Post on 04-Jun-2018
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Measurement Systems Analysis
MeasurementsChapter Four
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Measurement
Measurement accuracy andprecision
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Measurement
Accuracy and precisionMeasurements are said to be accurate iftheir tendency is to center around theactual value of the entity beingmeasured.Measurements are precise if they differ
from one another by a small amount.What can you say about the causesrelating to each type?
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Measurement
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 errorMeasurement error is considered to bethe difference between a value measuredand the true value.
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Types of gage variationSystematic variation
Accuracy - improper calibration
Reproducibility - different persons usingsame equipment with different techniques
Periodic variation
Stability - wear, deterioration, environmentRandom variation
Repeatability (unable to locate part to be
measured)
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Types of measurementvariation Accuracy
Stability
Reproducibility
Repeatability
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Measurement
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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Measurement
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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Measurement
RepeatabilityThe random variation in measurements when oneoperator uses the same gage to measure the samepart several times.
True Average
Observed Average
Repeatability
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Measurement
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