A method for processing a hyperbolic skin part with local features

Abstract

The application relates to a hyperbolic skin part machining method with local features, and belongs to the technical field of aviation sheet metal manufacturing. The combined forming machining method of the hyperbolic skin local feature part ensures the consistency of skin stretch forming and local forming under the same positioning condition, ensures the shape precision of the skin in the manufacturing process, and improves the accuracy of skin forming machining and measurement. The increased pneumatic forming structure device on the tool structure breaks through the process of completing machining of one skin part by two sets of independent toolings. The two-time forming and one-time positioning mode can accurately manufacture the width and depth of the complex structure, can realize uniform stress of the complex structure surface of the skin, and can realize smooth transition of the surface quality.

Description

Technical Field

[0001] This invention relates to a method for processing hyperbolic skin parts with local features, belonging to the field of aerospace sheet metal manufacturing technology. Background Technology

[0002] In the aerospace industry, sheet metal parts are a major component of the shape and structure of modern aircraft. They are characterized by their variety, complex structure, large dimensions, and low rigidity, constituting a significant proportion of the overall aircraft. Generally, using a large number of sheet metal parts, while meeting the functional and shape requirements of the parts, can significantly reduce the weight of aircraft and aero engines. Furthermore, sheet metal parts are low-cost, can be mass-produced efficiently, and offer substantial economic benefits. Because aircraft need to operate for extended periods under harsh high-altitude conditions, high demands are placed on the comprehensive material properties of their components. Currently, most aerospace companies rely on their own experience to develop production processes for sheet metal parts, with limited innovation, resulting in low production yields and high costs. The most significant factor affecting the cold forming performance of sheet metal is its inherent formability. Therefore, developing processing techniques without understanding the sheet metal's formability and basic parameters not only fails to fully utilize its maximum formability but also negatively impacts the quality of the formed parts.

[0003] The surface of aircraft curved skin parts has several local features. These features are assembled with the finished product. To meet the installation requirements of the finished product, these features need to be designed as raised reinforcing ribs. During the skin forming process, these raised areas cannot be formed in one step; the raised areas are stepped, with a transition zone width of 26–47 mm. During stretch forming, bulging and warping deformation occur in this area. After pressing with an upper pressure device, the transition zone depth is insufficient, and visual inspection reveals almost no stepped shape. The conventional method is for workers to use wooden mallets to hammer out the stepped shape of the reinforcing ribs. However, this manual forming results in waviness and poor surface quality, affecting the overall appearance of the skin. The machining tolerance requirement is 0-0.3 mm, which represents a three-fold improvement in inspection precision compared to the traditional ±1 mm tolerance for aircraft skin parts.

[0004] This invention proposes a method for machining hyperboloid skin parts with local features. It focuses on the local features of the hyperboloid skin part, conducting thorough analysis of material properties, external structural characteristics, and forming schemes. The method investigates the influence of stretch forming process parameters on the tensile stress, shear stress, and springback of the part. Based on the part's external features, tooling is designed and manufactured. Using two sets of tooling, the hyperboloid shape and pressed protrusions can be manufactured quickly, accurately, and efficiently with only one positioning with the part. This achieves rapid and accurate positioning, and the transition zone width meets the requirement of 26–47 mm, with a smooth, indentation-free transition zone surface. Summary of the Invention

[0005] This invention addresses a series of problems in hyperbolic skin parts, including the inability to quickly and accurately position local features, the inability of the transition zone width to meet the requirement of 26-47 mm, and the inability to achieve a smooth, indentation-free surface quality in the transition zone. A combined forming method is proposed, which, by adding a pneumatic local feature forming structure, achieves precise positioning in a single step of the combined forming process. This ensures the accuracy of both the hyperbolic skin stretching and local feature forming under the same positioning conditions, and also guarantees the shape accuracy of the hyperbolic skin during manufacturing. A typical example is the forming of a hyperbolic skin with local features.

[0006] The technical solution of the present invention is as follows:

[0007] The processing method comprises a combination of transverse stretching forming, drilling positioning holes, and local feature forming. The stretching forming principle involves the sheet metal being clamped at both ends by the stretching machine clamps, and then contacted by the stretching die lifted by the worktable. This generates uneven planar tensile strain, causing the sheet metal to adhere to the stretching die. This method is commonly used for forming hyperboloid skins. Transverse stretching involves clamping the sheet metal at both ends laterally, and under the force of the stretching die lifted by the worktable, the sheet metal adheres to the stretching die. This method is generally used for forming skin parts with large transverse curvature. Drilling positioning holes refers to the operation of machining holes on the workpiece using a drill bit positioned by a drill template. Its advantages are high precision, high speed, and high efficiency. Local feature forming utilizes the flow and pressure of gas to generate power to drive the local feature forming die. This allows for the processing of complex parts. To complete the forming of local features of a hyperboloid skin part, a pneumatic local feature forming die is required to achieve the processing of the skin part's local features.

[0008] The typical part processing flow is as follows: Figure 5 As shown.

[0009] The tooling designed for this combined processing method is a set of combined forming dies, including a stretch forming die and a pneumatic local feature forming die. The local feature forming die is used in conjunction with the stretch forming die, and its function is to accurately, quickly and efficiently process local features of the hyperbolic skin.

[0010] The stretch forming die includes: a base, a movable drilling template, and a lifting rod.

[0011] The pneumatic local feature forming mold includes: a Π-shaped fixing frame, a threaded positioning pin, a threaded connecting plate at both ends, a forming mold, a force-applying block, a pressure-applying nut, a support frame, a cylinder, an air inlet, and an air outlet. The base is made of SAM900 resin, a non-metallic material characterized by excellent chemical stability, electrical insulation, corrosion resistance, good adhesion, and high mechanical properties. It also possesses good machinability, with surface strength, wear resistance, and impact resistance very close to those of metallic materials. Three movable drill templates are located on the upper surface of the base, arranged in a triangular pattern along different edges of the part, used for drilling positioning holes. During use, one side of each movable drill template is connected to the base via... Pin connection, with a diamond-set sleeve on the other side. hole, The holes are part positioning holes. Four lifting rods, 80mm in diameter and 350mm in length, are used to move the base. They are placed in pairs, symmetrically positioned at the midpoint of the short side of the base. Two symmetrical Π-shaped fixing brackets are attached, each connected to the long side of the base by six bolts. Two symmetrical threaded connecting plates connect the Π-shaped fixing brackets and the forming mold, secured by threaded locating pins. The forming mold rests on the upper surface of the base, above the hyperbolic skin feature, and its lower surface has the same shape as the hyperbolic skin feature. A pressure block, identical in shape and size to the outermost edge of the hyperbolic skin feature, is placed on the upper surface of the forming mold and is used to apply pressure. The pressure nut connects to the pressure block on its lower surface and to the cylinder on its upper surface. The support frame primarily supports the cylinder. Compressed air enters the cylinder through a polyurethane hose, passes through the air inlet, and generates power by utilizing changes in gas pressure. The power is transmitted to the forming mold through the pressure nut and force block, forming the local features of the skin. After forming is completed, the compressed air is exhausted through the polyurethane hose and the air outlet. For better and more convenient adjustment of air pressure, an air regulating valve can be installed. This finished part can be purchased separately.

[0012] A method for machining hyperbolic skin parts with local features, comprising the following steps:

[0013] Step 1: Install the dedicated platform. Use a rail-mounted crane to place the dedicated platform required for stretch forming in the center of the stretching machine.

[0014] Step 2: Install the base. Use professional lifting ropes to attach to the four lifting rods. Use a rail-mounted crane to place the base in the center of the platform, ensuring that the base is securely fixed to the platform. Position the base according to the stretching direction, aligning it with the jaw direction.

[0015] Step 3: Clamp the billet. Place both ends of the billet in the jaws of the stretching machine, so that the ends are in contact with the bottom of the jaws, and ensure that the clamping amount of both jaws is not less than 50 mm.

[0016] Step 4: Close the jaws and clamp the billet. Check the clamping status of the jaws to ensure that the jaws are fully clamped to the billet.

[0017] Step 5: Lateral pre-stretching forming. The base rises and contacts the blank, and the material begins to bend. When the material bends to contact the base × 1200mm = 1800mm... 2 Due to the skin structure, the two sides are nearly perpendicular in the final stage of stretching, resulting in the maximum machine tool force. Let α = 90°, where α is the wrap angle of the sheet metal on the base. Therefore, the tensile force required for the sheet metal in the stretching process is approximately 460 N.

[0018]

[0019] Step 6: Heat treat the pre-stretched billet. Take the pre-stretched billet out of the jaws and quench it in an air furnace. The quenched pre-stretched billet is in the "W" or "AQ" state.

[0020] Step 7: Second stretching and forming. Repeat steps 3 and 4 for a second stretching and forming. Clamp the pre-stretched blank with the jaws completely covering the working surface of the base until the blank and base are fully in contact. Use a wooden mallet or rubber beater to tap the surface of the part to check if the part is in contact with the base. Control the tension during the stretching process to ensure that the stretching rate does not exceed 6%. The blank is stretched in the freshly quenched state, and the tensile strength is taken as σ. b =337MPa, the tensile force required for stretching a newly quenched billet is approximately 764N.

[0021] Step 8: Install the movable drill template at the designated location on the base and begin drilling. Positioning holes, remove the movable drill template, and drill in... Insert the positioning pin into the positioning hole, open the jaws, remove excess material from both sides of the blank, and take care to protect the surface of the part to prevent scratches or abrasions.

[0022] Step 9: Install the Π-shaped fixing bracket, threaded connecting plates at both ends, and forming mold in sequence. Connect the compressed air using a polyurethane hose, connecting the inlet and outlet. After installation, open the compressed air regulating valve. Through air pressure transmission, the force block provides power to the forming mold, thereby processing the local feature shape.

[0023] Step 10: Mill the outline using five coordinates. To ensure manufacturing positioning accuracy, use the same... Position the part using the positioning holes, and then use a five-coordinate CNC milling machine to mill all edges and inner holes of the part according to the verified CNC machining program.

[0024] Step 11: Inspect the shape and edge accuracy of the parts. The outer surface fit tolerance is ±0.5 mm, and the edge manufacturing accuracy tolerance is ±0.2 mm.

[0025] Advantages of this invention:

[0026] The combined forming method for local feature parts of the hyperbolic skin ensures consistency between skin stretching and local forming under the same positioning conditions, guaranteeing the shape accuracy of the skin during manufacturing and improving the accuracy of skin forming and measurement. The pneumatic forming structure added to the tooling structure breaks through the conventional requirement of two independent tooling sets to complete the processing of a single skin part. Employing a two-step forming and one-step positioning method, it is possible to precisely manufacture the width and depth of complex structures, achieving uniform stress on the complex surface of the skin and a smooth surface transition. Attached Figure Description

[0027] Figure 1 Front view of the combined forming mold;

[0028] Figure 2 Top view of a modular forming mold;

[0029] Figure 3 Top view of pneumatic local feature forming;

[0030] Figure 4 3D view of a pneumatic local feature forming mold;

[0031] Figure 5 Typical part machining process flow chart.

[0032] In the diagram: 1. Base, 2. Movable drilling template, 3. Hanging rod, 4. Π-shaped fixing frame, 5. Threaded positioning pin, 6. Threaded connecting plate at both ends, 7. Forming mold, 8. Force application block, 9. Pressure nut, 10. Support frame, 11. Cylinder, 12. Air inlet, 13. Air outlet. Detailed Implementation

[0033] Based on the structure of local features of hyperbolic skin, a composite forming process flow is formulated. Taking a typical local feature of hyperbolic skin as an example, the implementation steps of the composite forming method are as follows:

[0034] Step 1: Install the dedicated platform. Use a rail-mounted crane to place the dedicated platform required for stretch forming in the center of the stretching machine.

[0035] Step 2: Install base-1. Use professional lifting ropes to attach to the four lifting rods-3 respectively. Use a rail-mounted crane to place base-1 in the center of the platform, ensuring that base-1 is well fixed to the platform. Position base-1 according to the stretching direction, so that it is aligned with the jaw direction.

[0036] Step 3: Clamp the billet. Place both ends of the billet in the jaws of the stretching machine, so that the ends are in contact with the bottom of the jaws, and ensure that the clamping amount of both jaws is not less than 50 mm.

[0037] Step 4: Close the jaws and clamp the billet. Check the clamping status of the jaws to ensure that the jaws are fully clamped to the billet.

[0038] Step 5: Lateral pre-stretching forming. The base-1 rises and contacts the blank, and the material begins to bend. When the material bends to contact the surface of the base-1, the jaws are released, and the pre-stretched blank is removed. The machine tonnage required for stretching can be 0.9σ per unit area of ​​the blank. b The stress calculation is performed, so the top force P on the worktable during horizontal tension is taken as the tensile strength σ of the billet during pre-tensioning. b =203MPa, the cross-sectional area of ​​the billet is taken as F = 1.5mm (billet thickness) × 1200mm (billet width) = 1800mm 2 Due to the skin structure, the two sides are nearly perpendicular in the final stage of stretching, resulting in the maximum machine tool force. Let α = 90°, where α is the wrap angle of the sheet metal on base-1. Therefore, the tensile force required for the sheet metal in the stretching process is approximately 460N.

[0039]

[0040] Step 6: Heat treat the pre-stretched billet. Take the pre-stretched billet out of the jaws and quench it in an air furnace. The quenched pre-stretched billet is in the "W" or "AQ" state.

[0041] Step 7: Second stretching and forming. Repeat steps 3 and 4 for a second stretching and forming. Clamp the pre-stretched blank with the jaws completely covering the working surface of base-1 until the blank is fully attached to base-1. Use a wooden mallet or rubber beater to tap the surface of the part to check if the part is attached to base-1. Control the tension during the stretching process to ensure that the stretching rate does not exceed 6% (the stretching rate is tested using elongation test paper). The blank is stretched in the freshly quenched state, and the tensile strength is taken as σ. b =337MPa, the tensile force required for stretching a newly quenched billet is approximately 764N.

[0042] Step 8: Install the movable drill template-2 at the designated position on the base-1 and begin drilling. Positioning holes, remove the movable drill template-2, and respectively in Insert the positioning pin into the positioning hole, open the jaws, remove excess material from both sides of the blank, and take care to protect the surface of the part to prevent scratches or abrasions.

[0043] Step 9: Install the Π-shaped fixing bracket-4, the threaded connecting plates at both ends-6, and the forming mold-7 in sequence. Connect the compressed air using a polyurethane hose, with the air inlet-12 and the air outlet-13. After installation, open the compressed air regulating valve. Through air pressure transmission, the force block-8 provides power to the forming mold-7 to process the local feature shape.

[0044] Step 10: Mill the outline using five coordinates. To ensure manufacturing positioning accuracy, use the same... Position the part using the positioning holes, and then use a five-coordinate CNC milling machine to mill all edges and inner holes of the part according to the verified CNC machining program.

[0045] Step 11: Inspect the shape and edge accuracy of the parts. The outer surface fit tolerance is ±0.5 mm, and the edge manufacturing accuracy tolerance is ±0.2 mm.

Claims

1. A modular forming mold for processing hyperbolic skin parts with localized features, characterized in that, Including stretch forming dies and pneumatic local feature forming dies; The stretch forming die includes: a base (1), a movable drilling template (2), and a lifting rod (3); The pneumatic local feature forming mold includes: a Π-shaped fixing frame (4), a threaded positioning pin (5), a threaded connecting plate at both ends (6), a forming mold (7), a force application block (8), a pressure nut (9), a support frame (10), a cylinder (11), an air inlet (12), and an air outlet (13). The movable drill template (2) consists of 3 pieces, located on the upper surface of the base (1), and distributed in a triangular pattern on different edges of the part. It is used to drill positioning holes for the part. When in use, each movable drill template (2) is connected to the base (1) on one side by 2-φ5 pins, and has a φ5 hole with a drill sleeve on the other side. The φ5 hole is the positioning hole for the part. The aforementioned lifting rod (3) is used to move the base (1). It has a diameter of 80 mm and a length of 350 mm. There are 4 rods, and each pair is placed in the middle of the short side of the base (1) in a symmetrical manner. Two threaded connecting plates (6) are symmetrical to each other. Their function is to connect the Π-shaped fixing frame (4) and the forming mold (7). They are fixed together by threaded positioning pins (5). The forming mold (7) is mounted on the upper surface of the base (1) and located above the local feature of the hyperbolic skin. The shape of the lower surface of the forming mold (7) is the same as the shape of the local feature of the hyperbolic skin. The force-applying block (8) is placed on the upper surface of the forming mold (7). Its shape is the same as that of the local feature of the hyperbolic skin, and its size is the same as that of the outermost edge of the local feature of the hyperbolic skin. It is used to apply pressure. The pressure nut (9) is connected to the force-applying block (8) on its lower surface and to the cylinder (11) on its upper surface. The Π-shaped fixing bracket (4) is in the form of two symmetrical brackets, each of which is connected to the long side of the base (1) by six bolts; The support frame (10) supports the cylinder (11). Compressed air enters the cylinder (11) through the inlet (12) via a polyurethane hose. Power is generated by the change in gas pressure. The power is transmitted to the forming mold (7) through the pressure nut (9) and the force block (8). The forming skin features are formed. After forming is completed, the compressed air is exhausted through the outlet (13) via the polyurethane hose.

2. The combined forming mold for processing hyperbolic skin parts with local features as described in claim 1, characterized in that, The base (1) is made of SAM900 resin material.

3. A method for processing hyperbolic skin parts with local features, characterized in that, The steps are as follows: Step 1: Install the dedicated platform. Use a rail-mounted crane to place the dedicated platform required for stretch forming in the center of the stretching machine. Step 2: Install the base (1), use professional hoisting ropes to hook the four hoisting rods (3) respectively, and use the rail crane again to place the base (1) in the center of the platform to ensure that the base (1) is well fixed to the platform. Position the base (1) in the direction of tension so that it is consistent with the direction of the jaws. Step 3: Clamp the billet. Place both ends of the billet in the jaws of the stretching machine, so that the ends are in contact with the bottom of the jaws, and ensure that the clamping amount of both jaws is not less than 50 mm. Step 4: Close the jaws and clamp the billet, check the clamping status of the jaws to ensure that the jaws are fully clamped to the billet; Step 5: Lateral pre-stretching and forming, the base (1) rises and contacts the blank, and the material begins to bend; when the material bends to contact the surface of the base (1), the jaws are released and the pre-stretched blank is removed; Step 6: Heat treat the pre-stretched billet. Take the pre-stretched billet out of the jaws and quench it in an air furnace. The quenched pre-stretched billet is in the "W" or "AQ" state. Step 7: Second stretching and forming. Repeat steps 3 and 4 to perform the second stretching and forming. The jaws hold the pre-stretched blank and completely cover the working surface of the base (1) until the blank and the base (1) are completely in contact. Use a wooden mallet or rubber beater to tap the surface of the part to check whether the part is in contact with the base (1). During the stretching process, control the tension to ensure that the stretching rate does not exceed 6%. Step 8: Install the movable drilling template (2) at the designated position on the base (1), drill 3-φ5.2 positioning holes, remove the movable drilling template (2), insert positioning pins into the 3-φ5.2 positioning holes respectively, open the jaws, remove excess material on both sides of the blank, and take care to protect the surface of the part to prevent scratches or abrasions. Step 9: Install the Π-shaped fixing bracket (4), the threaded connecting plates (6) at both ends, and the forming mold (7) in sequence. Connect the compressed air with a polyurethane hose, the air inlet (12), and the air outlet (13). After installation, open the compressed air regulating valve. Through air pressure transmission, the force block (8) gives power to the forming mold (7) to process the local feature shape. Step 10: Five-coordinate milling of the outer shape. To ensure manufacturing positioning accuracy, use the same 3-φ5.2 positioning holes for positioning. Use a five-coordinate CNC milling machine to mill all edges and inner holes of the part according to the verified CNC machining program. Step 11: Inspect the shape and edge accuracy of the parts. The outer surface fit tolerance is ±0.5mm, and the edge manufacturing accuracy tolerance is ±0.2mm.

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