Method for testing bending forming characteristics and forming precision of double-curvature part

Abstract

The invention discloses a method for testing bending forming characteristics and forming precision of a double-curvature part in the technical field of metal pressure testing, forming of the double-curvature part is completed, and bidirectional bending characteristics and forming precision can be better evaluated. One end face of the material sheet is provided with a convex die, the convex die and the carbon paper act together to print and dye the area in contact with the convex die in the bending process, the other end face is engraved with slight scale marks through a laser marking machine, the scale marks are perpendicular to one another, grid circles are engraved at the intersection points, and the deformation size and the material flowing condition of different areas of the material sheet can be analyzed. According to the method, the point cloud data of the intersection points are collected through the three-coordinate measuring machine after bending, the mutually perpendicular scale lines are further measured, the forming precision of the double-curvature part can be evaluated, and the method has very high innovativeness.

Description

Technical Field

[0001] The present invention relates to the technical field of metal pressure testing, and specifically provides a testing method for the bending forming characteristics and forming accuracy of double-curvature parts. Background Art

[0002] Double-curvature bending forming is a process in which a sheet metal undergoes two-way deformation along two mutually perpendicular curvatures under the action of a blank-holding force or without a blank-holding force, and finally forms a part with the same or different curvatures in two mutually perpendicular directions. Such parts are mainly used in the double-curvature wall panels of large pressure vessels and the skins of airplanes. Under the action of the blank-holding force, during double-curvature bending forming, due to the action of tensile force, the springback of the bent part is relatively small, similar to the principle of stretch bending forming.

[0003] For the FLC curve for measuring the forming limit of sheet metal, the method of surface engraving grids is usually also adopted. However, the measurement directions are for measuring the strains of the major axis and minor axis. For example, in Patent 202310764928.6, square grids are printed on the surface of the blank, and then the deformation of the grids with the largest deformation is measured. Based on the volume invariance, the maximum thinning degree is calculated, and at the same time, the forming limit is measured based on the grid measurement system. In Patent 202210037897.X, the forming limit FLC of sheet metal is measured based on the grid method to evaluate the forming safety margin for stamping die acceptance.

[0004] In the above patents and similar patents, the grids are only used to evaluate the strains of the major axis and minor axis of the grids near the fracture area of the blank. During the double-bending process, due to different curvatures in two directions, the bending degrees are inconsistent. Generally, single-curvature bending similar to a certain direction is first carried out, and after reaching a certain degree, double-bending occurs simultaneously. After the midpoint of the blank edge touches the bottom of the die, reverse warping will occur, and the deformation is relatively complex. In addition, double-bending is generally carried out in a closed space, making it difficult to observe the evolution law of the contact area between the die and the blank. Generally, the directions of the major axis and minor axis with the largest grid deformation are not the same as the two curvature directions described in this patent. Therefore, the deformation of the major axis and minor axis makes it difficult to accurately analyze the material flow law of the blank bending process along the two curvature directions, and it is even more difficult to judge the springback changes of the bent part along the two curvature directions.

[0005] Based on this, the present invention designs a testing method for the bending forming characteristics and forming accuracy of double-curvature parts to solve the above problems. Summary of the Invention

[0006] The purpose of the present invention is to provide a testing method for the bending forming characteristics and forming accuracy of double-curvature parts, which solves the problems of difficult understanding of the deformation law of sheet metal and difficult evaluation of forming accuracy during the blank bending process.

[0007] To achieve the above object, the present invention provides the following technical solution: A test method for the bending forming characteristics and forming accuracy of a double-curved part, comprising the following steps:

[0008] S1. First, cut a forming blank suitable for the bending die. Coat one end face of the blank with paint, and use a laser marking machine to determine the center of the blank on the other end face.

[0009] S2. Print scale lines perpendicular to each other along the length and width directions. Print tangent circles with the intersection of the scale lines as the center, and print the diagonals passing through the center of the blank respectively.

[0010] S3. Using the clearance fit method between the guide frame and the female die, place the female die in the guide frame. After offsetting the guide frame upward by an appropriate distance relative to the female die, fix the position of the guide frame with fixing bolts. Then, fix it to the ejector rod of the punch head of the blank forming testing machine by relying on the counterbore on the lower end face of the female die. Use the testing machine to stretch and release to move the ejector rod downward, driving the female die and the guide frame to move downward to an appropriate position. Place the blank on the four small platforms of the female die for centering positioning, with the painted surface facing up, and place a carbon paper of the same size as the blank on it.

[0011] S4. Fasten the male die, spacer block, and flange with bolts. Using the clearance fit between the guide frame and the male die, install the male die and other assemblies into the guide frame together, and make the curved surface of the male die contact the carbon paper.

[0012] S5. Use the testing machine to set the limit load and limit displacement. The limit load value does not exceed 50% of the maximum range of the load sensor of the testing machine. The limit displacement increases upward at a fixed interval within the range of 2 - 4 mm each time. Start the experiment. Then, when any condition triggers and reaches the limit value, the experiment terminates. Use the testing machine to stretch and release to move the male die, female die and other devices downward to an appropriate position, remove the male die and the fixing device, and then take out the blank.

[0013] S6. Judge the evolution law of the contact area according to the painted surface and the blue surface printed on the carbon paper. Judge the material flow of the blank along the length and width directions according to the grid change and diagonal change of the grid surface. Infer the two-way bending deformation characteristics of the blank according to the evolution of the contact area and the material flow law.

[0014] S7. Use a coordinate measuring machine to measure the coordinate information of the center points of the scale lines on the grid surface of the bent part, perform surface fitting, and evaluate the curvature radii of the intersection points of the scale lines along the two curvature directions respectively, and evaluate the forming accuracy of each point.

[0015] In a further solution, in step S1, the four corners of the rectangular blank are all rounded, the radius of the rounded corners ranges from 1 - 3 mm, the length range of the rectangular size of the blank is 20 - 60 mm, the width range is 20 - 60 mm, and the thickness range is 0.5 - 2 mm.

[0016] In a further solution, in step S2, a laser marking machine is used to print scale lines perpendicular to each other along two length and width directions on one end face of the specimen. The distance between adjacent scale lines in any direction is equal, and the distance range is 2 - 5 mm. Taking the intersection point as the center, a tangent grid circle is printed. The radius of the grid circle is the same as the distance between adjacent scale lines. At the same time, taking the center intersection point of the sheet as the center, two diagonal lines are printed. The depth of any grid line is 0.02 - 0.1 mm.

[0017] In a further solution, in step S3, the outer peripheral surface of the female die and the inner peripheral surface of the guiding frame are both square, with a clearance fit, and the single-sided clearance is not greater than 0.05 mm.

[0018] In a further solution, in step S3, the assembled bodies such as the punch and the flange are placed on the sheet through the guiding frame after being installed, so that the curved surface of the punch contacts the carbon paper. The guiding frame is used to accurately position the punch and the female die. The outer peripheral surface of the punch and the inner peripheral surface of the guiding frame are both square, with a clearance fit, and the single-sided clearance is not greater than 0.05 mm.

[0019] In a further solution, in step S5, the flange is divided into a small round table on the upper part and a large round table on the lower part. The inner hole diameter of the fixing frame matches the diameter of the small round table of the flange, with a clearance fit, and the single-sided clearance is not greater than 0.05 mm. By controlling the upward movement of the punch pressure rod of the testing machine, the whole assembly of the female die, the guiding frame, and the punch device is driven to move upward until the round table surface of the flange completely enters the inner hole surface of the fixing frame of the testing machine. At this time, the cooperation between the upper round table surface of the flange and the inner hole surface of the fixing frame plays a role in guiding. When the annular surface formed by the large and small round tables is in complete contact with the lower end surface of the fixing frame, the displacement of the punch is restricted. When continuing to move upward slowly at a speed of 0.1 - 0.5 mm / min, the sheet is forced to undergo slight deformation. When the load recorded by the load sensor increases rapidly significantly, stop moving upward and zero the displacement and the load.

[0020] In a further solution, in step S5, on the control interface of the testing machine, the stroke is set with a fixed increment in the range of 2 - 4 mm, and at the same time, a limit load of 200 KN is set. Start the experiment. The sheet is pressed into the female die cavity under the restriction of the punch and the female die. When one of the displacement or the load reaches the set value, stop the experiment.

[0021] In a further solution, in step S6, the whole assembly of the punch, the female die, and the guiding frame is moved downward through tensile release. Remove the punch and its supporting devices, and then take out the sheet. Analyze the evolution of the contact area according to the coating surface and the blue surface printed on the carbon paper. The sheet after bending has deformed to a certain extent along its length and width directions. Use a three-dimensional stereomicroscope to measure the deformation of the grid circle and the change of the diagonal line formed after the sheet is bent, and analyze the material flow of the sheet in two directions.

[0022] In a further embodiment, in step S7, the displacement limit conditions for the next bending test can be set at fixed intervals within the range of 2 - 4 mm in sequence, while the load limit conditions remain unchanged, and steps S6 - S7 are repeated.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] By pre - treating the two end faces of the sheet material in this method, that is, spraying and brushing a coating on one end face, which can print out the contact area with the punch during the bending process in cooperation with carbon paper, and using a laser marking machine to engrave relatively light scale lines on the other end face. The scale lines are perpendicular to each other, and grid circles are engraved at the intersection points, which can analyze the deformation magnitude of different regions of the material and the material flow situation;

[0025] By using a coordinate measuring machine to collect the point cloud data of the intersection points after bending and further measuring the perpendicular scale lines in this method, the forming accuracy of the double - curvature part can be evaluated, which has strong innovation.

[0026] This method evaluates the two surfaces of the sheet material at the same time. One end face is used for engraving grids to measure the grid changes in two directions to evaluate the material flow in two directions.

[0027] This method uses the method of brushing a coating on the other end surface and printing with carbon paper, and conducts a two - way bending experiment in the form of different displacement intervals. By combining the printed area and bending degree of the end face of the sheet material at different displacement stages, the bending deformation behavior of the sheet material can be better analyzed.

[0028] This method can measure the elongation deformation of each grid circle along any curvature direction, realize the analysis of the material flow behavior and bending behavior of the sheet material along two bending directions, and can judge the forming accuracy of each position of the bent part. Combining the judgment of the deformation behavior of the bent part under different displacement increments, this method can comprehensively analyze the deformation behavior and forming accuracy of the sheet material at different displacement stages of two - way bending from the two surfaces of the bent part, and has good practicality and operability. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for describing the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0030] Figure 1 Schematic diagram of the forming device of the present invention;

[0031] Figure 2 Diagram of the painted surface of the part after bending of the present invention;

[0032] Figure 3 This is the printed grid surface diagram after the bending of the present invention. Specific embodiments

[0033] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0034] The present invention provides a technical solution: The forming device of the present invention includes: a punch 1, a die 2, a guiding frame 3, a flange 4, a cushion block 5, a fixing bolt 6 and a fastening bolt 7.

[0035] The test method of the present invention includes the following steps:

[0036] Step 1: Specimen preparation; Select the TA series of titanium alloys as the materials for studying the biaxial bending, and process rectangular test pieces with dimensions suitable for the bending punch and the bending die. Use a laser marking machine to print scale lines perpendicular to each other along the two length and width directions on one end face of the specimen. The distance between the scale lines in any direction is 3 mm. With the intersection point as the center, print a tangent grid circle with a diameter of 3 mm, and print two diagonal lines passing through the center of the material piece. Spray white paint on the other end face.

[0037] Step 2: Install, debug and prepare for the experiment on the device;

[0038] Step 2.1: Place the die 2 in the guiding frame 3. After moving the guiding frame upward to an appropriate distance, use the fixing bolt 6 to fix the guiding frame 3 on the upper side of the die, and install it on the tensile ejector rod of the testing machine using the counterbore on the lower end face of the die 2;

[0039] Step 2.2: Use the fastening bolt 7 to fasten the flange 4, the cushion block 5 and the bending punch 1 together;

[0040] Step 2.3: Use the tensile release of the testing machine to lower the tensile ejector rod, and move the adjusting die 2 and the guiding frame 3 downward together to a lower position. Place the material piece on the four small platforms of the die, with the grid surface facing downwards towards the die. Place a piece of carbon paper of the same size as the material piece on the painted surface of the material piece. Place the punch, flange, etc. installed in Step 2.2 above the material piece through the guiding frame, making the curved surface of the punch contact the carbon paper, and use the guiding frame to achieve the accurate positioning of the punch and the die;

[0041] Step 2.4: Control the punch rod of the testing machine to move upward, driving the forming device to move upward until the round platform on the flange completely enters the inner hole surface of the fixed frame of the testing machine. At this time, the round table surface on the flange and the inner hole surface of the fixed frame cooperate to play a guiding role. When the annular surface of the flange and the inner hole table surface of the fixed frame are in complete contact, the punch is restricted. When continuing to move upward, the sheet metal undergoes slight deformation, and when the load recorded by the load sensor increases rapidly significantly, stop moving upward and zero the displacement and load.

[0042] Step 2.5: On the control interface of the testing machine, set the limited displacement within the range of 2 - 4 mm and the limited load of 200 KN simultaneously, and start the experiment. The sheet metal is pressed into the concave mold surface under the restriction of the mold. When one of the displacement or load reaches the set value, stop the experiment.

[0043] Step 3: Forming characteristics and precision analysis of the bent part

[0044] Step 3.1: Press the "tensile release" button to lower the mold, remove the punch and its device, and then take out the sheet metal.

[0045] Step 3.2: Judge the evolution of the contact area according to the coating surface and the blue surface printed by the carbon paper; judge the material flow of the material in two directions according to the change of the grid on the grid surface.

[0046] Step 3.3: Use a coordinate measuring machine to measure the coordinate information of the center points of the grid circles on the sheet metal, perform surface fitting, evaluate the curvature radii at different positions along two curvature directions respectively, and evaluate the forming precision of each point. By measuring the diagonal deformation of the bent part, infer the material flow law during the bending process.

[0047] According to the description in Step 2.5, the limited displacement can be set to increase upward at fixed intervals within the range of 2 - 4 mm in sequence, and then the bending deformation characteristics and material flow conditions of the sheet metal at different displacement loading stages can be studied.

[0048] In this method, the rectangular length range of the formed sheet metal is 20 - 60 mm, and the width range is 20 - 60 mm. The diameter range of the cylindrical end of the punch is 42 - 47 mm, and the diameter range of the cylindrical part of the concave die is 45 - 50 mm.

[0049] In this method, the curvature radius range of the punch bending die is R15×R15 mm - R108×R108 mm, and the curvature radii in two directions can be arbitrarily combined. The sheet metal thickness is 0.5 - 2 mm, and the two-way radius value of the bending concave die is equal to the sum of the punch radius and the sheet metal thickness, and can be arbitrarily combined according to the specific radius size of the punch.

[0050] Take the punch and concave die with unequal radii in two curvature directions as an example to perform double-curvature bending forming.

[0051] The double-curvature bending forming method is as follows:

[0052] 1) First, use a laser marking machine to locate the center of the sheet metal, and engrave perpendicular scale lines. The distance between the scale lines in any direction is 3 mm. Taking the intersection of the perpendicular scale lines as the center, engrave a grid circle with a diameter of 3 mm, and at the same time engrave the diagonal line. Spray paint on the other end face.

[0053] 2) First, place the female die in the guiding frame and place them together on the working table. Lift the guiding frame up by an appropriate distance, use the fixing bolts to screw into the threaded holes on both sides of the guiding frame and make close contact with the side of the female die, so that the guiding frame and the female die no longer slide relative to each other, and place them together on the ejector rod of the tensile punch of the testing machine.

[0054] 3) Connect the flange, spacer block and male die together with bolts.

[0055] 4) Place the test piece on the table surface of the female die, and use the four small positioning table surfaces opened on the female die to position the sheet metal. Ensure that the grid surface faces downwards towards the female die surface, and place the carbon paper with the same size as the sheet metal on the painted surface of the sheet metal. Use the ejector rod of the tensile adjustment of the testing machine to move down by an appropriate distance, install the male die together with the flange, etc. into the guiding frame, and use the guiding function to make the male die contact the carbon paper in the center.

[0056] 5) On the control interface of the testing machine, set the limit displacement within the range of 2-4 mm and the limit load of 200 KN at the same time, start the experiment, and stop the experiment when one of the displacement or load reaches the set value.

[0057] 6) Through the tensile release button on the control interface, make the female die return, remove the male die and flange, etc., and use tweezers to take out the bent part.

[0058] 7) Evaluate the deformation characteristics of the sheet metal according to the color area printed on the painted surface of the sheet metal by the carbon paper; use a three-dimensional stereomicroscope to measure the change of the grid circle, measure the change of the perpendicular scale lines and the diagonal line, and evaluate the flow of the sheet metal along the two curvature directions.

[0059] 8) Use a coordinate measuring machine to measure the intersection of the perpendicular scale lines, obtain the point cloud coordinates, and perform reverse measurement through reverse software to obtain the curvature radius of any scale line in the two directions, and evaluate the forming accuracy of the bent part.

[0060] In this method, the length range of the middle rectangle of the formed sheet metal is 20-60 mm, and the width range is 20-60 mm. The bending radius range of the male die is R15×R15 mm - R108×R108 mm, and the curvature radii in the two directions can be combined arbitrarily. The sheet metal thickness is 0.5-2.0 mm, and the double-direction radius value of the bending female die is equal to the sum of the male die radius and the sheet metal thickness, and can be combined arbitrarily according to the specific radius size of the male die.

[0061] According to the description in 5), the limit displacement can be gradually increased at a fixed interval within the range of 2 - 4 mm in sequence, so that the deformation characteristics and material flow conditions of the blank in different displacement loading stages can be studied.

[0062] Specific example: The bent blank is made of industrial pure titanium TA3, with a length and width of 54 mm and a thickness of 1 mm. The double curvature radius of the bending punch is R36×R36 mm, the double curvature radius of the bending die is R37×R37 mm, and the length and width of the curved surfaces of the bending punch and die are both 60 mm.

[0063] First, utilize the clearance between the die and the guide frame, place the guide frame on the die, and use the fixing bolts to position the guide frame in a certain position. After fixing, it is installed on the punch ejector rod of the testing machine through the counterbore opened at the lower end of the die. Control the punch to move downward through the testing machine, driving the die to move downward to a suitable position. Place the blank test piece on the small platform of the die, with the grid surface of the blank facing the die surface and the coating surface facing upward. Then, place the carbon paper on the coating surface of the blank. Use the fastening bolts to fasten the flange, spacer, and punch in sequence, and then place this assembly on the carbon paper by using the guiding function of the guide frame. Use the testing machine to move upward, driving the entire assembly upward until the convex surface of the flange contacts the inner hole surface of the fixed frame of the testing machine, restricting the displacement of the punch. Set the speed of the testing machine to 0.5 mm / min and continue to move the punch upward, driving the punch to move upward. When the load shown changes rapidly, clear the values such as the load and displacement displayed on the testing machine. Then, set the upward limit displacement of the testing machine to 2 mm, the speed to 0.5 mm / min, the limit load to 200 KN, and start the experiment.

[0064] After the experiment, control the testing machine to move the overall die device downward to a suitable position, remove the punch, flange and other assemblies, and use tweezers to take out the blank. Analyze the evolution law of the contact area between the blank and the punch according to the printing area of the coating surface, and analyze the material deformation characteristics and material flow law by using the changes of the scale lines, grid circles, and diagonals on the grid surface. Collect the point cloud of the intersection points of the scale lines by using a coordinate measuring machine, and analyze the curvature radius of each position point through reverse reconstruction to analyze the forming accuracy of the blank.

[0065] The bent part is as follows Figure 2 and 3 shown, where Figure 2 is the sprayed coating surface, Figure 3 is the printed grid surface.

[0066] In the description of this specification, the descriptions referring to terms such as "one embodiment", "example", "specific example", etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0067] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. Obviously, many modifications and variations can be made according to the content of this specification. These embodiments are selected and specifically described in this specification in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can well understand and utilize the present invention. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A test method for the bending forming characteristics and forming accuracy of double-curved parts, characterized in that, It includes the following steps: S1. First, cut a forming sheet suitable for the bending die. Coat one end face of the sheet with paint, and use a laser marking machine to determine the center of the sheet on the other end face; S2. Print scale lines perpendicular to each other along the length and width directions. Print tangent circles with the intersection point of the scale lines as the center, and print the diagonals passing through the center of the sheet respectively; S3. Use the clearance fit method between the guide frame and the female die. Place the female die in the guide frame. After offsetting the guide frame upward by an appropriate distance relative to the female die, fix the position of the guide frame with fixing bolts. Then, fix it to the ejector rod of the punch head of the sheet forming testing machine by relying on the counterbore on the lower end face of the female die. Use the testing machine to stretch and release to move the ejector rod downward to drive the female die and the guide frame to move downward to an appropriate position. Place the sheet on the four small platforms of the female die to achieve centering positioning, with the painted surface facing up, and place a carbon paper with the same size as the sheet on it; S4. Fasten the male die, spacer block, and flange with bolts. Use the clearance fit between the guide frame and the male die to install the male die and other assemblies into the guide frame together, and make the curved surface of the male die contact the carbon paper; S5. Use the testing machine to set the limit load and limit displacement. The limit load value does not exceed 50% of the maximum range of the load sensor of the testing machine. The limit displacement increases upward at a fixed interval within the range of 2 - 4 mm each time. Start the experiment. Then, when any condition triggers and reaches the limit value, the experiment terminates. Use the testing machine to stretch and release to make the male die and female die device move downward to an appropriate position. Remove the male die and the fixing device, and then take out the sheet; S6. Judge the evolution law of the contact area according to the painted surface and the blue surface printed on the carbon paper. Judge the material flow of the sheet along the length and width directions according to the grid change and diagonal change of the grid surface. Infer the two-way bending deformation characteristics of the sheet according to the evolution of the contact area and the material flow law; S7. Use a coordinate measuring machine to measure the coordinate information of the center points of the scale lines on the grid surface of the bent part, perform surface fitting, and evaluate the curvature radii of the intersection points of the scale lines along the two curvature directions respectively, and evaluate the forming accuracy of each point.

2. The test method for the bending forming characteristics and forming accuracy of a double-curved member according to claim 1, wherein: In step S1, the four corners of the rectangular sheet are all rounded, and the radius of the rounded corners ranges from 1 - 3 mm. The length range of the rectangular size of the sheet is 20 - 60 mm, the width range is 20 - 60 mm, and the thickness range is 0.5 - 2 mm.

3. A test method for the bending forming characteristics and forming accuracy of a double-curved member according to claim 1, characterized in that: In step S2, use a laser marking machine to print scale lines perpendicular to each other along the two length and width directions on one end face of the sample. The distance between adjacent scale lines in any direction is equal, and the distance range is 2 - 5 mm. Print tangent grid circles with the intersection point as the center. The radius of the grid circle is the same as the distance between adjacent scale lines. At the same time, print two diagonals with the intersection point of the center of the sheet as the center. The depth of any grid line is 0.02 - 0.1 mm.

4. A test method for the bending forming characteristics and forming accuracy of a double-curved part according to claim 1, characterized in that: In step S3, the outer peripheral surface of the female die and the inner peripheral surface of the guide frame are both square, showing a clearance fit, and the single-sided clearance is not greater than 0.05 mm.

5. A test method for the bending forming characteristics and forming accuracy of a double-curved part according to claim 1, characterized in that: In step S3, the installed punch and flange assembly are placed on the sheet through the guide frame, making the punch surface contact the carbon paper. The accurate positioning of the punch and die is achieved by using the guide frame. The outer peripheral surface of the punch and the inner peripheral surface of the guide frame are both square, with a clearance fit. The unilateral clearance is not more than 0.05 mm, ensuring the precise alignment of the punch, die, and sheet.

6. A test method for the bending forming characteristics and forming accuracy of a double-curved part according to claim 1, characterized in that: In step S5, the flange is divided into a small round table on the upper part and a large round table on the lower part. The inner diameter of the fixing frame matches the diameter of the small round table of the flange, with a clearance fit. The unilateral clearance is not more than 0.05 mm. By controlling the upward movement of the punch rod of the testing machine, the whole assembly of the die, guide frame, and punch device is driven upward until the round table surface of the flange completely enters the inner hole surface of the fixing frame of the testing machine. At this time, the cooperation between the upper round table surface of the flange and the inner hole surface of the fixing frame plays a guiding role. When the annular surface formed by the large and small round tables is in full contact with the lower end surface of the fixing frame, the displacement of the punch is restricted. When continuing to move upward slowly at a speed of 0.1 - 0.5 mm / min, it forces the sheet to undergo slight deformation. When the load recorded by the load sensor shows a significant rapid increase, stop moving upward and zero the displacement and load.

7. A test method for the bending forming characteristics and forming accuracy of a double-curved part according to claim 1, characterized in that: In step S5, on the control interface of the testing machine, set the stroke with a fixed displacement increment within the range of 2 - 4 mm, and at the same time set a limit load of 200 KN. Start the experiment. The sheet is pressed into the die surface under the restriction of the punch and die. When either the displacement or the load reaches the set value, stop the experiment.

8. A test method for the bending forming characteristics and forming accuracy of a double-curved part according to claim 1, characterized in that: In step S6, the whole assembly of the punch, die, and guide frame is moved downward by tensile release. Remove the punch and its supporting device, and then take out the sheet. Analyze the evolution of the contact area according to the coated surface and the blue surface printed on the carbon paper. The bent sheet has deformed to a certain extent along its length and width directions. Use a three-dimensional stereomicroscope to measure the deformation of the grid circles and the change in the diagonal of the sheet after bending, and analyze the material flow of the sheet in two directions.

9. A test method for the bending forming characteristics and forming accuracy of a double-curved part according to claim 1, characterized in that: In step S7, sequentially set the displacement limit condition for the next bending test with a fixed increment within the range of 2 - 4 mm, and the load limit condition remains unchanged. Repeat steps S6 - S7.

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