PREDICTION OPERATIONAL ACCURACY OF THE CNC MACHINING CENTRE

The main research topic of this dissertation thesis deals with the influence of the working accuracy related to large CNC machines on the required workpiece properties. The state-of-the art has constantly been developed and the customer’s requirements directed to manufacturers of the machine tools are increasingly demanding. There is required a high accuracy, reliability, shortening delivery times etc. The manufacturer is expected to fulfill all the customer’s requirements. The machining accuracy is dependent, among others, on the positioning accuracy of a machine tool that represents the position of the cutting tool relative to a workpiece. The accuracy of a particular machine tool is therefore a limiting factor concerned with reaching the highest accuracy and quality of a workpiece. Acceptance test procedures are initially realized within a manufacturing shop floor and then within a customer environment. The main subject of the testing procedures is to verify. basic properties of a machine tool such as basic dimensions of a machine, strokes in all respective coordinates, machine travels and spindle speeds. A machining of a workpiece with a desired precision specified by the customer is also performed. In this thesis, a new methodology to calculate and determine errors concerned with machining on vertical lathes is proposed with the aim of predicting the machining accuracy of CNC machine tools. The objective of this work was to determine the influence of machining accuracy of a CNC machine in connection with a subsequent geometrical accuracy of a workpiece. Considered position of a tool relative to a workpiece will be affected by errors concerned with a kinematical inaccuracy, geometrical errors and errors related to machining forces. A vertical lathe SKIQ 30 produced by TOSHULIN, a.s. has been used as a testing machine on which the proposed methodology has been verified. During processing various calculations in this thesis there has been found out that the machining inaccuracy of the machine – main topic of interest – is influenced by the machining conditions such as cutting conditions, material to be machined and even the setting of working positions of the machine. For instance, a position of the cross slide is determined by a workpiece height and a position of the lathe carriage that is determined by a workpiece diameter and a travel of the slide that is also determined by a workpiece height. It can be stated then that to predict the machining accuracy of work pieces, it is important to know the behavior of the machine in its all range of working envelope. Based on the information acquired about the behavior of the machine, it can be determined an appropriate position for a workpiece setting that allows utilizing the maximum reachable accuracy with respect to the stiffness and geometrical accuracy of the machine tool. An appropriate setting of initial working position may influence the subsequent geometrical accuracy of a workpiece