“High performance, light weight, precision, high efficiency, reliability, economy and environmental protection are the development trends of a new generation of aerospace products in the 21st century. At present, my country’s aerospace industry has entered a period of rapid development, the application scope of aerospace technology has become wider and wider, and the market demand has shown a trend of rapid growth. Rapid and low-cost launch, long-term in-orbit flight, outstanding maneuverability, and high payload have become The urgent need for the development of aerospace industry.Faced with this demand, aerospace product parts materials need to develop in the direction of high strength and low density, the structure needs to develop in the direction of integration and thin-walling, and the processing methods need to be high-efficiency, low-cost, energy-saving and environmentally friendly.
High performance, light weight, precision, high efficiency, reliability, economy and environmental protection are the development trends of a new generation of aerospace products in the 21st century. At present, my country’s aerospace industry has entered a period of rapid development, the application scope of aerospace technology has become wider and wider, and the market demand has shown a trend of rapid growth. Rapid and low-cost launch, long-term on-orbit flight, outstanding maneuverability, and high payload have become The urgent need for the development of aerospace industry. Faced with this demand, aerospace product parts materials need to develop in the direction of high strength and low density, the structure needs to develop in the direction of integration and thin-walling, and the processing method needs to develop in the direction of high efficiency, low cost, energy saving and environmental protection.
The large number of applications of difficult-to-machine materials, the structural complexity and high material removal rate caused by their poor processing performance and structural integrity have brought great challenges to the processing of aerospace thin-walled complex structural parts, and also put forward suggestions on manufacturing equipment, process technology, etc. higher requirements. For aerospace thin-walled complex structural parts, especially large weakly rigid curved structural parts, thin-walled revolving body parts, and thin polyhedral parts, the flexible tooling system design is carried out. Efficient and precise machining of aerospace precision and complex structural parts provides a stable tooling system.
1. Flexible tooling design requirements
The machining process system is composed of machine tools, tools, workpieces and fixtures. There are two ways to improve the processing efficiency of parts, namely, reducing the cutting processing time and reducing the auxiliary preparation time. The application of high-speed cutting technology can greatly reduce the cutting processing time; and the application of advanced tooling can greatly reduce the auxiliary preparation time and improve the efficiency of workpiece positioning and clamping, adjustment and loading and unloading.
Aerospace thin-walled complex structural parts have common features 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, rudder surfaces and airfoils, etc. The positioning and clamping rules of these parts Strong sex. In the thin-walled integral structure, the rigidity of the part changes with the removal of a large amount of blank material during the cutting process, and the structural rigidity is low and complex. Therefore, it is objectively required that the workpiece clamping force during processing should be adjusted in real time to adapt to the changes in the overall dynamic rigidity of the part; more Point auxiliary support to improve the local rigidity of the processing part and reduce the thin-wall deformation.
Flexible tooling that comprehensively embodies electromechanical-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 components are common components, with good interchangeability; positioning and clamping position It can be adaptively adjusted; the magnitude, direction and clamping sequence of the clamping force can be automatically controlled; the drive actuator is an electromechanical-hydraulic integrated component; displacement, force and piezoelectric sensor elements are applied.
Flexible tooling technology can make a set of fixtures meet the installation requirements of parts of various sizes and specifications. It not only has the flexibility of mechanically adjustable fixtures and combined fixtures, but also has the efficiency of special special fixtures. It is suitable for CNC machining equipment and can make The performance of the high-speed CNC machining machine is more fully exerted, and the auxiliary preparation time is greatly reduced.
2. Large-scale weakly rigid curved structural parts
The launch vehicle propellant storage tank is made of large-size, thin-walled, honeycomb meshed high-strength aluminum alloy wall parts after milling and welding. It is the largest structural component in the rocket body structure and affects the safety and reliability of the launch vehicle. The key components of the rocket account for 60% of the total mass of the rocket and 2/3 of the total length of the arrow.
Large and complex aluminum alloy storage tank grid Panel is the basic part welded to become the storage tank. The panel can be divided into shell segment panel and cylinder segment panel according to different structures, and different types of shell segment panel or cylinder segment panel. The plate structure again varies, as shown in Figure 1. According to the design requirements, the siding should be as lightweight as possible while maintaining sufficient stiffness and strength, so its model has unique structural characteristics. The siding is manufactured by rolling a whole aluminum plate and then performing five-axis milling. The entire processing system and process have characteristics that are different from other conventional structural parts. These characteristics mainly include: irregular honeycomb grid structure, bosses The features such as mouth and frame are interlaced, the overall similarity and local differences coexist; the macroscopic large size is combined with the local variable rigidity; the complex transformation law under the multi-stress coupling condition causes the macroscopic warping deformation of the wall plate and the deformation of different grid positions. The local deformation increases the inhomogeneity of different mesh wall thicknesses.
In view of the high-efficiency and high-precision machining requirements of the tank wall, the vacuum adsorption clamping technology is adopted, and the parts are adsorbed and clamped by the vacuum adsorption fixture, so that they are subjected to the clamping force of the evenly distributed load, thereby reducing the deformation of the parts caused by the clamping force. Improve the machining accuracy of parts. The main components of the vacuum adsorption flexible clamping device include: a large vacuum adsorption device for machining the inner profile of the wall panel, a large vacuum adsorption device for the machining of the outer profile, a vacuum generation system, and a platform integrated control system. Among them, the main components of the vacuum adsorption device include a cast tire, a rotary arm cylinder and a vacuum suction cup, a valve block module, a pressure sensor, a vacuum pipeline, a quick connector, a manual stop valve, and a sealing strip, as shown in Figure 2.
Figure 1. Launch vehicle panel structure
(a) Shell section wall plate (b) Tank section wall plate
Fig. 2 Vacuum adsorption device of tank wall plate
(a) Vacuum adsorption device for shell wall plate (b) Vacuum adsorption device for cabin wall plate
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 suck the workpiece. The composition of the vacuum generation system includes: vacuum pump, muffler, electromagnetic differential pressure vacuum valve, high vacuum diaphragm valve, vacuum trap, high vacuum manual butterfly valve, vacuum meter, control system, etc. The important performance parameters in the vacuum generation system are the The ultimate vacuum obtained and the effective pumping speed of the vessel.
3. Thin-walled rotary parts
Structural parts such as cabin body and end frame are typical thin-walled rotary parts. The CNC milling flexible tooling of such structural parts can be used for milling, drilling and boring of circumferential holes, slots, mouth frames and cavities of parts. , and the clamping range in the length direction and the diameter direction can be adjusted within a certain range, and the clamping force range of the tooling system can also be adjusted, so as to meet the clamping needs of a variety of products with similar structures. All face turning fixtures have soft jaw chuck clamping function to meet the small deformation clamping requirements of thin-walled structures.
Under the traditional clamping conditions, the thin-walled rotary parts mostly use the combined clamping method of mechanical pressure plate and stuffy cover. The clamping time is long, and the reliability of the clamping depends entirely on the attitude of the workers, work standardization, and the size and consistency of the clamping force. Can not guarantee. According to the characteristics of thin-walled rotary parts, the hydraulic flexible tooling system is designed to form a flexible clamping technology with adjustable axial clamping position, combination of clamping and floating support, and multi-point automatic centering, so as to meet the requirements of rotation of different diameters and different lengths. The clamping requirements of body parts, the schematic diagram of the flexible fixture is shown in Figure 3.
Figure 3 Schematic diagram of flexible tooling for CNC milling of thin-walled rotary parts
Various types of thin-walled revolving structures such as cabins and end frames can be clamped by the same set of clamps shown in Figure 3. The axial and radial travel of the flexible clamps can be adjusted, and the axial clamping position can be adjusted. It changes with the position of the cabin shape processing to solve the problem of processing interference. The hydraulic station is used to control the system pressure and clamping force, and finite element simulation is used to analyze the clamping deformation of parts under different clamping force conditions, so as 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 rotary structures such as the cabin.
In the machining of the outer circle and inner cavity of cabin parts, the six-jaw or eight-jaw chuck is specially designed for multi-point clamping of thin-walled parts and easily deformed workpieces, as shown in Figure 4. The base jaws of the multi-jaw chuck are connected two by two and can be clamped by floating centripetal, so that the force directions of multiple clamping points are all directed to the center to ensure 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.
Figure 4 Typical six-jaw chuck
Fourth, thin polyhedron parts
As an important part to maintain the flight attitude and control the flight direction, thin polyhedron parts have high aerodynamic requirements, so the structure design is complex and the surface processing quality is high. Such structural parts are mainly multi-slope and complex in structure. The thinnest local wall thickness of the cutting edge is less than 0.5mm, and the material removal rate of the parts is more than 70%. Typical parts include rudder surfaces, airfoils, cover plates, etc.
In the traditional clamping mode, for thin polyhedral parts, the rudder is clamped with a mechanical pressure plate, which takes a long time to clamp, and the reliability of the clamping is completely dependent on the experience and work specification of the workers, and the size and consistency of the clamping force cannot be guaranteed. According to the characteristics of thin polyhedron parts, the hydraulic flexible tooling system is designed. Through reasonable distribution of clamping points, combined with automatic pressing and pressing force control, a flexible tooling system suitable for various types of rudder surfaces and airfoil parts is formed.
The rudder wing needs to be processed on both sides during the processing, so it is necessary to design two sets of flexible tooling systems to complete the processing of the front and the back respectively. The schematic diagram is shown in Figure 5. Taking the airfoil part as the frontal clamping of the airfoil part blank, six hydraulic corner down-pressing cylinders are used to complete the six-position pressing, and the blank placement base adopts a hollow design to prevent the base from interfering during parts 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 clamping deformation of the parts under different clamping force conditions, and the optimal clamping force is determined.
After the front side processing is completed, the front side clamping process is used, and the back side clamping uses six hydraulic corner pressing cylinders to complete the same six positions as the front side. put one’s oar in.
Figure 5 Typical six-jaw chuck
In view of the development requirements of NC machining flexible tooling for aerospace thin-walled complex structural parts, it is necessary to comprehensively study the cutting force-thermal coupling law of thin-walled complex structural parts, the dynamic change of the rigidity of the part, and the influence law of the machining path, so as to optimize the clamping position and clamping. The distribution of force can balance the cutting force through the flexibility of the clamping force and reduce the local cutting deformation. Based on the similarity of geometrical features of aerospace thin-walled complex structural parts, a flexible tooling for automatic positioning and clamping is developed, which is used for the positioning and clamping of large weakly rigid curved structural parts, thin-walled revolving structural parts and thin polyhedral structural parts cabins , to achieve the same set of flexible fixtures for similar structural parts, fast and automatic clamping and disassembly, which can greatly reduce the auxiliary preparation time for processing and effectively improve the processing quality and production efficiency.