SPC Modified With Percent Tolerance Precontrol Charts

Article

Vermani, S.K.   (2000, ASQ)   ASQ

Quality Progress    Vol. 33    No. 10
QICID: 13990    October 2000    pp. 43-48
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Article Abstract

The Military Aircraft and Missile Systems Group (A&M) of Boeing in St. Louis analyzed years of product acceptance data in 1994 and discovered that receiving inspection and source inspection at the facilities of suppliers was not adequate for predicting the quality of the suppliers' products. The group took a new approach that assured and improved the quality of supplier products by helping them create and improve their manufacturing processes. The focus of supplier quality management (SQM) at Boeing A&M changed from product inspection to process validation, and Boeing became both a partner and process consultant to its suppliers. The process involved periodic process validation assessments (PVA) at the facilities of suppliers by quality professionals. The periodic validation assessments were designed to monitor the health of suppliers' processes, compare process elements of suppliers with current industry standards, and recommend process monitoring and improvement tools to suppliers. The approach taken by Boeing A&M succeeded in reducing cycle time and providing cost savings for both A&M and its suppliers. In the past decade, however, the implementation of traditional SPC for monitoring and controlling manufacturing processes became more difficult because of drastic reductions in defense spending, which led to lower production rates and smaller lot sizes. Supplier quality management had to modify SPC concepts to adapt to the new environment. Working with one of A&M's suppliers, SQM developed an effective approach for monitoring the manufacturing processes of Essex Corporation, the supplier. Since maintenance of a large number of control charts proved burdensome and provided insufficient data, an innovative approach based on precontrol and short run charts was used. This approach, called "percent tolerance precontrol chart" or PTPCC, combines the principles of both and works well for high-capability processes. Precontrol charts are the focus of some controversy, but the simplicity of the percent tolerance precontrol chart proved beneficial in this environment.

Keywords

Process analysis,Process management,Quality assurance (QA),Short runs,Control charts,Tolerances,Aerospace industry,Case study,Statistical process control (SPC)


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