Study On Eliminating Failures Initiatively System Of CNC Machine Tools

With the rising of domestic industrial structure and the rapid development of economic, the demand number of CNC machine tools increased every year, and the market demand of CNC machine tools turned to high-end products. As an important measure of NC machine tools and the focus of competition in the market, reliability was respected. Because of the gap of reliability between domestic NC machine tools and foreign products, reliability turns to be the weakness of market competition. Frequent failure is a main performance of domestic NC machine tools’ low reliability. So the study of fault diagnosis and how to eliminate faults accurately at the right time is an effective mean to improve the inherent reliability of NC machine tools.This paper is supported by the National Major Scientific and Technological Special Project for “High-grade CNC and Basic Manufacturing Equipment”(NO.2013ZX04005-012). This paper established a system of eliminating failures initiatively based on the study of a certain series domestic horizontal machining tool center. The main parts of this system are as follow:① Product definition analysisThe main purpose of product definition is to understand the structure, functions, technical specifications, working environment of product, and these information are the premise of fault symptoms and performance degradation studying. These information can be found in product instructions. The other purpose of this phase is to analysis the function of each part of this product, and find out all potential failure modes of it then sequent them base on harmfulness. It can provide reference for the select of analysis objects later. It also can provide reference for work phases such as design, manufacturing, using and so on. This purpose can be reached by FMECA method.② Reliability testSubject of reliability test is one of the failure modes that produced by FMECA method. Purpose of reliability test here is to inspire the fault symptom and reflect the performance degradation trajectory of product. It can provide reference for fault symptom analysis and performance degradation trajectory predicting. Stress types and stress levels can be determined based on the working condition spectrums and load spectrums of product. Then reliability test can be implemented after choice right performance parameters to detect and the comprehensive stress profile has been determined. The data of reliability test can provide reference for the analysis of fault symptom later.③ Fault symptom analysisAfter the fault symptom was inspired and performance degradation data was obtained by reliability test, qualitative analysis of the fault symptom can be implemented to find out the fault reasons and quantitative analysis of performance degradation data can be implemented to predict performance degradation trajectory. Then fault time can be obtained based on the predetermined threshold value. Fault can be eliminated based the fault time and fault reasons, and the purpose of eliminating failures initiatively before the appearing of them can be reached. This is the key point of this system, and it includes both qualitative analysis and quantitative analysis.Qualitative analysis: Firstly, analysis the fault reasons of potential failure mode by fault tree analysis method; Then the relative weights of fault reasons to failure mode should be obtained using analytic hierarchy process method because we do not know which reason will lead to the appearing of failure before it appears. The results of qualitative analysis can provide reference for fault eliminating.Quantitative analysis: Firstly, fitting trajectory of performance degradation and predict the failure time of product under a set of stress level using time series analysis method based on support vector method; Then obtain the relationship between failure time and stress levels using artificial neural network method. Then the purpose of obtain failure time when we know any stress levels can be reached.④ fault symptom eliminatingAfter obtained the failure time by fault quantitative analysis and fault reasons by fault qualitative analysis, fault reasons can be isolated at the right time from design(structure design, material selection, purchased components selection), manufacturing(machining crafts, assembling crafts, working environment), using and maintenance(working conditions, working skills, maintenance) etc. Then heavy losses caused by forced shut down can be avoid, operational efficiency can be increased, maintenance cost can be reduced and inherent reliability can be increased.