| NC machine tools are the basic device to modern manufacturing industry, which are themanufacturers with the high-technology. Whether the advanced functional machine tools thathave the high machining precision and reliability are developed reflects the level ofmanufacturing industry of one country. Comparing to NC machine tools made by Europeancountries, Japan, America, etc, Chinese ones have relatively low level of reliability, and stillit is a huge gap. The markets for high and mid-grade machine tools productions and keyfunctional parts, especially high-end NC machine tools, are secured by foreign brands in along period. Market share for NC machine tools made by china is low, high and mid-grademachine tools productions and key functional parts mainly depend on import. The majorreason in the low market share is the relative lower reliability level of machine tools, and thisrestrains the development of machine tools industry in our country. Therefore, it is necessaryto improve the reliability of domestic machine tools and key functional parts.To carry out the reliability design for machine tools and key functional parts is onemain way to improve the reliability of machine tools, and load spectra are the foundation ofreliability design. However, there is no scientific and accurate load spectra because of thatdata are deficient in many companies, so workers always design production by choosingsafety factors based on their experience, and productions designed by this method may betoo larger. However, NC machine tools cutting load and strength of materials are notconstant, they are random variables. And those machine tools designed by experientialmethod doesn’t have the high inherent reliability and it make machine tools low level ofreliability and congenital defect. In the last century90’s, Jilin Technique Institute developeda serials of research on load spectrua for NC machine tools, but the method of these did notconsider the load influence to machine tools spectra during dry run, and the machiningcutters wear influence to machining load, which leaded the load spectra that built was notcompleted. Meanwhile, with the development of control, mechanics and sensing technology,machine tools have been improved a lot, especially the structure of machine tools have beenmore simplified comparing to productions made in last century. Feed system is driven byservo motor, so the chain of transmission becomes more shorter and AC or DC control unitmakes spindle rotation contiuously variable, so it abandoned the former step-by-step variable speed transmission. As well as, stiffness of machine tools improves a lot in order to realizehigh precision. The changes of structure of machine tools make machining load transfer lawsdifference to the former machine tools and the machine tools’ working conditions alteredmuch as well. Hence, load spectra cannot be applied to the reliability design of machinetools anymore now. To sum up, according to the project of “the Technology of Reliabilitydesign and Performance Testing for High speed/Precision NC machine tools” and project of“Research on Reliability Testing of NC machine tools”, which projects are founded byNational science and technology major special projects, failures analysis for machine toolsare analyzed, then obtained that the reliability design is the main reason that cause machinetools fail from that study. Research on machining load tranfer laws, the cutting loadinfluence by machining cutter wear and the establishing method of load spectra have beenconducted. The detailed work are as follows:(1) The development of machine tools, the technolgy of reliablity for machine tools andload spectra are analyzed, emphasizing the state and the hot research spots of reliability ofmachine tools in China. In this part, the reliability modelling method of machine tools,failure analysis technology, reliability design and tests are reviewed and analyzed. The maindynamic states of reliability technology study are summarized, the existing problems areproposed. The technology roadmap of machine tools reliability is presented.(2) The method of reliability data collection is drawn up,then the field reliabilitytestings are carried out and lots of failure data were collected. Based on the principles offunctional independences, functional shares and the conventions, the machine tools aredivided into several subsystems. Then the subsystems with high frequent failures areobtained by eareto diagram method. The criticality effect factors don’t take into accout whenFMECA and it is difficult to give a quantify for those factors, so the analysis result is notreasonable. To overcome this issue, the triangle-shape mbership functions are used toperform the fuzzy evaluation of researchers, the weight of factos are determined byfuzzy-DEA method. Based on this, a new criticality evaluating method is executed and thekey subsystems which have main effect on reliabiltiy are got. And then failure mechanism isanalyzed briefly.(3) The performance and structures of abundant NC machine tools are investigated,based on this, the load transfer laws during dry running, turning and drilling of NC machinetools are studied, then the laws of force and torque transfer are obtained and models of forceand torque are developed. The force models include force of axis bearing, force of leadscrew and pressure of guide rails. The torque models include torque of spindle motor and screw motor. Similarly, the load trasfer laws of machining center are obtained.(4) The load data collections of machine tools is determined. Based on this process thefield load tests were conducted, then the lots of load information were collected. Withstudying and analyzing on laws of machining force, the cutting force is also influenced bywear of tools. Then, the experiment of machining force affected by wear of tools wereperformed using the orthogonal experiment method. Finally, the experiential formulation ofmachining force are got by adopting multiple linear regression model.(5) The development of load spectra of machine tools are studied. According tofunctional performance of NC machine tools, the methods of establishing load spectrum arestudied. The key is to obtain data of machining load, accounting of circulation of machiningload and selecting of load model. Take the ratio of feeding length to the feed speed ascirculation amount, and the ratio of machining force or torque to the maximum machiningforce and torque as the relative load. On the basis of this, the relative load is the variable of xaxis and the relative circulation is the variable of y axis. After that, the data are fitted tolognormal distribution, Weibull distribution and the distribution of Beta and gamma. Finally,the best model is determined by data envelopment analysis.This study summarized the reliability characteristic of machine tools and gave the studydynamic spots. As shown there, the new FMECA considered the critically effects weightswas proposed and it would be a great complements to FMECA. Moreover, empirical formulaconsidered the wear of cutters was developed and then the load spectra establishing methodwas presented based on the new load transfer laws. Through this study, it would provide anestablishing load spectra method and a foundation for reliability design. |
PDF Full Text Request