Design for Limit Variability in Quality of Industrial Products: A Case Study of Cutix Cable Manufacturing Company, Nnewi, Nigeria

Abstract

This paper presents numerical approaches to assessment of process capability and product quality standards for production processes. One
hundred cables produced by cable production process of CUTIX factory
were sampled in order to access their insulation quality. Twenty-five samples were used from bathes in stock and each of the samples has sample size of four. The statistical method was used to establish the mean of distribution, the average standard deviation of samples and the average range of samples to establish the process control limits. The distribution of means of samples was normalized in order to ascertain the conformity of the distribution to normal distribution. The distribution was confirmed normal and transformed to standard normal distribution so that the area under the normal curve applies in the analysis of the process distribution. The area under the normal curve of the distribution was evaluated as 0.94 falling within acceptable limit for processes in control. Process control was established using classical relations and analogies to establish process capability and Process capability index. Control charts were developed for the mean and range of samples for the process monitoring period prior to the evaluation of the process population mean as 5.2, the average standard deviation of the process as 0.60, action limits 6.1 and 4.3 for the mean and for the range 2.3 and 0.0. The coefficient of variation was found to be 11% (0.11), indicating low variability of process. The CUTIX process for cable production therefore
produces within specification. Above all, the probability of any sample
observation being in the warning limit is 0.76 while the probability of any
sample observation being within the control limit is 0.94, showing that the
process in control.

Keywords: limit variability, quality of products, control limits, process
capability