4 aa process capability.ppt

7/27/2019 4 aa process capability.ppt

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Process Capab i li ty

Process Capabi l i ty is the Ab i l ity o f aProcess to Consis tent ly Meet Specif iedCustom er-Driven Requ irements

Speci f icat ion Lim its are Set by Management

in Response to Customers Expectationsand needs

The Upper Specif icat ion Lim it (USL) is theLargest Value that Can Be Obtained and Sti ll

Conform to Customers Expectation The Lower Speci f icat ion Lim it (LSL) is theSmallest Value that is Stil l Con form ing


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Range of natural var iabi l ity in pro cess

Measured w ith con tro l chart for

indiv iduals.

Process cannot meet speci f icat ions i f

natu ral variabi l i ty exceeds tolerances

3-sigma qual ity

Specif icat ions equal 3 variat ionf rom the mean

6-sigma qual ity

Specif icat ions equal 6 variationf rom the mean


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Capabi l i ty Versus Con tro l

Control

Capability

Capable

Not Capable

In Control Out of Control

IDEAL


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(b) Design specificationsand natural variation thesame; process isjustcapable of meetingspecifications most thetime.

DesignSpecifications

Process

(a) Natural variationexceeds designspecifications; processis not capable of

meeting specificationsall the time.


Process

LSL USL


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(c) Design specificationsgreater than naturalvariation; process iscapable of always

conforming tospecifications.


Process

(d) Specifications greaterthan natural variation,but process off center;capable but some outputwill not meet upperspecification.


Process


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Capab i l ity Ind ices

Aggregate Measures of a Process Abilityto Meet Speci f icat ion Lim its

The larger (>1) the values, the more capable a

proc ess is of meeting requirements Measure o f Process Potent ia l Performance

Cp>1 impl ies that a process has the po tential ofhaving m ore than 99.73% of outcomes with inspeci f icat ions

2specification spread

process spread6 /p

USL LSLC

R d


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MeasuresProcess Capabi l i ty Ratio

Cp =

=

tolerance range

process range

upper specification limit -lower specification limit

6


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Compu ting Cp

Net weight specification = 9.0 oz 0.5 ozProcess mean = 8.80 oz

Process standard deviation = 0.12 oz

Cp =

= = 1.39

upper specification limit -lower specification limit

6

9.5 - 8.5

6(0.12)


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Measures of Actual Process Performance

For one-sided speci f icat ion l imi ts

CPL (CPU)>1 impl ies that the process mean is mo re

than 3 standard deviat ions away from th e low er

(upp er) specif icat ion l imit

(continued)

23 /X LSLCPL

R d

23 /USL X

CPUR d


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For two -sided speci fication l imi ts

Cpk= 1 ind icates th at the process average is3 standard deviat ions away from the closest

speci f ication l imi t

LargerCpkind icates larger capabi l i ty of

meet ing the requirements

(continued)

min ,pkC CPL CPU


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MeasuresProcess Capabi l i ty Index

Cpk= minimum

x- lower specification limit

3=

upper specification limit - x

3=

,


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Comput ing Cpk

Net weight specification = 9.0 oz 0.5 ozProcess mean = 8.80 oz

Process standard deviation = 0.12 oz

Cpk= minimum

= minimum , = 0.83

x- lower specification limit

3=

upper specification limit - x

3

=

,

8.80 - 8.50

3(0.12)

9.50 - 8.80

3(0.12)


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Est imat ing Process

Capabil i ty Mus t Have an In-Con tro l Process First

Est imate the Percentage of Produc t or

Servic e Within Specif icat ion

Assum e the Populat ion ofX Values is

Approx imately Normal ly Dis t r ibuted wi th

Mean Est imated by and Standard

Deviation Est imated byX

2/R d


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Key Idea

Process capability calculations make littlesense if the process is not in statisticalcontrol because the data are confoundedby special causes that do not representthe inherent capability of the process.


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Process Capability

Process capability refers to the performance of

the process when it is operating in control.

Two graphical tools are helpful in assessingprocess capability:

Tolerance chart (or tier chart)

Histogram


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Process Capability


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Process Capability


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Process Capability


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Process Capability


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Remedial act ion fo r Cp < 1(fo r b i lateral specif icat ion s case)

1. a) X = (U+L)/2

b) We must maintain control charts

with high value of n so that controlchart is more sensitive and we get a

quick indication that process has gone

out of control


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Remedial act ion for Cp < 1(fo r b i lateral specif icat ions case)

2. Sometimes spec. limits are fixed arbitrarily and no

time/ effort is spent on their justification.

THEREFORE REVIEW AND REVISE THE

SPECIFICATION LIMITSIf spec. limits can be relaxed then problem can be

solved to a certain extent

Note: While fixing the spec. limits the designer

must give due considerations to Functional needs

Process Capability


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3. Reduce (process dispersion)a) can be done to some extent by

Employing better and skilled persons

Better training & supervision

Material of better quality

b) If we many different machines withvarying process capability, then jobs

requiring narrow tolerances should beassigned to those machines capable ofholding narrow tolerances


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c ) Make a fundamental change in the productionprocess in o rder to reduce the whichimplies we can go for a process of higher

precision but this change will be a matter of

cost in terms of additional investment inhigher precision equipment.

NPV

IRR

Payback periodEvaluating investments

on financial criteria


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Remedial ac t ion for Cp < 1(fo r b i lateral specif icat ion s case)

4. The wors t so lu t ion w i ll be to con tinuethe process as such and segregate the

defect ives thru 100% inspect ion


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Sing le speci f icat ion case

23 /X LSL

CPLR d

23 /USL X

CPUR d


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Single speci f icat ion case (L )

Al l the i tems p rodu ced w i l l be non defect ive i f

X - 3 LThree possible cases are

1. X - 3 > L Cp l > 12. X - 3 ~ L Cp l ~ 13. X - 3 < L Cp l < 1


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Sing le spec i f icat ion case (L )

Add i t ional remedial act ions forCp l < 1

Increase the process average

Review and revise the specif icat ion

l imits

Reduce dispers ion of the process


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LIMITATIONS OF X BAR R & CHARTS

1. Can be used on ly fo r var iab le q lt y. Ch.

Some ql ty. ch . can no t be measu red or

quant i f ied

---- appearance, cracks , damages, scratches

etc.

Such qlty . Ch. Can only be viewed as

attr ibute.

Each inspected i tem can on ly beclassi f ied as defect ive or non defect ive

In such cases charts fo r var iables can

no t be used


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2. Measurements fo r above char ts are

expens ive and take more t ime.

Somet imes to reduce time and cost

someq lty. ch . are measu red as

attr ibutes.GO / NO GO GAUGES

3. A Separate con tro l char t is needed fo r

each qual i ty char. This is a ser ious

l imitat ion/disadvantage.therefore we use above charts for

cr i t ic al variab les caus ing

subs tant ia l Rework or Reject ion .


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4. Use of X BAR R has to be justified.

Used when

a. U L b. Mass production item

c. Cs & Crare substantial

5. Economic Justificationa. Savings --- in reduction of defectives

b. Cost Cost of data collection and charting

Cost of interpretation of chart (specialist required)

Actual troubleshooting based on evidence from control

chart because this may involve loss in production or

shutdown

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4 aa process capability.ppt