Innovative Low-Stress Fixturing Solutions for Thin-Walled Split Components

This study addresses the issue of machining deformation in large split thin-walled parts by proposing a low-stress flexible clamping method. This method aims to minimize the effects of cutting forces and clamping stresses. During the machining process, hydraulic and pneumatic controls are used to loosen the flexible clamping and support points between operations. This approach helps relieve the internal stress within the workpiece, thereby reducing machining deformation caused by that stress. The effectiveness of this method was demonstrated in the machining of workpieces longer than 1 meter, achieving a dimensional accuracy of 0.2 mm for the large end diameter, which meets production requirements.

 

01 Introduction

Large split thin-walled parts are common in aerospace products, but they often have weak rigidity. During the machining process, these parts are influenced by several factors, including cutting forces, clamping stress, and the release of internal stress. As a result, machining deformation is unavoidable, making it challenging to maintain the dimensional accuracy of the workpiece.

To address this issue, a low-stress flexible clamping method has proven effective in controlling the machining deformation of thin-walled parts. This approach involves designing a flexible clamp that loosens between processes to relieve the internal stress of the workpiece. During machining, the workpiece is clamped adaptively, achieving low-stress clamping. This technique has been widely applied in managing machining deformation for large aviation plate parts.

However, large split thin-walled parts in aerospace applications often have a conical shape, which complicates the clamping process compared to flat parts. This paper proposes a low-stress flexible processing method specifically for large split thin-walled parts. It develops two sets of special fixtures and verifies the effectiveness of this method through practical processing trials.

 

02 Low-stress flexible clamping method for large split-type thin-walled parts

2.1 Part structure and process characteristics
The large split-type thin-walled parts, as illustrated in Figure 1, are conical split structures with a length exceeding 1 meter, a minimum wall thickness of 5 mm, and a maximum end diameter of approximately 500 mm (φ500 mm). The material used for the workpiece is the cast magnesium alloy ZM6. To ensure stable clamping, eight process chucks are evenly distributed on both sides of the casting blank. These chucks will be milled off during the final fine milling process.

The blank has a single-side allowance of 5 mm, and all parts include allowances requiring processing. The key dimensions of the workpiece are as follows: the flatness of the docking plane is 0.15 mm, the accuracy of the large end diameter is ±0.4 mm, the wall thickness of the lightening groove should be (5 ± 0.5) mm, and the accuracy of the circumferential hole angle is ±2′.

The main processing procedures of large split thin-walled parts are as follows.
- Perform rough milling of the inner cavity, docking plane, inner step, and boss, leaving a 1.5mm allowance.
- Carry out rough milling of the outer wall and concave, also leaving a 1.5mm allowance, and milling of the upper surface while the chuck is clamped.
- Conduct semi-finishing milling of the inner cavity lightening groove, boss, docking plane, and inner step, with a 1mm allowance.
- Execute semi-finishing milling of the outer wall and concave, again with a 1mm allowance.
- Complete the finishing milling of the inner cavity lightening groove, boss, docking plane, and drilling operations.
- After removing the chuck, finish milling of the outer wall and concave, along with drilling circular holes.

The processing accuracy of large split thin-walled parts can be challenging to achieve as per the specified index requirements. The primary processing issues include the following two points.

 

1) Poor structural stiffness.
Due to the large size of the parts and the low elastic modulus of magnesium alloy (approximately 45 GPa), the overall structural stiffness is relatively low. During processing, most parts are suspended and lack adequate support, making them prone to deformation during clamping. Additionally, the cutting force during milling can lead to “cutting deformation.” After processing, the release of internal stress in the parts can result in overall warping deformation. Currently, the size error of the large end diameter of the parts before and after rough and fine processing can be as high as 0.6 mm and 0.5 mm, respectively.

 

2) High clamping difficulty.
During the casting process, four process chucks are attached to both sides of the docking plane of the half cover. During processing, the inner and outer surfaces of the components are shaped by pressing against the process chucks. It is necessary to turn the components over four times to ensure that all eight support points are pressed evenly, reducing the likelihood of warping. Currently, the clamping is done manually, which takes approximately 30 hours. Therefore, there is a need to improve clamping efficiency.

Given the structure and process characteristics of large, split thin-walled parts, it is essential to develop automated, low-stress flexible tooling. This will enhance clamping efficiency and help minimize deformation during processing, which can be caused by cutting forces, clamping stress, and internal stress.

 

2.2 Low-stress flexible clamping method
A flexible clamping method designed to minimize stress is proposed for the processing of large, split thin-walled parts, as illustrated in Figure 2. During the processing of the inner cavity, eight flexible clamping points are positioned based on the locations of eight process chucks on the blank. Each flexible clamping point consists of a pair of hydraulically controlled vertices and a pressure plate.

 

b) Low-stress flexible clamping method for outer wall processing Figure 2 Low-stress flexible clamping method for large split-type thin-walled parts

 

The four-point coplanar over-positioning method is applied due to the large size of the parts and the symmetrical structure. This method utilizes four flexible clamping points at the front and rear, which are adjustable in height through hydraulic control. Hydraulically controlled indexable positioning points are integrated at these clamping points, ensuring that the height error remains below 0.02 mm after calibration.

 

In operation, the hydraulic cylinder pushes the vertices of the flexible clamping points upward, moving the top chuck until it makes contact with the workpiece’s docking plane and the indexable positioning point. At this moment, the hydraulic pressure locks the indexable positioning point, stopping the movement of the top chuck. Once the hydraulic cylinder controlling the vertices is locked, a hydraulic pressure plate presses against the upper surface of the chuck, allowing the indexable positioning point to rotate away from the workpiece docking plane. The four passive flexible clamping points then secure the other four chucks. To prevent excessive clamping stress, these passive clamping points are equipped with contact sensors. When the vertices of the flexible clamping points make contact with the lower surface of the chuck from the bottom up, they stop moving, and the pressure plate clamps the chuck in place.

 

Next, the machine tool probe is used to take measurements at both ends of the workpiece, aligning the workpiece axis by rotating the machine tool table. It’s important to note that in the initial process, the workpiece is machined from a single-side allowance of 5 mm down to 1.5 mm. Despite any imperfections in the docking plane due to it being a blank surface with low flatness, the positioning error from the four-point coplanar over-positioning method remains acceptable. The docking plane and other features are then machined while the workpiece is in this clamped state. In the third and fifth processes, the workpiece continues to be in the clamping state depicted in Figure 2a, but at this stage, the CNC aluminum milling has improved the flatness of the docking plane, ensuring the over-positioning method does not induce significant error.

 

For processing the outer wall, the tooling is depicted in Figure 2b and does not feature flexible clamping points; instead, it processes a tooling plane with high flatness. During this process, the docking plane of the previously machined workpiece serves as a reference, and it is placed directly onto the tooling plane, given the high flatness of both surfaces. Thus, there is no need for further alignment or clamping, and in this state, none of the chucks are clamped. The inner step of the workpiece is secured by a hydraulically controlled pressure plate. Again, the machine tool probe is used to take measurements at both ends of the workpiece, aligning its axis while rotating the machine tool worktable. In this clamping state for the outer wall machining, all chucks remain unclamped, and the upper surface of the chuck is machined to ensure high flatness. This guarantees optimal contact between the chuck and the vertices of the flexible clamping points when the workpiece is turned over and clamped for the next process.

 

Flexible support points are arranged underneath the workpiece to serve as auxiliary support. These support points can be locked and released using air pressure control. Each flexible support point is equipped with multiple ejectors, as illustrated in Figure 3, and features a pneumatic locking device housed within the support point.

Inside each ejector pin, a spring is installed. When there is no air pressure input, the pneumatic locking device is released, allowing the ejector pin to be compressed under external force, while the spring causes it to rebound once the external force is removed. Conversely, when air pressure is applied, the pneumatic locking device secures the ejector pin, keeping its position fixed.

 

The selected ejector pin for the flexible support point has a stroke of 20mm, and the locking air pressure is set at 0.7 MPa. When this air pressure is applied, each flexible support point can sustain a locked position under an external force of 1500 N. Therefore, during the installation of the workpiece, no air pressure is input into the flexible support points, causing each ejector pin to be compressed against the workpiece for conformal support.

During the processing phase, air pressure is applied, locking the pneumatic device. At this stage, the ejector pin maintains its position of conformal support, effectively acting as rigid support. This design enhances the overall rigidity of the processing system and minimizes deformation resulting from cutting forces.

 

It is important to note that to achieve low-stress clamping, the initial position of the flexible support point is aligned with the contour of the workpiece. By adjusting the overall height of the flexible support point, the ejector pin can be slightly compressed, preventing excessive support stress from being introduced by the spring within the ejector pin.

During machining, the flexible clamping points and support points are loosened between processes. This allows the workpiece to release internal stress and fully deform. Before starting the next process, the workpiece is re-clamped, and the deformed section is cut off in the following step. This approach minimizes machining deformation that results from the release of internal stress.

 

03 Design of flexible support point layout

The number and arrangement of flexible support points influence the static stiffness and dynamic characteristics of the process system. Higher static stiffness results in less machining deformation from the cutting force. Dynamic characteristics describe the overall vibration performance of the process system when subjected to dynamic loads. Generally, it is understood that a higher natural frequency indicates better dynamic characteristics, as it enhances the system’s ability to suppress cutting vibrations and improves the quality of the machined surface. Consequently, the layout of flexible support points must be carefully designed.

To analyze this, we utilize the finite element simulation software ABAQUS to conduct a modal analysis on the parts without support. This analysis provides the positions of each mode and the maximum amplitude of the parts. Flexible support points are then strategically positioned at the points of maximum amplitude to minimize vibration caused by dynamic loads. Additionally, we use the linear perturbation analysis step in ABAQUS to perform modal analysis on the machining system without auxiliary support. The lower surface constraints of the eight chucks simulate the actual clamping conditions, where the chosen constraint for the lower surfaces is “displacement/rotation.” Only the two translational degrees of freedom of the chuck in the horizontal direction are retained, as illustrated in FIG4.

The first three modes of the workpiece, obtained through finite element analysis, are shown in Figure 5. The workpiece exhibits larger amplitude positions in the middle, at the large end, and on the side.

Considering the size of the flexible support points and the findings from the finite element analysis, the support points are positioned in areas with higher amplitudes in the modal analysis results whenever possible. The finite element analysis indicates that the weak areas of the process system are the middle section, the large end, and the sides of the workpiece. Therefore, the flexible support points are strategically placed in these locations, as illustrated in Figure 6.

04 Test verification

The developed fixture was utilized for testing, as illustrated in Figure 7. The total clamping time for the entire processing operation was under 2 hours, significantly enhancing clamping efficiency. After inspection of the processed precision turned parts, it was found that the error in the large end diameter was 0.2 mm.

05 Conclusion

This paper presents a low-stress flexible clamping method and introduces two unique fixtures designed to address the challenges of machining deformation in large, split, thin-walled components. These specialized fixtures enable low-stress clamping, enhance the rigidity of the processing system, and alleviate the internal stress of the workpiece by loosening the clamp between processes. This approach effectively reduces machining deformation caused by clamping stress, cutting forces, and inherent material stresses.

Machining tests demonstrate that the low-stress flexible clamping method successfully controls the deformation of the parts. For components over 1 meter in length, the dimensional accuracy of the large end diameter achieved after machining can reach 0.2 mm, thus meeting the required precision standards. This method can serve as a valuable reference for machining other large, specially shaped thin-walled structures in the aerospace sector.

 

 

 

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