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STATISTICALPROCESS CONTROL

TOTAL QUALITY MANAGEMENT

Introduction to S.P.C.

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What is meant byStatistical Process Control?

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Thisisacontrolsystemwhichusesstatisticaltechniquesforknowing,allthe

time,changesintheprocess.

Itisaneffectivemethodinpreventing

defectsandhelpscontinuousquality

improvement.

Statistical Process Control (S.P.C.)

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STM QualityLimitedhat does S.P.C. mean?

Statistical:

Statisticsaretoolsusedtomake

predictionsonperformance.

Thereareanumberofsimplemethods

foranalysingdataand,ifapplied

correctly,canleadtopredictionswithahighdegreeofaccuracy.

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Process:

Theprocessinvolvespeople,machines,

materials,methods,managementand

environmentworkingtogetherto

produceanoutput,suchasanend

product.

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STM QualityLimitedhat does S.P.C. mean?

Control:

Controllingaprocessisguidingitand

comparingactualperformanceagainst

atarget.

Thenidentifyingwhenandwhat

correctiveactionisnecessarytoachievethetarget.

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STM QualityLimited.P.C.

Statisticsaidinmakingdecisionsaboutaprocessbasedonsampledataand

theresultspredicttheprocessasa

whole.

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STM QualityLimitedhe Process

People Machines Material

Management Methods Environment

Output

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S.P.C.isstatisticalanalysis

ofthepredictabilityandcapacityofaprocesstogive

auniformproduct.

Definition

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The Aim of S.P.C.- Detection Strategy

Detection:

Thisfocusesonidentificationof

problemsafterproduction,by100%

inspectionorbycustomercomplaints.

Itisahistorically-basedstrategy.

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The Aim of S.P.C.- Detection Strategy

DetectionDrawbacks:

Productionisalreadymade.

Customerdissatisfaction.Inflatedcosts-rework;inspection.

Repetitiveproblems.

Neglectedimprovements.

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The Aim of S.P.C.- Prevention Strategy

Prevention:Thisfocusesonin-processproduction

andidentificationofproblemsthrough

analysisofprocesscapability.

Itisafuture-orientatedstrategy.

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The Aim of S.P.C.- Prevention Strategy

PreventionBenefits:Improveddesignandprocess

capability.

Improvedmanufacturingquality.

Improvedorganisation.

ContinuousImprovement.

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TheS.P.C.hastobelookedatasastagetowardscompletelypreventing

defects.

Withstableprocesses,thecostofinspectionanddefectsaresignificantly

reduced.

S.P.C. as a Prevention Tool

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Assessesthedesignintent.Achievesalowercostbyprovidingan

earlywarningsystem.

Monitorsperformance,preventingdefects.

Providesacommonlanguagefordiscussingprocessperformance.

The Benefits of S.P.C.

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ProcessElement VariableExamplesMachine.Speed,operating temperature,feedrate

Tools..Shape,wearrateFixtures..Dimensionalaccuracy

MaterialsComposition,dimensions

OperatorChoiceofset-up,fatigueMaintenanceLubrication,calibration

EnvironmentHumidity,temperature

Process Variations

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Noindustrialprocessormachineisabletoproduceconsecutiveitemswhichare

identicalinappearance,length,weight,

thicknessetc.Thedifferencesmaybelargeorvery

small,buttheyarealwaysthere.

Thedifferencesareknownasvariation.

Thisisthereasonwhytolerancesare

used.

Process Variations

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Commoncausesarethemanysourcesofvariationthatarealwayspresent.

Aprocessoperateswithinnormalvariation

wheneachelementvariesinarandommanner,withinexpectedlimits,suchthatthe

variationcannotbeblamedononeelement.

Whenaprocessisoperatingwithcommon

causesofvariationitissaidtobestable.

Stability

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Theprocesscanonlybetermedunder

controlifitgivespredictableresults.

Itsvariabilityisstableoveralong

periodoftime.

Process Control

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Graphsandchartshavetobechosenfortheirsimplicity,usefulnessand

visibility.

Theyaresimpleandeffectivetoolsbasedonprocessstabilitymonitoring.

Theygiveevidenceofwhethera

processisoperatinginastateof

controlandsignalthepresenceofany

variation.

Process Control Charts

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STM QualityLimitedata Interpretation

Considerthese50measurementsBore Diameter 36.32 0.05mm (36.27 - 36.37mm)

1 36.36 11 36.37 21 36.34 31 36.35 41 36.36

2 36.34 12 36.35 22 36.37 32 36.35 42 36.37

3 36.34 13 36.32 23 36.34 33 36.36 43 36.37

4 36.33 14 36.35 24 36.35 34 36.37 44 36.35

5 36.35 15 36.34 25 36.34 35 36.34 45 36.37

6 36.33 16 36.34 26 36.35 36 36.36 46 36.36

7 36.33 17 36.35 27 36.36 37 36.38 47 36.35

8 36.34 18 36.33 28 36.33 38 36.34 48 36.34

9 36.35 19 36.32 29 36.36 39 36.35 49 36.35

10 36.35 20 36.35 30 36.38 40 36.35 50 36.34

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STM QualityLimitedata Interpretation

Asasetofnumbersitisdifficulttoseeanypattern.

Withinthetable,numbers30and37

wereoutsidethetolerancebutweretheyeasytospot?

Awayofobtainingapatternistogroup

themeasurementsaccordingtosize.

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STM QualityLimitedata Interpretation Tally Chart

Thetallychartgroupsthemeasurements

togetherbysizeas

shown.

Thetwopartsthat

wereoutoftolerance

arenoweasierto

detect(36.38mm).

36.39

36.38

36.37

36.36

36.35

36.34

36.33

36.32

36.31

36.30

36.29

36.28

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STM QualityLimitedally Chart - Frequency

Thetallychartshowspatternsandwecan

obtaintheRANGE-

36.32mmto36.38mm.

Themost

FREQUENTLY

OCCURRINGsizeis

36.35mm.

36.39

36.38

36.37

36.36

36.35

36.34

36.33

36.32

36.31

36.30

36.29

36.28

2

6

7

16

12

5

2

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STM QualityLimitedally Chart - Information

Thetallychartgivesusfurtherinformation:Thenumberofboresateachsize;

Thenumberofboresatthemostcommon

size;Thenumberofboresaboveandbelowthe

mostcommonsize(36.35mm)-

numberabove36.35mmis7+6+2=15numberbelow36.35mmis12+5+2=19

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STM QualityLimitedistogram

Wecanredrawthefrequencychartasabar

chartknownasahistogram:

16

14

12

10

8

6

4

2

0 36.31 36.32 36.33 36.34 36.35 36.36 36.37 36.38 36.39

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STM QualityLimitedmoothed Frequency

Ifwenowdrawasmoothcurvethroughthetop

ofeachboxwegetabell-shapedpattern:

16

14

12

10

8

6

4

2

0 36.31 36.32 36.33 36.34 36.35 36.36 36.37 36.38 36.39

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Thebell-shapedpatternisfairlytypicalofmostindustrialprocesses.

Thereisacentralvalueatthehighestpointof

thecurveandthepatternofresultsspreadoutequallyonbothsidesofthecentralvalue.

Thefurtherwemovefromthecentralvalue,

thefewervalueswewillfind.

Thisbell-shapedpatternisknownasthe

NORMAL DISTRIBUTION.

Distribution

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Thebell-shapedpatternknownasNormalDistributionisonethatwewouldexpectto

seewheretheprocessisrunninginastable

condition.

Whereyouworkinamanufacturing

environment,whynotperformtheexerciseon

50partsforyourselfandseeifyourprocessis

inastablecondition!

Normal Distribution

l

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S.P.C.isaveryeffectivemethodin

preventing_______.

Self-test (1 of 9)

defects

S.P.C.usesstatisticaltechniquesfor

knowing_______________.processvariation

S Q l

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Controllingaprocessisguidingitandcomparing________________againsta

targetandidentifyingwhenandwhat______________isnecessarytoachievethetarget.

Self-test (2 of 9)

actualperformance

correctiveaction

STM Q li

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Theprocessmayinvolveacombinationofpeople,machines,________,methods,managementand

___________workingtogethertoproduceanoutputsuchasanendproduct.

Self-test (3 of 9)

materials

environment

STM Q li

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uniform

predictability

S.P.C.isastatisticalanalysisofthe

__________andcapacityofaprocess

togivea_______product.

STM Q li

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historicallyalreadymade

Disadvantagesof100%detection

inspectionarethatproductionis

___________andthatitis__________based.

STM Q lit

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future

quality

Anadvantageofpreventioninspectionisthat_______isimproved.

Itisastrategythatis_____orientated.

STM Q lit

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variation

identical

Noindustrialprocessisableto

produceconsecutiveitemswhichare

_______.Therearealwaysdifferences,known

as_______.

STM Q lit

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stable

predictableTheprocesscanonlybetermedunder

controlifitgives__________results.

Itsvariabilityis______overalong

periodoftime.

STM Q lit

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STM QualityLimitedelf-test (9 of 9)

normal

histogram

Afrequencychartdrawnwithbarsis

knownasa_________.

Asmoothcurvedrawninabell-shaped

patternisknownasthe______distributioncurve.