(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 machin- ing 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 re- grinding 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 applica- tion, 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 machin- ing 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 require- ments, 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 manufac- turing 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 tool- positioning 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。