The Effect of Concrete Foundations and Machine Thermal Conditions on the Accuracy of Large Machine Tools

This thesis comprises of eighteen publications which describe research work which has been carried out into two aspects regarding the accuracy of large machine tools. The first six papers discuss the concrete foundations on which the machines are supported and the remaining twelve papers describe the consequences of thermal effects on machines. The publications are all ones of joint authorship, with the candidate being one of the named authors.

Manufacturing industry and the precision requirements of large machine tools:
The thesis reviews the range of errors and their causes that effect large machine tools and then specifically addresses what are considered to be two of the most significant contributors to these inaccuracies, namely those caused by the concrete foundations which support the machines and those caused by thermal effects. Both can cause problems that can be very difficult and extremely expensive to resolve. After a concrete foundation has been cast, if it is found to be inadequate it cannot, of course, be easily modified and thermal effects such as environmental temperature changes for instance can require the use of factory air conditioning systems which are expensive to install, operate and maintain. The publications included in the thesis show a systematic contribution to significantly improving the accuracy of large machine tools with respect to the two mentioned subjects.

Modern components:
Components used in modern equipment such as cars, trains, aircraft, robots, household appliances etc. require good functionality, reliability, long life, light weight etc. A significant contributor to these requirements is the high degree of precision [1] that these components are manufactured to in terms of their dimensions, form and surface finish. Production of virtually all of these components is achieved either directly or indirectly by use of machine tools. Therefore the machine tools need to perform extremely accurately, even more so than the components they produce since other factors such as fixtures, tooling etc. cause additional inaccuracies.

Machine tool Accuracy:
To achieve high accuracy performance, the machines need to move in a precise manner in terms of their straightness of movement, positional accuracy, rotational orientation and the relationship of one axis to another etc. It is also important that the accuracy is repeatable in order to guarantee the level of performance at all times.

Large machine tools:
Large machine tools can weigh hundreds of tonnes, have traverses of eighty metres length or more and are used for machining a vast variety of components such as nuclear reactor vessels, marine diesel engines, valve bodies, rolls for steel mills, aircraft wing components such as skins, ribs, stringers and spars and large undercarriage components.

Foundations:
Because the machines being considered are large and heavy their influence on the concrete foundations that support them is considerable and as a machine moves through its traverses the position of the center of gravity changes. The consequence is the foundation changes both shape (bends) and vertical position (sinks and rises). The six foundation papers describe the research and developement of techniques that can be used to ensure the distortions and deflections values are acceptable such that a machine will be capable of performing to its intended functionality. Also described is the research, development and performance testing of a taut wire and optical senors instrumentation for efficiently measuring the change of shape of the foundation.

Thermal Effects:
The machines and the workpieces they produce, can be constructed from a variety of materials such as steel, aluminium, carbon fibre, concrete etc all of which have coefficients of linear expansion that when subjected to modest temperature changes can result in contractions and expansions that can cause workpiece to cutting tool relative positions to vary more than the workpiece dimensional tolerances. The twelve thermal publications describe the research and developement that has been carried out into machine tool thermal characteristics and the positional compensation techniques that have been developed to minimise the resulting positional errors. Using artifical intelligence methods this has been achieved so successfully that the errors can be reduced by 95%.