Measurements Chapter Four Measurement Systems Analysis Measurement accuracy and precision Measurement Systems Analysis Accuracy and precision Measurements are said to be accurate if their tendency is.
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Measurements
Chapter Four Measurement Systems Analysis
Measurement accuracy and precision
Measurement Systems Analysis
Accuracy and precision
Measurements are said to be accurate if their tendency is to center around the actual value of the entity being measured.
Measurements are precise if they differ from one another by a small amount.
What can you say about the causes relating to each type?
Measurement Systems Analysis
Measurement Systems Analysis
2
observed
2
product
2
gage
Measurement Systems Analysis
Measured Value Measured Value = ƒ(TV + Ac + Rep + Rpr) TV = true value Ac = gauge accuracy Rep = gauge repeatability Rpr = gauge reproducibility
Measurement Systems Analysis
Measurement system components
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 Measurement Systems Analysis
Measurement error
Measurement error is considered to be the difference between a value measured and the true value.
Measurement Systems Analysis
Types of gage variation
Systematic variation • Accuracy - improper calibration • Reproducibility - different persons using same equipment with different techniques Periodic variation • Stability - wear, deterioration, environment Random variation • Repeatability (unable to locate part to be measured) Measurement Systems Analysis
Types of measurement variation
Accuracy Stability Reproducibility Repeatability Measurement Systems Analysis
Accuracy Difference between the true average and the observed average.
(True average may be obtained by using a more precise measuring tool) Accuracy or mean bias True average Observed average
Measurement Systems Analysis
Stability The difference in the average of at least 2 sets of measurements obtained with a gage over time.
Stability Time 1
Measurement Systems Analysis
Time 2
Reproducibility Variation in average of measurements made by different operators using the same gage measuring the same part.
True Average Operator B Operator C Operator A
Measurement Systems Analysis
Repeatability The random variation in measurements when one operator uses the same gage to measure the same part several times.
Observed Average Repeatability
Measurement Systems Analysis
True Average
How do we improve gage capability?
Reproducibility • operator training, or • more clearly define measurement scale available to the operator Repeatability • gage maintenance • gage redesign to better fit application Measurement Systems Analysis