Binbin Xu

A new failure mode and effects analysis model of CNC machine tool using fuzzy theory

Failure mode analysis is important in CNC machine tool. In order to assess the failure mode of CNC machine tool, this paper described a new fuzzy FMEA model integrating with fuzzy linguistic scale method. The model proposed a risk-space diagram to explicit the relationship of S,O and D. On the basis of the risk-space diagram, risk priority number is calculated by weighted Euclidean Distance formula and centroid defuzzification based on Alpha-level. This study is used to analysis a type of CNC lathe. Compared with the criticality ranking about another similar type of CNC lathe, the risk ranking of FMEA model revealed that the method is basically same with the actual situation. The results indicated that the fuzzy FMEA used in CNC lathe is a reasonable method corresponding to the manufacturing, and it is a validity foundation for constructing reliability design model or supporting control plan of manufacturing.

Bayesian reliability modeling and assessment solution for NC machine tools under small-sample data

Although Markov chain Monte Carlo(MCMC) algorithms are accurate, many factors may cause instability when they are utilized in reliability analysis; such instability makes these algorithms unsuitable for widespread engineering applications. Thus, a reliability modeling and assessment solution aimed at small-sample data of numerical control(NC) machine tools is proposed on the basis of Bayes theories. An expert-judgment process of fusing multi-source prior information is developed to obtain the Weibull parameters’ prior distributions and reduce the subjective bias of usual expert-judgment methods. The grid approximation method is applied to two-parameter Weibull distribution to derive the formulas for the parameters’ posterior distributions and solve the calculation difficulty of high-dimensional integration. The method is then applied to the real data of a type of NC machine tool to implement a reliability assessment and obtain the mean time between failures(MTBF). The relative error of the proposed method is 5.8020×10-4 compared with the MTBF obtained by the MCMC algorithm. This result indicates that the proposed method is as accurate as MCMC. The newly developed solution for reliability modeling and assessment of NC machine tools under small-sample data is easy, practical, and highly suitable for widespread application in the engineering field; in addition, the solution does not reduce accuracy.

Reliability Evaluation of NC Machine Tools considering Working Conditions

Reliability evaluation is the basis for reliability design of NC machine tools. Since traditional reliability evaluation methods do not consider the working conditions’ effects on reliability, there is a great error of a result of a traditional method compared with an actual value. A new reliability evaluation model of NC machine tools is proposed based on the Cox proportional hazards model, which describes the mathematical relation between the working condition covariates and the reliability level of NC machine tools. Firstly, the coefficients of working condition covariates in the new reliability evaluation model are estimated by the partial likelihood estimation method; secondly, the working condition covariates which have no effects on the reliability of NC machine tools are eliminated by the likelihood ratio test; then parameters of the baseline failure rate function are estimated by the maximum likelihood estimation method. Thus, the reliability evaluation model of NC machine tool is obtained under different working conditions and the reliability level of NC machine tools is obtained. Case study shows that the proposed method could establish the relation between the working condition covariates and the reliability level of NC machine tools, and it would provide a new way for the reliability evaluation of NC machine tools.

A fuzzy maintainability allocation method for NC machine tools based on interval analysis

In the process of maintainability allocation of numerical control (NC) machine tools, many influence factors should be considered, and some of which are difficult to analyze quantitatively. In order to solve the above-mentioned problem, this paper proposed a new method for maintainability allocation, which comprehensively applied interval analysis, fuzzy comprehensive evaluation, and analytic hierarchy process (AHP). The allocation model is built which can make use of the test information and experts experience comprehensively. Meanwhile, as the information carried by a single real number is always incomplete, interval number is used instead of real number to construct judgment matrix. Finally, the method is illustrated with an example, and the maintainability indexes of all subsystems are obtained as well. The analysis results show that the method is a feasible method for the maintainability allocation of NC machine tools. It could solve the uncertainty problem of maintainability allocation for NC machine tools effectively.

Posterior probability prediction on the key subsystem of machining center

In order to predict the fault probability when the MC is faulty or going to be faulty, a posterior probability method is proposed. Failure Mode, Effects and Criticality Analysis(FMECA) is used for finding out the key subsystem which affects the Machining Centers reliability most seriously. Then, the empirical models of the key subsystem and the whole Machining Center are built based on the fault time, under the assumption that the Machining Center is a Series System, the posterior probability of the key subsystem is obtained by Bayesian theory. The case example shows that the fault probability of the key subsystem changes as the whole system fault probability changes. At a given time t, the fault probability of the key subsystem can be calculated by the empirical models of the key subsystem and whole system

Research on imperfect preventive maintenance strategy for turret system of the CNC lathe

The turret system is one of the key functional components of the CNC lathe. Therefore it is important that the reliability of the turret system is optimised. In this paper, an imperfect preventive maintenance strategy, based on the Weibull distribution, for the turret system of the CNC lathe is proposed. The restoration and failure intensity increase factors are consistent with a uniform distribution. However, the cost of preventive maintenance increases with the preventive maintenance frequency being increased. Then maintenance time should also be considered. A flexible preventive maintenance model is described in this paper. The aim is to minimize the total maintenance cost overall. The preventive maintenance interval of the model was calculated using the limited reliability value, in order to find the optimal flexible preventive maintenance strategy. Finally, the optimal preventive maintenance results under different cost parameters are discussed.

Reliability degradation monitoring of CNC machine tools based on the SPC method with the Metropolis-Hastings algorithm

After the random failure period, CNC machine tools will enter the wear-out failure period as a result of abrasion, fatigue and aging. Therefore, it is extremely important to monitor the changing trend of operation state and thus to construct rational maintenance policy or determine when to scrap it. In this paper, the mean time between failures (MTBF) is chosen to be the characteristic variable of reliability degradation and is estimated from a Weibull process model. A statistical process control (SPC) chart is then developed using the Metropolis-Hastings (MH) algorithm for condition monitoring. The availability and sensitivity of the proposed method are illustrated through analyzing the field data of a CNC machining center.