GD&T and Reliability

Don Day
Don Day co-founded Tec-Ease, Inc. over a dozen years ago to provide software as well as training materials and classes to help improve the design and manufacture of mechanical products. He has helped improve the design and production of numerous products from sump pumps to cell phones and satellites. In January of 2001 he became President and Chief Engineer of Inter-Mec Systems Manufacturing, more


Low process capability indices of Cp and Cpk cause poor product performance, early life failures and lost market share. The numerator of the Cp and Cpk indices depends upon the specified tolerance. Often Design Engineering "holds back" tolerance on the drawing. Sometimes this is because Design has not taken the time to perform adequate tolerance analysis. Other times it is simply because they do not understand how to specify tolerances in accordance with the ASME Y14.5 standard on dimensioning and tolerancing which establishes the rules and concepts for Today's engineering drawings.This session will illustrate how GD&T (Geometric Dimensioning and Tolerancing) can increase the design spec width and thereby increase the Cp and especially the Cpk values. The result is a more robust design in the factory and in the hands of the customer. A case study illustrating how to improve Cpk without ever changing the process will be presented. In this case study the original drawing requirement was too tightly toleranced to allow the supplier to meet the required Cpk. By examining the real design requirements and making use of the tools found in the Y14.5 standard, the specification was increased and the Supplier was able to produce parts that met the required Cpk.

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