Process Capability Analysis for Mega-Scale Spun Foundation Rings

Large-diameter conical-bottomed rings present distinct challenges compared to conventional spherical and ellipsoidal structures in terms of datum coordination, clamping stiffness, and deformation control. To optimize allowance coordination margins during the turning process, three-coordinate laser measurement and on-machine measurement techniques were employed for datum coordination. However, significant discrepancies in allowance data were frequently observed. By analyzing practical phenomena alongside scientific methods, we identified the causes of datum variations related to the conical-bottomed structure, weight distribution, and handling. These findings provide valuable insights for machining similar workpieces.

 

1 Introduction

The conical-bottomed rings of the new generation of launch vehicles differ significantly from traditional spherical and ellipsoidal structures. This innovative design poses considerable challenges in terms of datum coordination, clamping stiffness, and deformation control. This article examines the causes of discrepancies in machining allowance data encountered during the development process. To maximize coordination margins during turning, both three-coordinate measurement and on-machine measurement techniques were employed for datum coordination. However, significant discrepancies between the actual machining allowances and the predicted allowances were repeatedly observed, creating confusion.

 

2. Problem Description

The spun cone bottom blank has a large end diameter of 5 µm and a small end diameter of 2 µm, with a bevel angle of 45°. The wall thickness allowance on one side is approximately 14 mm. This blank is made from aerospace aluminum alloy, and its dimensions are illustrated in Figure 1.

Due to its simple structure, machining the inner and outer surfaces at a 45° angle results in significant real-time deformation, leading to overcutting. The machining allowance for the 45° bevel of the spun cone bottom is critical. With limited machining allowance, achieving smaller roundness is preferable. For instance, with a roundness of 14 mm, disregarding the diameter means the allowance only covers half the wall thickness (15 mm). When considering the diameter, the machining allowance for the inner and outer surfaces is not evenly distributed, which results in insufficient machining allowance and the presence of a black skin (refer to Figure 2).

Since both the diameter and machining allowance must be factored into actual machining, the spun cone bottom blank is measured using a three-dimensional laser to obtain accurate machining allowance data.

 

Before machining, the allowances and roundness of the workpiece are measured and estimated. In the spinning workshop, the process begins with the workpiece being placed flat on the worktable. A three-dimensional coordinate measurement is conducted at multiple points along the circumference of the conical workpiece, and the data is then analyzed. The maximum roundness measured is 14.3 mm, which meets the requirement for a machining allowance of 15 mm on each side.

In the CNC machining workshop, measurements are taken on a large vertical lathe with the workpiece blank oriented downward (refer to Figure 3). The circumferential markings used during the spinning process serve as a reference for leveling the workpiece. The upper surface of the smaller end is machined to establish a rough datum, which will act as a fine datum for subsequent machining. Prior to machining the fine datum for the smaller end of the cone, it is essential that the machining allowances on both the inner and outer surfaces are equal to or greater than the roundness value to avoid the formation of black skin.

The operator carefully positions the workpiece, symmetrically aligning it at eight points and adjusting the roundness to minimize any deviations. To achieve this, 16 generatrices are evenly spaced around the circumference of the cone, and each generatrix is further divided into 12 points. The diameter at each of these 192 points is measured using CNC machine tool coordinates. The resulting roundness is found to be 15.7 mm, which is insufficient for the required size during machining, and as a result, black skin will remain on the surface of the cone.

 

The data provided by the spinning shop indicated that the machining allowance was adequate to prevent black skin from forming after the turning process. However, this conclusion differed from that of the CNC machining shop, which found a slight discrepancy upon measurement. As a result, the workpiece was removed, and the process was repeated twice, which proved to be time-consuming. Ultimately, the data showed minimal difference, and the conclusion remained unchanged.

 

3 Analysis of the Reasons for the Difference in Measurement Results

For both three-dimensional laser measurement and on-machine point measurement, the cone base should be placed flat on the worktable with the large end facing downward.

When conducting three-dimensional laser measurement, the operator places the workpiece on the platform with the large end down. They then perform point measurement and fitting analysis to obtain the necessary data.

In the case of on-machine point measurement, the turning operator positions the large end of the workpiece flat on the turntable. They horizontally level it according to the reference lines provided during the spinning process and vertically align it using eight designated points on the large end. The roundness is adjusted to minimize deviation—this requires a high level of skill from the operator. Subsequently, points are measured one by one to gather the required values.

It is important to note that the large end surface of the cone base, after spinning, is uneven. This results in multiple contact points with the work platform, causing the part to remain largely suspended. To facilitate analysis, the circumference of the large end surface (with a 5-meter diameter, resulting in approximately a 15-meter circumference) is expanded (see Figure 4). This expansion clearly illustrates the irregular multiple contact points with the machine platform.

Although we might consider the part to be in a free state, its actual state is nearly free or approaching that condition. The difference is significant, as the cone’s generatrix in the suspended position has indeed deflected.

 

To level the vertical lathe turntable, refer to the guidelines provided by the spinning shop or measure the height of specific points accurately. This process will help determine the optimal position of the part for coordinated machining allowances.

At this stage, the large end face of the workpiece requires multi-point shimming adjustments. This means adjusting the lowest point to align it with the highest point. It’s important to ignore disruptive factors, such as significant axial runout in certain areas of the cone caused by heat treatment, at this time; focusing on leveling the majority of points is sufficient.

In Figure 5, the blue numbers indicate the measured vertical heights at eight equally spaced points. The highest point is at 12 o’clock, while the lowest point is at 5 o’clock. To achieve proper alignment, raise the 5 o’clock position to 95mm, matching the height of the 1 o’clock position. This adjustment effectively completes the positioning of the cone.

 

During the leveling process, the operator uses a crowbar to tilt the workpiece and insert shims for support, as illustrated in Figure 6. Adjustments are made carefully and gradually to achieve the desired state. The primary objective of leveling is to tilt the workpiece to an optimal angle that aligns with its original position after spinning. Fine horizontal adjustments are then performed to balance the internal and external machining allowances.

The most significant feature to address is the lowest point, which needs to be raised. The height difference between this point and its symmetrical counterpart (the point where it contacts the machine tool’s rotary table) should match the desired value. A smaller height difference indicates better leveling, meaning the workpiece is closer to being perfectly level.

 

(1) Reason 1:

The ideal condition for the large end face of the workpiece is a perfect circle. However, due to various stress factors, such as spinning and heat treatment, this face can become deformed into a shape that deviates from a perfect circle. For the sake of analysis, we will overlook these contributing factors. We will also disregard the position changes at the mutation points, meaning that the highest and lowest point positions are in close proximity and cannot be adjusted. It is generally accepted that when the circumference of the large end face deforms, it tends to adopt an elliptical shape (see Figure 7). This results in the phenomenon of a long axis and a short axis, or an overall trend of elliptical transformation.

 

When the large end of a workpiece is leveled downward to machine the smaller end face as a reference for subsequent operations, the workpiece makes contact with the platform at very few points. As a result, its own weight causes the circumference of the large end to deform into an elliptical shape. In this scenario, point A, which is in contact with the bed, is considered immovable. In contrast, point B, which is elevated, becomes stretched in the horizontal direction, resembling a major axis. This induces a tendency for a minor axis phenomenon, or a tendency to shorten, in the vertical direction.

 

At this point, the generatrix of the cone at the elongated diameter (considered the major axis) rotates outward, as illustrated by the green line in Figure 9. The larger end face not only moves outward but also rises, causing the leveling mark on it to shift in the same direction. In contrast, the generatrix at the minor axis, represented by the red line in Figure 9, experiences the opposite effect: its end face moves slightly downward and inward, while the leveling mark on it shifts noticeably downward. This phenomenon mirrors actual leveling operations, where raising a particular point often results in the 90° point dropping. It is evident that, despite its large size, the overall cone ring exhibits weak rigidity.

 

(2) Reason 2:

The large end face of the CNC lathe machining parts tends to change from a circular shape to an elliptical one, which negatively impacts the control of its roundness. When the operator levels the workpiece using a pry bar, this can exacerbate the tendency for the shape to become elliptical. As the workpiece is pried upward, the contact point between the pry bar and the workpiece not only moves upward but also shifts outward horizontally (see Figure 10). This results in the long axis of the workpiece being stretched and altered. For instance, if the workpiece is lifted by 20 mm in the vertical direction (Z direction), it also moves outward by 2.8 mm in the horizontal direction (X direction). If the pad is raised by 25 mm, the horizontal movement increases to 4.3 mm. This demonstrates that the prying operation, particularly when using padding, causes a significant outward movement of the prying point, resulting in the elliptical deformation of the large end face and an outward rotation of the cone generatrix at that location.

 

The factors involved cause the major axis generatrix of the tapered surface to rotate outward, while the minor axis generatrix rotates inward (as shown in Figure 11). This leads to a change in the flatness of the small end face, which is used to position the cone. The outward rotation of the major axis generatrix (depicted by the red line in Figure 11) results in a concave deformation of the small end face, whereas the inward rotation of the minor axis generatrix (shown by the green line in Figure 11) leads to a convex deformation. Consequently, the small end face of the cone experiences elastic deformation (illustrated in Figure 12). The red areas indicate concavity, while the green areas indicate convexity. The actual distribution of this deformation is influenced by factors such as material thickness, and the location and nature of the deformation can be quite complex. In practice, it is common to continue machining the small end face under these conditions and to use it as a reference for subsequent machining operations.

 

When turning a small end face under elastic deformation, the elastic deformation is immediately released once the anodized aluminum parts is removed from the worktable, allowing it to return to a free state. As a result, the previously convex areas become concave, while the concave areas turn convex. This transformation of the end face is illustrated in Figure 13. The original angles of the workpiece’s major and minor axis generatrix are partially restored. Consequently, the flatness of the small end face is altered, becoming convex along the major axis and concave along the minor axis.

 

4. Result Verification

The flatness of the small end face was assessed by comparing it to the side surface of a 2-meter caliper. A flashlight was used to illuminate and examine the gap (see Figure 14). It was observed that the small end face, in its free state, exhibited a noticeable concavity along the original long axis. The maximum gap measured with a feeler gauge was greater than 2.6 mm. Additionally, there was a slight concavity in the middle of the short axis, although it was not very pronounced. Near the outer circumference, the surface was convex before becoming concave again. Overall, the appearance of the small end face displayed a slight convexity, which was entirely consistent with the analysis.

 

5. Result Analysis

The above analysis indicates the following situations:

1) The weight of the 5-meter integrally spun cone bottom ring can cause deflection in the cone generatrix, resulting in cone deformation. Greater deformation occurs when the large end face of the workpiece is uneven and has fewer contact points with the platform.

2) When an operator pries the cone bottom ring, it can lead to outward deflection of the cone generatrix, causing additional deformation.

3) The cone structure is more prone to deformation compared to a spherical structure.

4) To improve positioning accuracy, the small end face—which serves as the precision reference for subsequent processing—should be turned as much as possible while minimizing deformation.

 

To avoid these issues, the following methods can be employed:

1) After rough machining the large end face, it can be used as a leveling pad. This approach simplifies leveling, as it is easier to select contact points compared to a blank surface. Using the large end face increases the contact area or number of contact points between the part and the platform, making pressure estimation and control more manageable. However, this method has significant drawbacks. Turning the large end face requires manual handling with multiple square boxes for securing the workpiece, leading to two additional turns, which can be labor-intensive and time-consuming.

2) To maintain contact between the workpiece surface and the platform, a shim (see Figure 15) can be wedged into the highest position for adjustment. Striking the shim horizontally with a hammer allows for fine adjustments, saving time and effort. Additionally, when using a pry bar, operators should minimize the “long axis” effect. Each prying lift should be small, and fine adjustments should be made in multiple steps.

 

3) The small end face of a cone is prone to deformation due to the deflection of the cone’s generatrix, which leads to magnification errors when the cone is used as a reference surface for finishing after it has been flipped. To mitigate the influence of the small end face and prevent the angle of the cone’s generatrix from changing outward, we can abandon the positioning of the small end face and instead use a 45° groove ring. This rigid ring will constrain the outward rotation of the cone’s major axis generatrix while suppressing the inward rotation of the minor axis generatrix (see Figure 16).

Additionally, a height adjustment system enables precise adjustments to the cone’s tilt angle, allowing for quick identification of the exact measurement at the designated reference point. Once the workpiece is placed within the 45° groove ring, any offset can be corrected by adjusting the tilt angle of the entire groove ring. The height adjustment system should be designed for simplicity and ease of use.

 

6 Conclusion

The 5-meter class integrally spun cone-bottom ring is the largest of its kind in the world. While its simple structure may suggest ease of manufacturing, the reality is quite the opposite. The large size, weight, and low rigidity present significant challenges during production. Any changes to the machining datum, which is essential for maintaining the precision of a part’s dimensions and shape, can lead to a loss of the limited adjustable margins available. Therefore, understanding the causes of these changes is critical. We hope this information will offer valuable insights and support for the machining of similar components.

 

 

 

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