2。2 Parallel kinematics machines

The 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 inpidual axes。 A major disadvan- tage 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    of

Fig。 7 In situ boring system inside a cylindrical vessel [7]

compound errors。 At present, parallel kinematics machine tool development has focused on manufacturing; however, these machine designs can also be adapted and miniaturised for in situ operation as their closed-loop structure can make for a compact machine with a high stiffness-to-mass ratio。 Research and development of PKMs has been reviewed, with a particular focus on hexapod type machines, from the perspective of the National Institute for Standards and Technology (NIST) [11]。 The development of PKM systems arose from the need to increase speed, precision and flexibility of machine tools to meet the demands of the modern manufacturing environment。 The first and most common design is the Stewart Platform or hexapod, first demonstrated as a machine tool in 1994; and this is the one on which much of NIST’s research has focused。 Figure 10a shows a PKM used in this  research。

It has been found that PKMs can offer a higher stiffness- to-mass ratio, lower moving mass, higher speeds greater

Fig。 8  In situ rail grinding system  [8]

accuracy and lower structural complexity than conventional machine tools。 However, the unconventional structures also pose some difficulties。 They use six degrees of freedom to perform multi-axis operations requiring the simultaneous movement of all six motion axes。 This makes for more complicated control, tool path planning and error compen- sation than are generally encountered in conventional machines。 PKMs also typically display more complex workspace envelopes than conventional machines。 NIST and other organisations are conducting a range of research projects to address these and other challenges associated to PKM technology。

Another approach to PKM design is exemplified by the Giddings and Luis Variax, Fig。 10b [12, 13]。 This hexapod, the first introduced as a machine tool, uses a different architecture to that tested by NIST, with the workpiece mounted on the fixed platform between the legs rather than having the hexapod suspended above the workpiece。 This machine was designed and built with a space frame construction and damping systems in the lower platform to eliminate the need for a heavy machine base to damp vibrations。 This design was intended to make the machine more mobile and quicker to re-locate and set up in flexible manufacturing facilities and could even be used by the military to perform machining operations in the field。 This concept could be miniaturised to create a stiff, light weight and highly flexible portable machining system。

From the selected examples of macro machine tools dedicated to in situ machining which have been presented, it can be observed that their designs are directed toward

Fig。 9 Orbital machining sys- tems: a with eccentric spindles [9]; b with 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 engage- ment necessitates bulky machine structures to provide the stiffness required to cope with the high cutting forces which result from such processing。