Analysis of the variation of the 5m-class integral spinning cone bottom ring reference

The large-diameter conical bottom ring, unlike conventional spherical or ellipsoidal shapes, poses significant challenges in terms of datum coordination, clamping stiffness, and deformation control. During turning operations, both coordinate measuring machine (CMM) and on-machine measurement methods were utilized to maximize the allowance for datum coordination. However, substantial discrepancies in the allowance data consistently arose. By examining the practical phenomena and combining them with scientific analysis, the causes of datum variations stemming from the structure, weight, and operating methods of the conical bottom part were identified. These findings provide valuable insights for machining similar workpieces.

 

1. Introduction

The new generation of launch vehicle conical bottom rings differs from traditional spherical or ellipsoidal designs. This innovative structure presents significant challenges related to datum coordination, clamping stiffness, and deformation control. This paper focuses on analyzing and explaining the discrepancies in the machining allowance data that arose during the development process.

To optimize the allowance for datum coordination during the turning process, both coordinate measuring machine (CMM) and on-machine measurement methods were used. However, substantial discrepancies in the allowance data were repeatedly observed, with the actual machining allowance differing significantly from the predicted allowance, which is quite perplexing.

 

2. Problem Description

The spun conical bottom blank features a large end diameter of 5 meters and a small end diameter of 2 meters, with a bevel angle of 45°. It has a single-sided wall thickness machining allowance of approximately 14 mm. The material used for the blank is an aerospace aluminum alloy, and its dimensions are illustrated in Figure 1.

Due to its simple structure, significant real-time deformation occurs during the machining of the 45° inner and outer surfaces, which can lead to overcutting. The machining allowance for the 45° bevel of the spun conical bottom is critical; with a limited allowance, achieving a smaller roundness is preferable. For instance, if the roundness is 14 mm and we do not consider the diameter, the allowance can barely cover half of the required wall thickness allowance (15 mm). When the diameter is taken into account, the machining allowance for both the inner and outer surfaces cannot be evenly distributed, resulting in insufficient allowance and the potential appearance of black skin in some areas (refer to Figure 2).

Therefore, actual machining must consider both the diameter and the machining allowance. To acquire accurate machining allowance data, the spun conical bottom blank is measured using a coordinate measuring machine (CMM).

Before turning, the machining allowance and roundness are measured and estimated.

The spinning workshop provides the following information: The workpiece is placed freely on the worktable, and the coordinate measuring machine measures values at a sufficient number of points on the circumference of the conical workpiece. This data is then fitted to obtain the final rounded values. The maximum roundness value recorded is 14.3 mm, which meets the requirement of a 15 mm machining allowance on each side.

In the CNC machining workshop, the following measurements are taken on the lathe: Machining is performed on a large vertical lathe with the larger end of the workpiece blank facing downward. The circumferential markings from the spinning process are used as a reference line for leveling, acting as a rough datum for machining the upper surface of the small end, which serves as a fine datum for subsequent machining.

Before machining the fine datum small end face, it is essential to ensure that the machining allowance on both the inner and outer surfaces of the cone is greater than or equal to the roundness value. This precaution helps to avoid the formation of black skin on the surface. The operator carefully adjusts the workpiece position, aligning it at eight symmetrical points to minimize the workpiece roundness.

First, 16 generatrices are evenly spaced around the circumference of the cone, with each generatrix divided into 12 points. The diameter at each point is then measured using the coordinates of the CNC machine tool, resulting in data from a total of 192 points. The roundness was found to be 15.7 mm, which is insufficient to meet the required specifications during machining, leading to the potential for black skin to remain on the cone’s surface.

 

The data from the spinning workshop indicated that the machining allowance was sufficient to prevent any black skin from appearing after turning, confirming that the allowance was adequate. In contrast, the CNC machining workshop concluded that there was a slight deficiency in the allowance. As a result, the workpiece was removed, and the operation was repeated twice. After investing a considerable amount of time, the data showed little difference, and the conclusion remained unchanged.

 

3. Analysis of the Reasons for Different Measurement Results

Both coordinate measuring machine (CMM) laser measurement and machine tool point marking measurement involve placing the workpiece flat on the worktable with the larger end of the cone facing downward.

In CMM laser measurement, the operator places the workpiece on the platform with the larger end facing down. They then perform point marking measurements and analyze the fitted data.

In turning machining, the operator positions the workpiece flat on the turntable, ensuring the larger end is also facing down. They level it horizontally according to a reference line from the spinning process and then align it vertically using eight reference points on the larger end. This step requires a high level of skill to minimize roundness discrepancies, after which the operator continuously marks and collects measurements.

It is important to note that the large end face of the spun cone is uneven, leading to multiple contact points with the work platform and creating a largely suspended state. For clarity and analysis, the circumference of the large end face—measuring 5 meters in diameter and approximately 15 meters in circumference—is unfolded (see Figure 4). This illustration clearly depicts its irregular multi-point contact with the machine tool platform.

At this stage, we consider the part to be in a free state; however, its actual condition is close to or approaching a free state. The distinction is significant because, in this suspended position, the generatrix of the cone has indeed deflected.

To level the part on the vertical lathe turntable, align it according to the reference lines provided by the spinning workshop or based on the precise height measurements of specified points. This will help determine the optimal position for coordinating the machining allowance.

At this stage, it is important to adjust the large end face of the machining component by raising it at multiple points. Essentially, you want to raise the lowest point so that it is level with the highest point. When leveling, disregard points with abrupt changes, such as significant axial runout in certain areas of the conical surface caused by heat treatment. Instead, focus on leveling the majority of the points.

In Figure 5, the blue numbers indicate the measured vertical heights at eight evenly distributed points. The highest point is point 1, and the lowest point is point 5, which are nearly symmetrical. To complete the adjustment of the cone’s position, raise point 5 to 95 mm so that it matches the height of point 1.

 

During the leveling process, the operator uses a pry bar to lift the workpiece and inserts shims for support, as illustrated in Figure 6. Adjustments should be made gradually and carefully, progressively moving closer to the desired state. The primary goal of leveling is to tilt the workpiece to an optimal angle, maintaining its original position after spinning, and then to make fine adjustments in the horizontal direction to balance both internal and external machining allowances.

The most significant feature to focus on is the lowest point of the workpiece. This point is raised until the height difference between it and its symmetrical point (the point in contact with the machine tool turntable) reaches the desired value. Ideally, this value should be as small as possible, meaning that a smaller height difference indicates a closer alignment to horizontal.

 

(1) Reason 1:
The ideal state of the large end face of a workpiece is a perfect circle. However, factors such as spinning and heat treatment can cause deformations, resulting in a non-circular shape. For the sake of analysis, we will ignore the specific reasons for these deformations, as well as the abrupt changes where the highest and lowest points are closely adjacent and cannot be adjusted. Generally, it can be assumed that when the circumference of the large end face is deformed, it tends to take on an elliptical shape (as illustrated in Figure 7). This results in the appearance of major and minor axes, indicating an elliptical trend in the deformation.

When the workpiece is positioned with its larger end facing downward (to prepare the smaller end for subsequent machining), there are very few points of contact between the workpiece and the platform. Its own weight will cause the circumference of the larger end to potentially deform into an elliptical shape. Specifically, point A is in contact with the bed surface and remains stationary, while point B, which is elevated, experiences elongation in the diameter direction, resembling a major axis. In the vertical direction, a minor axis effect is observed, leading to a trend of shortening.

At this stage, the generatrix of the cone at the elongated diameter position, which can be considered the position along the major axis, rotates outward (see the green line in Figure 9). As a result, the large end face not only moves outward but also upward, causing the leveling lines on it to shift in the same direction. In contrast, the generatrix at the minor axis position (see the red line in Figure 9) moves slightly downward and inward, with the leveling lines on it moving noticeably downward. This observation aligns with the phenomenon seen during actual leveling, where raising a specific point results in a lowering of the 90° position point. Despite the overall size of the conical ring being relatively large, it demonstrates a lack of rigidity.

(2) Reason 2:

The large end face of the workpiece often takes on an elliptical shape, which is detrimental to controlling the roundness of the workpiece. When an operator levels the workpiece using a pry bar, this action can worsen the tendency for the shape to become elliptical. As the workpiece is pried upwards, the contact point of the pry bar not only moves vertically (in the Z direction) but also shifts horizontally (in the X direction). For instance, if the workpiece is lifted by 20mm vertically, it may additionally move outward by 2.8mm horizontally. If lifted by 25mm, the horizontal movement could increase to 4.3mm. This prying action significantly stretches the large end circle of the workpiece and causes the generatrix of the cone at that point to rotate outward, further compromising its shape.

The factors mentioned above lead to the outward rotation of the generatrix along the cone’s major axis, while the generatrix along the minor axis rotates inward (refer to Figure 11). This results in changes to the flatness of the small end face used for positioning the cone. The outward rotation of the generatrix along the major axis (indicated by the red line in Figure 11) causes a concave deformation of the small end face. Conversely, the inward rotation of the generatrix along the minor axis (shown by the green line in Figure 11) results in an upward convex deformation of the small end face.

The small end face of the cone experiences elastic deformation (as illustrated in Figure 12), with red areas representing concave regions and green areas depicting upward convex regions. The actual distribution of the deformation is influenced by factors such as material thickness, and the location and extent of the deformation can be quite complex. During actual machining, it is often possible to continue working on the small end face under these conditions and use it as a reference for subsequent machining processes.

 

When a small end face is machined under elastic deformation, the deformation is immediately released once the workpiece is removed from the worktable, causing it to return to its original free state. This results in originally convex areas becoming concave, and originally concave areas becoming convex. The deformation of the end face is illustrated in Figure 13. The angles of the original major and minor axes of the workpiece are partly restored. Consequently, the flatness of the small end face shifts to be convex in the direction of the major axis and concave in the direction of the minor axis.

 

4. Results Verification

The flatness of the small end face was evaluated by comparing the side of a 2-meter caliper with the surface of the end face. A flashlight was used to illuminate the gap for better visibility (see Figure 14). In its free state, the small end face exhibited a noticeable concavity along its original major axis, with the maximum gap measured using a feeler gauge exceeding 2.6 mm. A slight, inconspicuous concavity was observed in the middle of the minor axis, followed by a gentle upward convexity near the outer circumference. This was then followed by another concavity, resulting in a small overall upward convexity that aligns perfectly with the analysis.

5. Results Analysis

The above analysis reveals the following:

1) The 5-meter class integral spun cone bottom ring can deform due to its own weight, leading to deflection in the cone’s generatrix. The degree of deformation increases when the large end face of the workpiece is uneven and when there are fewer contact points with the platform.

2) Operators can pry the cone bottom ring, causing the cone generatrix to deflect outwards and deform the cone.

3) Conical structures are more prone to deformation than spherical structures.

4) The small end face, serving as the precision reference for subsequent machining, should be machined as much as possible while minimizing deformation to enhance positioning accuracy.

 

The following methods can be used to avoid the above situations:

1) After roughly machining the large end face, it can serve as a leveling pad. This method simplifies the leveling process, as it allows for easier selection of the padding position compared to using a blank surface. Additionally, the contact area or number of contact points between the part and the platform increases, making it simpler to estimate and control the pressure. However, there are notable disadvantages. When turning large end faces, several square boxes must be manually used to secure the workpiece, and the workpiece has to be turned over twice, which can be labor-intensive and time-consuming.

2) Alternatively, with the blank surface in contact with the platform, shims (see Figure 15) can be inserted at the highest point for adjustment. A hammer can be used to gently tap the shims horizontally for fine adjustments, which saves time and effort. Furthermore, when the operator uses a pry bar, it is important to avoid the practice known as “stretching the long shaft.” Instead, it is advisable to lift just a small height at a time and perform multiple fine adjustments.

3) The small end face of the cone is prone to deformation due to the deflection of its generatrix. Using this deformed surface as a reference for finishing can lead to magnification errors when the cone is flipped. To eliminate the impact of the small end face and to prevent outward angular changes in the cone’s generatrix, we can forgo using the small end face for positioning and instead utilize a 45° beveled ring. This rigid ring restricts the outward rotation of the cone’s long-axis generatrix while also controlling the inward rotation of the short-axis generatrix (see Figure 16).

An adjustable height system facilitates modifications to the cone’s tilt angle, allowing for the quick positioning of a specific precision reference point. Once the CNC milling products is placed in the 45° beveled ring, its offset can be adjusted by altering the tilt angle of the entire bevel ring. The height adjustment system should be designed for ease of use.

 

6. Conclusion

The 5-meter-class integral spun conical bottom ring is currently the largest of its kind in the world. Despite its seemingly simple structure, machining this component is far from easy. Its large size, substantial weight, and reduced rigidity create significant challenges. Any alterations in the machining datum, which is essential for ensuring the accuracy of the part’s dimensions and shape, can lead to a loss of the very limited adjustable allowance available. Therefore, it is crucial to understand the reasons behind changes to the datum. This research aims to provide insights and support for the machining processes of similar components.

 

 

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