Evaluation and comparison of a large machine tool structure with ISO standard alignment tests

In order to satisfactorily machine precision components it is necessary for machine tools to be able to achieve extremely high levels of geometric accuracy. This requires their structures to be extremely rigid such that deflections caused by self weight and traversing of the moving elements do not induce distortions that exceed the required tolerances of the components which are manufactured by the machines. To aid this aspect of machine tool technology, a range of standards have been systematically created which specify in great detail a variety of geometric tests which can be applied to the various configurations of machine tools currently in use today. Certain machine tool companies will use national or ISO standards, others will create their own, often based upon the ISO but tailored to suit their specific machine configuration and characteristics. However, achieving the required tolerances can be extremely difficult due to a number of reasons such as geometric errors, thermal distortions causing errors to change as temperatures change and load errors causing the errors to change due to the variation in load magnitude and the changes in positions of the loads. One major source of load errors on large machine is caused by the machine's own weight and its re-distribution as the machine is traversed through its own working stroke, whereas another significant source is caused by the variety in component weights and their variation in position, either on static or moving table machines. In some cases the tolerances specified in standard tests can be tighter than grade "A" straightedge tolerances and achieving this accuracy under conditions which apply to large and heavy machine tool installations can cause significant problems and considerable unanticipated costs. A review is made here of how these difficulties might be overcome by use of modern technology and adopting the outlined planned approach to the design and build procedure used of large and heavy machine tools.