USE OF SHEWHART CONTROL CHARTS TO ENSURE PRODUCT QUALITY AND OPTIMIZE THE MAINTENANCE SCHEDULE FOR CNC METAL-CUTTING MACHINES. CASE STUDIES

Purpose. On the example of a CNC machine DAEWOO PUMA 600M, using statistical methods of quality management, in particular, Shewhart charts, a scheme for maintenance and repair of machine tools in order to reduce operating costs was work out. Research methods. For the case under study, from each batch of 30 Wheel units, the quality control department monitored the deviation from the nominal value of the most critical accuracy parameter, the mounting diameter for the rolling bearing outer ring, for five random products. A total of 60 such samples were taken. The mounting surface diameter was monitored using three-point precision intalometer. Thus, for a total sample of 300 units, Shewhart control charts for the center and range positions were built and statistical analysis was performed the purpose of which is to identify special trends. In addition, the errors correction log entries were analysed in a similar way to determine whether a positioning error was present. Results. It has been shown that the use of Shewhart’s control charts allows assessing the actual state of the machine tool equipment. Based on this fact, a model of maintenance and repair of CNC machines using statistical data analysis is proposed. The application of the proposed model to the entire maintenance and repair cycle can significantly extend the inter-repair period for CNC machines. The number of repair activities was reduced by 35–50 %. At the same time, the cyclicity of operations provided by the manufacturer's technological routine for this type of machine tool remains unchanged, the equipment utilisation rate increases, and it is possible to reduce the required number of repair service personnel. Scientific novelty. The relationship between the results of the cutting process, which are determined by Shewhart's control chart, and the state of equipment is established to formulate maintenance and repair measures. Practical value. The proposed scheme for organizing the maintenance and repair of CNC machines based on statistical analysis using Shewhart charts provides a significant reduction in the cost for their operation.