Global market competitiveness imposes to many companies a high level of flexibility to allow for the production of a large variety of products. To limit work-in-process, decision makers periodically schedule according to a make-to-order management strategy the production of finite batches of the same product code. Scheduling calls for frequent set-up activities which reconfigure a manufacturing process to switch between different codes. This can limit the production horizon of one product code to a few hours or shifts. In this context, efficient on-line quality control monitoring by means of control charts is strategic to eliminate scrap or rework and to meet the demand on time according to the production plan. Design of control charts in processes with limited production horizon is a challenge tackled by Statistical Process Control (SPC) practitioners and scholars. Under this framework, this paper investigates the issues related to the implementation of the Variable Sampling Interval (VSI) Shewhart control chart in a process with finite production horizon. When the production horizon is finite, the statistical properties of a control chart are known to be a function of the number of scheduled inspections. In the case of a VSI control chart, the quality practitioner cannot fix the number of inspections a priori due to the stochastic nature of the sampling interval selection. Therefore, the aim of this paper is to propose a new Markov chain approach for the exact computation of the statistical performance of the VSI control chart in processes with an unknown and finite number of inspections. The proposed approach is general and does not depend on the monitored sample statistic. With reference to the process mean monitoring, an extensive numerical analysis compares the performance of the VSI X chart to the Variable Sample Size (VSS) and Fixed Sampling Rate (FSR) z charts. Numerical results show that the VSI X chart outperforms other charts for moderate to large shift sizes. An illustrative example shows the implementation of the VSI X chart in a short run producing a finite batch of mechanical parts.
|Titolo:||The Variable Sampling Interval control chart for short production runs|
|Data di pubblicazione:||2014|
|Appare nelle tipologie:||1.1 Articolo in rivista|