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TRANSCRIPT
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Measurement Systems Analysis
Measurements
Chapter Four
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Measurement Systems Analysis
Measurement accuracy andprecision
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Measurement Systems Analysis
Measurement SystemsAnalysis
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gage product observed
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Measurement Systems Analysis
Measured Value
Measured Value = ƒ(TV + Ac + Rep + Rpr)
TV = true value
Ac = gauge accuracyRep = gauge repeatabilityRpr = gauge reproducibility
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Measurement Systems Analysis
Measurement systemcomponents Equipment or gage
• Type of gage
•
Attribute: go-no go, vision systems(part present or not present)
• Variable: calipers, probe, coordinate
measurement machines• Unit of measurement - usually at least 1/10
of tolerance
Operator and operating instructions
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Measurement error
Measurement error is considered to be
the difference between a value measured
and the true value.
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Types of gage variation
Systematic variation
• Accuracy - improper calibration
•
Reproducibility - different persons usingsame equipment with different techniques
Periodic variation
•
Stability - wear, deterioration, environment Random variation
• Repeatability (unable to locate part to be
measured)
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Types of measurementvariation Accuracy
Stability
Reproducibility
Repeatability
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AccuracyDifference between the true average and theobserved average.
(True average may be obtained by using a more precisemeasuring tool)
True average
Observedaverage
Accuracy ormean bias
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Stability
Time 1 Time 2
The difference in the average of at least 2 sets ofmeasurements obtained with a gage over time.
Stability
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Reproducibility
Operator B
Operator A
Operator C
True Average
Variation in average of measurements made bydifferent operators using the same gage measuringthe same part.
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How do we improve gagecapability? Reproducibility
• operator training, or
•
more clearly define measurement scaleavailable to the operator
Repeatability
•
gage maintenance• gage redesign to better fit application