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 advanced machine 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。

Fig。 11   Second prototype micro machine tool [14]

Fig。 12  Diagram of micro lathe turning system  [17]

An example of a high-precision three-axis meso scale milling  machine  having  overall  dimensions  (200 ×300×

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 counter- weight 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 high- speed 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 (350 × 240×320 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 to support a range of lenses with different magnifications。 The

Fig。 13  Diagram and photograph of a three-axis meso milling machine   [18]

machine was tested performing a range of one-, two- and three-dimensional cutting operations in aluminium。 It was concluded that the machine was capable of effective machining on the micro scale, although potential for improvement and elimination of errors was identified。

Another versatile approach is described by Kurita et al。 [20] in an article on development of a compact multi- functional machine tool for producing micro scale products, in particular, micro-moulds。 The system has dimensions of (507 ×604× 655 mm) and weighs 80 kg, with a working envelope (200 × 100×100 mm)。 The X and Z axis stages are mounted on the granite frame, while the Y axis stage is mounted on the machine base。 The system has five interchangeable machining heads to enable a range of machining operations without re-mounting the workpiece。 The machining heads include three spindles with different speed ranges, a laser machining head and a piezoelectric actuator head。 These make the machine capable of conven- tional machining, laser machining, electro-discharge ma- chining and electro-chemical machining。 This machining system has been designed to specialise in the high-precision manufacture of micro scale moulds, while the modular nature of its machining head makes it highly versatile with ability to perform a wide range of machining tasks。