The article deals with accuracy of machine tools. The aim was to determine the accuracy with which already operates used machine – HAAS CNC Milling machine MiniMill. For evaluation of accuracy of the machine tool was used Renishaw QC20-W Ballbar system, which has the Department of Machining, Assembly and Engineering Metrology of VŠB – TUO. The theoretical part describes the measurement principle using a telescopic Ballbar system. The principle of measurement is the comparison of the actual deviation from the ideal circle which is programmed. Experimental part is focused on the diagnosis of the whole process by the direct method of measurement during changing feed rate. Specifically, the determination of circularity deviation at machine tool was in accordance with international standards, e.g. ISO 230-4.

This work is supported by: CZ.1.07/2.3.00/20.0037 financed by Structural Founds of Europe Union;  Students Grant Competition SP2016/172 and SP2016/174 financed by the Ministry of Education, Youth and Sports and Faculty of Mechanical Engineering VŠB-TUO