Machine selection for improved precision and scrap reduction in manufacturing using PCIs: a case study

Abstract

To improve work precision on finished products and reduce scrap in manufacturing, manufacturers must, as a rule, carry out precision tests on their equipment, which can be done using Process Capability Indices (PCIs). This case study presents a situation where a manufacturing company has two automatic steel rod-cutting machines of different models, which we refer to as machines A and B. Four steel rods of length 2.5m and thickness 16mm were required for the study. Two lengths of steel rod were fed into each piece of equipment to be cut into a desired length of 40mm with tolerance limits of 40±2mm. The Minitab 20 statistical software package was used to investigate the stability of the cutting process for both equipment using the Individuals and Moving Range (I-MR) chart and also to carry out a process capability analysis. Both machines were found to be stable; however, machine B was found to be incapable of operating within the desired tolerance limits as its upper control limit was outside the upper specification limit of 42mm. Machine A has superior PCIs and the best performance expected overall of 0.46 in parts per million. Based on these findings, machine A was recommended for usage when tolerance limits are well within ±2 mm, and machine B may be used only when wider tolerance limits are permissible. This helps to improve the precision of elements cut for manufacturing and also helps to reduce the scrapping of materials.