Abstract: In minimal repair, in order to describe the failure process of numerical control(NC) machine tools experiencing both early failures and deterioration phenomena, and operating so long that the intensity function approaches a finite asymptote as the system operating time grows, a new four-parameter non-homogeneous Poisson process model is proposed. The characteristics of the model are discussed, and the physical meaning of its parameters is given, calculating formula of point estimation for model parameters and reliability indices such as the minimum intensity value and the time of minimum intensity are all derived, then a method of model selection for reliability assessment of two NC machine tools is provided based on the likelihood ratio statistic, and the model evaluation criterion is given based on the index of goodness-of-fit test. The real failure data of two NC machine tools are analyzed by using the proposed method, the results show that this method can quantitatively evaluate the machine tools’ lasting time of early failure period, it is suitable for describing the failure process of NC machine tools with bounded and bathtub shaped failure trend, and can provide a theoretical basis for eliminating as much early failures as possible before the NC machine tools leave the factory.

Key words: four-parameter non-homogeneous poisson process(4-NHPP);minimal repair;bathtub shaped failure intensity;numerical control machine tools;reliability analysis