Analysis

Based on the structural characteristics of the part and the problems encountered in the first-piece machining, a further analysis reveals the following key machining challenges:

1) Easy Deformation During Machining.

Due to the significant ratio of the part’s dimensions to its thickness, and with a material removal rate as high as 95%, improper control of the milling path and allowances can easily lead to cavity deformation. This deformation can negatively impact the fit of the cover plate during installation.

 

2) Prone to Vibration During Machining.

The bottom of the part is a suspended thin-walled plate, making it prone to vibration during machining, resulting in vibration marks and substandard surface quality.

 

3) Low Accuracy in Repeated Clamping and Positioning.

As illustrated in Figure 2, the connection of the arm demands high precision. The wall thickness of the connecting hole is only 3 mm. If improper clamping occurs while machining one end to the correct position, it can easily create a gap in the connecting hole at the opposite end, resulting in the part becoming unusable.

 

05 Optimized Process Scheme

Through process analysis, the optimized process flow is outlined as follows:

- Rough milling of the outer shape
- Milling of the back cavity (leaving a process table)
- Milling of the back hole system
- Milling of the inner cavity and end face (leaving an allowance around the perimeter)
- Stress relief aging
- Milling of the arm connection hole
- Milling of the inner holes of the four legs
- Milling of the four side heat dissipation grooves
- Removing the allowance around the inner cavity
- Removing the back process table

The optimized process route advances the back machining forward. A process table is maintained during the milling of the back cavity, serving as the positioning reference for the roughing of the large back surface and the residual machining surface following cavity milling. This approach also enhances the rigidity of the part during positioning. The tool path for milling the back cavity (while leaving a process table) is illustrated in Figure 3.

2) To control deformation, a 3-5mm allowance is left around the perimeter when milling the inner cavity. This reduces the deformation of the part itself and increases the rigidity of the part during clamping in subsequent milling processes. The actual milling and clamping is shown in Figure 4.

 

To control deformation, an aging process is incorporated after a significant amount of material has been removed from the inner cavity. This process helps to further alleviate machining stress.

3) To reduce vibration marks, a shim of appropriate thickness is added to the back cavity, as illustrated by the green rectangle in Figure 3.

4) The pinholes at the arm connection require high precision. To enhance repeatability and clamping accuracy, a V-shaped positioning fixture can be designed using the 90° angle formed by the adjacent sides of the cavity. During 5 axis machining, this fixture will be used for positioning and clamping, thus improving clamping accuracy and efficiency while minimizing errors that may arise from positioning with the small end face of the arm connection. Figure 5 shows the positioning before and after the optimization of the fixture.

06 Implementation Results

By optimizing the process route and tooling design, we were able to control the flatness of the open end face to within 0.05 mm during the reprocessing of trial-produced CNC turning components. Additionally, we kept the error from repeated clamping and positioning within ±0.05 mm. The parts successfully passed inspection, confirming the feasibility of the process flow. Compared to outsourcing machining to five-axis or horizontal machining centers, our processing costs were reduced by nearly 50%. Furthermore, the implementation of auxiliary clamping and positioning fixtures significantly improved the efficiency of repeated clamping and lessened the labor intensity for operators, eliminating the need for frequent dial indicator calibration.

 

07 Conclusion

Complex irregularly shaped parts are often processed using five-axis or horizontal machining centers to minimize the number of clamping operations and ensure machining accuracy. However, in actual production settings—especially in small and medium-sized enterprises or research institutions—equipment limitations are significant, as they typically only have access to three-axis machining centers.

This paper focuses on the center compartment of a large-cavity, thin-walled part as a case study, demonstrating how to achieve integrated milling processing on a three-axis machining center. By implementing improvements such as optimizing the process route, adjusting machining allowances, and designing auxiliary tooling, we effectively addressed issues related to part deformation, as well as poor repeatability in clamping and positioning accuracy. These enhancements ensured the overall quality of the machining process for the part.

 

 

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