Improving Reliability of GD&T Evaluations Using Portable Measuring Systems

Abstract

GD&T is an engineering drawing language establishing consistent communication between design intent, manufacturing and inspection.  The standard does not address measurement variation and uncertainty, or physical gage fabrication errors.  This paper evaluates several measurement strategies to improve the reliability of GD&T evaluations when using portable measurement equipment. These systems have small, non-zero measurement variation.  The impact of this variation on determining conformance / non-conformance is often dependent on how the measured values are determined.  The goal of this paper is to examine several methods of mitigating measurement variation, thereby improving the reliability of the inspection process.  This paper looks at several methods for determining measured values – data set size, averaging of multiple measurements, guard banding, and localized measurements.  The paper concludes that localized averaging within feature boundaries is probably the best, though not foolproof, method for inspection.