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。

2 Special-purpose macro machine tools

At 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 of these 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。1 Serial kinematics machines

Serial 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。1 SKM for manufacturing

A 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,