Combining attribute and variable data to monitor process variability: MIX S 2 control chart

• Published in: International journal of advanced manufacturing technology 2016-12, Vol.87 (9-12), p.3389-3396
• Main Authors: Ho, Linda Lee, da Costa Quinino, Roberto
• Format: Article
• Language: English
• Subjects: Control charts; Mathematical analysis; Phase control; Project specifications
• ISSN: 0268-3768; 1433-3015
• DOI: 10.1007/s00170-016-8702-5

The necessity to monitor process variability to maintain similar, but more economical, performance to that of the S2 control chart in terms of ARL1 is the motivation of this paper. In general, a sample of four to six units is used to build an S2 control chart. Reducing the sample size to two or three units is a possibility, but the performance will be poor (in terms of ARL1), and the project specification may not be met. Additional cost would be required to remove the nonconforming units before shipment to the customers. This would compromise the quality of products in an environment of high worldwide competitiveness. The aim of this paper is to present the MIX S2 control chart, which employs a two-phase control chart to monitor the variability. At every interval of h hours, each unit is sequentially classified as approved or rejected by employing a Go/No Go ring gauge. A unit is approved if its dimension lies in [LDL; UDL], where L(U)DL is the lower(upper) discriminant limit; otherwise, it is rejected. If A1-approved items are first observed, then the production continues. However, if B1-rejected items are first observed, then n (n