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Measurement Systems Analysis
- Gages should be both accurate (return, on average, the actual dimension of the part or standard) and precise (return the same measurement consistently). Accuracy is ensured by calibration while precision is measured by MSA
- Gage capability is a function of repeatability (random error inherent in the gage) and reproducibility (inspector dependence). The gage's capability is measured in terms of the precision to tolerance (P/T) ratio
- Recognize other influences such as linearity and bias and the effect of environment (temperature in particular) on measurements
- Know how gage variation affects the estimate of the process capability or process performance index,reduces the ability of SPC charts to detect process shifts and affects the risk of rejecting borderline acceptable product and accepting borderline nonconforming product
- Know the procedure for performing an R&R study
- Instructions will be provided for StatGraphics, Minitab and Excel
- Know potential remedies for non-capable gages such as replication of measurements (where practical) and guard banding
- An appendix in the handout (will not be covered in the presentation) provides additional technical detail
Overview of the webinar
Measurement systems analysis (MSA) quantifies gage precision, or the ability of an instrument to return consistent measurements, in terms of (1) repeatability or random variation, and (2) reproducibility, or inspector dependence. The result is gage reproducibility and repeatability (GRR), which is expressed in terms of standard deviation, and in the units being measured. As an example, if the measurement is in microns, GRR will be expressed in microns. The precision/tolerance (P/T) ratio is then proportional to GRR and inversely proportional to the specification width. Less is better, with 10% or less being highly desirable.
A gage study is a designed experiment in which two or more inspectors measure a sufficient number of parts (at least twice per part per inspector) to obtain estimates of repeatability and reproducibility. Gages will ideally not depend on the inspector; as an example, people may read a dial gage somewhat differently but everybody will get the same result from a digital output. Inspector-related dependence may also result, however, from different perceptions of how a digital micrometer lines up with a product feature, or the definition of "finger tight." The influence of the environment on measurements (especially temperature when coefficient of expansion is an issue) cannot be overemphasized.
Remedies for non-capable gages include replicate measurements which, when practical, suppress repeatability, and guard banding which protects the customer at the expense of rejection of some borderline acceptable product. Attendees will receive d2.xls (the d*2 factors used for manual calculations in the average and range method) and also the Excel spreadsheet and calculations used in the slides.
Who should attend?
- Quality Engineer
- Quality Technician
- Quality Inspector
- Maintenance Technician
- Quality Manager
Why should you attend?
Gage calibration is a mandatory activity under ISO 9001:2015 (clause 7.1.5, Monitoring and Measuring Resources), but measurement systems analysis (MSA) is not required explicitly as it is in the automotive industry. This does not mean it can or should be overlooked because non-capable gages and instruments risk (1) shipment of nonconforming work to customers and (2) rejection of in-specification work. They also make it more difficult for statistical process control (SPC) charts to detect process shifts. The procedure for MSA is however well established, generally accepted throughout industry, and very straightforward. The necessary calculations can be handled easily with off-the-shelf statistical software or on a spreadsheet if one wants to take the time to program it.
Faculty - Mr.William A. Levinson
William A. Levinson, P.E., is the principal of Levinson Productivity Systems, P.C. He is an ASQ Fellow, Certified Quality Engineer, Quality Auditor, Quality Manager, Reliability Engineer and Six Sigma Black Belt. He is also the author of several books on quality, productivity and management, of which the most recent is The Expanded and Annotated My Life and Work: Henry Ford's Universal Code for World-Class Success.