3 Miniature and micro machine tools for special-purpose applications

In 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-precision

machining 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 ma- chine tools for maintenance/repair work。 These machines can be pided 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    fabricate

Fig。 10 Parallel kinematics machine tools; a PKM used at NIST [11, 28]; b Giddings  and

Luis 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 solu- tion 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 three- dimensional 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 approach can offer the advantages of reduced 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 of micro 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 milli- metres。 These could be used to produce a second generation

of 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 120× 160×85 mm and workspace envelopes of 20× 20× 35 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。