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Dynamic Time Reliability Modeling of Machining Center

Author: LiXiaoBing
Tutor: YangZhaoJun
School: Jilin University
Course: Mechanical Manufacturing and Automation
Keywords: Machining Center Time Dynamic Reliability Posterior Probability FMECA Availability
CLC: TG659
Type: PhD thesis
Year: 2012
Downloads: 334
Quote: 2
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Abstract


As the main apparatus of CNC machine tools, the reliability of Machining Center(MC)has become one of the key indicators to measure machine performance. Currently, themiddle and top grade MC depend on imports mainly, the total amount of importedmachining center occupies about70%of domestic market, which has seriously hamperedChina’s industrial development. With the development of China’s car, power generationequipment, high-speed railway, shipbuilding, aerospace technology, the MC demand willcontinue to rise. However, the domestic user prefer to the expensive imported MC as thehigh reliability, which lead to the market lose directly. How to effectively improve thereliability level of the machining center to obtain the domestic market share, has become animportant work of the entire machine tool industry.Reliability modeling and assessment is the basic work to MC reliability research, suchas reliability management, reliability growth, maintenance and so on. However, the currentassessment methods focus on the time interval between the faults, the obtianed functionscan only describe the reliability status in one time interval. This method always supposesthat the MC can be repaired to new when fault occurs, and the cutting load is ignored,which causes the estimation error directly. In order to solve the problems, a new dynamictime reliability and instantaneous availability modelling method considering the cutting loadwas proposed which build up a complete theory for MC reliability modelling. The detailedwork is as follows:1. The CNC machine tool and MC reliability development status at home and abroadwas studied, the conclusion that the current MC reliability modelling method was biased.And a new time reliability modelling method considering the cutting load was proposed.2. As the traditional reliability models based on the time between faults can notdescribe the reliability of the MC at any time, a modeling method based on the failure timewas proposed. By means of empirical modeling method, the reliability models of fault timeand each fault time were built. The obtained modes could not only descripe the reliability atany time, bu also distinguish the different when the fault time interval was identical whilethe fault time was not. The distributions of each fault time depict the probability of eachfault time, combined with the distribution of fault time, it can descripe the reliability statusof MC at any time. Then, the correlation between the time was studied, and the caseexample showed that the correlation was significant, which proved that the time series can’t be changed.3. The reliability is different when the cutting load changes, under the presumption thatthe reliability was lower when the cutting load was larger, the acceleration model was builtbased on the average cutting circumferential force. Two parameters Weibull ditributions waschose for the fault time under each cutting circumferential force, the Van-Montfort andBartlett methods were used for the fit goodness and shape parameter consistency test. Then,a case example was given, the acceleration model and the different ditributions for faulttime under each cutting circumferential force were obtained. In order to obtain the optimalacceleration model, the fitting correlation coefficient which was greater than0.85wasdefined. With the least squares method, the optimal model which can be used forreliability adjustment was obtained. The case example showed that the time between failureunder the various cutting load in accordance with the two-parameter Weibull distribution.Besides, the obtained shape parameters showed that the acceleration force did not changethe fault mechanism, the correction factor was credible. The Comparative results showedthat the differences was significant if not consider the cutting load when modeling.4. The machining center is a complex electro-hydraulic system, generally it can beregarded as a series system, and the fault is generally caused by one or more subsystems. So,how to determine the key subsystem which affect thewhole system reliability most seriousis important for its maintenance management and reliability status detection. When the MCis going to have a fault, the key subsystem may be the most possible fault source. TheFMECA method was used to find out the key subsystems and the posterior was obtainedbased on the Bayesian methods. The case example showed that under the given cutting load,the key subsystems of the MC were spindle syytem, automatic tool change system,electrical system, ABC-axis system and so on. The posterior probability of each subsystemwas increasing at first. As the time increased to2000hours, the posterior probabilityreached to the maximum, and the it decreased.5. The instantaneous availability modeling of MC was studied. The existing methodshave many deficiencies, for example, most of the Markov process modelling methodassumed that the fault time obeys the exponential distribution. The simulation methods aregenerally based on the accurate subsystems reliability information which can hardly obtainin a short time. The Empirical modeling method has the decoupling problem when used. So,focus on these shortcomes, a new way for calculating the instantaneous availability wasproposed based on Functionnal Data Analysis method. It introduced the Quartic Bernstein Polynomial into the smoothing method firstly for the reliability which was estimated bymedian rank method and estimated the fitting parameters by least square method. Then, theFourier baisis function was chosen based on the trend after the time section adjustment forthe estimated maintainability. And the non-linear least squares were selected for caculatingthe fitting parameters. Finally, it bulit the instantaneous availability model of MC based onthe functional linear regression method, and a case exampleof18MCs was given. It showedthat the proposed modelling method could evaluate the instantaneous availability of MCeffectively.These innovations of the work are as follows:(1) The cutting load were considered in the reliability nodeling. By means of theaverage cutting circumferential force to establish acceleration model under different cuttingforce conditions, it can convert the time between fault and modify the reliability underdifferent cutting load.(2) A new modeling method based on fault time was proposed, compared with thetraditional method, the obtained model can descripe the reliability of MC at any time. Also,it could descripe the difference when the fault time interval is identical while the fault timeis not.(3) The functional data analysis methods was introduced for availability modeling ofmachining center, the established instantaneous availability model overcome theshortcomings of decoupling difficulties.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metal cutting and machine tools > Program control machine tools, CNC machine tools and machining
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