Introduction to SPCadibucch/2WS10/IntroductionSPC.pdf · 2008. 2. 5. · 0 4 8 12 16 20 14,4 14,2...
Transcript of Introduction to SPCadibucch/2WS10/IntroductionSPC.pdf · 2008. 2. 5. · 0 4 8 12 16 20 14,4 14,2...
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Introduction to SPC
2WS10 Applied Statistics
A. Di Bucchianico
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What is Statistical Process Control (SPC )?
The set of operational and m
anagerial activities that a company uses
to ensure that the quality characteristics of a product are at the
required level??
A series of control charts??
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Statisticalm
ethods
•acceptance sampling (AS)
•statistical process control (S
PC)
•design of experiments (DOE)
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Dimensions of Quality
Perform
ance
Reliability
Durability
Serviceability
Aesthetics
Features
Perceived Q
uality
Conform
ance to standards
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Definitions of Quality
Fitness for use
–design
–conform
ance
Inversely proportional to variability
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Why study history of quality control?
•understand and foresee trends and future
developments
•companies m
ay still be w
orking w
ith concepts from
the past
/kHistory of Quality control
•craftsmen: quality is w
hat producer and consumer agree
about
•start of mass-production
–parts should be exchangeable
–all the same
–no variability
/kHistory: in the beginning
Quality really became an issue during the Industrial
revolution (19th century).
Inspection-oriented quality w
as adopted in the emerging
factories (usually through inspection department)
F.W
. Taylor startedresearch on quality and m
anagement in
the 1890’s (for more inform
ation, read the introductionto the
Taylor collection at the Stevens Institute of Technology)
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output
input
processes
specifications scrap
customer
History part I: inspection
•inspection just before shipping of product
•related definition of quality:
–conform
ance to specifications
/kProblems with total inspection
•mistakes have already been m
ade
–“one cannot inspect quality into a product”
•does not yield insight in production process
•very expensive
•inspection is not water-tight
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How many times does the letter foccur ?
Finished files are the result of years of scientific
study combined w
ith the experience of many
years.
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How many times does the letter foccur ?
Finished files are the result ofyears of
scientific study combined w
ith the
experience ofmany years.
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Example of acceptance sampling
sim
plest form
:
–lot size N
–sample size n
–acceptance number c (lot is accepted if number of defectives in sample
is ≤c)
OC (Operating characteristic)-curve gives
P(acceptance of lot | percentage defectives)
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input
processes
output
scrap
customer
specifications
sampling plans
History part II: Acceptance sampling
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History: W.A. Shewhart
Shewhart developed at Bell Labs in the 1920’s a process
oriented control approach.
Instead of inspection, production process is m
onitored.
Sampling plans w
ere used to achieve acceptable quality levels
(AQL).
Techniques employed in quality control were statistical,
basically sampling m
ethods and control charts
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History part III: Monitoring process output
output
input
OCAP
control
charts
limits
single process
/kSPC: Statistical process control
•new important tool: control chart
•measurements of production process during production
•prevention instead of detection afterw
ards
•monitoring variance behaviour of production
•corresponding definition of quality:
–variation of process fits within tolerances
X-b
ar C
har
t fo
r yie
ld
Subgro
up
X-bar
04
812
16
20
13,6
13,814
14,2
14,4
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History: World War II
Increase of production during W
orld W
ar II caused a large
expansion of quality control activities in both the U.S.A. and
Britain.
Statistical techniques w
ere so effective that they w
ere
classified as m
ilitary secrets during W
orld W
ar II!
After World W
ar II, the U.S.A. only had m
ajor production
facility left.
Lackof competition andenorm
ous post-war demand caused
disinterest in quality control.
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History: Japan
Before W
orld W
ar II, Japanese products w
ere cheap but of
low quality.
After World W
ar II, Japan had to rebuild its factories
(including the m
anagement structure).
American consultants (in particular Deming and Juran) were
brought in to teach quality.
A national centre for quality control was established (JU
SE)
and m
any national activities w
ere organized.
Japanese companies have extensive training programme for
“everyone”in the company.
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History: W.E. Deming
Deming applied Shewhart’s
principles of quality control at
various places
From 1950 on, he often visited Japan as lecturer and
consultant (the Japanese honoured him
by naming the highest
Japanese quality award after him
)
In spite of this, his fame in the US did not really start until
1980.
Deming puts m
uch emphasis on m
anagement rather than
production floor.
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History: J.M. Juran
Early Japanese efforts in quality control overemphasised
statistical methods under the influence of Deming.
Juran’s 1954 visit to Japan initiated a change of focus on
technology to a focus on m
anagement (especially
involvement of top-m
anagement).
Juran also emphasised the importance of education and the
cost of quality.
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History part IV: Problem solving tools
7 problem solving tools (‘M
agnificent Seven’)
–fishbone/Ishikawa diagram
–Pareto chart
–run chart
–defect concentrationdiagram
–Box-andWhiskerplot
–...
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History: K. Ishikawa
Ishikawa w
as very important to the quality revolution in
Japan because of his m
any activities (consulting, member of
various committees, ...).
Ishikawa stressed that quality control must try to m
eet the
requirements of customers.
Technical contribution: the cause-and-effect (or fishbone)
diagram.
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Process thinking
•all deviations from product specifications have causes in production
process
•outcome production process is caused by m
any process factors
Methods/
adjustments
People
Environment
Tools
Mach
ine
Materials
deviation
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Design of Experiments
•preferably
designphase(cf. EVOP)
•off-line
•team
•detect causes of variation
•optimise process
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History: G.E.P. Box
•IntroducedDOE in industrialenvironment sinceearly1950’s
•Developedresponse surfacemethodsat Im
perial Chemical
Industries
•Movedto US in 1960 and initiatedCenterfor Quality and
Productivity Im
provementin W
isconsin
•ASQC history: http://w
ww.asq.org/join/about/history/box.htm
l
•George Box award:
http://w
ww.enbis.org/barcelonaconference/gepbox.htm
l
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History: Taguchi
Systematic experimentation is necessary for product
development that leads to high quality products
Taguchi developed a system of statistical experimentation
that meet the needs of the engineer working in industry and
is easier to understand than the classical statistical methods
Taguchi introduced the concept of “loss function”(i.e., how “
much ”
do I conform
to specifications?) and “robust design”.
/kConform
ance to specifications
Why is product just outside specifications w
orse than product just
inside specifications?
classical: yes/no
LS
LU
SL
LS
LU
SL
modern: Taguchi loss function
/kConform
ance
to specificationsII
specificationsof characteristic1
specificationsof characteristic2
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History: US rediscovers quality control
In the 1970’s and 1980’s Japan established itself as w
orld leader
in quality and became an economic superpower
Several American industries could not compete w
ith Japanese
industries, because of their neglect of quality control
Quality control starts to prosper again in the 1980’s in the US
under the guidance of Deming, Juran and new names like
Feigenbaum
and Crosby.
Examples of successful American companies include Ford
( QS9000) and M
otorola (Six Sigma).
ASQ: American Society forQuality
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History: Europe
Interest in quality control in Europe did not start until the
1990’s.
Much emphasis in Europe on ISO 9000certification.
European quality control initiatives like EFQM
focus m
ainly
on m
anagement issues.
In 2000, ENBIS
(EuropeanNetw
ork
forBusiness and
Industrial Statistics) was founded. InitiativebyBisgaard,
thensuccessorof Box in W
isconsin.
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History part V: Monitoring process input
•Control of process factors
•Prevention of disturbances
•Tools: SPC, TPM, Poka Yoke, ...
•corresponding definition of quality:
–variation of process fits w
ithin tolerances
PROCESS
output
specification
s
input
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History: new developments
An important new development is the Six Sigma approach
developed by M
otorola (one of the first winner of the Baldrige
Award)
Six Sigma approach uses known statistical and m
anagement
concepts to come to an overall approach to quality:
–use statistical tools (DOE, regression analysis) to understand
relations betw
een input and output parameters of processes
–use control charts on inputs
instead of outputs
–direct link of effortswithreductionof costs
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His
tory
part
VI: Q
ualit
y b
y d
esig
n
Actions in design phase:
–prevent quality problems in production phase
–prevent quality problems w
hen customer uses product
Typical tools: Design of Experiments, Robust Design
Corresponding definition of quality:
–quality is conform
ance to customer requirements and
expectations
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The m
ain line of history to be remembered is
•total inspection
•acceptance sampling
•control charts on output
•control charts on input
•technical solutions/ TPM / ...
•quality by design
Sum
mary
of his
torica
l deve
lopm
ent
insp
ecti
on
pre
ven
tion