Characteristics of motion errors of 3–Axis CNC vertical milling machines

Abstract

This project presents a systematic approach to understand the different types of motion errors exist in Computer Numerical Controlled (CNC) machines. This research focused on five units of identical CNC milling machines, that is, the same brand, model and years of operation. As the increasing number of CNC machines in manufacturing field, there is a need to ensure the machines are in good running condition. Machine tools such as CNC vertical milling machines are meant to produce precise workparts, therefore is important to maintain their accuracy. Periodic and preventive maintenance of the machine is crucial, whereby motion error inspection is one of predictive maintenance activities in ensuring CNC machines are in good working condition. The purpose of this project is to find out any abnormalities to the machine performance. The motion errors of CNC machines were analysed by using Double Ball Bar (DBB) measuring device. This equipment is designed to analyse the machine performance by measuring the accuracy of its movements. This device having two identical size balls at the end of the bar was mounted on spindle and onto a special holder which is mounted on the working table. The machine is made to move in a circular motion, clockwise and anti-clockwise in three planes, XY, YZ and ZX. Any deviation from the standard data will represent the imperfection of machine condition especially mechanical components such as slide bearing, spindle bearing or servo motor responses. Based on the study, DBB equipment has successfully captured the motion errors of five units of 3-axis CNC vertical milling machines. Diagnosis has been done and several error origins have been presented in this study. By knowing the most significant error origin, the corrective countermeasures can be carried out.