Process Capability Control Charts for Monitoring Process Accuracy and Precision

Process capability index (PCI) is a convenient and useful tool of process quality evaluation that allows a company to have a complete picture of its manufacturing process in order to prevent defective products while ensuring the product quality is at the required level. The aim of this study was to develop a control chart for process incapability index Cpp, which differentiates between information related to accuracy and precision. Index Cia measures process inaccuracy as the degree to which the mean departs from the target value, while index Cip measures imprecision in terms of process variation. The most important advantage of using these control charts of Cpp, Cia, and Cip is that practitioners can monitor and evaluate both the quality of the process and the differences in process capability. The Cia and Cip charts were instead of Shewhart's X over bar and S chart since the process target values and tolerances can be incorporated in the charts for evaluation as a whole, which makes the charts capable of monitoring process stability and quality simultaneously. The proposed Cpp, Cia, and Cip control charts enable practitioners to monitor and evaluate process quality as well as differences in process capability. The control charts are defined using probability limits, and operating characteristic (OC) curves used to detect shifts in process quality. The method proposed in this study can easily and accurately determine the process quality capability and a case is used to illustrate the application of control charts of Cpp, Cia, and Cip.