Ch04 Measurements

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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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Measurement

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

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Ch04 Measurements