Thermal distortion in machine tools

A method of detecting distortion of the machine base of a machine tool for machining a workpiece, the machine tool comprising:

• a machine base;
• a first support provided by a first rotational machine axis, the first rotational machine axis being rigidly mounted on the base in an immovably fixed position relative to the base and comprising a first drive operable to rotate the first support about a first axis of rotation;
• and
• a second support provided by a second rotational machine axis, the second rotational machine axis being rigidly mounted on the base in an immovably fixed position relative to the base and comprising a second drive operable to rotate the second support about a second axis of rotation, wherein the second axis of rotation of the second rotational machine axis is parallel to and spaced laterally from the first axis of rotation of the first rotational machine axis, and the first and second axes of rotation lie in a longitudinal reference plane,
• wherein the method comprises the steps of:
• detecting a change in the distance between a first pair of reference points on the machine base which lie on a first reference line with at least one linear distortion sensor;
• and
• detecting a change in the distance between a second pair of reference points on the machine base which lie on a second reference line with the at least one linear distortion sensor,
• the first and second reference lines are transverse with respect to the first and second axes of rotation, and are spaced apart along the direction of the first and second axes of rotation, and the first and second pairs of reference points are located on and spaced from the same side of the longitudinal reference plane.