This part of ISO 10791 specifies a method of checking the contouring performance of machining centres (or numerically controlled milling machines, etc., where applicable) by conducting circular tests in the three coordinate planes (XY, YZ and XZ) and by evaluating the radial deviation, F, and circular deviation, G, in accordance with ISO 230-4. NOTE Measurements of circular movements can be carried out by different methods as described in 6.6 of ISO 230-1:1996. These methods consist of using a rotating one-dimensional probe and a test mandrel, a circular masterpiece and a two-dimensional probe, or a telescoping ball bar. Other test methods may be applied if the accuracy of the equipment is the same as (or better than) the accuracy of the methods described. Influences of typical machine deviations on circular paths are shown in annex B of ISO 230-4:1996. The purpose of the checks by circular tests described in this part of ISO 10791 is to verify the circular movement — only in one position per coordinate plane of the machine, — at only one feed rate, and — to repeat the test once in the opposite contouring direction. The purpose of this part of ISO 10791 is not to provide a means of analysing the causes for the measured circular deviations, but to offer the user a method for periodic checking of the machine tool. It is recommended to carry out the tests of this part of ISO 10791 once the machine tool has satisfactorily undergone the acceptance tests and to use these results as a basis for comparison when carrying out periodic checks. Therefore, acceptable deviations from the initial results shall be decided by the user. If the tests are used for machine acceptance purposes, the supplier/manufacturer and the user shall agree on specific diameters, feed rates and tolerances for the tests. NOTE ISO 10791-6, which deals with the accuracy of feeds, speeds and interpolations, also includes as test K4 a circular test. Here, the purpose is to check the mutual behaviour of two linear axes (generally X and Y) at two defined feed rates for one defined diameter. This is more or less a diagnostic test with respect to the circular interpolation, whereas the circular tests of this part of ISO 10791 give information on the overall contouring performance of the machine.