The accuracy and the repeatability of tool positioning in relation to the work piece is one of the most important features in contemporary NC lathes. Precise lathes are particularly required in the automotive industry. The applied designs of the NC lathe slides do not always assure the required thermal stability of work piece dimensions. It has been observed that during the continuous work time of the one of the world’s famous lathes, the X and the Z axes have been significantly deformed by means of thermal influence. Diagnostic investigations concerning the positioning accuracy have been performed according to the ISO 230/3 standard [7]. While investigations, unpermitted values of thermal deformations in both slide axes, have been observed. The design of the slides has been analyzed and an attempt has been made to minimize the positioning errors that are influenced by thermal deformations. Particular attention has been paid to the errors of the X axis as they influence directly upon the accuracy of the work piece cross dimensions. A new method of deformation compensations for the X axis has been elaborated. The method is based on the elongation measuring of the ball screw applied in cross slides and then programmed compensation in the X axis control system. The practical effectiveness of the introduced compensation method has been confirmed and a five times increase of the machining accuracy has been obtained.