Six Sigma Quality Engineering Week 5 Chapters 5 (Measure Phase) Variable Gauge R&R “Data is only as good as the system that measures it. If you can’t measure it, you can’t manage it.”
The Science of Measurement “I often say that when you measure what you are speaking about and express it in numbers, you know something about it.” LORD KELVIN, 1891 He clearly stressed that little progress is possible in any field of investigation without the ability to measure. The progress of measurement is, in fact, the progress of science.
Objectives • Measurement Systems Analysis • Key Terminology • Variable Gauge R&R • A tool for estimating measurement system error • How to conduct a gauge R&R • Minitab Output • Gauge R & R Study Exercise
Definitions • Variable Data • Continuous measurements such as length, voltage, viscosity • Repeatability • Variation in measurements obtained with one gage when used several times by one appraiser. • Reproducibility • Variation in the average of the measurements made by different appraisers using the same measurement system.
What is GR&R? Measurement Systems Analysis How good is our measurement system? 2T = 2p + 2m 2T = Total Variance 2p = Process Variance 2m = Measurement Variance GRRRRRRR!!!
Observed Process Variation Actual Process Variation Measurement Variation Long-term Process Variation Short-term Process Variation Variation within a Sample Variation due to Gage Variation due to Operators Repeatability Reproducibility Linearity Accuracy Stability Possible Source of Variation
Variable Gauge R&R - What’s Involved? 3 Appraisers 1 Gauge 10 Parts
How to set up a Variable GRR Study • Preparation & Planning • 1 Gauge • 3 Operators (Appraisers) • 10 Parts • 3 Trials • Randomize the readings • Code the parts (blind study) if possible • 3 Ops x 10 parts x 3 trails = 90 Data Points • 4 Ops x 10 parts x 3 trails = 120 Data Points
Minitab Gage R&R Graphical Output The number of distinct categories of parts that the process is currently able to distinguish (Must distinguish at least 5 types of parts)
Acceptability Criteria • R&R Indices • 10% Acceptable Measurement System • 10% - 30% May be acceptable based upon application, cost of measurement device, cost of repair, etc. • 30%Not acceptable. Measurement system needs improvement. • Number of Distinct Categories Index • 1 Unacceptable. One part cannot be distinguished form another. • 2 -4 Generally unacceptable • 5 Recommended Module 0025