3rd ICAI 2024

International Conference on Automotive Industry 2024

Mladá Boleslav, Czech Republic

used to control the process using statistical methods may be the resolution of the instrument. The resolution must meet the following conditions: 1. Enable process analysis by detecting process variability 2. Enable process control by detecting variability due to special causes. The basic rule of thumb for selecting the appropriate resolution of the instrument used in industrial practice is the rule of 1 to 10. This principle is included in the guidelines for the methods of analysis of measurement systems issued by AIAG [1]. “ The increments of measure should be small relative to the process variation or specification limits for the purpose of measurement. The commonly known Rule of Tens, or 10-to-1 Rule, states that instrument discrimination should divide the tolerance (or process variation) into ten parts or more .” This sentence introduces some inaccuracies due to the use of the word or with respect to the tolerance field and the width of the variation of the process. The quoted principle suggests that one of the measures can be freely adopted, e.g. the one related to the product for which the tolerance field is defined or related to the process for which the dispersion measure in the form of e.g. standard deviation or variance must be known. Difficulties in applying this principle may arise at the stage of designing a measurement system for the monitoring of new processes for which the use of statistical quality control methods is planned. Without appropriate measurements with a properly selected measuring instrument, it will most likely not be possible to determine the width of the range of variation measured by standard deviation. The deviation can only be calculated based on the tests carried out and the analysis of the measurement results, and the possible results of the measurements can hardly be considered appropriate without being sure whether a device with the appropriate resolution has been selected to carry them out. In such cases, at the stage of selecting the appropriate measuring instrument, we must use the width of the tolerance area as a reference measure for the application of the “1 to 10” rule. When planning the construction of a measurement system, you need to ask yourself what the measurement results will be used for. The main application of the measurement results is the monitoring of the process status with the use of e.g. control charts and process capability indicators from the family of indicators type Cp. The quality capability index of the Cp process is calculated as the ratio of the tolerance area of the tested feature to the interval of “normal” variability of this feature (in accordance with the three-sigma rule)

The question arises, is there any way to select the resolution of the measuring device depending on the assumed value of the Cp index and the known value of the tolerance field, so that the obtained results represent the variability of the process and are not “masked” by insufficient resolution of the instrument?

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