Nowadays, difficult-to-process materials are widely used in the manufacturing industry. Their poor processing performance and the structural complexity and high material removal rate brought by the structural integration have brought great challenges to the processing of thin-walled complex structural parts, and also put forward higher requirements for manufacturing equipment, process technology, etc. In particular, large weak rigidity curved surface structural parts, thin-walled rotating body parts, thin polyhedron parts and other parts urgently need breakthroughs in clamping technology.
1. Flexible tooling design for thin-walled parts processing
The thin-walled complex structural parts of aerospace have common characteristics such as weak rigidity and similar shape and structure. At the same time, the types of models show the characteristics of serial development, such as cabins and end frames, rudders and wing surfaces, etc. The positioning and clamping regularity of these parts is strong. The rigidity of the thin-walled integral structure changes with the removal of a large amount of blank materials during cutting processing. The structural rigidity is low and complex. Therefore, it is objectively required that the clamping force of the workpiece during processing should be adjusted in real time to adapt to the changes in the overall dynamic stiffness of the parts; multi-point auxiliary support is required to improve the local stiffness of the processing part and reduce thin-wall deformation. ▲Flexible tooling with sensors Flexible tooling that comprehensively embodies mechatronic and hydraulic integration technology and multi-sensor information fusion technology is an advanced equipment technology that has emerged in recent years. The technical characteristics of flexible tooling are that the positioning and clamping elements are universal elements with good interchangeability; the positioning and clamping positions can be adjusted adaptively; the clamping force size, direction and clamping sequence can be automatically controlled; the driving actuator is a mechatronic and hydraulic integration component; displacement, force and piezoelectric sensor elements are applied. Flexible tooling technology can enable a set of fixtures to meet the installation requirements of parts of various sizes and specifications in a series. It has the flexibility of mechanical adjustable fixtures and combined fixtures, and the high efficiency of special fixtures. It is suitable for CNC processing equipment, which can make the performance of high-speed CNC processing machine tools more fully utilized and greatly reduce the auxiliary preparation time.
2. Processing method of large curved surface structural parts
The grid wall panels of large and complex aluminum alloy tanks are the basic parts welded into tanks. The wall panels can be divided into shell wall panels and barrel wall panels according to different structures. The structures of shell wall panels or barrel wall panels of different models are different. According to the design requirements, the wall panel needs to be as lightweight as possible while maintaining sufficient rigidity and strength, so its model has unique structural characteristics. ▲Wall panel structure The wall panel manufacturing adopts a whole piece of aluminum plate roll bending and then five-axis milling processing. The entire processing technology system and processing process have characteristics different from other conventional structural parts processing. These characteristics mainly include: irregular honeycomb grid structure, staggered features such as bosses and mouth frames, coexistence of overall similarity and local differences; macroscopic large size and local variable rigidity characteristics are combined; complex transformation laws under multi-stress coupling conditions, which cause the wall panel to undergo macroscopic warping deformation and local deformation at different grid positions, increasing the unevenness of different grid wall thicknesses. ▲Wall panel vacuum adsorption device In view of the high-efficiency and high-precision processing requirements of tank wall panels, vacuum adsorption clamping technology can be used. The parts are adsorbed and clamped by vacuum adsorption fixtures, so that they are subjected to the clamping force of uniformly distributed loads, thereby reducing the deformation of parts caused by clamping force and improving the processing accuracy of parts. The main components of the vacuum adsorption flexible clamping device include: large vacuum adsorption device for milling the inner surface of the wall panel, large vacuum adsorption device for milling the outer surface, vacuum generation system, platform integrated control system. Among them, the main components of the vacuum adsorption device include casting mold, swing arm cylinder and vacuum suction cup, valve block module, pressure sensor, vacuum pipeline, quick connector, manual stop valve, sealing strip, etc. The main function of the vacuum generation system is to provide a continuous and stable air pressure difference to ensure that the suction cup can firmly absorb the workpiece. The components of the vacuum generation system include: vacuum pump, muffler, electromagnetic pressure difference vacuum valve, high vacuum diaphragm valve, vacuum trap, high vacuum manual butterfly valve, vacuum gauge, control system, etc. The important performance parameters in the vacuum generation system are the ultimate vacuum degree it can obtain and the effective pumping speed of the container.
3. Processing of thin-walled rotating parts
Structural parts such as cabins and end frames are typical thin-walled rotating parts. The flexible CNC milling tooling for such structural parts can be used for milling, drilling and boring of circumferential holes, slots, mouth frames and cavities of parts. The clamping range in the length and diameter directions can be adjusted within a certain range, and the clamping force range of the tooling system can also be adjusted to meet the clamping needs of multiple varieties of similar structural products. Its external cylindrical turning fixtures, inner cavity and end face turning fixtures all have soft claw chuck clamping functions to meet the small deformation clamping needs of thin-walled structures. Under traditional clamping conditions, thin-walled rotating parts are mostly clamped by a combination of mechanical pressure plates and blind covers. The clamping time is long, and the clamping reliability depends entirely on the attitude and work standardization of the workers. The size and consistency of the clamping force cannot be guaranteed. According to the characteristics of thin-walled rotating body parts, a hydraulic flexible tooling system is designed to form a flexible clamping technology with adjustable axial clamping position, combined clamping and floating support, and multi-point automatic centering, so as to meet the clamping needs of rotating body parts of different diameters and lengths.▲Flexible tooling for milling of thin-walled rotating parts Various types of thin-walled rotating body structural parts such as cabins and end frames can be clamped with the same set of fixtures. The axial and radial travels of the flexible fixtures can be adjusted, and the axial clamping position can be changed with the position of cabin body shape processing to solve the problem of processing interference. The system pressure and clamping force are controlled by the hydraulic station, and the deformation of parts clamping under different clamping force conditions is analyzed by finite element simulation to determine the optimal clamping force. The bottom of the fixture adopts a 360-degree turntable, which can realize the rotation and processing of different positions of rotating body structural parts such as cabins.▲Schematic diagram of clamping the outer circle of thin-walled parts
In the outer circle and inner cavity processing of cabin parts, six-jaw or eight-jaw chucks are specially designed for multi-point clamping of thin-walled parts and easily deformed workpieces. The base jaws of the multi-jaw chuck are connected in pairs and can be clamped in a floating centripetal manner, so that the force direction of multiple clamping points is directed to the center, ensuring that the workpiece is not easily deformed. At the same time, this design makes it possible to use traditional jaws directly on the chuck, and it also has centrifugal force compensation.
4. Processing of thin polyhedral parts
Thin polyhedral parts generally have higher aerodynamic requirements, so the structural design is complex and the surface processing quality requirements are high. This type of structural parts is mainly multi-slope, complex structure, the local thinnest wall thickness of the cutting edge is less than 0.5mm, and the part material removal rate is more than 70%. Typical parts include rudders, wing surfaces, cover plates, etc. In the traditional clamping mode, the rudder of thin polyhedral parts is clamped by a mechanical pressure plate, which takes a long time to clamp. The clamping reliability depends entirely on the experience and work standardization of the workers, and the clamping force size and consistency cannot be guaranteed. According to the characteristics of thin polyhedral parts, a hydraulic flexible tooling system is designed. By reasonably distributing the clamping points, combined with automatic clamping and clamping force control, a flexible tooling system suitable for various types of rudder and wing parts is formed. ▲Schematic diagram of flexible tooling Rudder wings need to be processed on both sides during processing, so two sets of flexible tooling systems need to be designed to complete the processing of the front and back sides respectively. The structural diagram is shown in Figure 5. Taking the wing surface parts as the front clamping of the wing surface parts blank, six hydraulic angle pressing cylinders are used to complete the clamping of six positions. The blank placement base adopts a hollow design to prevent the base from interfering during part processing. The clamping force of the six clamping points is controlled by the pressure of the hydraulic station control system. The finite element simulation is used to analyze the deformation of the parts under different clamping force conditions to determine the optimal clamping force. After completing the front processing, the front clamping process is used, and the back clamping uses six hydraulic angle pressing cylinders to complete the same six positions as the front. The blank placement base adopts a hollow design to prevent the base from interfering during part processing. Many advanced technologies are generally gradually transferred from high-precision industries such as military and aerospace to ordinary industries. Flexible tooling for CNC machining of complex structural parts in aerospace also provides a verifiable processing solution for similar parts. The main idea is to first study the cutting force of thin-walled complex structural parts, the dynamic changes of part rigidity, and the influence of the processing path, so as to optimize the clamping position and the distribution of the clamping force, balance the cutting force through the flexible adjustment of the clamping force, and reduce the local cutting deformation. At the same time, the design of the tooling should be as flexible as possible, and the main goal should be to automate positioning and clamping, so as to achieve fast and effective automatic clamping and disassembly.
