A forming tool and forming method for a double-curvature skin part with a bent edge

Abstract

The application provides a forming tool and forming method for a double-curvature skin part with a hole and a bent edge, and belongs to the technical field of aviation aluminum alloy aircraft skin sheet metal processing. The tool comprises a stretching die body and left and right side die bodies connected to two ends of the stretching die body, and the three together form a part profile. A core hole die is arranged on the stretching die body and is used for stamping forming a hole and a bent edge. First, the stretching die body is used to form an overall contour by stretching, and then the part is formed on both sides by roll bending. After the left and right side die bodies are installed on the two ends of the stretching die body, the part after roll bending is checked, the part is fixed on the tool, and the core hole die is used to stamp form a hole and a bent edge. The tool is used in two processing methods, reduces the knocking marks caused by manual forming, improves the surface quality and processing efficiency of the part, and uses the core hole die to stamp the middle hole at one time, so that the surface quality of the formed hole is good and the height of the bent edge is accurate.

Description

Technical Field

[0001] This invention belongs to the field of aerospace aluminum alloy aircraft skin sheet metal processing technology, and relates to a forming fixture and forming method for a double-curvature skin part with an inner hole and a bent edge. Background Technology

[0002] Aircraft skins are mostly made of double-curvature or single-curvature aluminum alloy parts, and the thickness of the parts is usually no more than 6mm. The common processing method for aircraft skins is stretch forming, which involves using a skin stretching machine to clamp the sheet metal with clamps on both sides, and then completing the part processing by wrapping the aluminum alloy sheet metal onto the tooling.

[0003] For some hyperbolic skin parts, due to assembly requirements, openings need to be made inside the skin parts, and the edges of the openings need to be bent downwards at 70-90°. The two sides of the skin bend upwards, opposite to the overall shape of the skin part. For this structure, conventional skin stretching and forming can only process part of the shape. The remaining reverse-bent parts need to be manually beaten with a rubber beater to bring the reverse-bent edges into place with the tooling. The opening in the middle of the skin part needs to be initially machined manually with scissors, then the circumference of the opening is hammered downwards with a wooden hammer to fit into the mold, and finally the end face is sanded with a file to complete the part processing.

[0004] The surface of the skin parts processed using the above process has large areas of trimming marks, requiring manual polishing to complete the surface treatment. The overall processing efficiency of the parts is low, and it takes about 3 hours to complete the processing of a skin part with a size of 1000X1800mm, resulting in low production efficiency. Summary of the Invention

[0005] The purpose of this invention is to provide a forming fixture and method for hyperbolic skin parts with curved inner holes, used for the combined processing of such parts. It achieves a composite processing method of skin stretching and roll bending by using a detachable mold. Furthermore, by using formula calculations, the corresponding relationship between the height of the curved inner hole and the opening size is clearly defined, enabling one-time punching forming and ultimately completing the processing of this type of skin part.

[0006] The present invention adopts the following technical solution:

[0007] A forming fixture for a double-curvature skin part with an inner hole and a bent edge, the forming fixture comprising a stretching die 1, a left die 2, a right die 3, a drilling template 4, an upper pressure plate 5, a core hole mold 6, a left mold pin 7, a right mold pin 8, and a positioning reference hole 9.

[0008] The flatness tolerance of the bottom surface of the stretching jig 1 is required to be no greater than 0.1mm. The forming tooling material is resin or metal, and its upper surface is the working surface, set according to the shape of the middle part of the target part, for the first step of stretching and forming of the target part. Multiple positioning holes are provided on both sides of the working surface of the stretching jig 1, and drilling templates 4 are provided in the holes for drilling positioning holes on the surface of the target part and fixing the target part. Fixed core hole molds 6 are detachably installed on the working surface of the stretching jig 1 corresponding to the opening position of the target part for forming the opening and bending edge on the surface of the target part. Left mold pins 7 and right mold pins 8 are respectively provided on the end faces of the working surface of the stretching jig 1 for connecting the left jig 2 and the right jig 3 respectively. Two positioning reference holes 9 are symmetrically installed on the surface of the stretching jig 1 for establishing the coordinate system during the tooling manufacturing process and for use in CNC machining tooling.

[0009] The left side body 2 and the right side body 3 are provided with pin holes at the connection end face, which are respectively engaged with the left side mold pin 7 and the right side mold pin 8, so that the two are installed at both ends of the working surface of the stretching body 1. The upper surface of the two is the working surface. After being installed at both ends of the stretching body 1, the working surfaces of the two are precisely connected with the working surface of the stretching body 1, and together they form a complete target part shape. The ends of the left side body 2 and the right side body 3 are provided with positioning holes and drilling templates 4 for drilling positioning holes on the surface of the target part.

[0010] The lower surface of the pressure plate 5 is set according to the working surface of the stretching body 1, and a hole is opened at the position of the core die 6. The pressure plate 5 presses the target part onto the stretching body 1, and is used to keep the position fixed and perpendicular during the downward bending process of the core die 6 to prevent it from becoming unstable.

[0011] The method for determining the connection positions between the stretching body 1 and the left and right body 3 is as follows: Using the position of the target part placed on the stretching body 1 as a reference, select point A at the end of the target part and point B near the end of the target part, with points A and B selected along the length of the target part. Draw a horizontal line from point A and a vertical line from point B, intersecting at point C to form a right triangle ABC. According to Table 1, determine the length relationship of line segment AC through the angle BAC, and then determine the position of point B. Based on the length of line segment AC, determine the segmentation design surfaces between the stretching body 1 and the left and right body 3.

[0012] Table 1. Relationship between the angle of angle BAC and the length of line segment AC.

[0013] Angle BAC Length of line segment AC (in mm) Less than 30° 201-250 ≥30° and <45° 175-190 45° or greater and less than 60° 100-150

[0014] For target parts with an angle BAC greater than 60°, the stretching body 1 does not need to be segmented. Its shape is set according to the complete shape of the target part, and the target part is formed by stretching the stretching body 1 in one go.

[0015] The core die 6 is available in multiple sizes with different outer diameters, which can be selected as needed. The required outer diameter is determined based on the thickness of the target part and the outer diameter of the opening on it, as shown in Table 2.

[0016] Table 2 shows the relationship between the outer diameter of the core mold 6 and the height of its forming bend.

[0017]

[0018] In the table, D is the outer diameter of the opening, d is the inner diameter of the opening (i.e., the outer diameter of the core die 6), and δ is the thickness of the part material.

[0019] A method for forming a hyperbolic skin part with an inner hole and a curved edge, based on the above-mentioned forming fixture, is provided. The forming method generally requires the use of three forming methods, including stretch forming, roll forming, and stamping forming. The specific steps are as follows:

[0020] Step 1: Use stretch forming to process the overall contour of the target part. Remove the left side jig 2 and the right side jig 3 from both ends of the stretch jig 1. Place the stretch jig 1 on the worktable surface of the skin stretching machine. Place the aluminum alloy sheet to be formed into the jaws on both sides of the skin stretching machine. Close the jaws and move the jaws on both sides to make the aluminum alloy sheet cover the stretch jig 1. During the covering process, the top of the worktable can be used to improve the mold fit.

[0021] Step 2: Use roll bending to process the reverse bending shape of the target part's end. Remove the sheet metal processed in Step 1, and use a hand band saw to cut off the parts held by the stretching jaws at both ends of the sheet metal. Use sandpaper to smooth the end faces, ensuring the roughness does not exceed Ra12.5. After completion, install the left jig 2 and the right jig 3. Connect them using the left-side die-cutting pin 7 and the right-side die-cutting pin 8 on both sides of the stretching jig 1. During the connection process, a wooden mallet can be used to tap the left-side jig 2 and the right-side jig 3 to ensure a tight connection with the stretching jig 1. After connection, use a feeler gauge to check, requiring the connection gap to be no greater than 0.1mm, meeting the tooling manufacturing tolerance requirements.

[0022] After the connection is completed, a three-axis roll bending machine is used to roll bend the two ends of the sheet metal. Specifically, the areas to be formed at the ends of the sheet metal are placed between the rollers, causing continuous plastic deformation and forming a reverse bend to obtain the formed sheet metal. After bending, the formed sheet metal is placed on the surface of the forming fixture for inspection. A feeler gauge is used to check the fit gap between the formed sheet metal and the forming fixture to ensure that the fit gap is no greater than 0.5mm. At this point, the parts on the left side body 2 and the right side body 3 are processed.

[0023] Step 3: Machining the positioning holes. Press the drilling template 4 on the forming fixture onto the surface of the forming sheet, and use a pneumatic drill to make the positioning holes. The diameter tolerance of the positioning holes should not exceed 0.1mm. Use a digital caliper to measure the hole diameter. Then, use positioning pins to position the positioning holes on the forming sheet and the forming fixture to ensure that the relative positions of the forming sheet and the forming fixture are correct.

[0024] Step 4: Hole Making. Place the upper pressure plate 5 on the stretching die 1 to press the formed sheet metal tightly. Use a marker to outline the center hole position using the holes on the upper pressure plate 5. Move the upper pressure plate 5 away and use a scriber drill to pre-drill the hole. The size of the pre-drilled hole is determined according to the preset settings. Reinstall the upper pressure plate 5 and use the core die 6 to punch downwards to form the center curved edge hole to the mold. After punching, remove the upper pressure plate 5 and the core die 6. Use a feeler gauge to check the mold gap, completing the forming of the target part.

[0025] The effects and benefits of this invention are as follows: By using composite forming tooling to process parts, one set of tooling can be used for two processing methods, reducing the repair marks caused by manual forming and improving the surface quality and processing efficiency of the parts. Simultaneously, for parts with curved openings on the skin surface, the opening size can be calculated using a formula, and the central opening can be punched in one pass using a core die, resulting in good surface quality and accurate curved edge height after forming. Attached Figure Description

[0026] Figure 1 This is a front view of the forming tooling.

[0027] Figure 2 This is a side view of the forming tooling.

[0028] Figure 3 This is a schematic diagram of the forming tooling and mold pins.

[0029] In the diagram: 1. Stretching tire body; 2. Left tire body; 3. Right tire body; 4. Drilling template; 5. Upper pressure plate; 6. Core hole mold; 7. Left mold pin; 8. Right mold pin; 9. Positioning reference hole. Detailed Implementation

[0030] To make the technical problems solved by this invention, the technical solutions adopted, and the technical effects achieved clearer, the invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0031] Taking a hyperbolic skin part of a certain type of aircraft as an example, the present invention will be further described. The forming fixture includes a stretching die 1, a left die 2, a right die 3, a drilling template 4, an upper pressure plate 5, a core die 6, a left mold pin 7, a right mold pin 8, and a positioning reference hole 9, as shown. Figures 1 to 3 .

[0032] The flatness tolerance of the bottom surface of the stretching jig 1 is no greater than 0.1mm. It is made of resin material, and its upper surface is the working surface, designed according to the shape of the middle part of the hyperbolic skin part. It is used for the first step of stretching and forming of the hyperbolic skin part. Multiple positioning holes are provided on both sides of the working surface of the stretching jig 1. Drilling templates 4 are provided in the holes for drilling positioning holes on the surface of the hyperbolic skin part and fixing the hyperbolic skin part. Fixed core hole molds 6 are detachably installed on the working surface of the stretching jig 1 at the opening positions of the hyperbolic skin part for forming the openings and bends on the surface of the hyperbolic skin part. Two left-side mold pins 7 and two right-side mold pins 8 are respectively provided on the end faces of the working surface of the stretching jig 1 for connecting the left jig 2 and the right jig 3 respectively. Two positioning reference holes 9 are symmetrically installed on the surface of the stretching jig 1 for establishing the coordinate system during tooling manufacturing and for use in CNC machining tooling.

[0033] The left and right tire bodies 2 and 3 are each provided with two pin holes on their connecting surfaces, which cooperate with the left mold pin 7 and the right mold pin 8 respectively, to fix the left and right tire bodies 2 and 3 at the end faces of the working surface of the stretching tire body 1. The upper surfaces of the two are the working surfaces. After being installed at both ends of the stretching tire body 1, the working surfaces of the two are precisely connected with the working surface of the stretching tire body 1, forming a complete surface of the double-curvature skin part. The ends of the left and right tire bodies 2 and 3 are each provided with positioning holes and drilling templates 4 for drilling positioning holes on the surface of the double-curvature skin part.

[0034] The lower surface of the pressure plate 5 is set according to the working surface of the stretching body 1, and a hole is opened at the position of the core die 6. The pressure plate 5 presses the double-curvature skin part onto the stretching body 1 to maintain the fixed position and verticality of the core die 6 during the downward bending process and prevent it from becoming unstable.

[0035] The process for determining the connection positions between the stretched tire body 1 and the left tire body 2 and right tire body 3 on both sides is as follows: Taking the position of the hyperbolic skin part placed on the stretched tire body 1 as a reference, point A is selected at the end position of the hyperbolic skin part, and point B is selected near the end position of the hyperbolic skin part. Points A and B are selected along the length direction of the hyperbolic skin part. Point A is drawn as a horizontal line, and point B is drawn as a vertical line, intersecting at point C to form a right triangle ABC. The angle BAC of the hyperbolic skin part is 45°. According to Table 1, the length of line segment AC is determined to be 190mm. Based on the length of line segment AC, the dividing design surface between the stretched tire body 1 and the left tire body 2 and right tire body 3 is determined.

[0036] In this embodiment, the thickness of the hyperbolic skin part is 3mm, the outer diameter of the opening is 15mm, and according to Table 2, the outer diameter of the core hole mold 6 is selected as 8.82mm.

[0037] The forming method for this hyperbolic skin part is based on the above-mentioned forming fixture, and the specific steps are as follows:

[0038] Step 1: Use stretch forming to process the overall contour of the hyperbolic skin part. Remove the left jig 2 and right jig 3 from both ends of the stretch jig 1. Place the stretch jig 1 on the worktable of the skin stretching machine. Place the aluminum alloy sheet to be formed into the jaws on both sides of the skin stretching machine. Close the jaws and move the jaws on both sides to make the aluminum alloy sheet cover the stretch jig 1. During the covering process, the top of the worktable can be used to improve the mold fit.

[0039] Step 2: Use roll forming to process the reverse bending shape of the end of the hyperboloid skin part. Remove the sheet material processed in Step 1, and use a hand band saw to cut off the parts held by the stretching jaws at both ends of the sheet material. Use sandpaper to smooth the end faces, ensuring the roughness does not exceed Ra12.5. After completion, install the left jig 2 and the right jig 3. Connect them using the left mold pin 7 and the right mold pin 8 on both sides of the stretching jig 1. During the connection process, a wooden mallet can be used to tap the left jig 2 and the right jig 3 to ensure a tight connection with the stretching jig 1. After connection, use a feeler gauge to check, requiring the connection gap to be no greater than 0.1mm, meeting the tooling manufacturing tolerance requirements.

[0040] After the connection is completed, a three-axis roll bending machine is used to roll bend the two ends of the sheet metal. Specifically, the areas to be formed at the ends of the sheet metal are placed between the rollers, causing continuous plastic deformation and forming a reverse bend to obtain the formed sheet metal. After bending, the formed sheet metal is placed on the surface of the forming fixture for inspection. A feeler gauge is used to check the fit gap between the formed sheet metal and the forming fixture to ensure that the fit gap is no greater than 0.5mm. At this point, the parts on the left side body 2 and the right side body 3 are processed.

[0041] Step 3: Machining the positioning holes. Press the drilling template 4 on the forming fixture onto the surface of the forming sheet, and use a pneumatic drill to make the positioning holes. The diameter tolerance of the positioning holes should not exceed 0.1mm. Use a digital caliper to measure the hole diameter. Then, use positioning pins to position the positioning holes on the forming sheet and the forming fixture to ensure that the relative positions of the forming sheet and the forming fixture are correct.

[0042] Step 4: Hole Making. Place the upper pressure plate 5 on the stretching body 1 to press the formed sheet metal tightly. Use a marker to outline the center hole position using the holes on the upper pressure plate 5. Move the upper pressure plate 5 away and pre-drill the hole using a scriber. Reinstall the upper pressure plate 5 and use the core die 6 to punch downwards to form the center curved edge hole to the mold. After punching, remove the upper pressure plate 5 and the core die 6. Use a feeler gauge to check the mold gap, completing the forming of the double-curvature skin part.

Claims

1. A forming tool for a double-cambered skin part with a bent edge in an inner hole, characterized in that, The forming fixture includes a stretching die (1), a left die (2), a right die (3), a drilling template (4), an upper pressure plate (5), and a core die (6); The upper surface of the stretching body (1) is the working surface, which is set according to the shape of the middle part of the target part and is used for the first step of stretching and forming of the target part. Multiple positioning holes are provided on both sides of the working surface of the stretching body (1), and drilling templates (4) are provided in the holes for drilling positioning holes on the surface of the target part and fixing the target part. A fixing core hole mold (6) is detachably installed on the working surface of the stretching body (1) corresponding to the opening position of the target part for forming the opening and bending edge on the surface of the target part. The end faces of the two ends of the working surface of the stretching body (1) are used to connect the left body (2) and the right body (3) respectively. The left side body (2) and the right side body (3) are detachably installed at both ends of the working surface of the stretching body (1). The upper surface of both is the working surface. After being installed at both ends of the stretching body (1), the working surfaces of both are precisely connected with the working surface of the stretching body (1) to form a complete target part shape. The ends of the left side body (2) and the right side body (3) are equipped with positioning holes and drilling templates (4) for drilling positioning holes on the surface of the target part. The lower surface of the pressure plate (5) is set according to the working surface of the stretching body (1), and it has a hole at the position of the core die (6). The pressure plate (5) presses the target part onto the stretching body (1) to maintain the position and verticality during the downward bending process of the core die (6).

2. The forming tool for a double-crowned skin part with a bent edge of an inner hole according to claim 1, characterized in that, The forming tooling is made of resin or metal.

3. The forming tool for a double-crowned skin part with a bent edge of an inner hole according to claim 1, characterized in that, Two positioning reference holes (9) are symmetrically installed on the surface of the stretching carcass (1) for establishing a coordinate system during the manufacturing process.

4. The forming tool for a double-crowned skin part with a bent edge of an inner hole according to claim 1, characterized in that, The working surfaces of the stretching body (1) are respectively provided with a left mold-matching pin (7) and a right mold-matching pin (8). The connecting surfaces of the left body (2) and the right body (3) are provided with pin holes, which cooperate with the left mold-matching pin (7) and the right mold-matching pin (8) respectively, so that the left body (2) and the right body (3) can be detachably installed at the working surfaces of the stretching body (1).

5. The forming tool for a double-crowned skin part with a bent edge of an inner hole according to claim 1, characterized in that, The method for determining the connection position between the stretching body (1) and the left body (2) and right body (3) on both sides is as follows: Taking the position of the target part placed on the stretching body (1) as the reference, point A is selected at the end of the target part, and point B is selected on the target part near the end area. Points A and B are selected along the length direction of the target part. Point A is drawn as a horizontal line, and point B is drawn as a vertical line, intersecting at point C to form a right triangle ABC. According to Table 1, the length relationship of line segment AC is determined by the angle of angle BAC, and then the position of point B is determined. Based on the length of line segment AC, the dividing design surface between the stretching body (1) and the left body (2) and right body (3) is determined: Table 1. Relationship between the angle of angle BAC and the length of line segment AC. For target parts with an angle BAC greater than 60°, the stretching body (1) does not need to be segmented. Its surface is set according to the complete surface of the target part, and the target part is formed by stretching the stretching body (1) once.

6. The forming fixture for a hyperbolic skin part with an inner hole and a bent edge according to claim 1, characterized in that, The core die (6) is provided with multiple outer diameters. The required outer diameter is determined according to the thickness of the target part and the outer diameter of the opening on it, as shown in Table 2: Table 2 shows the relationship between the outer diameter of the core mold (6) and the height of its forming bend. In the table, D is the outer diameter of the opening, d is the inner diameter of the opening (i.e., the outer diameter of the core mold (6), and δ is the thickness of the part material.

7. A method for forming a hyperbolic skin part with an inner hole and a bent edge, implemented based on the forming tooling described in any one of claims 1-6, characterized in that, The forming method includes the following steps: Step 1: Remove the left side tire body (2) and the right side tire body (3) from both ends of the stretching tire body (1), place the stretching tire body (1) on the worktable surface of the skin stretching machine, put the aluminum alloy sheet to be formed into the jaws on both sides of the skin stretching machine, and move the jaws on both sides to make the aluminum alloy sheet cover the stretching tire body (1). Step 2: Remove the sheet material processed in Step 1, cut off and grind off the tensioning parts at both ends of the sheet material; after completion, install the left side body (2) and the right side body (3), and check the connection gap; After the connection is completed, the two ends of the sheet metal are rolled and shaped using a three-axis roll bending machine to obtain the shaped sheet metal; after bending and shaping, the shaped sheet metal is placed on the surface of the forming fixture, and the fitting gap is checked to be no more than 0.5mm; Step 3: Press the drilling template (4) onto the surface of the formed sheet, drill positioning holes and measure and inspect the hole diameter, and then fix the positioning holes of the formed sheet with the positioning holes on the forming fixture. Step 4: Place the upper pressure plate (5) on the stretching body (1) to press the forming sheet tightly. Use the opening on the upper pressure plate (5) to draw the outline of the center opening position. Move the upper pressure plate (5) and pre-open the hole. The size of the pre-opening hole is determined according to the preset setting. Install the upper pressure plate (5) again and use the core hole mold (6) to punch downward to form the center curved edge opening to the mold. After punching, remove the upper pressure plate (5) and the core hole mold (6), check the mold gap, and complete the forming of the target part.

8. The method for forming a hyperbolic skin part with an inner hole and a bent edge according to claim 7, characterized in that, In the second step, the gap between the forming sheet and the working surface of the forming tool is no more than 0.5 mm.

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