variability and spc lesson52

8/6/2019 Variability and Spc Lesson52

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Statistical Process Control 04/03/96

What is Variation?What is Variation?

Less VariationLess Variation

==

Higher QualityHigher Quality


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What is Variation?What is Variation?How the Loss Function Affects ProductHow the Loss Function Affects Product

Both are within specBoth are within specWhich is more desirable?Which is more desirable?

LSLLSL USL USL


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Examining VariationExamining VariationDefinitionDefinition

AA Stable ProcessStable Process has the samehas the same

normal distribution at all times.normal distribution at all times.A stable process isA stable process is In ControlIn Control

A stable process still has variation A stable process still has variation


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Examining VariationExamining VariationStable ProcessStable Process

Normal distribution at all timesNormal distribution at all times

Prediction

Time


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Examining VariationExamining Variation

Common CausesCommon Causes

The cause of variations in a stable The cause of variations in a stableprocess is called aprocess is called a CommonCommon

CauseCause ..

A common cause is a natural cause of A common cause is a natural cause of variation in the system.variation in the system.


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Machine vibrationMachine vibration Temperature fluctuations Temperature fluctuationsSlight variation in raw materialsSlight variation in raw materialsHuman variation in setting controlHuman variation in setting controldialsdials

Common Cause ExamplesCommon Cause Examples


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Examining VariationExamining Variation Tools for Examining Stability Tools for Examining Stability

Trend ChartTrend Chart : A plot showing the: A plot showing thebehavior of a process over time.behavior of a process over time.

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T h i c k n e s s

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Time


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Examining VariationExamining Variation Tools for Examining Stability Tools for Examining Stability

HistogramHistog ram : A barchart showing the: A barchart showing thedistribution of the process.distribution of the process.

P e r c e n t a g e

0

5

10

1520

25

30

35

1 0

2 0

3 0

4 0

5 0

6 0

7 0 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

1 4 0

1 5 0

1 6 0

1 7 0

1 8 0

1 9 0

2 0 0

Thickness


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T h i c k n e

s s

Activity: Comparing stable processesActivity: Comparing stable processes

Which process has better quality?Which process has better quality?

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T h i c k n e s s

Sequence0 5 10 15 20 25

Sequence0 5 10 15 20 25

A A BB


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Examining VariationExamining VariationActivity: Comparing stable processesActivity: Comparing stable processes(contd)(contd)

Which process has better quality?Which process has better quality?

T h i c k n e

s s

1501401301201101009080706050

1501401301201101009080706050

T h i c k n e s s

Sequence0 5 10 15 20 25

Sequence0 5 10 15 20 25

A A BB


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Examining VariationExamining VariationUnstable ProcessUnstable Process

Any process that is not stable is called anAny process that is not stable is called anunstableunstable oror out-of-controlout-of-control process.process.

Prediction

Time

??


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Examining VariationExamining VariationKinds of Instability:Kinds of Instability: ExcursionsExcursions

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T h i c k n e s s

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Time


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Examining VariationExamining VariationKinds of Instability:Kinds of Instability: ShiftsShifts

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T h i c k n e s s

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Time


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Examining VariationExamining VariationKinds of Instability:Kinds of Instability: DriftsDrifts

200180

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T h i c k n e s s

200180

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Time


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Examining VariationExamining VariationKinds of Instability:Kinds of Instability: CyclesCycles

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T h i c k n e s s

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Time


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Examining VariationExamining VariationKinds of Instability:Kinds of Instability: ChaosChaos

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T h i c k n e s s

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Time


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Special CausesSpecial Causes

Anything that causes variations thatAnything that causes variations thatare not part of the stable processare not part of the stable process

is called ais called a special causesp ecial cause ,,

assignable causeassig nable cause , or, or unnaturalunnaturalcausecause ..


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Batch of defective raw materialBatch of defective raw materialFaulty set-upFaulty set-upHuman errorHuman error

Incorrect recipeIncorrect recipeBlown gasketBlown gasketEarthquakeEarthquake

Examples of Special CausesExamples of Special Causes


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Reducing VariationReducing Variation

Centering at TargetCentering at Target

Reducing Common Cause VariationReducing Common Cause Variation

Improving a Stable ProcessImproving a Stable Processwo strategies for improving a stable proceswo strategies for improving a stable proces


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Reducing VariationReducing VariationCentering at TargetCentering at Target

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40200

T h i c k n e s s

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Time


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Reducing VariationReducing VariationReducing Common Cause VariationReducing Common Cause Variation

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Structured problem solvingStructured problem solving

Planned experimentsPlanned experiments

Reducing Variation in a StableReducing Variation in a StableProcessProcess

Make Permanent ChangesMake Permanent ChangesChanges are based on the scientificChanges are based on the scientific

approachapproach

Examples: new equipment, equipment upgrade, new procedure, new machine

settings, better raw material


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Do not Do not ignore special causes.ignore special causes.

DoDo quickly detect special causequickly detect special causevariations.variations.DoDo stop production until the process isstop production until the process isfixed. (Reactive)fixed. (Reactive)DoDo identify and permanently eliminateidentify and permanently eliminatespecial causes. (Preventive)special causes. (Preventive)

Reducing Variation in anReducing Variation in anUnstable ProcessUnstable Process


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DetectDetect the special cause variation.the special cause variation.

IdentifyIdentify the special cause.the special cause.FixFix the processthe process Remove the special cause, orRemove the special cause, or

Compensate for the special cause.Compensate for the special cause.PreventPrevent the special cause fromthe special cause fromoccurring againoccurring again

Improving an Unstable ProcessImproving an Unstable ProcessFour Step ProcessFour Step Process


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Reducing VariationReducing VariationImproving an Unstable ProcessImproving an Unstable Process

ReactiveReactive200180

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40200

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40200

Time

Detect HereNot Here

Detect Here


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Detecting VariationDetecting Variation

How can we decide if How can we decide if variation is the result of variation is the result of common or special cause?common or special cause?


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Detecting VariationDetecting Variation Tool: Control Chart Tool: Control Chart

Benefit: Prevents tampering or ignoringBenefit: Prevents tampering or ignoring

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Detecting VariationDetecting VariationControl Chart for Detecting VariationControl Chart for Detecting Variation

Control ChartControl Chart

Trend Chart Trend Chart Center LineCenter Line Control LimitsControl Limits++ ++

Upper Control Limit

Lower Control Limit

Center Line


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Detecting VariationDetecting VariationCreating a Control ChartCreating a Control Chart

Turn the distribution on its side Turn the distribution on its side

Upper Control Limit

Lower Control Limit

Center Line


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What is the Center Line?What is the Center Line?Process mean, based on historicalProcess mean, based on historical

datadataor or

Process TargetProcess Target

Creating a Control ChartCreating a Control Chart


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Creating a Control ChartCreating a Control ChartSelecting the Center LineSelecting the Center Line

The center line should be the The center line should be thetarget, unless we aretarget, unless we are unableunable ororunwilling to control the processunwilling to control the processto target.to target.

Since the target is zero defects,Since the target is zero defects,the center line is the processthe center line is the processmean.mean.

Measurements:Measurements:

Defects:Defects:


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Control LimitsControl Limits

Based onBased onperformance of performance of thethe process process ..

Tell us when to Tell us when totake action ontake action onthethe process process ..

Spec LimitsSpec Limits

Based onBased onperformance of performance of thethe product product ..

Tell us when to Tell us when todisposition thedisposition the product product ..

Control Limits vs. Spec LimitsControl Limits vs. Spec Limits


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Focus OnFocus On

Control LimitsControl Limits

Improve ProcessImprove ProcessQualityQuality

Spec LimitsSpec Limits

Improve ProductImprove ProductQualityQuality

Control Limits vs. Spec Limits


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Measurement Capability Measurement Capability Have you ever been bitten by aHave you ever been bitten by a

measurement system?measurement system?

METROLOGYSYSTEM

True TrueDataData

ObservedObservedDataData

black box


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Measurement Capability Measurement Capability A Measurement ProcessA Measurement Process

Measurement tools themselvesMeasurement tools themselves hardwarehardware

softwaresoftware

All the procedures for using the toolsAll the procedures for using the tools which operatorswhich operators set-up/handling proceduresset-up/handling procedures

off-line calculations and data entryoff-line calculations and data entry calibration frequency and techniquecalibration frequency and technique


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Measurement Capability Measurement Capability Why Do Measurements Vary?Why Do Measurements Vary?

NOTE: Not all of these will necessarily beNOTE: Not all of these will necessarily besignificant sources of variation for everysignificant sources of variation for every

measurement system.measurement system.

MeasuremenVariation

Work Methodsease of data entry

operator training calibration frequencyoperator technique maintenance of standards

standard procedure sufficient time for work

Environment

line voltagevariation

temperaturefluctuation

vibrationhumidity

fluctuation

cleanliness

Tool

mechanicalinstability

electricalinstability

wearalgorithm

instability


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Measurement Capability Measurement Capability

Assumptions We Often MakeAssumptions We Often Make

Metrology tools are perfectly accurateMetrology tools are perfectly accurate

No day-to-day variation in performanceNo day-to-day variation in performance

No operator-to-operator variationNo operator-to-operator variation


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Measurement Capability Measurement Capability MCA Tells Us:MCA Tells Us:

How big is the measurement error?How big is the measurement error?What are the sources of measurement error?What are the sources of measurement error?

Is the tool stable over time?Is the tool stable over time?Is the tool capable of making theIs the tool capable of making themeasurements for this project?measurements for this project?Is the tool capable of making theIs the tool capable of making themeasurements for this process?measurements for this process?What needs to be done to improve theWhat needs to be done to improve themeasurement process?measurement process?


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Measurement Capability Measurement Capability Capability vs. CalibrationCapability vs. Calibration

CalibrationCalibrationProcedure to compare readings from aProcedure to compare readings from a

tool with a standard and then correcttool with a standard and then correctfor any deviations.for any deviations.Statistically: centering the mean of theStatistically: centering the mean of the

distribution of readings on the truedistribution of readings on the truevalue (obtained from a standard).value (obtained from a standard).


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Measurement Capability Measurement Capability Capability vs. Calibration (contd)Capability vs. Calibration (contd)

CapabilityCapabilityProcedure to identify and quantify sources of Procedure to identify and quantify sources of

variation in readings and then eliminate them.variation in readings and then eliminate them.

Statistically: fitting the model to the readingsStatistically: fitting the model to the readingsso that the components of variance can beso that the components of variance can be

estimatedestimated ..Both work together to keepBoth work tog ether to keep

measurement tool performing optimally.measurement tool p erforming optimally.


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Concepts and Vocabulary Concepts and Vocabulary Sources Of VariationSources Of Variation

Process Variation

Measurement Variation

Total Variation

+

=


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IntroductionIntroduction Total Variation Total Variation

ProductProduct MeasurementMeasurementSystemSystem

AccuracyAccuracy PrecisionPrecision


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Concepts and Vocabulary Concepts and Vocabulary

AccuracyAccuracy The degree to which a process The degree to which a process

mean is on targetmean is on target

Related TermsRelated Terms

True Value True ValueBiasBias


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PrecisionPrecision The degree of variability in a process The degree of variability in a process

Related TermsRelated TermsRepeatabilityRepeatability

ReproducibilityReproducibility


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Concepts and Vocabulary Concepts and Vocabulary BiasBias

ObservedAverage

TrueValue

bias


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Precision Says Nothing AboutPrecision Says Nothing AboutHow Close The MeasurementsHow Close The Measurements

Are To The Truth.Are To The Truth.

Accuracy Says Nothing AboutAccuracy Says Nothing AboutHow Close Measurements Are ToHow Close Measurements Are To

Each Other.Each Other.


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Concepts and Vocabulary Concepts and Vocabulary PrecisionPrecisionCan be separated into repeatability and reproducibilityCan be separated into repeatability and reproducibility

Total TotalVariationVariation

ProductProduct MeasurementMeasurementSystemSystem

AccuracyAccuracy PrecisionPrecision

These characteristics have the relationship: These characteristics have the relationship: 22 msms == 22 rptrpt ++ 22 rpdrpd

RepeatabilityRepeatability ReproducibilityReproducibility


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Concepts and Vocabulary Concepts and Vocabulary RepeatabilityRepeatability

Variation that results when repeatedVariation that results when repeatedmeasurements are made of the same parametermeasurements are made of the same parameter

underunder absolutely identical conditionsabsolutely identical conditions ..

Same operatorSame operatorSame set-up procedureSame set-up procedureSame partSame part

Same environmental conditionsSame environmental conditionsRepeatability (Repeatability ( 22 rptrpt ) is usually much smaller) is usually much smaller

(better) than the precision of the system.(better) than the precision of the system.


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Concepts and Vocabulary Concepts and Vocabulary ReproducibilityReproducibility

The variation that results when different The variation that results when differentconditions are used to make theconditions are used to make the

measurement.measurement.Different OperatorsDifferent OperatorsDifferent Set-Up ProceduresDifferent Set-Up ProceduresDifferent Measurement ToolsDifferent Measurement Tools

Different Environmental ConditionsDifferent Environmental ConditionsDifferent DaysDifferent Days

Reproducibility (Reproducibility ( rpdrpd ), is approximately the standard), is approximately the standarddeviation of the averages of measurements from differentdeviation of the averages of measurements from different

measurement conditions.measurement conditions.

variability and spc lesson52

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