Recent progress in flexible supporting technology for aerospace thin-walled parts: A review

Yan BAO, Bin WANG, Zengxu HE, Renke KANG, Jiang GUO

Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, Daliann 116024, China

KEYWORDS Aerospace;Chatter;Deformation;Flexible support;Low rigidity;Thin-walled

Abstract Thin-walled parts are widely used in the aerospace industry owing to their light weight and high specific strength.However,due to the low rigidity of thin-walled parts,elastic deformation and chatter easily occur,which seriously affect the machining accuracy and workpiece surface quality.To solve this problem,several supporting technologies have been reported in recent years.This paper reviews the recent research progress of flexible supporting technologies in the aerospace field by classifying them based on different principles and characteristics.The principle,progress,advantages, and limitations of the technologies are expounded by systematic comparison and summarized. Finally, the challenges and future development trends of flexible supporting technology,which will provide guidelines for further research, are discussed.

1. Introduction

To reduce the weight of aircraft and increase its load capacity and maneuverability, an increasing number of thin-walled parts are being adopted in the aerospace field.Low rigidity,complex shape, and high precision are typical features of thin-walled parts,as shown in Fig. 1. The thin-walled parts are usually processed by milling,but the milling force during the milling process can easily cause machining deformation and vibration, which seriously affects the machining accuracy and surface quality.Therefore, using supporting technology to improve the rigidity of thin-walled parts and reduce machining deformation and vibration has become the main method of thin-walled parts machining.

Fig. 1 Typical thin-walled parts used in aerospace field.3

After decades of development,a variety of supporting technologies have been developed for different thin-walled structures and machining requirements. In the early stage, rigid molds were used to support workpieces. For each workpiece,a corresponding mold needs to be designed, which requires a long preparation time and high manufacturing costs.However, as aircraft parts are being replaced more frequently, the problems of long manufacturing cycles and high manufacturing costs are increasingly prominent.In addition,the accuracy of the mold seriously affects the machining accuracy of the workpiece. To improve machining accuracy and efficiency,flexible supporting technologies capable of adapting to various complex shapes have become the main supporting methods for the machining of thin-walled parts in aerospace fields. At present, flexible supporting technologies can be divided into three categories according to the supporting principle and supporting characteristic.The classifications and principles of flexible supporting technologies are shown in Figs. 2 and 3,respectively. Entirely self-adaptive flexible supporting technology,mainly including temperature rheological material(TRM)supporting technology and magnetorheological fluid (MRF)supporting technology,is characterized by filling other materials to support the entire machining area. Multi-point discrete flexible supporting technology is characterized by using numerous discrete supporting heads to support the workpiece.Symmetrical follow-up flexible supporting technology, including mirror milling supporting technology and jet supporting technology, is characterized by using a supporting head that moves with the cutter symmetrically to support the workpiece.

Entirely self-adaptive flexible supporting technology used on the basis of flexible fixtures provides good supporting effects for small complex thin-walled parts.At present,entirely self-adaptive flexible supporting technology has been widely used in the machining of small complex thin-walled parts such as web structures and honeycomb structures. However, to improve the comprehensive performance of advanced aerospace products,thin-walled parts of large structures are widely used,the size of which is usually 1–2 m or even more than 10 m. Large scale thin-walled parts have the characteristics of large size, complex structure, and poor rigidity. For the machining of large thin-walled parts, the use of entirely selfadaptive flexible support not only requires a considerable material consumption but also results in a low machining accuracy and efficiency. Therefore, many European and American machine tool companies have developed multi-point discrete flexible supporting technology,which is combined with numerical control machining technology for the manufacturing of large thin-walled parts such as fuselages and wings.Afterwards,to further improve the machining accuracy and machining efficiency,mirror milling supporting technology integrating pocketing, trimming, and hole-making was developed, which has been successfully applied to the manufacturing of large thin-walled parts such as aircraft skin.However, the hard supporting head easily scratches the surface of the workpiece.In addition, when impurities and chips are embedded in the soft supporting head, the surface of the workpiece can also be easily scratched.To achieve scratch-free milling, some scholars have proposed the concept of jet supporting technology, which uses a water or air jet to support the workpiece.However, jet supporting technology is not yet well developed and still needs further investigation. Aerospace thin-walled parts manufacturing technology,as one of the six crucial technologies for aircraft body manufacturing, has continued to be a challenge for the aviation industry.As an important method to improve the machining quality of thin-walled parts,flexible supporting technology has a significant effect on solving deformation and chatter in the machining process. Therefore, in recent years, flexible supporting technology has been developed and widely used in the field of aerospace manufacturing and has broad development prospects.

Fig. 2 Classifications of flexible supporting technologies.

Fig. 3 Schematic diagram of supporting principles.

This paper aims to review the research progress over the past ten years on flexible supporting technologies in aerospace manufacturing. First, the flexible supporting technologies that are commonly used in aerospace manufacturing are classified, and the basic principles of different flexible supporting technologies are illustrated in detail. In addition, the research progress of flexible supporting technologies is introduced, with a focus on the major issues of different supporting technologies. Furthermore, the advantages and limitations of different flexible supporting technologies are summarized and compared in detail. Finally, the challenges and possible development trends of flexible supporting technologies are proposed, which could provide a reference for future research.

2. Entirely self-adaptive flexible supporting technology

Entirely self-adaptive flexible supporting technology can support the whole contact area of thin-walled parts by solidifying the liquid supporting material under the stimulation of external conditions, such as temperature and magnetic field.Because the supporting material in the liquid state can arbitrarily change its shape to adapt to the shape of the workpiece,this supporting technology is suitable for machining thinwalled parts with complex structures. At present, according to the differences in external stimulation conditions, entirely self-adaptive flexible supporting technology can be divided into TRM supporting technology and MRF supporting technology.

2.1. TRM supporting technology

TRM supporting technology can provide support for workpieces with different shapes and sizes by controlling the temperature to change the physical state of the supporting material.The principle of TRM supporting technology is illustrated by the instance of using paraffin to support the honeycomb core, as shown in Fig. 4.First, melted paraffin is poured into the honeycomb core fixture. Then, by decreasing the temperature of the paraffin, the melted paraffin is transformed into a solid to strengthen the honeycomb core. After milling, the honeycomb core can be removed successfully by using hot water to melt the paraffin.

Initially, TRM supporting technology was widely used for flexible fixtures.To adapt to the development of individuation and diversification of products, Gandhi et al. proposed the concept of flexible fixtures.In flexible fixtures, phase change material fixtures can change their physical state under certain external conditions to adapt to different shapes of workpieces and enhance the rigidity of thin-walled parts; flexible fixtures are used in the machining of complex parts.Zhong et al.introduced the principles, advantages and disadvantages of phase change fixtures such as paraffin, ice and low-melting-point alloy and summarized the performance requirements and selection principles of various TRMs, which greatly promoted the development of TRM supporting technology.In the early days,low-melting-point alloys were widely used to support turbine blades for precision machining.In addition, paraffin is used for the machining of aircraft web structures because of its low price,low melting point and easy recycling.The honeycomb structure widely used in aerospace can be filled with ice to strengthen the rigidity, which greatly improves the machining accuracy. Due to its good adaptability to the workpiece shape and low machining cost, TRM supporting technology is widely used in precision machining of complex thin-walled parts.

TRM supporting technology was introduced as an early supporting technology and has been used for auxiliary machining of various complex thin-walled structures. Many scholars have studied the properties of various TRMs,such as the melting point,strength and binding ability,to select a TRM with a better support performance for a certain workpiece. At the same time, the machining effects of various TRMs were compared. Currently, ice, paraffin, and low-melting-point alloys are mainly used to support various complex structures.

Fig. 4 Principle of TRM supporting technology12.

In recent years, ice has mainly been applied to the machining of honeycomb structures. The wall thickness of a honeycomb structure is generally 0.05–0.1 mm. Therefore, a honeycomb structure has a low rigidity in the radial direction,and it is easy to deform under radial milling force, which makes it difficult to machine. According to the characteristics of honeycomb structures, Han et al. used the method of highspeed milling to cut aluminum honeycomb cores with the support of ice. The results show that ice can effectively improve the in-plane strength of the aluminum honeycomb core.Wang et al.used ice to support honeycomb structures in ultralow temperature environments. The experimental results show that this method can increase the in-plane compressive strength of honeycomb structures and reduce the stress and deformation. At the same time, this method can eliminate machining defects of thin-walled honeycomb structures, such as burrs, curls, and edge collapse, and has a significant effect on improving the machining accuracy.

Fig. 5 Measuring results of thickness.9

Since paraffin is relatively inexpensive and easy to obtain and remains solid at room temperature,it has become a widely used supporting material. Zha et al. used paraffin to support thin-walled double-sided freeform components during the milling process to increase their rigidity and thereby improve machining accuracy. The results show that the PV values and RMS can be clearly improved for both concave and convex surfaces.Ge et al.used paraffin to strengthen thin-walled titanium alloy web structures. The surface roughness of a workpiece after machining was significantly reduced, and the thickness consistency was good. The results are shown in Fig.5.By analyzing the vibration characteristics of workpiece with paraffin reinforcement,as shown in Fig.6,it can be seen that the vibration amplitude after paraffin reinforcement is significantly lower than that without paraffin reinforcement.Therefore, machining vibration can be effectively suppressed.To further improve the machining effect of thin-walled titanium alloy members in paraffin reinforcement, Gao et al.added a small amount of stearic acid to paraffin. The results show that when stearic acid is added to paraffin wax,the binding force with titanium alloy can be improved. The results of finite element prediction have shown that the machining effect of filling with paraffin containing stearic acid is significantly better than that of pure paraffin.

The low-melting-point alloy has excellent physical properties and can provide great supporting force, which is widely used in practical production. The main components of the low-melting-point alloy are Sn,Pb and Bi,some of which contain elements such as Cd, In, Zn, and Hg. The melting points vary from 40°C to 200°C depending on the chemical composition.Obare et al.used a low-melting-point alloy to support thin-walled samples such as honeycombs, cross bellows, and propellers.Wang et al.used a low-melting-point alloy to support the outer surface of 7075 aluminum alloy with a complicated shape.The experimental results show that the workpiece with the support of a low-melting-point alloy has high dimension accuracy and a low removal rate. As the volume of the low-melting-point alloy increases during solidification, it may lead to the deformation of parts. To solve this problem,Saito et al.proposed an improved method to support turbine blades by combining a low-melting-point alloy with an elastomer, as shown in Fig. 7. The experimental results show that using a low-melting-point alloy with elastomer to support turbine blades can ensure machining stability and improve machining accuracy.

Fig. 6 Vibration spectrum comparison.9

Ice is inexpensive and can provide a large supporting force.The entire machining area is in a low-temperature environment, which can reduce the cutting temperature to improve the life of the machining tool. For honeycomb materials, ice support is also effective for suppressing defects such as burrs as well as improving the quality of the machined surface.However,it is not suitable for paper honeycomb materials.In addition, since the melting point of ice is low, it is necessary to control the machining temperature to prevent the ice from melting and separating from the workpiece, which consumes considerable energy. Paraffin has many advantages, such as a low cost,a low melting point,and convenient recycling. However,compared with other TRMs,its supporting force is small.In addition, under the action of cutting heat, paraffin easily produces odor smoke, which is not environmentally friendly.In addition, low-melting-point alloy in the liquid state has good fluidity, and the phase changes rapidly. However, lowmelting-point alloys have high densities and heavy weights,which may cause large deformations and difficulties in removing the alloy from the workpiece.At the same time,gas or dust generated by alloying elements such as Pb may cause environmental pollution. In addition, the cost of low-melting-point alloy is relatively high. Table 1 summarizes and contrasts several common TRMs based on the introduction above. Each TRM has its own advantages and disadvantages, which need to be selected according to the actual machining situation.

Clearly, common TRMs, such as ice, paraffin, and lowmelting-point alloys, still have shortcomings. To improve the supporting performance of TRM supporting technology,some scholars have tried to find other materials with excellent supporting properties. To improve the performance of the supporting material, Xu et al. used polyethylene glycol as a matrix and added modifiers such as rosin and gypsum to adjust the viscosity, shrinkage, and hardness of the filler material.Liu et al. used polyurethane foam to assist in strengthening titanium alloy thin-walled structures to improve the rigidity of the workpiece. Due to the large damping coefficient and small specific gravity of polyurethane foam, this material can buffer vibration absorption, which can significantly improve the machining stability and the surface quality of the workpiece.

Fig. 7 Supporting method using low-melting-point alloy and elastomer.23

The TRM supporting technology is particularly suitable for the machining of complex thin-walled parts such as honeycomb structures, titanium alloy web structures and engine blades. The machining equipment is simple, the cost is low,and the operation is easy. However, the use of TRM supporting technology requires extra machining processes, which decreases the machining efficiency. In addition, after machining, it is difficult to remove the supporting material that may remain on the surface of the workpiece.Furthermore,this supporting technology consumes a considerable amount of energy and may cause environmental pollution.

2.2. MRF supporting technology

MRF is a kind of intelligent material that consists of small magnetic particles with high magnetic permeability, nonmagnetic liquid and stabilizer. Its physical state can be controlled by the magnetic field.MRF supporting technology is designed based on a magnetorheological effect: under the action of a magnetic field, the MRF can be rapidly converted from a liquid state to a solid state within several milliseconds. When the magnetic field is removed,it can be rapidly converted from the solid state to the liquid state,and the transformation process is reversible. The principle of MRF supporting technology is similar to that of TRM supporting technology and shown in Fig. 8.Initially, the MRF is in a liquid state and can automatically fill the gap entirely based on the shape of the workpiece. During the machining processes, under the action of a magnetic field, the shear strength of the MRF increases sharply so that the MRF obtains a supporting rigidity similar to that of a solid.

MRF was studied as an early supporting material. In the 1940s, Rabinnow in the United States discovered MRF;however, its shear yield stress was small and could not meet the design requirements of fixtures. Afterwards, to study the phase change principle of MRFs, Tang et al. studied the chain-forming mechanism of MRFs under extrusion. Using the extrusion enhancement characteristics of MRF, they obtained MRF with a shear stress close to 800 kPa, which can meet the required supporting force during the machining processes.At present, the application of MRF is mainly based on three working modes: flow mode, shear mode, and extrusion mode, as shown in Fig. 9.MRF is widely used in many engineering fields such as shock absorbers,dampers,precision polishing,etc. At an early stage, Liu et al. successfully used MRF to support thin-walled parts.Under a magnetic field, the MRF instantaneously solidifies to support the valve disc to offset part of the machining deformation and improve the machining accuracy.Afterwards,some scholars improved the MRF supporting device for different parts and studied the damping characteristics of the MRF support. MRFs have been applied and developed in the support of thin-walled parts due to their advantages, such as a short phase change time, a controllable morphology, and no pollution.

The MRF supporting technology controls the supporting force by the magnetic field intensity. The required magnetic field intensity can be reversed according to the milling force to meet the support requirements. Liu et al. proposed the derivation formula of the magnetic field intensity as follows:

Table 1 Comparison of several TRMs.

Fig. 8 MRF supporting principle.

Fig. 9 Basic operational modes for controllable MRF devices.30

In Eqs. (1) and (2), Fis the milling force, τis the yield stress, Ris the cutter radius, h is the thickness of the MRF,η is the viscosity coefficient, v is the extrusion speed, Cis the magnetorheological coefficient related to the liquid medium, φ is the volume fraction of the magnetic particles, tanh is the hyperbolic tangent function, and H is the magnetic field intensity.

MRF supporting technology, which is a relatively new method,is not well developed.The research of this supporting technology mainly focuses on the supporting device and the optimized design of the magnetic field. In addition, the damping characteristics of MRFs can effectively suppress machining chatter, which has attracted the attention of some scholars.

Designing MRF fixtures to support thin-walled parts can increase the rigidity and improve the machining quality of the workpieces. Xiao et al. designed an MRF fixture for auxiliary milling of thin-walled cylindrical parts. Due to the large clamping area, the force on the workpiece is uniform, which can reduce the machining deformation and improve the machining accuracy.For a thin-walled spherical shell with a wall thickness of less than 1 mm, Kong et al. designed a smart fixture based on MRF to increase the rigidity of a workpiece.Liu et al. found that when the working state of the MRF is in squeeze mode, its elastic modulus can be increased by 1 to 2 orders of magnitude compared to the shear mode,which can provide greater supporting force.In MRF supporting technology, the design of the magnetic field is crucial.Based on electromagnetic principles, Kong et al. designed a special magnetic field and optimized the magnetic field distribution.Ma et al. analyzed the MRF fixture by finite element method (FEM) to obtain the relationship among the current, the number of coils turns, the magnetic field, and the rheological state of the MRF, which provided theoretical support for optimizing supporting structural.Wang et al. proposed a novel magnetic source connected to the machine tool spindle.The magnetic source generates a specific magnetic field in the machining area,where it has the strongest MRF supporting ability.MRF has excellent damping characteristics and can effectively suppress chatter. Ma et al. studied the displacement response of the thin-walled plate at the selected point in the two cases of damped and undamped control. The results show that MRF can provide enough additional damping for a machining system to suppress mechanical vibration.Jiang et al. analyzed the effects of MRF volume, magnetic field intensity, and location on vibration suppression.

At present, only a few scholars have proposed the MRF supporting method, which is seldom used in the actual manufacturing of thin-walled parts. However, MRF has broad application prospects due to its good adaptability and controllability and can increase the damping of the workpiece to reduce the machining vibration. MRF supporting technology has the following advantages:1)the solid–liquid phase change time is short,which can be completed on the order of milliseconds;2)the rigidity of the MRF support can be controlled by the magnetic field,and the controllability of the magnetic field is strong; 3) the MRF can phase change at room temperature and will not generate heat;4)the MRF can be reused to reduce costs; and 5) neither environmental pollution nor surface workpiece debris will be produced. 6) MRF can increase the damping of the workpiece and suppress the vibration of the workpiece. The advantages of MRF supporting technology are obvious. However, MRF cannot bear large loads due to the limitation of the magnetic field intensity. In addition, the design and control of the magnetic field is complicated.

3. Multi-point discrete flexible supporting technology

At present, for large thin-walled parts, entirely self-adaptive flexible supporting technology corresponds to a low supporting rigidity and a complex machining process, which cannot meet machining precision and efficiency requirements. Therefore,multi-point discrete flexible supporting technology is proposed to machine large thin-walled parts.

The basic idea of multi-point discrete flexible supporting is to divide the supporting surface of the traditional supporting tooling system into several supporting points to fit the complex curved workpiece surface. The principle is shown in Fig. 10.Each supporting head is equivalent to an independent flexible supporting unit,which is mainly composed of a lifting mechanism and vacuum adsorption device.According to the contour shape of the workpiece,the supporting head can be moved to a predetermined height by a cylinder or a ball screw.To adapt to the curved workpiece,the vacuum adsorption device is usually connected by a universal ball, which can achieve a large angle of rotation and adaptively change the direction of contact with the workpiece.Finally,the supporting head and the workpiece can be tightly adsorbed by the vacuum adsorption device to support the workpiece.

Fig. 10 Principle of multi-point discrete flexible supporting.46

To improve the machining accuracy of large thin-walled parts and reduce the number of molds, many European and American machine tool manufacturing companies started developing flexible tooling systems in the late 1980s. The supporting head of the flexible tooling system should be able to move flexibly to meet the machining requirements of workpieces with different curved surfaces. The supporting head of the flexible tooling system based on the POGO unit developed by CAN Manufacturing Systems Inc in the United States can realize telescopic movement in the Z direction to adjust the height according to the shape of the workpiece. To increase the flexibility of the supporting head,the TORRESTOOL flexible tooling system developed by Martinez in Spain adopts a bent structure,and each supporting head is independently controlled by a motor, which can realize movement in the X,Y,and Z directions.Their flexible tool system can effectively improve the rigidity of the workpiece and has been widely used in the manufacture of large thin-walled parts by aircraft manufacturers such as Boeing and Airbus.However, since the supporting heads are separately driven and controlled,the volume of each supporting unit is large, limiting the increase in the number of supporting heads. To simplify the structure of the flexible tooling system, Zhan et al. developed a set of flexible tooling systems driven by robots.As the volume of each support unit decreases,the density of the supporting heads can be increased to increase the rigidity of the workpiece and reduce the machining deformation. Since the proposal of flexible tooling systems by several institutions,multi-point discrete flexible supporting technology has developed rapidly and has been widely used in actual manufacturing.

The research on multi-point discrete flexible supporting technology is mainly focused on the development of multipoint flexible supporting systems, theoretical research and the use of the FEM to reduce machining deformation by optimizing the distribution of the supporting points.In addition,a few scholars have introduced intelligent algorithms to optimize these supporting positions and achieve good results.

The theoretical research of multi-point discrete flexible supporting technology is the premise of flexible tooling system development.Zhou et al.conducted research on key technologies such as multi-point positioning theory, supporting algorithms and synchronous motion control.For a curved thin-walled part,the position of the supporting head is adjusted according to the profile of the workpiece.The NURBS surface equation is usually used to represent the contour of a thinwalled workpiece surface. The p×q order NURBS surface equation containing m×n control vertices is as follows:

In Eq. (3), Pis the control vertex, forming a control network in double directions,and wis the corresponding weight of the control vertex. The B-spline basic functions N(u) and N(v)along the direction of the knot vectors can be written as recursive formulas:

Based on the geometry of the workpiece,the position coordinates of the target supporting point required on the workpiece surface are obtained using the NURBS curve equation.Because the supporting ball in the vacuum head is tangent to the supporting surface of the target point during the machining processes, the height of the supporting ball can be written as:

In Eq.(5),ris the radius of the supporting ball,and n(u,v)is the unit normal vector of the NURBS surface at the tangent point.

The number, position, and clamping force of the supporting heads in the multi-point discrete flexible supporting technology directly affect the rigidity of the machining area,thereby affecting the machining deformation.Many scholars used FEM to analyze the deformation of the workpiece and optimized the number,position,and clamping force of the supporting points to reduce machining deformation.Miao studied the deformation of a rectangular thin metal plate with a multipoint flexible support under its own weight with the finite element software Aabqus.Hu proposed a quantitative evaluation method for the dynamic rigidity of the flexible fixture system by using the FEM, which provided a reference for the quantitative analysis of the multi-point supporting position layout.Liu et al. provided a multi-point positioning optimization method based on FEM to determine the number,position, and clamping force of supporting heads, as shown in Fig. 11.However, when using the FEM to design the fixture layout for large thin-walled parts,hundreds or even thousands of finite element analyses are needed to determine the optimal location of the positioning point, which takes considerable time and has a great impact on the product manufacturing cycle.

With the development of computer technology and artificial intelligence, some intelligent algorithms, such as neural network algorithms, genetic algorithms, and ant colony algorithms, have shown good comprehensive optimization capabilities and have high efficiency in solving complex problems.Some scholars have combined intelligent algorithms with FEM to optimize the layout of multi-point supporting fixtures.Li et al. proposed a global optimization algorithm combining the FEM and a genetic algorithm.The optimization process is shown in Fig.12.Yu et al.used a genetic algorithm to optimize the supporting layout globally. The experimental results show that the maximum deformation and average deformation of the multi-point supporting layout optimized by the genetic algorithm are lower than those of a uniform layout.Based on the geometry of the workpiece, Do et al. used the genetic algorithm method to find the optimal position of a workpiece relative to the fixture system to maximize the workpiece supporting capacity within 30 seconds, as shown in Fig. 13.Zhou et al. proposed a fast design method for fixture layout based on hybrid particle swarm optimization. Compared with the uniform distribution method and iterative FEM, this method has a higher efficiency while maintaining accuracy.

Fig. 11 Optimization of number and locations of supporting heads.56

The multi-point discrete flexible support was introduced as an early supporting technology and is well developed. Many companies have developed corresponding supporting equipment, which has been widely used in actual aerospace manufacturing. The multi-point discrete flexible supporting technology can adjust the position of the supporting point according to the actual contour of the workpiece in a flexible way and has strong adaptability, allowing this approach to be used for machining thin-walled parts with different curvatures. The multi-point discrete flexible supporting technology uses a computer combined with a variety of sensors to control the supporting position. It has high positioning accuracy and automation. However, there is a gap between the supporting heads, which may lead to deformation or even chatter during the machining processes. Therefore, the multi-point discrete flexible support technology has medium machining efficiency and precision.In addition,limited by the supporting structure,only particular parts such as simple planes or curved surfaces can be supported.Furthermore,multi-point supporting equipment is expensive and only suitable for the machining of large thin-walled parts.

4. Symmetrical follow-up flexible supporting technology

Because the suspended area between the discrete supporting heads is prone to elastic deformation, the multi-point discrete flexible supporting technology cannot achieve precise control of the workpiece thickness. Symmetrical follow-up flexible supporting technology, which can ensure that the supporting head moves synchronously with the tool to support the workpiece and compensate for machining errors in real time, provides a new way to machine thin-walled parts. Symmetrical follow-up flexible supporting technology can be divided into mirror milling supporting technology and jet supporting technology according to the characteristics of the supporting head.

4.1. Mirror milling supporting technology

The principle of mirror milling supporting technology is shown in Fig. 14.The milling head, which is equipped with a tool,performs machining on one side of the workpiece, while the supporting head moves synchronously with the milling head on the other side of the workpiece for support.The supporting head improves the rigidity of the machining area and offsets the milling force, thereby reducing machining deformation of the workpiece.

Fig. 12 Optimization procedure combining the FEM and a genetic algorithm.58

Fig. 13 Results of optimal supporting locations.60

For the machining of large thin-walled parts, Dufieux Industrie in France first developed a new generation of skin mirror milling system. To achieve efficient and accurate machining of thin-walled parts, the supporting head and the milling head always maintain a mirror-symmetric relationship to offset machining distortion.Afterwards, M.Torres developed its own mirror milling system,which was successfully put into use in the manufacturing aircraft fuselage by Airbus.To improve the machining efficiency of cylindrical thin-walled parts, Shanghai TOP Numerical Control Technology Co.,Ltd., independently developed a multihead mirror milling device that can machine several areas in rocket fuel storage tanks at the same time.Xiao et al. used a dual-robot structure to implement mirror milling support.In addition,some scholars have tried to simplify the support structure to reduce the cost of mirror milling. Mahmud simplified the supporting device and reduced equipment costs by using a magnetically adsorbed supporting head to move with the milling head at the same time.In addition, to eliminate the use of two robots, Rasoulimir proposed a C-clamp end effector to complete the task of milling large flexible panels with complex shapes and double curvature.Mirror milling technology is the key to the efficient, accurate and green machining of large thin-walled parts, and it is gradually replacing traditional skin chemical milling and computer numerical control precision milling supporting technologies.

Fig. 14 Principle of mirror milling support.61

At present, research on mirror milling supporting technology is mainly focused on the design of supporting devices,wall thickness error compensation and chatter suppression technology.

The design of the supporting device is the key to the mirror milling supporting technology, which directly affects the machining accuracy.At present,the mirror milling supporting head can be categorized into three types of head: sliding supporting head, rolling supporting head, and hydrostatic supporting head. The sliding supporting head generally supports the workpiece with a flat plate and has a sufficient rigidity but lower hardness than the workpiece. To reduce the scratches caused by sliding friction between the supporting plate and the workpiece, Wang et al. adhered cotton fabric to the surface of the hard supporting plate. However, this method inevitably reduces the stiffness of the supporting head,causing dimensional errors of the workpiece. In addition, the sliding supporting head cannot fully fit a complex curved surface.Rolling supporting heads are widely used in mirror milling. Li et al. used a single-point rigid supporting head to support a workpiece.Due to the high rigidity of the supporting head,the deformation of the workpiece was reduced compared to that without supporting technology.Wang et al. invented a multi-point flexible rolling supporting head, which is connected by spring to absorb the vibration of the workpiece for the purpose of suppressing chatter.Due to the fixed rigidity of the spring, Xiao et al. proposed a multi-point flexible follow-up supporting head, which can adjust the supporting force during mirror milling by adjusting the cylinder pressure to change the damping of the system.For the rolling supporting head, the number and position of the support points affect the stiffness of the workpiece in the support area. Bao et al. studied the influence of the number and position of support points on the deformation of the workpiece. The results show in the Fig. 15 that when the distribution of support points allows the change in the stiffness of the workpiece to be consistent with the change in the milling force,the thickness of the workpiece is well maintained.To achieve scratchfree support,Li et al.proposed a hydrostatic supporting head.Because there is a layer of liquid film between the supporting head and the workpiece,the supporting head does not directly contact the workpiece, which effectively solves the problem of surface scratches in the process of mirror milling.

The remaining wall thickness of the large thin-walled parts is critical for determining the weight and strength of the part.However, the cutting deformation may greatly decrease the accuracy of the remaining wall thickness and needs to be compensated.Xiao et al. invented a mirror milling supporting head with a real-time thickness measurement function, which used an ultrasonic thickness measurement probe to measure the wall thickness of thin-walled parts in real time.However,the online compensation method that directly compensates for measurement wall thickness errors by adjusting the tool path has a time-lag problem.Wang et al. proposed a fast convergence algorithm for iteratively adjusting the cutting depth based on the secant line method and introduced it into the real-time error compensation.Zhang proposed a real-time thickness compensation strategy based on a modified Smith predictor (MSP) and disturbance observer (DOB), as shown in Fig.16.Bi et al.adopted a compensation that decomposes the wall thickness error into spatial error and temporal error,which can ensure that the final wall thickness error is within 0.05 mm.

For the machining of thin-walled parts, chatter has limited the improvement in productivity and part quality.Bo et al.studied the influence of the supporting force on the stability of mirror milling. During the mirror milling process, multiple springs,masses and dampers were used to simulate the impact of the supporting head on the workpiece, as shown in Fig.17,and the dynamic model of the three-degree-of-freedom system can be written as follows:

Fig. 15 Measured surface profiles of workpiece at support coordinates.61,73

Fig. 16 Overview of the online measurement and compensation system.62

Through finite element modal analysis of the mirror milling process,Wang et al.obtained the modal vibration mode results at different machining positions and predicted the stability of different machining positions, as show in Fig. 18.Liu et al.proposed a new chatter index Q factor to identify signal components related to chatter and quantify chatter levels.Based on the Q factor and support vector machine,Wang et al.developed a chatter recognition method to realize the diagnosis of the machining state.The Q factor can be expressed as follows:

Fig. 17 Machining dynamic model of thin-walled parts.81

In Eq.(7),fis the center frequency,Bwis the bandwidth of an impulse signal, n is the number of signals, and m is the number of spectra.

Fig. 18 Stability lobes and machining parameters selection.82

Mirror milling supporting technology is a key technology for the accurate and efficient machining of large thin-walled parts and has been widely used by airline companies.The mirror milling supporting technology has the following advantages: 1) High degree of integration. The supporting device integrates online thickness measurement, position feedback,and chatter detection functions,which can ensure dimensional accuracy in the thickness direction of the workpiece, suppress chatter, and improve surface quality. 2) High machining efficiency. Mirror milling supporting technology can realize a variety of machining procedures, such as trimming,hole-making, and pocketing of skin parts, simplifying the machining process of parts, shortening the machining cycle and improving production efficiency. 3) Green and environmental protection.With the new generation of green manufacturing technology, no environmental pollution problem arises from the machining process.4)High machining accuracy.Mirror milling supporting technology can complete multiple machining procedures at one time, reduce the clamping error of the workpiece and improve machining accuracy. In addition,advanced error compensation strategies are used to compensate for wall thickness errors according to the accurate measurement of the wall thickness of the workpiece using a variety of sensors. However, mirror milling equipment is relatively large,priced on the order of 10 million U.S.dollars,and it is difficult to control the synchronous movement of two fiveaxis machine tools. In addition, the use of a rigid supporting head is likely to result in scratches on the workpiece surface.Moreover, limited by the supporting structure, mirror milling is only applicable to the machining of large-scale thin-walled complex curved surfaces such as aircraft skin.

4.2. Jet supporting technology

The rolling supporting head usually used in mirror milling supporting technology can easily scratch the surface of the workpiece to reduce the surface quality. To solve this problem, jet supporting technology using a water or air jet instead of a rigid rolling supporting head is proposed.

Jet support is also a symmetrical follow-up flexible supporting technology. Its principle is shown in Fig. 19.The jet device is used to generate a jet impact force equal to the milling force in the opposite direction to offset the milling force during the machining process and suppress the vibration of the process system. It is an ideal supporting method to decrease machining deformation and improve machining accuracy.

Water jet technology was first proposed in the early 19th century and has been mainly used in the fields of coal mining,oil drilling, and industrial cutting. Many scholars have conducted in-depth research on the flow field characteristics of water jets,laying a theoretical foundation for the development of jet supporting technology. Ye et al. applied water jet technology to support thin-walled parts by using a pulsed water jet device to generate a jet impact force to offset the deformation of the workpiece, and the feasibility of this method was verified experimentally.Thongkaew et al. established an abrasive waterjet impact model using CFD analysis method to study the effect of stagnation on the jet and particle impact characteristics.Later, Liu used an air jet instead of a water jet to support thin-walled structural parts of titanium alloys,which has widened the range of jet support.

Fig. 19 Principle of jet supporting technology.86

At present, jet supporting technology is still in its infancy,mainly focusing on the analysis of the characteristics of the jet flow field and the effect of jet supporting technology on machining thin-walled parts.The analysis of the characteristics of the jet flow field is the premise of designing and optimizing the jet supporting device. Many scholars have conducted a considerable amount of research on this topic.Without considering the air resistance,the impact force of the water jet generated by the nozzle perpendicular to the target plane is as follows:

Leu et al.divided the jet into different regions by analyzing the structure and flow field characteristics of the water jet.The jet medium is an important parameter of jet supporting technology and affects the supporting effect. Liu et al.studied the impact pressure distribution of high-pressure pure water and high-pressure abrasive jets against rigid walls. The results show that the addition of an abrasive has a significant strengthening effect on the impact pressure of the jet.In addition,the nozzle is the crucial component to realize jet supporting technology,the structural parameters of which directly determine the performance of the jet support.Wen et al.studied the effects of different nozzle shapes on jet impingement through computational fluid dynamics simulations and experiments.

During the milling process, the impact force of the jet is used to provide an active supporting force for the workpiece to reduce machining deformation by offsetting the milling force, which is an effective supporting method. Xu et al. proposed a water jet supporting method for the machining of thin-walled parts, considering the dimensional error caused by milling force.The precise control of the jet impact force can offset the deformation caused by the milling force. Zhou proposed a water jet-assisted supporting method for thinwalled parts to increase the process rigidity of the machining system and decrease the effect on the surface quality generated by vibration.The machining error after machining is shown in Fig. 20. Liu et al. used an air jet to process titanium alloy thin-walled parts. The experimental setup is shown in Fig. 21.By analyzing the vibration acceleration and milling force signals under different cutting parameters, the results show that the air jet-assisted supporting method can improve stability during the milling process,thereby improving the surface quality and reducing the deformation of the workpiece.

Jet supporting technology is a novel support method that can achieve scratch-free machining and improve the surface quality of the workpiece,but its supporting equipment and relevant theory still have flaws.The advantages of jet support are as follows:1)The impact force of a liquid or gas is used to support the workpiece without inducing mechanical damage such as scratches.2)The media of the jet has the functions of cooling, lubricating, and cleaning, which can reduce the cutting temperature.3)Controlling the jet pulse frequency can reduce the vibration of the system. Jet supporting technology is a potential supporting method for milling, but how to control the jet supporting force to offset the milling force is a difficult problem since the milling force changes throughout the milling process.In addition,due to the addition of a hydraulic system,which needs to be controlled to coordinate with the computer numerical control machine tool system,the control difficulty is increased. Due to the imperfect theory and supporting equipment of jet supporting technology,it is still in the experimental stage,and its machining efficiency and accuracy are low,which need to be further developed.

5. Summary and development trends

This article outlines the latest advances over the past ten years in flexible supporting technology for manufacturing aerospace thin-walled parts. After decades of development, a variety of flexible supporting technologies have been proposed and continuously improved for different machining needs, which have been widely used in practical applications. The above flexible supporting methods are summarized and compared in Table 2 in terms of their principles, advantages, applications, and limitations.

Although flexible supporting technology has been widely used in the manufacturing of aerospace thin-walled parts,low rigidity, workpiece deformation, and vibration are still the main problems in manufacturing.For different supporting technologies, to improve the machining quality of airspace thin-walled parts, some challenges remain. For entirely selfadaptive supporting technology, regardless of whether TRM or MRF is used,the rigidity of the solidified supporting material is low. Although the vibration absorption effect is good,when the cutting force is large,the workpiece will still produce large deformation, resulting in low machining accuracy. For multi-point discrete flexible supporting technology, the supporting rigidity of the area near the supporting head is high,but the supporting rigidity of the suspended area between the supporting heads is low, so deformation and chatter are easily produced when machining the suspended area.For mirror milling supporting technology,the supporting head is usually a plurality of rolling metallic sphere, and its supporting area is smaller than the machining area, resulting in deformation during the machining process. In addition, when the processing parameters are not selected properly, chatter easily occurs.For jet supporting technology,the jet impact force cannot change synchronously with the milling force, and machining deformation will inevitably appear. In the above flexible supporting technologies, the deformation of the workpiece is inevitable, which will cause thickness error and restrict the improvement in the dimensional accuracy. At the same time,during the machining process, vibration will affect the surface quality of the workpiece. Therefore, precision thickness control technology and chatter suppression technology are the main challenges in manufacturing thin-walled parts.

Fig. 20 Comparison of machining errors of thin-walled parts with or without jet support.86

To meet the increasingly strict processing requirements of thin-walled parts,many aspects of each supporting technology need to be further explored. The future research trends are as follows:

(1) At present,the main problems of TRM supporting technology are low machining efficiency, low machining accuracy, and environmental pollution. Therefore, high efficiency and green manufacturing have become the future development trend by adding other substances or looking for other high-performance TRMs to improve the machining accuracy of parts. In addition,the properties of different materials such as stiffness and damping should be studied systematically to improve the machining quality.

(2) The core of MRF supporting technology is the optimal layout and control of the magnetic field.The structure of the magnetic source needs to be further optimized. The influence of the magnetic field on the supporting rigidity needs to be studied further. At the same time, the magnetic field intensity can be adjusted to adapt to different machining conditions.In addition,the supporting rigidity and damping characteristics of MRF are affected by the properties of the magnetorheological material,so the influences of the particle size, particle shape, and additives on the performance of MRFs need to be studied to produce MRFs that can meet the application requirements.

(3) The multi-point discrete flexible supporting system design involves many parameters, such as the number of supporting heads, the position of the supporting head, and the suction pressure of the suction cup. This multi-objective optimization process is complicated.Using FEM combined with intelligent algorithms to optimize multi-point flexible support with multiple targets has broad prospects for development. In addition,a multi-point discrete flexible supporting system can integrate a variety of measuring devices to obtain machining information online, such as the thickness,deformation, and vibration signals of the workpiece,automatically adjust machining parameters, and improve machining accuracy.

Fig. 21 Schematic view of experimental setup.89

Table 2 Comparison of flexible supporting technologies.

(4) Before mirror milling, it is necessary to use laser scanning to obtain the actual profile of the skin because there is still a large deviation between the skin blank and the design model. However, to improve the efficiency of scanning and data processing, the actual scanning position is limited, resulting in an error in the supporting head position.The supporting head with a surface adaptive function is the future development trend.Moreover,to precisely control the thickness of the skin, the mirror milling supporting head should have the functions of real-time online thickness measurement,feedback,automatic compensation,etc.One of the main factors affecting the machining quality of thin-walled parts is cutting chatter.The mirror milling supporting head should also have the function of suppressing chatter. In the future,the mirror milling supporting head will be developed into a support system with multiple functions.

(5) For jet supporting technology, since the milling force changes periodically, the actual cutting process is more complicated. To ensure that the jet impact force can counteract the milling force, it is necessary to model the milling process under jet support, optimize the process parameters, and achieve matching between the milling parameters and the jet hydraulic system parameters to reduce the machining error. In addition, it will be an important research direction to develop a jet supporting device that can quickly respond to the change in milling force and that has the function of compensation.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This research is supported by National Natural Science Foundation of China(No.51975096,No.51905075),China Postdoctoral Science Foundation (No. 2019M661090) and LiaoNing Revitalization Talents Program(No.XLYC1807230).