Arial Compilation Of Cutting Force Spectrum Of CNC Lathe And Its Application In Reliability Test

The inherent reliability of the products is primarily determined by its design and the main method to improve the reliability of the CNC machine tool is conducting reliability design for the whole-machine and its key components,which must take load spectrum as its objective basis.And reliability test is the major technical approach to obtaining failure data,executing failure analysis and establishing reliability model,while the load spectrum of CNC machine tool is the foundation to conduct reliability test,which can simulate the real working condition,in the laboratory.Therefore,putting forward a scientific,reasonable and complete load spectrum compilation method of CNC machine tool,and accomplish the load spectrum of CNC machine tool is of great theoretical and application value.In recent decades,due to the rapid development of direct drive technology,servo technology and digital control technology,CNC machine tool has changed greatly in the structure,function,performance indicators and the application filed.For the reason above,the static cutting force spectrum that build with the empirical formula is no longer suitable for the reliability design of the whole machine and reliability test of the key function parts nowadays.Besides,compiling a load spectrum that can reflect the real dynamic cutting load of the CNC machine tool requires a lot of process load data.What’s more,a long time load measurement test based on the typical working condition and process is necessary.All these are time and money consuming and it is the reason why many researchers stops their research about this filed.Based on the Key Laboratory of CNC Equipment Reliability Technique of Machinery Industry of Jilin University,combining many projects,this paper studies the compilation method of the cutting force spectrum and its application in reliability test.Based on an in-deep reading of related literatures,this paper systematically study the compilation method and process of CNC machine tool load spectrum,including the selection of typical process,acquisition and process of load data,determination of sample length,extrapolation of load spectrum and compiling of cutting force spectrum of CNC machine tool.Besides,a reliability test based on the load spectrum compiled has been conduct in the laboratory.The main research work is as follows:(1)In order to ensure the authenticity and validity of the load spectrum data sources,the field cutting process parameters and machine running conditions of the sample CNC lathes was collected according to the field data record form designed;according to the working condition and process feature of CNC lathe,this paper built the selection principle of CNC lathe’s typical test conditions and process.Then a method to determine the typical test process based on interval of cutting parameters has been proposed.Besides,a CNC lathe cutting force testing system was developed,based on which,the measurement of cutting load of typical test conditions and process has been carried out.(2)The noise sources in the process of cutting force test was analyzed.As to the cutting force signal with low SNR,an improved Multi-wavelet Neighboring coefficients de-noising method was proposed.This de-nosing method can make the Multi-wavelet decomposition layers,wavelet function and the length of neighboring coefficient blocks more reasonable by removing singular points in load signal using the statistical variance smoothing method and introducing correlation dimension and BDS index to identify the noise in load signal.By de-noising the four synthetic signals with different SNR,the proposed method is proved to be more valid than the classical wavelet threshold de-noising method.Then the de-noising results of the cutting force signals were compared from the perspective of the time domain waveform,frequency domain energy and rainfall domain damage;What’s more,in order to ensure that the load data collected can replace the general information,a load sample length calculation method based on the cutting force characteristic was proposed.(3)According to the load characteristics of the constant speed cutting condition of CNC lathe,a POT extrapolation method based on the improved time domain load expansion was proposed.For the traditional time domain load expansion ignoring the extreme load time interval,this paper proposed an improved MCMC time-domain extension method,which can make time-domain extension of each load data group according to specified loading time.Secondly,the range of extreme load threshold is obtained through MEF diagram.And the GPD fit was carried out for the overshoot value of each threshold.Then the optimal load threshold and extreme value distribution were selected by analyzing the grey correlation degree.By merging the matrix of the mean-frequency and amplitude-frequency of 30 groups of load data in the constant speed cutting process,the cutting force spectrum of CNC lathe in the constant speed cutting process was built.(4)According to the load characteristics of CNC Lathe in variable speed cutting,a parameter extrapolation method based on mixture Weibull distribution is proposed.30 groups of load data are sorted and processed rain flow counting by the means of improved MCMC time-domain extension method according to their use time ratio.According to the mean and amplitude value characteristic of the merging of 30 groups load data,the mixture Weibull distribution was built for the mean and amplitude value.The number of basic functions is obtained by the grey correlation and the parameters are estimated with the improved EM method.Through correlation analysis of the mean and amplitude of the loads,the candidate Copula functions have been got.By calculating square Euclidean distance,the optimal Copula function is obtained and the joint distribution function of mean and amplitude is established.Meanwhile,the cutting force program loading spectrum in the variable speed cutting process is compiled.(5)Taking the CNC turret as example,the eight-level cutting force programed loading spectrum of the CNC turret in the process of “cutting-changing-cutting” is compiled.The loading angle,loading frequency of dynamic cutting forces and frequency of tool changing of the CNC turret corresponding to each step of the programed loading spectrum during the reliability test are determined.According to the reliability testing program of the CNC turret based on load spectrum the reliability test has been conducted in the laboratory.For the small sample data obtained in the reliability test,a Bayesian reliability modeling method based on fusing multi-sources prior information is proposed.At last,according to the reliability test data,the reliability model of the CNC turret is established.In this paper,a set of methods which are used in the measurement,compilation and application of cutting force spectrum for CNC Lathe are proposed.This methods can provide theoretical foundation for the subsequent reliability test,accelerated life testing,life prediction and reliability design of key functional units of CNC lathes.