model-based measurement uncertainty (2)

Model-Based CMM Measurement Uncertainty

Jon Baldwin, MetroSage LLC

Copyright 2008 MetroSage LLC

Outline• Task-specific uncertainty

– What it is– Why you should care– Why is it difficult?

• Influence variables in CMM metrology

• Simulation in the face of incomplete information


NIST on measurement uncertainty“A measurement result is complete only

when accompanied by a quantitative statement of its uncertainty. The uncertainty is required in order to decide if the result is adequate for its intended purpose and to ascertain if it is consistent with other similar results.” - The NIST Reference on Constants, Units and Uncertainty

©Copyright 2008 by MetroSage LLC


What is Task-specific Uncertainty?

It is: Specific to a particular measurand. Specific to a particular level of confidence. Sample Statement: “The uncertainty of

the diameter of the maximum inscribed cylinder that will just fit inside this nominally 3-inch diameter hole, measured with this particular CMM, under these specific conditions is ±0.0008 inches at 95% confidence.”


Why should I Care?- I

Measurement Traceability – National & international standards require it. For example, ISO 17025 states, “Calibration certificates and/or reports shall…state the traceability to…standards of measurement and shall provide the measurement results and associated uncertainty of measurement…”


Why should I Care?- I I• Profitability

– ASME B89.7.3.1, for example, provides guidance for decision rules to govern the acceptance or rejection of articles of commerce, and provides the basis for economic penalties for failure to conform to specifications.

– There are real costs associated with shipping bad product and for rejecting or reworking product that is incorrectly judged to be bad. For example, the cost of maintaining the warranty pool.

• Intangible costs: Reputation, customer perception, etc.

• Profitability– ASME B89.7.3.1, for example, provides

guidance for decision rules to govern the acceptance or rejection of articles of commerce, and provides the basis for economic penalties for failure to conform to specifications.

– There are real costs associated with shipping bad product and for rejecting or reworking product that is incorrectly judged to be bad. For example, the cost of maintaining the warranty pool.

• Intangible costs: Reputation, customer perception, etc.


Why is Task-specific CMM Uncertainty difficult?

• Generic CMM performance tests (B89.4.1, ISO 10360) will not do the job; they estimate single point error in the CMM volume and do not completely describe CMM errors.

• Many different measurands• Almost unlimited measurement conditions.• Unknown interactions between influence

variables.


CMM Influence VariablesCMM Errors

Feature FormErrors

ProbeSystemErrors

SamplingPatternsEnvironmental

Factors

Fitting Algorithms


CMM uncertainty evaluation methods (ISO 15530 draft)

• Sensitivity analysis• Expert judgment• Substitution• Simulation• Measurement history


Uncertainty Evaluation Methods - Comparison

Evaluation Method

Desirable Attributes of an Uncertainty Evaluation Method

TractableComprehensive

Detects Measure-ment Bias

Detects Measure-ment Variability Versatile Predictive Economical

Sensitivity Analysis ????? ????? Strong Strong Weak Strong Weak

Expert Judgment Strong ????? ????? ????? Weak Strong ?????

Substitution Strong Strong Strong Strong Weak Weak Weak

Computer Simulation Strong ????? Strong Strong Strong Strong ?????

Measurement History Strong Strong Weak ????? Weak Weak Weak


How PUNDIT/CMM WorksMeasurement Dataforming Bounding Measurement Set

Population of good virtual CMM states

One-to-many mapping viaData fitting to Kinematic Eqns

Point Coordinate Errors

One-to-one mapping via Kinematic Eqns

Substitute Geometry Errors

One-to-one mapping viaFitting algorithm forSubstitute Geometry

Substitute Geometry Uncertainty

Population of PossibleSubstitute Geometry Errors is Expressed asMeasurand Uncertainty


Some questions best answered with task specific uncertainty

• Will the CMM I intend to purchase meet the accuracy requirements for my specific measurement problems?

• What is the uncertainty of my current measurement results?

• What will be the effect of changing some specific aspect of my measurement procedure?

• Which of my available CMMs is best used to measure a this specific part?

• Where will I get the most bang for my CMM improvement bucks?

• What are the risks and costs associated with incorrect measurements?


Additional Benefits• Ensure tolerancing is complete, correct &

unambiguous.• Evaluate potential CMM purchases.• Choose the optimum CMM from your

stable.• Find the weak link in your measurement

process.• Economically train operators &

programmers in CMM best practices.

model-based measurement uncertainty (2)

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