This part of ISO 10791 specifies, with reference to ISO 230-3, tests for the evaluation of thermal distortions of the machine structure and positioning system, up to 2 000 mm in length, of machining centres. It gives three test methods: — environmental temperature variation error; — thermal distortion caused by a rotating spindle; — thermal distortion caused by moving linear axes. There is no intention of assigning any numerical tolerances associated with the tests specified.