Evaluating Measurement Systems and Manufacturing Processes Using Three Quality Measures

Abstract

Manufacturers and suppliers use quality measures calculated from dimensional data to make informed decisions regarding measurement systems and product quality. Many manufacturers and suppliers use the precision-to-tolerance ratio—scaling the standard deviation of gage error (σ_g) by the design tolerance—to approve a measurement system. Manufacturers and suppliers also use one or more measures of process capability, such as C _p that scales the tolerance by the standard deviation of the product (σ_p), to approve a manufacturing process. A measure used to assess the ability for two parties to communicate via dimension data is the correlation in repeat measurements that we derive as a function of σ_g and σ_p. By plotting the precision-to-tolerance ratio and the correlation in repeat measurements on the σ_g and σ_p axes, acceptable and nonacceptable regions for measurement systems are defined. When we add C _p (a measure of process capability) to the mix, a relationship between the three measures suggests a method for determining an acceptable level for the correlation criterion and defines additional regions. This approach of plotting the quality measurement criteria in the σ_g and σ_p axes precisely defines the quality situation and lends to improvement suggestions.

Keywords:

• Measurement system analysis
• Process capability
• Precision-to-tolerance ratio
• Correlation
• Supplier quality

ACKNOWLEDGMENT

The first author was supported in part by NSF grant SBR-9712997 to The University of Michigan.