Effect of Excessive , Understanding the sources of variation of a measurement system is important for all measurement applications but becomes even more critical where there is significant within-part variation. Within-part variation, such as taper or out-of-round, can cause the measurement system evaluation to provide misleading results. This is because unaccounted within-part variation affects the estimate of repeatability, reproducibility, or both. That is, the within-part variation can appear as a significant component of the measurement system variation. Understanding the within-part variation present in the product will result in a more meaningful understanding of the suitability of the measurement system for the task at hand.
Examples of within-part variation which may be encountered are: roundness (circular runout), concentricity, taper, flatness, profile, cylindricity, etc. P73F It is possible that more than one of these characteristics may be present at the same time within the same part (composite error). The strength of each characteristic and their interdependencies may compound the data and the resultant understanding of the measurement system. Recognition and analysis of these additional sources of variation during a measurement system study is paramount to understanding the actual measurement system variation. A D.O.E., ANOVA or other more sophisticated statistical technique may be required to analyze this situation. Whatever methodology is chosen should be approved by the customer supplier quality representative.
Also, decisions that product design engineers make can unwittingly affect how a part is measured, how a fixture may be designed, and the result can affect the outcome of measurement error. An example might be a plastic part that has a critical feature on the parting line (a parting line typically has excess plastic material where the two halves of the mold join, and thus is an uncontrolled surface). These factors ought to be considered during a Design FMEA.
Once the within-part variation components are understood, it may be possible to control these factors within the measurement system (e.g., by redesigning the gage or using different fixturing methods/equipment) so that future data is not confounded.
source of Recognizing the Effect of Excessive Within-Part Variation: Analysis of measurement systems
