Technical note
Failure probabilistic model of CNC lathes

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Abstract

A field failure analysis of computerized numerical control (CNC) lathes is described. Field failure data was collected over a period of two years on approximately 80 CNC lathes. A coding system to code failure data was devised and a failure analysis data bank of CNC lathes was established. The failure position and subsystem, failure mode and cause were analyzed to indicate the weak subsystem of a CNC lathe. Also, failure probabilistic model of CNC lathes was analyzed by fuzzy multicriteria comprehensive evaluation.

Introduction

During the last decade, computerized numerical control (CNC) lathes have increasingly been introduced into mechanical machining process. As a result of their considerable inherent flexibility, stable machining accuracy and high productivity, CNC lathes are of immense interest to the users. However, as the breakdown of a single CNC lathe may result in the production of an entire workshop being halted and repairs are more difficult and expensive when a breakdown occurs [1], CNC lathes are capable of creating lot of troubles to the users.

In the meantime, the manufacturers are also needed to improve continuously the reliability of CNC lathes to sharpen their competitive edge in the marketplace. So the reliability of CNC lathes has an increased significance and paramount importance both to the manufacturers and the users.

The need to improve the reliability of CNC lathes forces the users and manufacturers to collect and analyze field failure data and take measures to reduce downtime. This paper discusses the studies on the failure mode and cause, failure position and weak subsystem, failure probabilistic model of CNC lathes.

Section snippets

General description of CNC lathes

A CNC lathe is a complex system, with high-level automation and complicated structure, which employs mechanics, electronics, hydraulics and so on. It is mainly composed of the mechanical system; CNC system and hydraulic or/and air feed system [2], [3], [4]. Fig. 1 is the system block diagram of a typical CNC lathe.

The mechanical system includes spindle and its transmission (fixed in a headstock), two slide axes (named X, Z or U, W in turns), carriage apron, turret or tool-holder, tailstock, bed

Data collection

It was difficult to collect reliable field failure data of CNC lathes several years ago, because users seldom kept adequate and complete maintenance records beyond the warranty period [1], [5], [6], [7]. Fortunately, a state administrative institution in this country had made a mandatory rule that all CNC machine tools users had to trace the CNC machine tools performance and keep and feedback the complete maintenance reports to the manufacturers and the related research institutions within and

Frequency analysis of failure position and subsystem

To find the weakest subsystem, we count the number of failures for each subsystem by retrieving the subsystem code defined in Table 2 from the data bank, then calculate the frequency of failures of each subsystem.

Table 2 is the code of failure position and subsystem and the frequency of failures of each subsystem for CNC lathes. Fig. 3 is the histogram of failure position and subsystem. It can be seen that the main failure subsystems are the electric and electronic system, turret, CNC system,

Probabilistic model of failure

Early work [1], suggested that the failure pattern could be best described using the Weibull [9] or lognormal distributions [10]. The parameters of these distributions were usually estimated using maximum likelihood method (MLM) or least square method (LSM), and Kolmogorov–Smirnov test statistic Dmax [11] or χ2 test statistic [12] was usually used to test the goodness of fit.

There is some fuzzy information while determining a distribution to describe the observed data. Generally, when a

Conclusion

  • 1.

    The lognormal or Weibull distributions provide suitable vehicles for the analysis of the failure patterns of CNC lathes. The lognormal distribution provides the best fit to describe time between successive failures. It is different from that of electronic equipment (failure rate is constant) and mechanical product (failure rate increases within lifetime) [15], [16].

  • 2.

    The lognormal distribution provides the best fit to describe repair times of CNC lathes. It shows that maintenance process of CNC

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