Research On Running Reliability Assessment And Growth Technology For Heavy-duty CNC Machine Tools

The Heavy-duty CNC machine tools is mainly used for the manufacturing of large, extra large parts, is the key equipment of the national defense and military, aviation, aerospace, shipbuilding, energy, transportation, metallurgy, machinery and other key state enterprises.The Heavy-duty CNC machine tools is a mechatronic products that integrates themechanical, electrical, hydraulic, pneumatic, microelectronics and information and many other technologies, withthe characters ofsmall sample size, complex structure, high weight, long life and various failure. Traditional reliability evaluation method (based on the failure-time data) requireslarge sample, and is difficult to assess the high reliability, long life product. Therefore, merely using the reliability assessment method based on failure-time is difficult to reflect the heavy-duty CNC machine tools’ real reliability comprehensively. To solve this problem, this paper proposesthe three different reliability assessment methods with the angles of’failure’,’processing stability’and’performance retention’ to comprehensively assess the reliability of the heavy-duty CNC machine tools, to provide the guiding significance reliability assessment results. On the basis of reliability assessment, this paper researches for reliability growth technology of heavy-duty CNC machine tools and proposes the maintenance strategy optimization method based on FMEA and the multi-objective optimization method of machining process paremeters based on the nonlinear dynamic model, in order to improve the reliability of the Heavy-duty CNC machine tools from the point of view of maintenance and control parameters.This paper builds the reliability assessment system based on failure-time, dynamic characteristic parameters and performance parameters degradation, and carry out the research of running reliability assessment and reliability growth technology for the Heavy-duty CNC machine tools. The specific content including:(1) This paper makes the definition ofrunning reliability of the heavy-duty CNC machine tools, establishes the comprehensive reliability evaluation index system from the angle of narrow reliability, performance reliability, maintenance reliability and availability. The same time, due to the structure complexity and failure diversity of the heavy-duty CNC machine tools, this paper discusses the reliability assessment problem from the there different angle of the’failure’,’processing stability’ and’ performance retention’, and give the research reviews at home and abroad of the three reliability assessment methods focused on different angle.(2) This paper firstly analyse the running reliability of the heavy-duty CNC machine tools focused on’failure’, assess the machine’s reliability based on failure-time data and obtain the machine’s reliability index. This assessment method draws the scatter plot of the probability density function and the cumulative probability density function of machine’s time between failure and maintenance time. By comparing and excluding, some distribution models will be chosen to meet the scatter plot of the probability density function and the cumulative probability density function of machine’s time between failure and maintenance time. We can obtain the final distribution model and calculate all reliability indexes by K-S hypothesis testing.(3) This paper sencondly analyse the machine reliability from the angle of ’processing stability’, and assess the machine reliability by reliability assessment method based on dynamic characteristic parameters. The assessment method builds the nonlinear dynamic model of the process according to vibration characteristics of machine-tool-workpiece system. Then, the frequency response function and cutting force will be obtained by experiment. The dynamic characteristic parameters and cutting force model parameters could be obtained by fitting the frequency response function and the nonlinear relationship between the cutting force and the feed rate. The all dynamic characteristic parameters and cutting force model parameters will be taken into the nonlinear dynamic model and the model will be solved to obtain the machine simulation vibration curve. After verifying the nonlinear dynamic model by comparing with actual vibration curve, the process reliability index value at the specified operating conditions could be obtained according to the mechine simulation vibration curve.(4) This paper analys the performance degradation phenomenon in the running process of the heavy-duty CNC machine tools, and assess the machine reliability from the angle of’performance retention’based on performance degradation data. The reliability assessment uses the hidden markov chain model to build the hidden relationship model between performance degradation course and machine running state. Considering the machine’s characteristics of small sample and multi-performance, this paper improve the standard hidden markov chain model to obtain the hidden markov chain model that can train multi-sequence and multi-performance parameter. Based on the improved model, we can obtain the probability transition curve of each state and reliability change curve.(5) This paper analyses the affection of maintenance to reliability growth, and proposes the FMEA analysis based on fuzzy failure affection plot, and based on the analysis result of FMEA, differet maintenance strategies will be used for different failure models after considering maintenance cost, time and other infactors. The maintenance strategies optimization method could reduce effectively the maintenance cost and time better besides improve the machine’s reliability.(6) This paper analyses the impact of different process parameters on the processing stability and tries to improve the machine’s process reliability by optimizing machining process parameters combination. The same time, this paper takes the account of productivity, cost of production, dimensional accuracy and surface roughness factors into the process of machining process parameter optimization, and build the multi-objective machining process parameter optimization model based on nonlinear dynamic model. Search the machining process parameter’s feasible region by Grid Optimization Algorithm and obtain the more stable, more low-costand faster machining process parameter combinations, so as to impove machine’s machining performance and stability.Finally, a summary of the full article is proposed and the future research directions is pointed out.