关于外国先进机床介绍的英语文献

Int J Adv Manuf Technol (2010) 50:843-857 DOI10.1007/s00170-010-2559-9ORIGINAL ARTICLEA review of recent developments in the design ofspecial-purpose machine tools with a view to identi^cation of solutions for portable in situ machining systemsJohn Allen • Dragos Axinte • Paul Roberts •Ralph AndersonReceived: 23 October 2009/Accepted: 31 January 2010/Published online: 21 February 2010 ©Springer-Verlag London Limited 2010Abstract With the ever increasing diversity ofmanufacturing and maintenance needs for modern products, the realm of machine tool design has expanded considerably in recent years. While machine tools for producing singular parts are widely documented, there is much less information available about machine tools for specialist tasks such as repair and maintenance. The paper aims to give an overview of the developments in the field of specialist machine tools and machining systems with a particular emphasis on maintenance operations and in situ machining. The difficulties of performing maintenance on large-scale systems are outlined along with the potential benefits of in situ machining in these applications. A number of examples of specialist machines for applications in various fields are described. The developing area of micro machine tools and micro factories is introduced along with examples of systems which have been developed or proposed. The possible advantages and draw-backs of the various technologies described are discussed. The paper goes on to comment on the potential for a new approach which combines these technologies to produce small in situ machine tools for performing macro-scale machining operations on large workpieces. These machining systems could offer increased versatility of in situ maintenance machining, compared to machines designed for a single repair operation, while reducing costs and environmental impacts.J. Allen • D. Axinte (*)Department of Mechanical,Materials and Manufacturing Engineering,University of Nottingham,Nottingham, UKe-mail: dragos.axinte@P. Roberts • R. AndersonRolls-Royce,London, UKKeywords In situ machine tools . Serial kinematics machines . Parallel kinematics machines .Miniature machine tools1IntroductionThe field of machine tools for generating singular products is well documented; however, the area of specialist machines for dedicated tasks such as repair/maintenance has received less attention. This paper explores the area of specialist machine tools with a focus toward in situ machining operations for repair and maintenance. In general, repair and maintenance machining has been carried out using large conventional machine tools which are, by necessity, installed in dedicated machine shops. This can pose problems, especially for work on large and potentially complex systems such as aircraft, ships, power stations, metal fabrication mills, heavy machinery, etc. In these cases, components which require re-conditioning work must be dismantled from the system and shipped to a machine shop for processing before they are returned and re-assembled into the system. This process is time-consuming, complex, potentially dangerous and expensive; in the case of very large systems (e.g. power stations), this type of maintenance may not be possible at all. Furthermore, this conventional maintenance process can result in prolonged downtime of the system. For these reasons, in situ machine tools which can be brought and attached to the component to be machined, while that component is still more or less installed, can offer obvious advantages. To address this niche domain of specialised machine tools, an overview of the development of such systems is presented with examples having applications in a range of industries. In addition, some other developments in specialist machining systems are discussed.The developing area of miniature or micro machine tools is also investigated. The motivation and philosophy which led to their development is explored, and a number of example machines are described. The concept of integrating several of these machines into micro factories is also introduced and some examples which have been produced or proposed are presented.A selection of specialist miniature in situ sampling and inspection machines are introduced. The potential advantages and disadvantages of these various machining systems are discussed.Finally, the potential for a new type of machine tool which integrates ideas from across these fields is suggested. The proposed concept is for a miniature in situ maintenance machine which uses micro scale systems to perform a range of operations on workpieces, potentially much larger than the machine, to which it can be temporarily attached. This concept should offer greater versatility than the present solutions, which are generally designed around a specific maintenance task.2Special-purpose macro machine toolsAt present, most manufacturing and maintenance work are undertaken using large-scale machine tools, generally having overall dimensions measuring in metres and using high-power spindles. These systems have their disadvantages, especially in the maintenance of large structures and systems such as aircraft, ships, power station plant, heavy industry and other heavy machinery; in these cases, the process of removing components and transporting them to a machine shop for repair is time-consuming, complex and expensive. There has been significant development, but little reported, in the field of in situ machine tools, which can be temporarily attached to large workpieces for assembly and maintenance tasks. These systems offer an alternative approach in which the machine can be brought to the component requiring machining, thus eliminating the need to disassemble it from the system in which it is installed. This approach should reduce downtime and maintenance costs. In the field of manufacturing, these machines offer more flexible solutions with the ability to work on different parts of a large structure. Some examples ofthese machines with applications in the aircraft, marine and power generation industries are discussed, along with some other specialist machine designs which can offer more flexible machining capabilities.2.1Serial kinematics machinesSerial kinematics machines (SKM) are those of more traditional design in which motion axes are generated by independent slides or stages stacked one on top of the other to provide multi-axis movement.2.1.1SKM for manufacturingA number of in situ machines have been developed for aircraft construction, two of these are given as examples. One machine devised for working on aircraft structures and other large workpieces comprises a series of index devices which can be temporarily mounted at known positions on the workpiece and are connected by an index member [1]. A machining device is mounted on the index member and can be moved along it by various means. The machining unit can carry drills, fastening inserters, probes and other devices: it is able to reference its location by a sequence of position indicators spaced along the index member. The position indicators may be optically or magnetically readable strips or may be small transmitters which transmit location data to the machining device. The index devices contain sensors which store information including their location on the workpiece, part geometry, and information on tasks to be performed in that zone of the workpiece. The machining device may also be in communication with an external controller. This system can be implemented in a range of embodiments, one of which is illustrated in Fig. 1.This machine is intended as an alternative to large fixed- base machines for use in continuous-flow production environments. It offers the advantage of a smaller, more compact and less expensive system, as well as making it possible to swap machines when maintenance or calibration are required in order to reduce production downtime. The versatility of this system may be limited by the method used to attach it to the workpiece as well as its inflexible structure.Another example of this type of in situ machine tool is mounted on a flexible rail system for temporary attachment to large and non-uniform workpieces, such as aircraft structures, which can perform a range of machining and manufacturing tasks [2]. Primary and secondary rails (22 and 38, respec-tively—Fig. 2) are set approximately parallel and joined by a spanned or cross rail (50). The cross rail is securely mounted on the primary rail, which serves as the dimensional reference, but is more loosely mounted on the secondary rail so it will not foul if they are not parallel. The tool head (12) is mounted on the cross rail and both are able to move to deliver X and Y axis movement. The tool holder may support additional axes. The system is mounted on an array of spacing pins (26) to keep it a uniform distance from the workpiece, and it is securedby Fig. 1 One embodiment of the in situ manufacturing system for aircraft structures [1]vacuum cups (28). This system is designed for aircraft manufacture, but could be applied to a much wider range of tasks. The system is computer controlled to perform manufacturing tasks with great accuracy; the controller can compensate for the shape of the workpiece when positioning tools if necessary. This machine offers greater flexibility as its use of vacuum clamping makes it able to attach anywhere without being dependent on mounting features such as holes in the workpiece. In addition, the flexible rail structure should enable it to work with a greater range of workpiece geometries.2.1.2SKM for maintenanceAlso in relation to the aerospace industry, a maintenance system has been reported for temporary attachment to the surfaces of aircraft and similar structures, in order to apply positive pressure to them [3]. This maintenance device, Fig. 3, is intended to aid with the application of adhesive composite patches to damaged aircraft skins and similar operations. The system comprises a frame (12) equipped with a number of vacuum cups (17) which are connected by vacuum lines (34) to a vacuum pump (32) mounted in theframe. When the vacuum is activated the force through the cups is sufficient to hold the system in place while it is applying pressure to the surface on which it is mounted by means of an inflatable pressure pad (26). The cups can be fitted with sensors (18) which monitor the vacuum and, through a controller, regulate the operation of the pump to maintain a constant vacuum. A composite patch can be applied to a damaged surface and the positive pressure generator mounted over it. The device can then apply pressure to the patch until the adhesive cures to ensure a good bond and surface finish. This is a highly specialised system designed for a specific maintenance task and so is not very versatile, although the fastening system can be used to attach a variety of manufacturing and repair systems onto large parts with complex geometry such as aircraft, heavy machinery, pressure vessels, etc.A range of specialist machines have been developed for in situ maintenance of marine engines and power systems as it is impractical, or in some cases almost impossible, to disassemble them and remove them from the ship for repair. Several examples of these in situ machines are described below; the first is for maintenance of large piston engines. This machine is designed for in situ re-conditioning of crank shafts and pins [4]. This machine is intended for maintenance of large internal combustion engines such as marine diesels without the time and expense of removing the crank shaft and shipping it to a machine shop. The design comprises a bracket system which temporarily attaches to the crank webs and supports a pair of orbital wheels with the machine tool mounted between them. This system enables the machine tool to move in an orbital fashion around the workpiece while machining. A number of variations on the basic design for machining parts of different sizes and configurations are described. This is a fairly simple design for a specific range of re-conditioning tasks; however, the use of different embodiments of the system for different workpieces could suggest a lack of flexibility and versatility of the design.Other in situ machines for marine applications are intended for working on marine power systems and steam turbine engines. Several specialist in situ machine tools have been developed for maintenance and material sampling operations on large structures such as marine power systems. One of these is a machine for dealing with seized studs in structures including pump and valve assemblies in systems such as marine power plants [5]. This powerful hydraulic machine tool can be mounted onto the workpiece (attachment method not specified) in line with the seized stud and used to drill it out. The machine can also be fitted with thread cutting tools to recover the thread, or in cases where the surrounding material has been damaged, prepare the hole for a thread insert. This machine is designed for work in challenging environments and variations can beFig. 2 An example flexible rail machine tool for temporary attachment to workpieces [2]Fig. 3 Portable system for in situpatching of aircraft skins [3]developed for working on specific components: one form of the system is illustrated in Fig. 4.A range of orbital cutting machines have been developed for cutting welds used to seal parts in high-pressure systems [5]. These machines are designed to cut out welds without damaging surrounding plant and can machine surfaces to the original weld preparation geometry ready for rewelding. One of these machines is illustrated in Fig. 5. The machine has a circular guide rail which allows the milling head to follow the form of the weld to be machined. The milling unit uses a hydraulic spindle driven by a separate hydraulic power pack.For positioning various end effectors in hostile and inaccessible environments different configurations of manip-ulators have been developed [6]. These devices can be used to position sampling systems cutting devices and welders deep inside power plants and similar systems to perform a range of tasks. One of these manipulators (Fig. 6a) takes the form of a hydraulically driven, computer controlled, modular snake-like robotic arm comprising a series of sections with elbow and wrist joints and standard connections. This modular construction enables manipulators to be configured to access a wide range of environments such as reactors or piping systems.Other designs include orbital and linear tracked manipulators, like that illustrated in Fig. 6b, for navigating their way through tube systems to position end effectors at desired locations. These systems have the advantage of being able to reach very inaccessible components buried deep inside systems withminimal need for their disassembly; these manipulators can accommodate small machining/inspection devices at their ends as the repair/maintenance work requires. However, their considerable length and number of joints may reduce their precision and stiffness at the working location, which could limit the operations which can be performed to those producing very low reactive forces.Also with reference to power systems and the like, another system has been devised for boring and facing operations inside large vessels [7]. This system is intended for re-machining of structures such as inlet ports in reactorFig. 4 In situ seized stud removal and thread recovery machine [5]. (A)Machine mount, (B) cutting unit, (C) demonstration workpieceFig. 5 In situ orbital milling machine for weld cutting and preparation of weld surfaces installed on a demonstration workpiece [5]: (A) hydraulic power pack, (B) machining head, (C) workpiecevessels and similar tasks. The boring machine is suspended from a support and positioning structure mounted above the work area, possibly sitting on the top of the vessel. The boring machine (18) is mounted in a cradle-type structure (10) having several outriggers (30, 32) extending from it and serving to brace it in position against the vessel walls. The boring machine can be set on a system of positioning stages (20) in the cradle and can be remotely controlled. The machine is illustrated in Fig. 7. This system should have the potential for good versatility, as different machining units could be mounted in the cradle. The means of mounting this system in the work area has the advantage that no modification or deformation of the workpiece is required, but it may limit the range of environments in which the system can be used.An example of a specialised maintenance machine is presented in the form of a system devised for in situ regrinding of rail bearing surfaces [8]. The machine is intended for maintenance of rails such as slew bearings in heavy earth-moving equipment. The system (Fig. 8) comprises a grinding wheel (22) driven by a motor (24) and held against and driven along the rail to be ground by a system of lead screws (28). The position of the lead screws is dictated by a set of displacement sensors (31) which follow guide rails (29) that define the profile to be ground. The system can be linked to an external controller for CNC operation. This is a highly specialised machining solution for a specific application, it is well suited to this task, but is unlikely to be readily adapted to other in situ machining operations.Several systems have been patented for orbital machining processes [9, 10]. Orbital machining enables tools to be used on holes of differing diameters; as the tool is in partial contact with the workpiece, the process offers significant reductions in torque and cutting forces. This has the inherent advantage of minimising the stiffness requirements, and consequently the mass of the machining system to which it is mounted. Orbital machining can also facilitate improved cooling, chip removal and tool life when compared to conventional drilling. These designs offer good versatility when the tool requires an orbital movement (e.g. spiral milling) by avoiding machining systems with orthogonal axes. These systems can be mounted on robot arms and other positioning structures for use in a wide range of manufacturing and maintenance applications. They are finding particular use for in situ drilling of large composite components, particularly in the aerospace industry, where de-lamination is a concern. The machining versatility, reduced cutting forces and simplicity of this approach could be of particular benefit in in situ machining applications.One of these systems [9] has a motor (28)-driven support (20) which rotates about a machining axis (22): a tool holder (30) is mounted on the support and driven by a second motor (5) to rotate about a tool-positioning axis (32) which is parallel to and offset from the machining axis. When both motors (28, 50) are driven at equal angular velocities, the tool moves in a machining circle of constant diameter; when the motors are driven at different angular velocities the diameter of the machining circle will vary continuously. Although not reported, the spindle which supports the tool is assumed to be offset from the toolpositioning axis in the same way it is offset from the machining axis. This system is illustrated in Fig. 9a.An alternative orbital machining system comprises two cylinders with eccentric holes in their ends [10]. The inner cylinder (24) is positioned in the eccentric hole (34) in the outer cylinder (36). The spindle (12) is mounted in the eccentric hole (26) in the inner cylinder (24); this arrangement is illustrated in Fig. 9b. The two cylinders are fitted with motors (44,50) so that they can rotate independently. The rotation of the outer cylinder provides the orbital motion, and the rotation of the inner cylinder serves to alter the radius of the machining circle.2.2Parallel kinematics machinesThe developments summarised so far have dealt with serial kinematics architectures, in which the motion axes are stacked one on top of the other and operate separately to achieve movement in individual axes. A major disadvantage of such machine structures is their susceptibility to compound errors, as an error in a lower stage will affect the accuracy of those stacked on it. An alternative machine architecture is the parallel kinematics machine (PKM), in which several actuators work in parallel to achieve motion in multiple axes: this approach overcomes the problem ofFig. 6 a Modular under water manipulator for in situ sampling and repair operations; b linear tracked manipulator [6]cylindrical vessel [7]compound errors. At present, parallel kinematics machine tooldevelopment has focused on manufacturing; however, thesemachine designs can also be adapted and miniaturised for in situoperation as their closed-loop structure can make for a compactmachine with a high stiffness-to-mass ratio. Research anddevelopment of PKMs has been reviewed, with a particularfocus on hexapod type machines, from the perspective of theNational Institute for Standards and Technology (NIST) [11].The development of PKM systems arose from the need toincrease speed, precision and flexibility of machine tools to meetthe demands of the modern manufacturing environment. Thefirst and most common design is the Stewart Platform orhexapod, first demonstrated as a machine tool in 1994; and thisis the one on which much of NIST’s research has focused. Figure10a shows a PKM used in this research.It has been found that PKMs can offer a higher stiffness- to-massratio, lower moving mass, higher speeds greateraccuracy and lower structural complexity than conventionalmachine tools. However, the unconventional structures alsopose some difficulties. They use six degrees of freedom toperform multi-axis operations requiring the simultaneousmovement of all six motion axes. This makes for morecomplicated control, tool path planning and error compensationthan are generally encountered in conventional machines. PKMsalso typically display more complex workspace envelopes thanconventional machines. NIST and other organisations areconducting a range of research projects to address these andother challenges associated to PKM technology.Another approach to PKM design is exemplified by theGiddings and Luis Variax, Fig. 10b [12, 13]. This hexapod, thefirst introduced as a machine tool, uses a different architecture tothat tested by NIST, with the workpiece mounted on the fixedplatform between the legs rather than having the hexapodsuspended above the workpiece. This machine was designed andbuilt with a space frame construction and damping systems inthe lower platform to eliminate the need for a heavy machinebase to damp vibrations. This design was intended to make themachine more mobile and quicker to re-locate and set up inflexible manufacturing facilities and could even be used by themilitary to perform machining operations in the field. Thisconcept could be miniaturised to create a stiff, light weight andhighly flexible portable machining system.From the selected examples of macro machine toolsdedicated to in situ machining which have been presented, it canbe observed that their designs are directed toward^-XI-Fig. 8 In situ rail grinding system [8] 省 Springera with eccentric spindles [9]; bwith nested eccentric cylinders [10]fulfilment of a restricted family of tasks. This makes them unsuitable for performing operations in other technical scenarios. Moreover, it should be noted that their use of conventional machining processes with large tool engagement necessitates bulky machine structures to provide the stiffness required to cope with the high cutting forces which result from such processing.3 Miniature and micro machine tools for special-purpose applicationsIn addition to the developments in the established field of conventional machine tools, a recent area of research and development is that of specialised miniaturised or micro machine tools, which have overall dimensions measuring in hundreds of millimetres or less and use compact low-power spindles. These systems can be considered as special- purpose machines as they mainly address high-precisionmachining of micro scale products. This section gives an overview of micro machine development and discusses some examples of micro machines and micro factory systems which have been developed with a view to highlighting a proposed approach to special-purpose machine tools for maintenance/repair work. These machines can be divided into two broad categories, those which are miniature versions of conventional machine tools and those which are designed around the task to be performed without holding onto traditional machine architectures.3.1 Miniature machines based on traditional (SKM) design In recent years, there has been a trend in many industries toward miniaturisation, most notably in the fields of electronics, medicine and aerospace [14-16]. It was observed that while the parts and tolerances have become smaller, the machine tools have not. Machines with greater accuracy and precision have been developed to fabricateFig. 10 Parallel kinematics machine tools; a PKM used at NIST [11, 28]; b Giddings andLuis Variax [13]micro scale products, but they still require considerable amounts of energy and factory space.To address this, there is a growing school of thought that as parts and products become smaller so should the machines which produce them. One well-developed solution is micro-electro-mechanical systems, this technology can produce micro structures on mass and at low cost. However, they are limited in terms of geometry and materials which can be processed and are consequently of little interest to the present investigation.The area undergoing considerable research is that of micro machines and micro factories; the aim being to produce miniature versions of conventional machine tools as a compact and cost effective means of fabricating threedimensional micro structures. These machines have reduced material, power and space requirements; while their use of high-speed spindles with micro cutting tools reduces cutting forces and torque. This makes for a lower machine mass and inertia allowing higher speed and acceleration. This approachcan offer the advantages ofreduced noise and vibration, easier waste and pollution management, and reduced susceptibility to vibration and thermal deformation. Additional benefits can include reduced capital cost and improved flexibility as these machines do not require a traditional workshop environment. A number of these micro machines could be integrated along with miniaturised part handling and assembly systems to create micro factories; with the potential to offer increased productivity and the ability to manufacture in a variety of locations including vehicles. It is this realm ofmicro machines which will be explored in this section with a focus on special- purpose machine tools and practical machine portability.It has been suggested that several successive generations of micro machine tools could be produced [14]: the first generation produced by conventional macro machine tools, and having overall dimensions in the hundreds of millimetres. These could be used to produce a second generationof smaller machines, and so on, theoretically culminating in machine tools measuring a few millimetres. However, in later generations, the machines’ dimensions can become so small that interaction with other equipment and users becomes impractical. Some prototype first-generation micro machines are described.A prototype PC controlled three- axis machine (Fig. 11) has overall dimensions of 12〇x 160x85 mm and workspace envelopes of 20x20x35 mm. The use of low-cost components is suggested, and the required precision should be achievable as a result of miniaturisation, smaller systems should have smaller errors, combined with adaptive control algorithms.Some of the difficulties of working with micro machines are outlined, in particular the need for greater automation of machine support activities. As parts and tools become smaller, loading and unloading of workpieces by hand along with manual tool changing are no longer feasible. In addition, more advancedmachine vision systems and other process and condition monitoring apparatus are required to monitor and inspect work on such a small scale.A three-axis micro lathe with overall size of 200 mm, illustrated in Fig. 12, has been developed and tested in Japan [17]. The machine could have been made smaller, but the larger size was selected to make it easier to handle by a human operator; however, a manipulator was implemented to handle the micro scale workpieces. This machine uses a single-point diamond micro cutting tool to machine brass and other materials. The machine is intended for use under a microscope, and is designed to keep all operations within its field of view.An example of a high-precision three-axis meso scale milling machine having overall dimensions (200x300x 200 mm) and a working envelope 20 mm3 has been reported [18]. The machine is equipped with a 60,000 rpm air spindle. This machine is arranged with the spindle mounted horizontally and the XY positioning system is mounted vertically in front of it. An air bearing counterweight is fitted to the Y axis to counter the force of gravity acting on it; the machine is illustrated in Fig. 13. The positioning stages use voice coil-type drives in X and Y, and permanent magnet linear motors with air bearings in Z; which, along with the high-speed spindle make the system capable of high-speed machining (HSM). HSM is a cutting method combining small cut depths with high feed rates to remove material quickly with minimal cutting force or heat generation. The tool clamping in this machine is achieved by a shape memory alloy holder which opens and closes in response to changing temperature. This machine was designed primarily as a research tool to explore the capability of micro linear motor stages and shape memory tool-clamping system. These are simple and compact systems with potential benefits for other compact highspeed machining systems.Another prototype micro machining centre has been developed and tested at the Georgia Institute of Technology [19]. This machine has a gantry architecture with a frame made from Invar 36 steel, this structure gives the machine overall dimensions of (350x240x320 mm). The machine uses a brushless DC motor powered spindle with ceramic ball bearings, capable of speeds up to 60,000 rpm, although faster versions are available. The positioning system comprised four axes, three linear and one rotational, although only the three linear were used in the tests described. The stages are driven by linear motors. The vision system for this machine is based on a basic video camera mounted on the machine frame with the ability tosupport a range of lenses with different magnifications. The。