Modeling and measurement of multi-axis machine tools to improve positioning accuracy in a software way

• Main Author: Rahman, M. (Mahbubur)
• Format: Doctoral Thesis
• Language: English
• Published: University of Oulu 2004
• Subjects: CNC; accuracy; compensation; measurement; modelling
• Online Access: http://urn.fi/urn:isbn:951427332X; http://nbn-resolving.de/urn:isbn:951427332X

Abstract Manufacturers are under tremendous pressure to improve product quality in terms of dimension while maintaining high productivity. To maintain product quality, it is necessary to know the accuracy level of machine tools so that defective parts can be prevented in manufacturing. Different machine tools deviate from their ideal situation to an error prone state over time. Even new machine tools may cause errors due to faulty installation, an extra heat source etc. Roll, pitch and yaw errors are common problems in machine tools for the manufacturing industries. The origins of these errors are kinematics parameter deviations resulting from manufacturing errors, assembly errors or quasistatic errors. By considering the geometric description of any machine tool, one should be able to predict the actual tool tip as compared to ideal tool tip for every controlled point in the machine's workspace. By counting the forward kinematics of the machine it is possible to predict the tool tip deviation for every point. A number of measuring methods can be adopted to describe the actual geometry of machine tools. Each method has it's own advantages and disadvantages. Often machine tool experts measure the machine with different types of measuring devices to obtain error traces based on its error sources and magnitude. In this thesis, a theoretical and practical relation has been established between static and dynamic measuring systems. These relations are important when we are measuring machine tools with different measuring devices to validate the measurement results. In this work, traces obtained by one measuring system have been compared and simulated with the traces obtained by other methods. A number of systematic mathematical models have been developed, and compared with the results obtained by other measuring methods. The outcome of this can lead to the development of a software system that can be used to validate measuring results obtained from different measuring systems and those can be compared with each other. The VM182 measurement result simulates closer than the laser measurement result when both are compared using the traces obtained by DBB measurement. Several methods for improving the positioning accuracy of machine tools have been studied. One of the methods is NC code modification. This method has been applied to develop an NC program processor based on the error found by the measurements. An aluminium test piece has been cut with the modified program to test the developed model. The finding of NC code modification is that for repeatable error, we can obtain a better dimensional accuracy for work pieces when we use a modified NC program based on the algorithms developed. The arch replacement technique has given a circularity improvement from 22 to 12 with DBB measurement, and circularity has been improved from 12.59 to 8.10 when it has been applied to cut aluminium work piece.