Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Figure 1. Diagram summarized the interaction of accuracy, repeatability, and reproducibility. Series 1 Series 2 Neither accurate nor precise. Random and systematic errors Random error controlled; both present. Precise but inaccurate. systematic error still present. Series 3 Accurate and precise. Both random and systematic errors under control. Figure 2. Diagram compares a highly repeatable series of measurements in the top series of bull's eyes and a nonreproducible series of measurements in the bottom targets. Think of each bull's eye as representing measurements of sequential batches. Series 1 Times 1 Series 2 Times 2 Series 3 Time 3 intervals. View this as three successive batches coming out of a process. The concepts in this article apply to a wide range of processes. The specific parameters being measured can be anything—current, voltage, pH, concentrations of specific chemistry, temperature, thickness, hardness, reflectivity. Controlling the quality of the work passing through a shop demands constant vigilance. It requires knowing what to test, and it requires knowing what balance of accura- cy, precision, and reproducibilty are required to meet customer expectations. 517

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