Three-dimensional curved surface forming method

Abstract

A three-dimensional curved surface forming method, by setting the stretching cylinder of the stretch forming machine on both sides of the mold drawing direction of the three-dimensional curved surface forming tool and the segmented shaft rotating stretch forming machine stretching cylinder sub clamp joint movement, to exert bending and stretching movement load on both ends of the sheet laid on the mold, so as to make the sheet adhere to the mold section by section; And the x direction profile cross section line with the maximum chord height of the mold is taken as the control profile cross section line, and then according to the curvature and arc length change of the control profile cross section line, the stretch forming area is segmented and divided, the dispersion of the extension rate of each stretch forming area is realized, finally the stretching is carried out according to the stretch forming extension rate of each stretch forming area, and then the three-dimensional curved surface with high contour precision is obtained. The present application proposes segmented stretch forming, effectively improves the uniformity of elastic and plastic deformation in each stretch forming range, realizes the deformation uniformity of overall stretch forming, and improves the performance of stretch forming process in stretch forming precision and stretch forming efficiency through stretch forming area division.

Description

Technical Field

[0001] This invention relates to the field of aircraft manufacturing technology, and in particular to a method for forming three-dimensional curved surfaces. Background Technology

[0002] The aerodynamic shape and internal cabin walls of aircraft are composed of a large number of hyperbolic or monobolic three-dimensional curved surface parts. These parts are made of thin metal sheets and have the structural characteristics of thin walls, large size and continuous irregular changes in curvature radius. The forming of their curved surface shape is a difficult point in the manufacturing process of aircraft body structure, and stretch forming is its main manufacturing method.

[0003] In the stretch forming process, stretching machine jaws positioned at both ends of the sheet metal to be stretched clamp the two ends of the sheet. The jaws apply a stretching motion to the sheet, causing it to gradually conform to the mold and form a three-dimensional curved surface with a specific shape and size. Traditional stretch forming processes often focus on a single-pass integral forming stage, where the sheet metal is directly bent to cover the entire mold surface before stretching. This allows the sheet metal to conform to the mold surface during bending, covering, and stretching, achieving the desired three-dimensional curved surface shape. Because the single-pass stretch forming process involves bending and covering the entire die surface in a single pass, the area of ​​the die surface involved in bending and covering is the same as the entire die surface area. This means that the uniformity of frictional force, the uniformity of the covering angle, and even the uniformity of the mechanical properties of the sheet metal being stretched cannot be guaranteed at all points on the die surface during the stretching process. This results in the formed three-dimensional surface being highly susceptible to defects such as wrinkling and arching, making it impossible to guarantee shape accuracy. Furthermore, the stretching process, where the sheet metal comes into contact with all parts of the die surface, can easily lead to an excessively large contact area, causing a sharp increase in frictional resistance that hinders plastic deformation. This places higher demands on the stretching force of the stretching machine, especially the downward pressure, directly increasing the manufacturing and maintenance costs of the stretching machine and hindering the establishment and expansion of a competitive market advantage.

[0004] To address the aforementioned issues, the paper "Research on Loading Trajectory of Aircraft Skin Pull Forming" proposes a pull forming method that uses equal division points on the mold surface cross-section line to divide the mold surface cross-section line into multiple segments. The sheet metal is then gradually wrapped around the mold in an equidistant manner to form a curved surface. While this method can overcome the problem of uneven pull forming deformation caused by an excessively large single-pass pull forming process, it still faces the challenge of matching the segment length with the curvature variation characteristics of the mold surface cross-section, a problem that the equidistant segmentation method cannot solve. Furthermore, due to the high degree of complexity of the aerodynamic shape of aircraft, the curvature of the mold surface cross-section line in different parts of the same length segment area often varies, resulting in inconsistent frictional resistance and other factors in the sheet metal during the pull forming process. This leads to uneven elastic and plastic deformation in different parts of the same length segment area during the pull forming process, and still cannot meet the requirements of high-precision three-dimensional curved surface pull forming.

[0005] More importantly, aircraft skin parts are typical examples of three-dimensional curved surfaces and are crucial to forming the aircraft's aerodynamic shape. Their manufacturing and assembly involves a complex process including skin stretching, skin mirror milling, rigid shell riveting, and attitude adjustment. The aerodynamic shape formed by skin stretching is transferred along the process route to skin mirror milling and even the assembly process, directly limiting the improvement of skin mirror milling machining accuracy and even the overall assembly accuracy. Therefore, from the perspective of the entire aerospace sheet metal skin processing production line, skin stretching's forming accuracy and processing efficiency have become one of the decisive factors determining the accuracy of the aircraft's aerodynamic shape and overall processing efficiency, and urgently need to be addressed. Summary of the Invention

[0006] The technical problem solved by this invention is to provide a three-dimensional curved surface forming method to solve the problems in the background art mentioned above.

[0007] The technical problem solved by this invention is achieved by the following technical solution:

[0008] A three-dimensional curved surface forming method uses a three-dimensional curved surface forming fixture to apply bending and tensile loads to both ends of a sheet metal, causing the sheet metal to fit into the mold segment by segment, thereby obtaining a three-dimensional curved surface with high dimensional accuracy. The specific steps are as follows:

[0009] Step 1) Assemble the three-dimensional curved surface forming fixture. First, place the mold (1) between the left frame (10) and the right frame (11) of the stretching machine. Then, install the left rotating shaft (18) and the left stretching cylinder (5) of the stretching machine on the left frame (10), and install the right rotating shaft (19) and the right stretching cylinder (6) of the stretching machine on the right frame (11). Then, install the left stretching cylinder clamp (3) of the stretching machine, which can rotate around the shaft in sections, on the end of the left stretching cylinder (5) facing the mold (1). The end of the right stretching cylinder (6) facing the mold (1) is equipped with a segmented rotating clamp (4) of the right stretching cylinder of the stretching machine. Finally, the sheet material (2) is laid on the mold (1), and one end of the sheet material (2) is clamped by the clamp (3) of the left stretching cylinder of the stretching machine, and the other end of the sheet material (2) is clamped by the clamp (4) of the right stretching cylinder of the stretching machine. The left stretching cylinder (5) and the right stretching cylinder (6) of the stretching machine apply bending and stretching loads to both ends of the sheet material (2), so that the sheet material (2) is gradually attached to the mold (1).

[0010] Step 2) Take the x-direction profile section line with the largest chord height of the mold (1) as the control profile section line (7). Based on the principle of uniform distribution of the stretching elongation according to the change of the normal angle of the stretching region, establish a functional relationship that makes the overall stretching elongation uniformly distributed according to the change of the normal angle of the stretching region.

[0011] (Step 3) Based on the curvature and arc length changes of the control profile section line (7), and based on the principle that the chord height of each stretching region does not exceed the threshold and the sum of the lengths of each stretching region covers the control profile section line (7), the control profile section line (7) is divided into stretching regions, the segmentation points of each stretching region are determined, and the stretching extension rate of each segment is calculated based on the change in the normal angle of each segment, so as to realize the discretization of the stretching extension rate of each stretching region.

[0012] (Fourth step) Based on the clamping of the stretching cylinder clamp of the stretching machine to hold the two sections of the sheet metal (2), the stretching cylinder clamp of the stretching machine rotates around the axis at different angles to bend the sheet metal (2) laterally, so that the end of the sheet metal (2) in the x direction is bent laterally to be consistent with the arc of the end corresponding to the mold (1). Then, the stretching and forming of each stretching area is completed in the following order: first, the sheet metal (2) is bent longitudinally to be in contact with the two ends of the stretching area on the control contour section line (7); then, the stretching is performed along the tangent direction of the sheet metal (2) according to the stretching elongation rate of the stretching area; and finally, the stretching and unloading are performed in the opposite direction of the stretching direction.

[0013] In this invention, to accommodate the conventional skin stretching machine which has two stretching cylinder clamps on the left and right, facilitating the use of the two stretching cylinder clamps to clamp both ends of the sheet metal (2) and complete the forming of the half-area respectively, before establishing the functional relationship between the stretching elongation rate and the change of the normal angle of the stretching area, the control profile section line (7) is divided into a left half-area and a right half-area, and the highest point of the control profile section line (7) is used as the dividing point between the left half-area and the right half-area, specifically:

[0014] Establish the curve equation of the control profile section line (7), obtain the highest point of the control profile section line (7) in the z direction, and then take the highest point in the z direction as the boundary point A between the left and right halves of the control profile section line, and divide the control profile section line (7) into the left half and the right half with the boundary point A between the left and right halves of the control profile section line as the center.

[0015] In this invention, in the second step, the profile section line of the mold (1) with the largest chord height in the x direction is determined in the following way:

[0016] Establish a plane perpendicular to the y-axis and intersect it with the surface of the mold (1) to obtain the intersecting profile section line. Create a straight line connecting the two ends of the intersecting profile section line and obtain the chord height from the straight line to the intersecting profile section line. Move the plane along the y-axis. When the chord height from the straight line to the intersecting profile section line reaches its maximum, stop moving the plane. Take the intersecting profile section line formed by the plane at this time and the surface of the mold (1) as the x-direction profile section line with the maximum chord height.

[0017] In this invention, in the second step, a functional relationship is established to uniformly distribute the overall elongation rate of the stretched shape according to the change in the normal angle of the stretched region, specifically as follows:

[0018] Obtain the normal vector N of the control profile section line (7) at the boundary point A of the left and right halves respectively. A And the left end point P of the control profile section line (7) in the left stretching direction (12). e0 The normal vector N at that location e0 The right end point P of the right-side pull-shaped direction (13) e1 The normal vector N at that location e1 Calculate the left and right end points P respectively. e0 and P e1 The normal vector N at that location e0 N e1 With normal vector N A The included angle θ e0 and θ e1 Given a pre-set total elongation of the drawing, the angle θ between the total elongation of the drawing and the aforementioned angle is obtained. e0 and θ e1The ratios K1 and K2 are then used as the slopes of the function relating the total elongation of the stretching in the left and right halves to the uniform distribution of the stretching region's normal angle. Based on these slopes and a univariate linear function model without intercept, the function relationships f1(θ1) and f2(θ2) for the uniform distribution of the total elongation of the stretching in the left and right halves to the uniform distribution of the stretching region's normal angle are established:

[0019]

[0020] In equation (1), θ1 and θ2 are the changes in the normal angles of the left and right half-regions of the stretching region, respectively. This achieves reasonable and uniform dispersion and distribution of the elongation rate of each region during the stretching process according to the angle change of the mold control contour section line, preventing uneven stretching elastic and plastic deformation caused by unreasonable distribution of stretching elongation rate in each region.

[0021] To ensure that the sheet metal does not produce material defects after stretching, the total stretching elongation is set to the elongation that does not cause material defects in the sheet metal after stretching. Specifically, the total stretching elongation is related to the mechanical properties of the sheet metal being stretched, and is 20% to 90% of the elongation corresponding to the tensile strength of the sheet metal (2) on the stress-strain curve.

[0022] In the third step of this invention, the control contour section line (7) is used to divide the stretching region, specifically as follows:

[0023] Multiple segmented calculation points are made on the control profile section line (7), and these points are set from the boundary point A of the left and right halves towards the left end point P of the control profile section line (7). e0 and the right end point P e1 By moving the points at different distances, lines are established connecting each adjacent point, including the boundary point A of the left and right halves and the aforementioned segmented calculation points. The chord height between the lines connecting the adjacent points and the control profile section line (7) is obtained. Based on a predetermined threshold, when the relationship between the chord height and the threshold satisfies a predetermined condition, the movement of the aforementioned segmented calculation points is stopped, and the left end point P, which is located at this time on the control profile section line (7), is moved to the next position. e0 and the right end point P e1 The segment calculation points between them are determined as the segment points of the stretch-shaped region; the control contour section line (7) is divided according to the segment points of the stretch-shaped region and the boundary point A between the left and right halves;

[0024] Furthermore, when the moving segment calculation points are stopped, the relationship between the line connecting each adjacent point, including the boundary point A of the left half and the right half and multiple segment calculation points, and the chord height and threshold of the control profile section line (7) should meet the predetermined condition that: the ratio between the above chord height value and the threshold is equal to M, wherein the ratio M is preset and less than or equal to 1, and the ratio M of chord height to threshold is optimally set to 0.6 to 1.

[0025] In this invention, in the third step, the elongation rate of each segment of the segmented stretching is discretely allocated based on the change in the normal angle of the stretching model surface contour as the criterion. Specifically:

[0026] Based on the segmentation of the control profile section line (7), the normal vectors of the control profile section line (7) at the two ends of each stretching region are established. Then, the angle between the normal vectors at the two ends of each stretching region is calculated. According to the left and right half of each stretching region, the angle between the normal vectors at the two ends is multiplied by the function relationship f1(x) and f2(x) that the total stretching elongation rate of the left and right half is uniformly distributed according to the change of the normal angle of the stretching region. The stretching elongation rate of each stretching region is obtained, so that the stretching elongation rate matches the curvature change of the stretching model surface, and the limited stretching elongation rate is distributed more to the region with a large change in the curvature of the stretching model.

[0027] In this invention, in step four), a segmented three-dimensional curved surface stretching forming is adopted with bending-stretching-relaxation as the basic cycle unit. This enables the stretching machine to complete the stretching cylinder clamp rotation and downward pressing under low load after each stretching segment is completed, reducing the requirements for the downward pressure of the machine tool jaws, reducing equipment costs, and broadening the scope of application. Specifically:

[0028] After completing the bending-stretching action, a relaxation and unloading step is added in the opposite direction of the stretching direction, so that the stretching force borne by the left frame (10) and the right frame (11) of the stretching machine is unloaded to less than the force that the stretching machine can output to make the left stretching cylinder (5) of the stretching machine rotate around the left rotation axis (18) of the stretching machine and the right stretching cylinder (6) of the stretching machine rotate around the right rotation axis (19) of the stretching machine.

[0029] In this invention, the three-dimensional curved surface forming fixture includes a mold (1), a sheet metal (2), a left stretching cylinder clamp (3), a right stretching cylinder clamp (4), a left stretching cylinder (5), a right stretching cylinder (6), a control contour section line (7), a left frame (10), a right frame (11), a left stretching direction (12), a right stretching direction (13), a left rotation shaft (18), and a right rotation shaft (19). The left frame (10) of the mold (1) is provided in the left stretching direction (12), and the right frame (11) of the mold (1) is provided in the right stretching direction (13). At the same time, a device for connecting with the left stretching cylinder 5 of the stretching machine is provided on the left frame (10). The left rotating shaft (18) of the stretching machine is connected, and the right rotating shaft 19 of the stretching machine is provided on the right frame (11) of the stretching machine for connecting with the right stretching cylinder (6) of the stretching machine; and the left stretching cylinder clamp (3) of the stretching machine that can rotate around the shaft in segments is installed at the end of the left stretching cylinder (5) of the stretching machine facing the mold (1), and the right stretching cylinder clamp (4) of the stretching machine that can rotate around the shaft in segments is installed at the end of the right stretching cylinder (6) of the stretching machine facing the mold (1). The sheet material (2) is laid on the mold (1), and one end of the sheet material (2) is clamped by the left stretching cylinder clamp (3) of the stretching machine, and the other end of the sheet material (2) is clamped by the right stretching cylinder clamp (4) of the stretching machine; the x-direction profile section line with the largest chord height of the mold (1) is the control profile section line (7).

[0030] Beneficial effects:

[0031] 1) This invention proposes segmented stretch forming, which discretizes the large overall deformation of a single pass covering the entire mold surface into multiple segments of small deformation. By improving the uniformity of elastic and plastic deformation within each stretch forming range, the overall stretch forming deformation becomes uniform, avoiding the problems of poor stretch forming accuracy caused by uneven material mechanical properties, uneven stretching friction, and stretching friction exceeding the limits of material mechanical properties in the traditional single-pass stretch forming process.

[0032] 2) This invention is based on the division of the drawing region by the chord height of the drawing model surface. By calculating the relationship between the chord height and a preset threshold, the drawing region can be divided into uniform or non-uniform regions. By reducing the length of each drawing region in the part of the drawing die with larger curvature and increasing the length of each drawing region in the part of the drawing die with smaller curvature, the purpose of reducing the elastic and plastic deformation of each drawing process and improving the drawing efficiency is achieved, thereby comprehensively improving the performance of the drawing process in terms of drawing accuracy and drawing efficiency.

[0033] 3) This invention proposes a method for discretely allocating the elongation rate of each segment of the stretching model by using the change in the normal angle of the stretching model surface as the criterion. This method matches the elongation rate of the stretching model with the curvature change of the stretching model surface, and allocates more of the limited elongation rate to the area where the curvature of the stretching model changes significantly. This is beneficial for the rational allocation of elongation rate resources and provides a good solution for obtaining three-dimensional surfaces with large curvature and high curvature change amplitude.

[0034] 4) This invention proposes a segmented three-dimensional surface stretching process using bending-stretching-relaxation as the basic cycle unit. A relaxation stage, which is opposite to the stretching loading direction, is added to each stretching step. This reduces the stretching force on the left and right stretching cylinder jaws and the frame of the stretching machine after each stretching step is completed to a level lower than the maximum output force of the stretching cylinder jaws pitching rotation. This reduces the requirement for pitching rotation force on the stretching machine, enabling skin stretching machines without vertical stretching cylinders to complete the stretching of three-dimensional surfaces that require large-angle jaw pitching rotation and downward pressure. This reduces the equipment cost required to manufacture complex three-dimensional surfaces, enhances the market price competitiveness of three-dimensional surface products, lays a good foundation for low-cost, high-precision manufacturing, and greatly expands the scope of application.

[0035] 5) This invention not only improves the efficiency of stretch forming of three-dimensional curved surface products, especially aerospace sheet metal skin parts, but also improves the forming accuracy. Furthermore, by improving and ensuring the processing accuracy of the stretch forming process, the dimensional transfer law can be utilized to transfer the dimensions from the skin stretch forming process to the subsequent skin mirror milling process and even the final assembly process. Based on the positive effects of improved skin forming accuracy on mirror milling and assembly processes in terms of part positioning, surface inspection, and processing accuracy control, the improvement of stretch forming accuracy can not only maintain the accuracy of the aerodynamic shape of the skin itself, but also comprehensively improve the processing accuracy and efficiency of skin mirror milling and skin assembly processes, laying a good foundation for the efficient and high-quality production of the entire skin processing production line. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of a three-dimensional curved surface forming tooling structure in a preferred embodiment of the present invention.

[0037] Figure 2 This is a schematic diagram illustrating the relationship between the overall stretching structure and its elongation rate, the change function of the normal angle, and the calculation of the stretching elongation rate of each stretching region in a preferred embodiment of the present invention.

[0038] Figure 3 This is a schematic diagram illustrating the determination of segmentation points and the segmentation of the control contour section line in a preferred embodiment of the present invention.

[0039] Figure 4 This is a schematic diagram of transverse bending of a sheet material in a preferred embodiment of the present invention.

[0040] Figure 5 This is a schematic diagram showing the angle between the normal vectors at the two ends of the stretch region in the preferred embodiment 1 of the present invention.

[0041] Figure 6 This is a schematic diagram of the initial unstretched state of the sheet metal in a preferred embodiment 1 of the present invention.

[0042] Figure 7 The first segment region (S) in the preferred embodiment 1 of the present invention 10 -A、AS 11 (Diagram showing the completed state of the pull-out shape)

[0043] Figure 8 The second segment region (S) in the preferred embodiment 1 of the present invention 20 -S 10 S 11 -S 21 (Diagram showing the completed state of the pull-out shape)

[0044] Figure 9 The third segment (S) in the preferred embodiment 1 of the present invention 30 -S 20 S 21 -S 31 (Diagram showing the completed state of the pull-out shape)

[0045] Figure 10 The fourth segment region (P) in the preferred embodiment 1 of the present invention e0 -S 30 S 31 -P e1 (Diagram showing the completed state of the pull-out shape)

[0046] Figure 11 This is a schematic diagram showing the completed state of all regions in the preferred embodiment 2 of the present invention.

[0047] Figure 12 This is a schematic diagram showing the completed state of all regions in the preferred embodiment 3 of the present invention.

[0048] Attached diagram labels: 1. Mold; 2. Sheet metal; 3. Left stretching cylinder clamp of the stretching machine; 4. Right stretching cylinder clamp of the stretching machine; 5. Left stretching cylinder of the stretching machine; 6. Right stretching cylinder of the stretching machine; 7. Three-dimensional curved surface forming trajectory control contour section line; 10. Left frame of the stretching machine; 11. Right frame of the stretching machine; 12. Left stretching direction; 13. Right stretching direction; 14. Left segment calculation point S. 10 The line connecting to the dividing point A, 15, the right segment calculation point S. 11 The line connecting to the dividing point A, 16, the left segment calculation point S 20 To the left segment calculation point S 10 The line connecting the points, 17. Calculation point S of the right segment.21 To the right segment calculation point S 11 The connecting lines are: 18. Left rotation axis of the stretch forming machine; 19. Right rotation axis of the stretch forming machine; 20. Left end point P of the control profile section line. e0 To the left segment calculation point S 20 The connecting line, 21, controls the right end point P of the profile section line. e1 To the right segment calculation point S 21 The connection. Detailed Implementation

[0049] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific illustrations.

[0050] A three-dimensional curved surface forming method uses a three-dimensional curved surface forming fixture to apply bending and tensile motion loads to both ends of a sheet metal (2), causing the sheet metal (2) to fit into the mold (1) segment by segment, thereby obtaining a three-dimensional curved surface with high shape accuracy. The specific steps are as follows:

[0051] Step 1) Assemble the three-dimensional curved surface forming fixture. First, place the mold (1) between the left frame (10) and the right frame (11) of the stretching machine. Then, install the left rotating shaft (18) and the left stretching cylinder (5) of the stretching machine on the left frame (10), and install the right rotating shaft (19) and the right stretching cylinder (6) of the stretching machine on the right frame (11). Then, install the left stretching cylinder clamp (3) of the stretching machine, which can rotate around the shaft in sections, on the end of the left stretching cylinder (5) facing the mold (1). The end of the right stretching cylinder (6) facing the mold (1) is equipped with a segmented rotating clamp (4) of the right stretching cylinder of the stretching machine. Finally, the sheet material (2) is laid on the mold (1), and one end of the sheet material (2) is clamped by the clamp (3) of the left stretching cylinder of the stretching machine, and the other end of the sheet material (2) is clamped by the clamp (4) of the right stretching cylinder of the stretching machine. The left stretching cylinder (5) and the right stretching cylinder (6) of the stretching machine apply bending and stretching loads to both ends of the sheet material (2), so that the sheet material (2) is gradually attached to the mold (1).

[0052] Step 2) Take the x-direction profile section line with the largest chord height of the mold (1) as the control profile section line (7). Based on the principle of uniform distribution of the stretching elongation according to the change of the normal angle of the stretching region, establish a functional relationship that makes the overall stretching elongation uniformly distributed according to the change of the normal angle of the stretching region.

[0053] (Step 3) Based on the curvature and arc length changes of the control profile section line (7), and based on the principle that the chord height of each stretching region does not exceed the threshold and the sum of the lengths of each stretching region covers the control profile section line (7), the control profile section line (7) is divided into stretching regions, the segmentation points of each stretching region are determined, and the stretching extension rate of each segment is calculated based on the change in the normal angle of each segment, so as to realize the discretization of the stretching extension rate of each stretching region.

[0054] Step 4) Figure 4 As shown, based on the clamping of the stretching cylinder clamp of the stretching machine to hold the two sections of the sheet (2), the stretching cylinder clamp of the stretching machine rotates around the axis at different angles to bend the sheet (2) laterally, so that the x-direction end of the sheet (2) is bent laterally to be consistent with the arc of the corresponding end of the mold (1). Then, the stretching and forming of each stretching area is completed in sequence according to the following steps: first, the sheet (2) is bent longitudinally to be in contact with the two ends of the stretching area on the control contour section line (7); then, the stretching is performed along the tangent direction of the sheet (2) according to the stretching elongation rate of the stretching area; and finally, the stretching is relaxed and unloaded in the opposite direction of the stretching direction. This improves the elasticity and plasticity uniformity of the sheet (2) while reducing the force required for the pitch rotation of the left rotating shaft (18) and the right rotating shaft (19) of the stretching machine, thereby reducing the requirement for the stretching force of the stretching machine and reducing equipment costs.

[0055] In this embodiment, to adapt to the left and right stretching cylinder clamp structure of conventional skin stretching machines, and to facilitate the use of the two stretching cylinder clamps to clamp the two ends of the sheet metal (2) respectively and complete the forming of the half area, before establishing the functional relationship between the stretching elongation rate and the change of the normal angle of the stretching area, the control contour section line (7) is divided into the left half area and the right half area, and the highest point of the control contour section line (7) is used as the dividing point between the left half area and the right half area, specifically:

[0056] Establish the curve equation of the control profile section line (7), obtain the highest point of the control profile section line (7) in the z direction, and then take the highest point in the z direction as the boundary point A between the left and right halves of the control profile section line, and divide the control profile section line (7) into the left and right halves with the boundary point A between the left and right halves of the control profile section line as the center.

[0057] In this embodiment, in the second step, the profile section line of the mold (1) with the largest chord height in the x direction is determined in the following way:

[0058] Establish a plane perpendicular to the y-axis and intersect it with the surface of the mold (1) to obtain the intersecting profile section line. Create a straight line connecting the two ends of the intersecting profile section line and obtain the chord height from the straight line to the intersecting profile section line. Move the plane along the y-axis. When the chord height from the straight line to the intersecting profile section line reaches its maximum, stop moving the plane. Take the intersecting profile section line formed by the plane at this time and the surface of the mold (1) as the x-direction profile section line with the maximum chord height.

[0059] In this embodiment, in the second step, to achieve a reasonable and uniform dispersion and distribution of the elongation rate of each segment during the drawing process according to the angular change of the mold control contour section line, a functional relationship is established to uniformly distribute the overall drawing elongation rate according to the change of the normal angle of the drawing region. Specifically:

[0060] like Figure 1 , 2 As shown, the normal vector N of the control profile section line (7) at the boundary point A of the left and right halves is obtained respectively. A And the left end point P of the control profile section line (7) in the left stretching direction (12). e0 The normal vector N at that location e0 The right end point P of the right-side pull-shaped direction (13) e1 The normal vector N at that location e1 Calculate the left and right end points P respectively. e0 and P e1 The normal vector N at that location e0 N e1 With normal vector N A The included angle θ e0 and θ e1 Given a pre-set total elongation of the drawing, the angle θ between the total elongation of the drawing and the aforementioned angle is obtained. e0 and θ e1 The ratios K1 and K2 are then used as the slopes of the function relating the total elongation of the stretching in the left and right halves to the uniform distribution of the stretching region's normal angle. Based on these slopes and a univariate linear function model without intercept, the function relationships f1(θ1) and f2(θ2) for the uniform distribution of the total elongation of the stretching in the left and right halves to the uniform distribution of the stretching region's normal angle are established:

[0061]

[0062] In equation (1), θ1 and θ2 are the changes in the normal angles of the left and right half-regions of the stretching region, respectively. This achieves reasonable and uniform dispersion and distribution of the elongation rate of each region during the stretching process according to the angle change of the mold control contour section line, preventing uneven stretching elastic and plastic deformation caused by unreasonable distribution of stretching elongation rate in each region.

[0063] To ensure that the sheet metal does not produce material defects after stretching, the total stretching elongation is set to the elongation that does not cause material defects in the sheet metal after stretching. Specifically, the total stretching elongation is related to the mechanical properties of the sheet metal being stretched, and is 20% to 90% of the elongation corresponding to the tensile strength of the sheet metal (2) on the stress-strain curve.

[0064] In this embodiment, in the third step, a method for dividing the stretching region based on chord height is used to achieve uniform or non-uniform division of the segmented stretching region, thereby solving the problem of uneven elastic and plastic deformation processes and poor shape accuracy in the prior art. To this end, the stretching region is discretized into multiple interconnected stretching regions, thus achieving segmented stretching region division. Specifically:

[0065] like Figure 3 As shown, the control profile section line (7) starts from the boundary point A between the left and right halves and extends towards the left end point P. e0 Right end point P e1 Calculate the first segment S in the direction 10 and S 11 Establish the first segment calculation point S 10 and S 11 The line S connecting point A and point S 10 A and AS 11 Thus, the above connection S is obtained. 10 A and AS 11 The chord height G of the control profile section line (7) 10 and G 11 , in the first segment calculation point S 10 and S 11 Left and right end point P e0 and P e1 During the continuous directional movement, the chord height G is continuously calculated. 10 and G 11 The relationship with a predetermined threshold, when the above chord height G 10 and G 11 When the predetermined conditions are met, stop moving the first segment calculation point S. 10 and S 11 And calculate the first segmentation point S at this time. 10 and S 11 , as the first stretching region segmentation point on the control contour section line (7); based on this, from the first stretching region segmentation point S at this time 10 and S 11 Starting from point P on the left, proceed to the left end point. e0 Right end point P e1 Calculate the second segment S in the direction 20 and S 21 Establish the second segmented calculation point S20 and S 21 S, respectively, with the segmentation point S of the first stretch region 10 and S 11 The line S between them 20 S 10 and S 11 S 21 Thus, the above connection S is obtained. 20 S 10 and S 11 The chord height G between S2 and the control profile section line (7) 20 and G 21 ; Calculate the second segment point S 20 and S 21 To the left end point P e0 and the right end point P e1 During the continuous directional movement, the chord height G is continuously calculated. 20 and G 21 The relationship with a predetermined threshold, when the above chord height G 20 and G 21 When the predetermined conditions are met, stop moving the second segment calculation point S. 20 and S 21 And calculate the second segment point S at this time. 20 and S 21 As the first segmentation point of the stretch region on the control profile section line (7); repeat the above steps until the segmentation calculation point moves to the left end point P of the control profile section line (7). e0 and the right end point P e1 They coincide, and the aforementioned left end point P is aligned. e0 and the right end point P e1 As the segmentation point of the final stretch region, according to P e0 S 20 S 20 S 10 S 10 S A AS 11 S 21 S 11 S 21 P e1 The segmentation scheme divides the control profile section line (7) into multiple stretch regions;

[0066] Furthermore, when the moving segment calculation points are stopped, the relationship between the line connecting each adjacent point, including the boundary point A of the left half and the right half and multiple segment calculation points, and the chord height and threshold of the control contour section line (7) should meet the following predetermined conditions: the ratio between the above chord height value and the threshold is equal to M, wherein the ratio M is preset and is less than or equal to 1. By appropriately increasing the ratio M, the length of each stretching region can be increased under the premise of meeting the stretching forming accuracy, thereby reducing the number of stretching region segments and achieving the purpose of improving stretching efficiency; the ratio M of chord height to threshold is set to 0.6 to 1.

[0067] Furthermore, the threshold is the key to maximizing the efficiency of drawing while meeting the forming accuracy. The value of the threshold is closely related to the drawing elongation and the required shape accuracy after drawing. The threshold is set as the chord height of the drawing area when the forming accuracy can reach the minimum requirement under the condition of drawing with a given drawing elongation. The minimum requirement of forming accuracy is preset. The given drawing elongation is the elongation corresponding to a certain area on the control contour section line (7) where the relationship between the chord height and the threshold needs to be judged. It is calculated by multiplying the change in the normal angle of the area between each adjacent point with the function relationship of the overall drawing elongation evenly distributed according to the change in the normal angle of the drawing area. By associating the threshold with the drawing elongation of the area where the chord height and threshold need to be judged and the maximum chord height that can guarantee the accuracy, the length of the drawing area in each pass can be maximized under the premise of ensuring the drawing accuracy, so as to improve the drawing efficiency.

[0068] Specifically, the threshold is obtained using a stretching experiment to ensure that the determination process of the threshold T closely reflects actual production.

[0069] In this embodiment, in the third step, a segmented stretching elongation discrete allocation method is used, with the change in the normal angle of the stretched model surface profile as the criterion, to achieve matching between the stretching elongation and the curvature change of the stretched model surface. Figure 2 As shown, the segmented calculation point S in the left half of the control profile section line (7) needs to be used to determine the relationship between the chord height and the threshold. (K+1)0 and S K0 In the region between, the given stretch elongation is controlled by point S on the profile section line (7). (K+1)0 and S K0 The normal vector N at that location (k+1)0 and N k0 The included angle θ K0 The result is obtained by multiplying the overall elongation rate of the stretching region in the left half by the functional relationship f1(θ1) which is uniformly distributed according to the change of the normal angle of the stretching region, i.e.:

[0070]

[0071] In equation (2), To control point S on the profile section line (7) (K+1)0 and S K0 Given the given stretching elongation rate corresponding to the region between, after determining the given stretching elongation rate, stretching experiments are conducted based on the control profile section line (7) in order of increasing chord height value, with the region on the control profile section line (7) of different chord height as the stretching region. In the stretching experiment, the goal is to ensure that the sheet material (2) being stretched meets the forming accuracy after stretching. As the chord height of the stretching experiment region gradually increases, the forming accuracy of the sheet material (2) gradually decreases. When it decreases to the minimum requirement of the pre-set forming accuracy, the stretching experiment is stopped and the chord height of the stretching experiment region at this time is taken as the threshold.

[0072] In this embodiment, in the third step, the elongation rate of each segment of the segmented stretching is discretely allocated based on the change in the normal angle of the stretching model surface contour as the criterion. Specifically:

[0073] Based on the segmentation of the control profile section line (7), the normal vectors of the control profile section line (7) at the two ends of each stretching region are established. The angle between the normal vectors at the two ends of each stretching region is calculated. According to the left and right half of each stretching region, the angle between the normal vectors at the two ends is multiplied by the function relationship f1(x) and f2(x) that the total stretching elongation rate of the left and right half is uniformly distributed according to the change of the normal angle of the stretching region. The stretching elongation rate of each stretching region is obtained, so that the stretching elongation rate matches the curvature change of the stretching model surface. This allows the limited stretching elongation rate to be allocated more to the area with a large change in the curvature of the stretching model, which is conducive to the rational allocation of stretching elongation rate resources.

[0074] In this embodiment, in step four), a segmented three-dimensional curved surface stretching forming is adopted with bending-stretching-relaxation as the basic cycle unit. This enables the stretching machine to complete the stretching cylinder clamp rotation and downward pressure under low load after each stretching segment is completed, reducing the requirements for the jaw rotation and downward pressure of the machine tool equipment, reducing equipment costs, and broadening the scope of application. Specifically:

[0075] After completing the bending-stretching action, a relaxation and unloading step is added in the opposite direction of the stretching direction to unload the stretching force borne by the left frame (10) and right frame (11) of the stretching machine to less than the force that the stretching machine can output to make the left stretching cylinder (5) of the stretching machine rotate around the left rotation axis (18) of the stretching machine and the right stretching cylinder (6) of the stretching machine rotate around the right rotation axis (19) of the stretching machine.

[0076] Furthermore, mold (1) is a skin-drawing mold;

[0077] Furthermore, the sheet metal is a ductile metal material;

[0078] Specifically, the sheet material is aluminum alloy with a thickness of 0.8mm to 6mm.

[0079] In this embodiment, the three-dimensional curved surface forming fixture includes a mold (1), a sheet metal (2), a left stretching cylinder clamp (3), a right stretching cylinder clamp (4), a left stretching cylinder (5), a right stretching cylinder (6), a control contour section line (7), a left frame (10), a right frame (11), a left stretching direction (12), a right stretching direction (13), a left rotation axis (18), and a right rotation axis (19). The left stretching direction (12) of the mold (1) is provided with the left frame (10), and the right stretching direction (13) of the mold (1) is provided with the right frame (11). At the same time, a device for connecting with the left stretching cylinder 5 of the stretching machine is provided on the left frame (10). The left rotating shaft (18) of the stretching machine is connected, and the right rotating shaft 19 of the stretching machine is provided on the right frame (11) of the stretching machine for connecting with the right stretching cylinder (6) of the stretching machine; and the left stretching cylinder clamp (3) of the stretching machine that can rotate around the shaft in segments is installed at the end of the left stretching cylinder (5) of the stretching machine facing the mold (1), and the right stretching cylinder clamp (4) of the stretching machine that can rotate around the shaft in segments is installed at the end of the right stretching cylinder (6) of the stretching machine facing the mold (1). The sheet material (2) is laid on the mold (1), and one end of the sheet material (2) is clamped by the left stretching cylinder clamp (3) of the stretching machine, and the other end of the sheet material (2) is clamped by the right stretching cylinder clamp (4) of the stretching machine; the x-direction profile section line with the largest chord height of the mold (1) is the control profile section line (7).

[0080] The following describes the specific implementation process using three molds with different controlled profile cross-sectional shapes as examples, so that the application of this technology can be more closely aligned with actual production scenarios.

[0081] Example 1

[0082] Based on the elongation requirements during the forming process in the 3D surface design drawings, a significant margin of mechanical properties needs to be retained after forming to extend the product's service life. Therefore, the total elongation of the stretching is set to 40% of the elongation corresponding to the tensile strength on the material's stress-strain curve. Simultaneously, to improve the forming accuracy of each segment, the ratio M of the chord height to the threshold is set to 0.7. The length of each stretching segment is constructed by increasing the number of segments to reduce the required forming distance for each segment. Under the above parameter settings, as... Figure 5 As shown, the left and right halves of the control profile section line (7) are each divided into four stretching sections, namely P e0 -S 30 S 30 -S 20 S 20 -S 10 S10 -A、AS 11 S 11 -S 21 S 21 -S 31 S 31 -P e1 .

[0083] like Figures 6-10 As shown, following the steps of bending-stretching-relaxing, the sheet (2) and the mold (1) are gradually brought into contact with each stretching zone as planned above, thus completing the final stretching step.

[0084] Example 2

[0085] like Figure 11 As shown, compared with Example 1, the curvature of the control contour section line (7) of the mold (1) changes more gently. In order to improve the efficiency of the stretching process, if the stretching deformation elongation rate is allowed by the design drawings, the total stretching elongation rate is set to 80% of the elongation rate corresponding to the tensile strength on the material stress-strain curve, and the ratio M of chord height to threshold is set to 0.9. Therefore, the left half and right half of the control contour section line (7) are each divided into two stretching areas, and the stretching is completed by setting this stretching segment.

[0086] Example 3

[0087] like Figure 12 As shown, the left half of the control contour section line (7) of the mold (1) is a region with a curvature much smaller than that of the right half and is approximately a straight line segment. When the total elongation rate of the stretching and the ratio M of the chord height to the threshold are set in the same way as in Example 1, the total chord height of the left half is much smaller than the threshold. Therefore, only one stretching segment is set in the left half, while four stretching segments are still set in the right half. The stretching is completed using this stretching segment setting scheme.

[0088] The above description is an illustrative embodiment of the present invention and is not intended to limit the scope of the invention. Any equivalent changes and modifications made by those skilled in the art without departing from the concept and principles of the present invention should fall within the scope of protection of the present invention. Furthermore, it should be noted that the components of the present invention are not limited to the overall application described above. Each technical feature described in the specification can be used individually or in combination as needed. Therefore, the present invention naturally covers combinations and specific applications related to the inventive points of this case.

Claims

1. A method for forming a three-dimensional curved surface, characterized in that, By having the stretching cylinders of the stretching machine, located on both sides of the mold stretching direction of the three-dimensional curved surface forming fixture, and the clamps of the stretching cylinders of the stretching machine, which can rotate around the axis in segments, move together to apply bending and stretching loads to both ends of the sheet metal laid on the mold, thereby causing the sheet metal to gradually fit into the mold. The x-direction contour section line with the largest chord height of the mold is used as the control contour section line. Then, based on the curvature and arc length changes of the control contour section line, the stretching area of ​​the control contour section line is divided into segments to achieve discrete elongation rates of each stretching area. Finally, stretching is performed based on the stretching elongation rates of each stretching area to obtain a three-dimensional curved surface with high shape accuracy. The specific steps are as follows: Step 1) Assemble the three-dimensional curved surface forming fixture; Step 2) Take the x-direction profile section line with the largest chord height of the mold (1) as the control profile section line (7). Based on the principle of uniformly distributing the stretch elongation according to the change of the normal angle of the stretch region, establish a functional relationship that makes the overall stretch elongation uniformly distributed according to the change of the normal angle of the stretch region, specifically: Obtain the normal vector N of the control profile section line (7) at the boundary point A of the left and right halves respectively. A And the left end point P of the control profile section line (7) in the left stretching direction (12). e0 The normal vector N at that location e0 The right end point P of the right-side pull-shaped direction (13) e1 The normal vector N at that location e1 Calculate the left and right end points P respectively. e0 and P e1 The normal vector N at that location e0 N e1 With normal vector N A The included angle θ e0 and θ e1 Given a pre-set total elongation of the drawing, the angle θ between the total elongation of the drawing and the aforementioned angle is obtained. e0 and θ e1 The ratios K1 and K2 are then used as the slopes of the function relating the total elongation of the stretching in the left and right halves to the uniform distribution of the stretching region's normal angle. Based on these slopes and a univariate linear function model without intercept, the function relationships f1(θ1) and f2(θ2) for the uniform distribution of the total elongation of the stretching in the left and right halves to the uniform distribution of the stretching region's normal angle are established. （1） In equation (1), θ1 and θ2 are the changes in the normal angles of the left and right half-regions, respectively. (Step 3) Based on the curvature and arc length changes of the control profile section line (7), and based on the principle that the chord height of each stretching region does not exceed the threshold and the sum of the lengths of each stretching region covers the control profile section line (7), the control profile section line (7) is divided into stretching regions, the segmentation points of each stretching region are determined, and the stretching extension rate of each segment is calculated based on the change in the normal angle of each segment, so as to realize the discretization of the stretching extension rate of each stretching region. The control contour section line (7) is used to divide the stretching region, specifically as follows: Multiple segmented calculation points are made on the control profile section line (7), and these points are set from the boundary point A of the left and right halves towards the left end point P of the control profile section line (7). e0 and the right end point P e1 By moving different distances, lines are established between adjacent points, including the boundary point A of the left and right halves and the aforementioned segmented calculation points. The chord height between the lines connecting the adjacent points and the control profile section line (7) is obtained. Based on a predetermined threshold, when the relationship between the chord height and the threshold satisfies a predetermined condition, the movement of the aforementioned segmented calculation points is stopped, and the left end point P, which is located at this time on the control profile section line (7), is moved to the next position. e0 and the right end point P e1 The segment calculation points between them are determined as the segment points of the stretch-shaped region; the control contour section line (7) is divided according to the segment points of the stretch-shaped region and the boundary point A between the left and right halves; When the moving segment calculation point stops, the relationship between the line connecting each adjacent point, including the boundary point A of the left half and the right half and multiple segment calculation points, and the chord height and threshold of the control profile section line (7) should meet the predetermined condition that: the ratio between the above chord height value and the threshold is equal to M, wherein the ratio M is preset and is less than or equal to 1. (4th step) The stretching cylinder clamp of the stretching machine rotates around the axis at different angles to cause the end of the sheet metal (2) in the x direction to bend laterally until it matches the arc of the end of the mold (1). Then, the stretching and forming of each stretching area is completed in the following order: first, the sheet metal (2) is bent longitudinally until it fits with the two ends of the stretching area on the control contour section line (7); then, the sheet metal (2) is stretched along the tangent direction according to the stretching elongation rate of the stretching area; and finally, the stretching and unloading are performed in the opposite direction of the stretching direction.

2. The three-dimensional curved surface forming method according to claim 1, characterized in that, The specific steps for assembling the 3D curved surface forming fixture are as follows: First, place the mold (1) between the left frame (10) and the right frame (11) of the stretch forming machine. Then, install the left rotating shaft (18) and the left stretching cylinder (5) of the stretch forming machine on the left frame (10), and install the right rotating shaft (19) and the right stretching cylinder (6) of the stretch forming machine on the right frame (11). Then, install the left stretching cylinder clamp (3) and the right stretching cylinder (6) of the stretch forming machine, which can rotate around the shaft in sections, on the end of the left stretching cylinder (5) facing the mold (1). A right stretching cylinder clamp (4) of the stretching machine, which can rotate around the axis in segments, is installed at the end facing the mold (1). Finally, the sheet material (2) is laid on the mold (1), and one end of the sheet material (2) is clamped by the left stretching cylinder clamp (3) of the stretching machine, and the other end of the sheet material (2) is clamped by the right stretching cylinder clamp (4) of the stretching machine. The left stretching cylinder (5) and the right stretching cylinder (6) of the stretching machine apply bending and stretching motion loads to both ends of the sheet material (2), so that the sheet material (2) is gradually attached to the mold (1).

3. The three-dimensional curved surface forming method according to claim 1, characterized in that, In the second step, before establishing the functional relationship between the elongation rate of the stretching section and the change of the normal angle of the stretching region, the control profile section line (7) is divided into a left half and a right half, and the highest point of the control profile section line (7) is used as the dividing point between the left and right half. Specifically: Establish the curve equation of the control profile section line (7), obtain the highest point of the control profile section line (7) in the z direction, and then take the highest point in the z direction as the boundary point A between the left and right halves of the control profile section line, and divide the control profile section line (7) into the left half and the right half with the boundary point A between the left and right halves of the control profile section line as the center.

4. The three-dimensional curved surface forming method according to claim 1, characterized in that, In the second step, the profile section line of the mold (1) with the largest chord height in the x direction is determined in the following way: Establish a plane perpendicular to the y-axis and intersect it with the surface of the mold (1) to obtain the intersecting profile section line. Create a straight line connecting the two ends of the intersecting profile section line and obtain the chord height from the straight line to the intersecting profile section line. Move the plane along the y-axis. When the chord height from the straight line to the intersecting profile section line reaches its maximum, stop moving the plane. Take the intersecting profile section line formed by the plane at this time and the surface of the mold (1) as the x-direction profile section line with the maximum chord height.

5. The three-dimensional curved surface forming method according to claim 1, characterized in that, In the third step, the total elongation of the stretching is 20% to 90% of the elongation corresponding to the tensile strength of the sheet (2) on the stress-strain curve.

6. The three-dimensional curved surface forming method according to claim 1, characterized in that, In steps three and four, a segmented three-dimensional surface stretching process with bending-stretching-relaxation as the basic cycle unit is adopted. This allows the stretching machine to complete the stretching cylinder clamp rotation and downward pressing under low load after each stretching segment is completed. Specifically: After completing the bending-stretching action, a relaxation and unloading step is added in the opposite direction of the stretching direction, so that the stretching force borne by the left frame (10) and the right frame (11) of the stretching machine is unloaded to less than the force that the stretching machine can output to make the left stretching cylinder (5) of the stretching machine rotate around the left rotation axis (18) of the stretching machine and the right stretching cylinder (6) of the stretching machine rotate around the right rotation axis (19) of the stretching machine.

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