A spinning forming method of a large thin-walled variable-taper rotary body member
By combining shear spinning and counterspinning techniques, a mold was designed and external and internal spinning wheels were used to form large thin-walled variable taper rotating components. This solved the problems of high core mold cost and insufficient forming accuracy in the existing technology, and achieved low-cost, high-efficiency and high-precision spinning forming effect.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HARBIN INST OF TECH
- Filing Date
- 2023-08-22
- Publication Date
- 2026-05-19
AI Technical Summary
Existing technologies for manufacturing large thin-walled tapered components suffer from problems such as high core mold costs, long mold manufacturing cycles, and insufficient forming accuracy. In particular, in spin forming technology without core mold support, the forming accuracy of the workpiece decreases during the tapered support process.
By combining shear spinning and counter-rotating wheel spinning technologies, and by designing a mold that includes a tail top and a supporting core mold, the first and second conical surfaces are formed by using an outer spinning wheel and an inner spinning wheel, respectively. The first conical surface is formed by shear spinning and the second conical surface is formed by counter-rotating wheel spinning. The supporting core mold only needs to be supported in the first spinning area, which reduces the size and cost of the supporting core mold and improves forming efficiency and accuracy.
It achieves low-cost, high-efficiency, and high-precision spinning forming, reducing manufacturing costs and cycle time, while ensuring the forming accuracy of the first conical surface and the forming quality of the overall component.
Smart Images
Figure CN117019977B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of rotary component processing technology, and in particular to a spinning forming method for large thin-walled variable taper rotary components. Background Technology
[0002] With the increasing demand for large-scale aerospace equipment, lightweight components are crucial for improving equipment performance. Therefore, achieving integrated manufacturing and reducing the number of components is an effective approach. Currently, many critical load-bearing components tend to be designed as large, thin-walled structures. The commonly used spinning process for manufacturing thin-walled, tapered components is conventional spinning, such as the composite forming method for a deep double-cone thin-walled part with a dome shape provided in patent CN106623611B. This method involves placing the blank in a shearing spinning mold for support, and then using a double-spinning wheel to spin the double-cone surface. This process requires a mandrel for support, but for large components, the mandrel is expensive and has poor versatility, resulting in a long mold manufacturing cycle. Alternatively, there are spinning forming technologies that do not use mandrel support, such as wheel spinning. This increases applicability, saves costs, and enhances the flexibility of spinning processing. However, because wheel spinning lacks mandrel support, the forming accuracy of the workpiece decreases during the tapered support process. Summary of the Invention
[0003] The purpose of this invention is to provide a spinning forming method for large thin-walled variable taper rotating components to solve the problems existing in the prior art. Combining shear spinning and wheel spinning techniques, it has the advantages of lower production cost, higher forming efficiency and better forming accuracy.
[0004] To achieve the above objectives, the present invention provides the following solution:
[0005] This invention provides a spin forming method for a large thin-walled variable taper rotary component, comprising the following steps:
[0006] Obtain the structural features of the rotating body component: the structural features include the first cone angle of the first cone surface and the second cone angle of the second cone surface;
[0007] Obtaining the sheet blank: The sheet blank has, from the center outwards, a clamping area, a first spinning area, and a second spinning area;
[0008] Mold design: The mold includes a tail top and a support core mold. The tail top and the support core mold coaxially clamp both sides of the sheet metal blank and can drive the sheet metal blank to rotate synchronously. One side of the tail top is clamped to one side of the clamping area. The side of the support core mold facing the sheet metal blank has a clamping surface and a first inclined support surface in sequence from the center to the outside. The size of the clamping surface is the same as the size of the clamping area. The inclined angle of the first inclined support surface is the same as the angle of the first conical surface.
[0009] Forming the first conical surface: An outer spinning wheel is set outside the first area to be spun and the first area to be spun is sheared and spun until the first area to be spun is attached to the first inclined support surface to form the first conical surface;
[0010] Forming the second conical surface: An inner spinning wheel is set in the second spinning zone and fed synchronously with the outer spinning wheel to spin the second spinning zone, so that the second spinning zone forms the second conical surface.
[0011] Preferably, in the step of designing the mold, a second inclined support surface is also provided on the outer side of the first inclined support surface on the supporting core mold; the second inclined support surface is used to support the connection between the second conical surface and the first conical surface.
[0012] Preferably, the slope angle of the second inclined support surface is smaller than the slope angle of the second conical surface.
[0013] Preferably, in the step of forming the first conical surface, when the outer spinning wheel shears and spins the first area to be spun, the inner spinning wheel supports the inner edge of the second area to be spun.
[0014] Preferably, in the step of forming the second conical surface, the outer rotating wheel and the inner rotating wheel perform 5-40 passes of wheel-to-wheel spinning on the second area to be spun.
[0015] Preferably, in the step of forming the second conical surface, the spinning angle before the forming angle of the second spinning area reaches 1 / 3 of the angle of the second conical surface is greater than the spinning angle after the forming angle of the second spinning area reaches 1 / 3 of the angle of the second conical surface.
[0016] Preferably, in the step of forming the second conical surface, after every 2-10 passes of wheel spinning, the second area to be spun is spun during the return stroke of the outer and inner spinning wheels.
[0017] Preferably, in the step of forming the second conical surface, before the final pass of the spinning process, the thickness of the second area to be spun has a margin of 3%-20%. Then, the inner spinning wheel is set as a flat wheel and cooperates with the outer spinning wheel to perform the final pass of spinning on the second area to be spun, so that the second area to be spun forms the second conical surface.
[0018] Preferably, in the step of forming the second conical surface, the gap between the outer rotating wheel and the inner rotating wheel is 5%-10% smaller than the initial thickness of the second area to be spun, the radius of the inner rotating wheel is 1.5-3.0 times the initial thickness of the second area to be spun, and the radius of the outer rotating wheel is 1.0-1.5 times the initial thickness of the second area to be spun.
[0019] Preferably, in the step of forming the first conical surface, the number of the outer rotating wheels is set to one or two and they are evenly distributed along the circumference of the first area to be spun; in the step of forming the second conical surface, the number of the inner rotating wheels is set to one or two and they are evenly distributed along the circumference of the first area to be spun; and the number of the outer rotating wheels and the number of the inner rotating wheels are the same.
[0020] The present invention achieves the following technical effects compared to the prior art:
[0021] The present invention provides a spinning forming method for large thin-walled variable taper rotary components. The first cone surface of the variable taper rotary component is formed by shear spinning, and the second cone surface is formed by roller spinning. Roller spinning has strong applicability. In this way, the supporting mandrel only needs to be supported on the first spinning area. Compared with forming a rotary component by shear spinning alone, which requires supporting the first and second spinning areas, the size of the supporting mandrel is reduced, manufacturing costs and manufacturing cycle are reduced, and forming efficiency is improved. In the forming method of combining shear spinning and roller spinning, the first cone surface can be supported by the supporting mandrel during the forming process of the second cone surface. Compared with the forming process of roller spinning alone, the forming of the second cone surface will interfere with the formed first cone surface, thereby ensuring the forming accuracy of the first cone surface and achieving high forming accuracy. Attached Figure Description
[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0023] Figure 1 This is a structural schematic diagram of the rotating component provided in Embodiment 1;
[0024] Figure 2 This is a schematic diagram of the supporting core mold provided in Embodiment 1;
[0025] Figure 3 This is a schematic diagram of the structure after the first conical surface is formed in the spinning method provided in Example 1.
[0026] Figure 4 This is a schematic diagram of the trajectory of the second conical surface (single inner spinning wheel and single outer spinning wheel) after forming in the spinning method provided in Example 1;
[0027] Figure 5 This is a schematic diagram of the reciprocating spinning trajectory after forming the second conical surface in the spinning forming method provided in Example 1.
[0028] Figure 6This is a schematic diagram of the final spinning step in forming the second conical surface in the spinning method provided in Example 1.
[0029] Figure 7 This is a schematic diagram of the trajectory of the second conical surface (double inner spinning wheel and double outer spinning wheel) after forming in the spinning forming method provided in Example 1.
[0030] Icons: 1-Rotating component; 11-First conical surface; 12-Second conical surface; 2-Sheet blank; 21-Clamping area; 22-First spinning area; 23-Second spinning area; 3-Tail top; 4-Supporting core mold; 41-Clamping surface; 42-First inclined support surface; 43-Second inclined support surface; 5-Outer spinning wheel; 6-Inner spinning wheel. Detailed Implementation
[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0032] The purpose of this invention is to provide a spinning forming method for large thin-walled variable taper rotating components to solve the problems existing in the prior art. Combining shear spinning and wheel spinning techniques, it has the advantages of lower production cost, higher forming efficiency and better forming accuracy.
[0033] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.
[0034] Example 1
[0035] This embodiment provides a spin forming method for a large thin-walled variable taper rotary component, including the following steps:
[0036] To obtain the structural features of the rotating body components, please refer to [link / reference]. Figure 1 The structural features include the first cone angle of the first cone surface 11. Figure 1 (a) and the second cone angle of the second cone surface 12 ( Figure 1 In part b), specifically, the structural features of the rotating component 1 must be determined before forming, and may also include thickness features, so as to facilitate the selection of the sheet metal blank 2 and the design of the mold.
[0037] To obtain sheet blanks, please refer to [link / reference]. Figure 3The sheet blank 2 has a clamping area 21, a first spinning area 22 and a second spinning area 23 in sequence from the center outwards. When selecting the sheet blank 2, the thickness and size should be considered so that the dimensional characteristics of the rotating body component 1 can be met after spinning.
[0038] For mold design, please refer to [link / reference]. Figure 2 and Figure 3 The mold includes a tail top 3 and a support core mold 4. The tail top 3 and the support core mold 4 coaxially clamp both sides of the sheet metal blank 2 and can drive the sheet metal blank 2 to rotate synchronously. Specifically, the tail top 3 and the support core mold 4 are set on a conventional spinning machine. One side of the tail top 3 is clamped to one side of the clamping area 21. The side of the support core mold 4 facing the sheet metal blank 2 has a clamping surface 41 and a first inclined support surface 42 from the center outward. The size of the clamping surface 41 is the same as the size of the clamping area 21, which fully clamps the sheet metal blank 2. The inclined angle of the first inclined support surface 42 is the same as the angle of the first conical surface, which facilitates the shearing and spinning forming of the first conical surface 11.
[0039] For forming the first conical surface, please refer to [link / reference]. Figure 3 An outer spinning wheel 5 is set outside the first spinning area 22 and shears and spins the first spinning area 22 until the first spinning area 22 is attached to the first inclined support surface 42 to form the first conical surface 11.
[0040] For forming the second conical surface, please refer to [link / reference]. Figures 4-6 An inner spinning wheel 6 is set in the second spinning zone 23 and fed synchronously with the outer spinning wheel 5 to spin the second spinning zone 23, so that the second spinning zone 23 is formed into the second conical surface 12.
[0041] The first conical surface 11 of the variable taper rotary component 1 is formed by shear spinning, and the second conical surface 12 is formed by wheel spinning. Wheel spinning has strong applicability. In this way, the supporting mandrel 4 only needs to be supported on the first spinning area 22. Compared with the rotary component 1 formed by shear spinning alone, which requires supporting the first spinning area 22 and the second spinning area 23, the size of the supporting mandrel 4 is reduced, the manufacturing cost and manufacturing cycle are reduced, and the forming efficiency is improved. In the forming method of shear spinning and wheel spinning, the first conical surface 11 can be supported by the supporting mandrel 4 during the forming process of the second conical surface 12. Compared with the forming process of wheel spinning alone, the forming of the second conical surface 12 will interfere with the formed first conical surface 11, thereby ensuring the forming accuracy of the first conical surface 11 and making the forming accuracy of the entire rotary component 1 high.
[0042] In the optional solutions of this embodiment, a more preferred approach is to refer to the mold design step. Figure 2 and Figure 6A second inclined support surface 43 is also provided on the outer side of the first inclined support surface 42 on the support core mold 4; the second inclined support surface 43 is used to support the connection between the second conical surface 12 and the first conical surface 11, and the second inclined support surface 43 supports the inner wall of the connection to ensure the forming accuracy of the connection.
[0043] More preferably, since the outer side of the connection is spun by the outer spinning wheel 5, the inner side of the connection cannot be spun by the inner spinning wheel 6. In order to ensure that the second conical surface 12 remains flat on the inner side of the connection after forming, the inner side of the connection needs to maintain deformability during the spinning process. Therefore, the slope angle of the second inclined support surface 43 is smaller than the second conical surface angle, such as slightly smaller by 1°-6°. Specifically, the slope angle of the second inclined support surface 43 can be slightly smaller than the second conical surface angle. The specific degree can be determined according to the thickness of the rotating body component 1, the first conical surface angle or the second conical surface angle, etc., or adjusted in actual production.
[0044] In the optional scheme of this embodiment, more preferably, during the step of forming the first conical surface, when the outer rotating wheel 5 shears and spins the first area to be spun 22, the inner outer edge of the second area to be spun 23 is supported by an inner rotating wheel 6. By simultaneously supporting the inner outer edge of the second area to be spun 23 during the forming of the first conical surface 11, the local instability and edge wrinkling of the second area to be spun 23 can be constrained and limited, ensuring forming accuracy. Moreover, the inner rotating wheel 6 can cooperate with the outer rotating wheel 5 to perform wheel spinning on the second area to be spun during the forming process of the second conical surface 12.
[0045] In the optional scheme of this embodiment, more preferably, in the step of forming the second conical surface, the outer rotating wheel 5 and the inner rotating wheel 6 perform 5-40 passes of wheel spinning on the second area to be spun 23. The outer rotating wheel 5 and the inner rotating wheel 6 are fed synchronously. The outer rotating wheel 5 mainly plays a forming role, and the inner rotating wheel 6 plays a supporting role. The spinning trajectory adopts a straight trajectory according to the shape of the spun part. Through multiple passes of wheel spinning, the second conical surface 12 is gradually formed. Specifically, the number of passes can be determined according to the structural characteristics and spinning angle, and can be 5 passes, 10 passes, 20 passes, etc.
[0046] In the optional scheme of this embodiment, more preferably, since there is no mold constraint during the forming process of the second conical surface 12, the second spinning area 23 will have obvious springback. In the early stage of spinning, the second spinning area 23 has a springback tendency to expand outward, while in the later stage of spinning, the second spinning area 23 has a springback tendency to contract inward. Therefore, in the step of forming the second conical surface, the spinning angle before the forming angle of the second spinning area 23 reaches 1 / 3 of the angle of the second conical surface is greater than the spinning angle after the forming angle of the second spinning area 23 reaches 1 / 3 of the angle of the second conical surface. The larger spinning angle in the early stage suppresses the outward expansion springback tendency, and the smaller spinning angle in the later stage suppresses the inward contraction springback tendency. Using a variable spinning angle can improve the component accuracy. Specifically, the change of spinning angle can be determined according to the specific situation.
[0047] In the optional solutions of this embodiment, a more preferred option is described in the following description: Figure 5 During the 12-pass spinning process of the second conical surface, in order to prevent excessive thinning, a reciprocating spinning trajectory can be adopted. That is, the outer spinning wheel 5 and the inner spinning wheel 6 are fed outwards synchronously first, and after the outer spinning wheel 5 and the inner spinning wheel 6 have finished spinning, they are fed inwards. This spinning trajectory can change the flow direction of the material, prevent the material from being excessively thinned, and improve the uniformity of the wall thickness of the blank. In the step of forming the second conical surface, after every 2-5 passes of spinning, during the return stroke of the outer spinning wheel 5 and the inner spinning wheel 6, the second spinning area 23 is spun. Specifically, reciprocating spinning can be performed once every 2, 3, or 5 passes, and can also be adjusted according to actual production experience.
[0048] In the optional scheme of this embodiment, more preferably, in order to ensure the accuracy of the forming surface and reduce springback, in the step of forming the second conical surface, the gap between the outer rotating wheel 5 and the inner rotating wheel 6 is 5%-10% smaller than the initial thickness (thickness before spinning) of the second spinning area 23, specifically 5% or 7%, which can increase the spinning force. The radius of the inner rotating wheel 6 is 1.5-3.0 times the initial thickness of the second spinning area 23, specifically 1.5 times or 2 times, and the radius of the outer rotating wheel 5 is 1.0-1.5 times the initial thickness of the second spinning area 23, specifically 1.0 times or 1.2 times, etc., and can also be adjusted according to the actual production. This can better constrain the deformation of the material, ensure deformation according to the trajectory, improve the accuracy of the workpiece, and also improve the uniformity of the wall thickness.
[0049] In the optional solutions of this embodiment, a more preferred option is described in the following description: Figure 6To ensure the accuracy and mechanical properties of the formed surface, in the process of forming the second conical surface, before the final pass of spinning, the thickness of the second area to be spun 23 has a margin compared to the thickness of the second conical surface 12, such as between 3% and 20%, or 3% or 10%. Then, the inner spinning wheel 6 is set as a flat wheel and works with the outer spinning wheel 5 to perform the final pass of spinning on the second area to be spun 23, so that the second area to be spun 23 forms the second conical surface 12. The inner spinning wheel 6 with rounded corners is replaced with a flat wheel, which acts as an inner support for surface finishing and cold spinning hardening, thereby improving the accuracy and mechanical properties of the formed surface of the second conical surface 12.
[0050] In the optional scheme of this embodiment, more preferably, in the first cone-shaped step, the number of outer rotating wheels 5 is set to one or two and evenly distributed along the circumference of the first spinning area 22; in the second cone-shaped step, the number of inner rotating wheels 6 is set to one or two and evenly distributed along the circumference of the first spinning area 22, wherein the inner rotating wheels 6 and outer rotating wheels 5 are used in pairs; specifically, it can be determined according to the structural characteristics of the rotating body component 1. If the size of the rotating body component 1 is small, and the diameter of the sheet blank is 1000-2000mm and the thickness is 5mm-35mm, one inner rotating wheel 6 and one outer rotating wheel 5 can be used in combination to avoid interference; if the size of the rotating body component 1 is large, and the diameter of the sheet blank is 2000-5000mm and the thickness is 35mm-80mm, two inner rotating wheels 6 and two outer rotating wheels 5 can be used in combination, making the forming process more stable and reducing the occurrence of defects. Please refer to [link to relevant documentation]. Figure 7 .
[0051] Specifically, the structural characteristics of a typical large thin-walled variable-tapered rotary body component are: the first cone angle α is between 5° and 25°, the second cone angle β has a wider range, between 5° and 60°, the diameter of the sheet metal blank is between 1000-2000 mm, and the thickness is between 5 mm and 35 mm. Specifically, the first cone angle α of the thin-walled variable-tapered rotary body component 1 is set to 10°, and the second cone angle α is set to 18°. The sheet metal blank 2 is selected with a diameter of 1500 mm and a thickness of 25 mm. Based on the shape and dimensions of the support core mold 4, the angle of the second inclined support surface 43 is 16° to prevent the second cone surface 12 from being uneven during the final mold application process. Then, the first area to be spun 22 is sheared and spun using an external rotating wheel 5. The mold gap is determined according to the sine law. The inner outer edge of the second area to be spun 23 is placed... An inner spinning wheel 6 is placed to constrain the wrinkling fluctuations of the sheet metal blank 2. Then, the second spinning zone 23 is spun multiple times. The inner spinning wheel 6 and the outer spinning wheel 5 are fed synchronously. The outer spinning wheel 5 mainly plays a forming role, while the inner spinning wheel 6 plays a supporting role. According to the shape of the spun part, the spinning trajectory adopts a straight trajectory. Based on the shape of this workpiece and experimental experience, 30-pass multi-pass spinning is adopted. The spinning angle changes from 5° in the early stage to 2° in the later stage. In order to prevent excessive thinning of the blank, a reciprocating spinning trajectory is used every 5 passes to improve the uniformity of the blank wall thickness. Finally, flat wheel spinning is performed. Before the final spinning, a certain allowance of about 5% is left. The blank angle is changed by about 2° by flat wheel spinning. Flat wheel cold spinning is used for shaping and cold work hardening to ensure the contour accuracy and strength of the rotating component 1.
[0052] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.
Claims
1. A method for spin forming a large thin-walled variable taper rotary component, characterized in that: Including the following steps: Obtain the structural features of the rotating body component: the structural features include the first cone angle of the first cone surface (11) and the second cone angle of the second cone surface (12); Obtaining sheet blanks: The sheet blanks (2) have a clamping area (21), a first spinning area (22), and a second spinning area (23) from the center outwards. Mold Design: The mold includes a tail top (3) and a support core mold (4). The tail top (3) and the support core mold (4) coaxially clamp the two sides of the sheet metal blank (2) and can drive the sheet metal blank (2) to rotate synchronously. One side of the tail top (3) is clamped to one side of the clamping area (21). The side of the support core mold (4) facing the sheet metal blank (2) has a clamping surface (41) and a first inclined support surface (42) from the center outward. The size of the clamping surface (41) is the same as the size of the clamping area (21). The inclined angle of the first inclined support surface (42) is the same as the angle of the first cone surface. A second inclined support surface (43) is also provided on the outside of the first inclined support surface (42) on the support core mold (4). The second inclined support surface (43) is used to support the connection between the second cone surface (12) and the first cone surface (11). The inclined angle of the second inclined support surface (43) is smaller than the angle of the second cone surface. Forming the first conical surface: An outer spinning wheel (5) is set outside the first spinning area (22) and shears and spins the first spinning area (22) until the first spinning area (22) is attached to the first inclined support surface (42) to form the first conical surface (11); when the outer spinning wheel (5) shears and spins the first spinning area (22), the inner outer edge of the second spinning area (23) is supported by an inner spinning wheel (6). Forming the second conical surface: An inner spinning wheel (6) is set in the second spinning area (23) and fed synchronously with the outer spinning wheel (5) to spin the second spinning area (23) to form the second conical surface (12); the spinning angle before the forming angle of the second spinning area (23) reaches 1 / 3 of the angle of the second conical surface is greater than the spinning angle after the forming angle of the second spinning area (23) reaches 1 / 3 of the angle of the second conical surface.
2. The spinning forming method for a large thin-walled variable taper rotary component according to claim 1, characterized in that: In the process of forming the second conical surface, the outer spinning wheel (5) and the inner spinning wheel (6) perform 5-40 passes of spinning on the second area to be spun (23).
3. The spinning forming method for a large thin-walled variable taper rotary component according to claim 2, characterized in that: In the process of forming the second conical surface, after every 2-10 passes of wheel spinning, the second spinning zone (23) is spun during the return stroke of the outer spinning wheel (5) and the inner spinning wheel (6).
4. The spinning forming method for a large thin-walled variable taper rotary component according to claim 2, characterized in that: In the step of forming the second conical surface, before the final pass of the wheel spinning, the thickness of the second spinning area (23) has a margin of 3%-20%. Then, the inner spinning wheel (6) is set as a flat wheel and cooperates with the outer spinning wheel (5) to perform the final pass of the wheel spinning on the second spinning area (23) so that the second spinning area (23) forms the second conical surface (12).
5. The spinning forming method for a large thin-walled variable taper rotary component according to claim 2, characterized in that: In the step of forming the second conical surface, the gap between the outer rotating wheel (5) and the inner rotating wheel (6) is 5%-10% smaller than the initial thickness of the second area to be spun (23). The radius of the inner rotating wheel (6) is 1.5-3.0 times the initial thickness of the second area to be spun (23), and the radius of the outer rotating wheel (5) is 1.0-1.5 times the initial thickness of the second area to be spun (23).
6. The spinning forming method for a large thin-walled variable taper rotary component according to any one of claims 1-5, characterized in that: In the step of forming the first conical surface, the number of the outer spinning wheels (5) is set to one or two and is evenly distributed along the circumference of the first spinning area (22); in the step of forming the second conical surface, the number of the inner spinning wheels (6) is set to one or two and is evenly distributed along the circumference of the first spinning area (22); and the number of the outer spinning wheels (5) and the inner spinning wheels (6) is the same.