All About “Poka-Yoke”

(Mistake Proofing)

CONTINUOUS IMPROVEMENT TRAINING

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 Learning Objectives
• U N DERSTAND D E F I NITION OF P OK A - YOKE A N D “ Z ERO D E F EC T ”
QUALITY.
• U N DERSTAND T H E T Y PES OF I N S PECTION A N D T H EIR E F F EC TIVENES S.
• K N OW T H E T HREE BA S I C F U NC TIONS OF A P OK A - YOKE D E V I CE.
• S E E E X AMPLES OF P OK A - YOKE D E V I CES.
• R EVI EW ST E P -BY - STEP A P PROACH TO P OK A - YOKE D E V I CE
I MPLEMENTATION.
• L EARN TO C H ALLENGE S OU RCES OF D E F EC TS A N D E LI MINATE T H EM.

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 What Poka-Yoke Means

P O K A - YOK E WA S D EV E LO P E D BY S H I G EO S H I N G O F R O M TOYOTA M OTORS

as a tool to achieve Zero Defects.

• T H E P R O C ES S O F Z E R O D E F EC TS I S A L S O K NOW N A S
“ M I S TA K E P R O OF I N G ” O R “ FA I L - SA F E ”.

• BY TA K I NG O V E R R E P E T I T I V E TA S KS O R A C T I O N S T H AT D E P E N D O N
V I G I L A NC E
O R M E MO RY, P O K A - YOK E C A N F R E E WO R KE RS ’ T I M E A N D
M I ND TO P U RS U E M O R E VA LU E A D D E D A C T I V I T I ES .

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 Why mistake-proofing is important
• L E A N P R AC T I C E D O ES N OT A L LOW E X T R A I N V E N TORY TO C O M P E N SAT E F O R
S C R A P.
• L E A N I D EO LO GY F O C U S ES O N S P E E D . . .
. .. SPEED CANNOT EXIST WHEN DEFECTS AND REWORK PREVAIL.

• C O ST P R ES S U R ES D O N ’ T TO L E R AT E C O N T I N U E D M I S TA K ES : S C R A P, R E WO R K ,
L AT E NES S .

A N D M O ST I MP O RTANTLY:
Our customers rightfully expect defect free products and …
Traditional 100% inspection won’t provide 100% defect-free
products.

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 Mistake-proofing Philosophy

THE MISTAKE -PROOFING PHILOSOPHY RECOGNIZES THAT PEOPLE

SOMETIMES FORGET AND MAKE ERRORS. THE APPROACH USES
COMMON-SENSE I DEAS AND METHODS I N BOTH PROCESS AND
PRODUCT TO EL IMINATE HUMAN AND MECHANICAL ERRORS.

• H I S TO R I C A L LY, G R EAT E F FO RT S H AV E B E E N M A D E I N A P P LY I N G T H ES E
C O NCE P T S
TO S A F E T Y A N D A C C I D E N T P R E V E N T I O N .

• T H E S A M E A P P R OAC H M U S T D R I V E I N TO L E R AN C E F O R S C R AP A N D R E WOR K .

Fact: People Make Mistakes.

A good Poka-Yoke device makes it impossible to make a mistake.

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 Some Background on Inspection
T R A D I T I O NA L LY, Q UA L I T Y I S I N S P EC T E D Q UA L I T Y .
• Judgment Inspection: Separating good from bad after processing.
• Informative Inspection: Inspect then take action to eliminate future
occurrence.
• 100% or Sampling inspection based on SPC (Statistical Process Control)
methodologies.

I N S P EC T I O N:
• assumes defects are inevitable.
• believes the process of inspection will reduce defects.
• can reinforce ‘quality’ is responsibility of the quality department.
• is not 100% effective.

All of the above tolerate some level of defects. How do we achieve 0 defects?

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 “Self-Check” Inspection
ROI ( R E L I A N C E O N O P E R ATO R I N S P E C T I O N ) E M E R G E D I N T H E 1 9 8 0 ’ S .
• A self-check occurs when the person performing the work checks the work before the work moves to
the next process step.

ON THE UPSIDE ..
• 100% coverage is likely.
• instant learning and correction are possible.
• potentially less resistance when self-discovered.

ON THE DOWNSIDE ..
• sometimes compromises are made & proper corrective action
skipped.
• blind to your own mistakes.
• compliance can be marginal if not part of standard work.
• key features, characteristics, interface relationships not always
understood.

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 “Successive-Check” Inspection
A SUCCESSIVE-CHECK OCCURS WHEN THE PREVIOUS
WORK IS COMPLETED AND THAT WORK IS CHECKED BY
THE NEXT PERSON IN THE PROCESS.
ON THE UPSIDE . .
• often catches errors overlooked by self-checking process.
• can build a sense of cooperation between process participants.

ON THE DOWNSIDE . .
• corrective action still required and happens after the point
of error.
• compliance can be marginal if not part of standard work.
• can create friction between workers.

Self-check and successive-check was required because the

process allowed errors to occur.

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 “Special Cause” Variation
SPECIAL CAUSE (ASSIGNABLE CAUSE) VARIATION SHOULD BE
AT TACKED FIRST. ( A S I N G L E FA C TO R , W I T H L I T T L E I N T E R A C T I O N ,
A C T I N G O N P RO C ES S C AU S I N G A L A RG E P E RT U R BAT I O N … )
• TOOL S WHICH A R E E F F EC TI VE I N R ES OLVING ‘ S P ECI AL C AU SES ’ H AVE
MI NIMAL I MPAC T ON ‘ COMMON C AU SES’.
- S P C , 7 B A S I C TO O L S ( FO C U S TO S TA B I L I Z E P RO C ES S )

• D I F FERENT TOOLS A N D ME THODS A R E R EQUIRED TO E F F EC TIVELY

I D ENTIFY A N D
I MPROV E COMMON C AU SE I S S UES.
- P F M EA , D O E ( F O C U S TO O P T I MI Z E P R O CES S )

Eliminate the common cause variation through repeated

application of Poka-Yoke.

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 Make No Mistake
AT TRIBUTES OF ZERO DEFECT QUALITY SYSTEM

• S O U R CE I N S P EC T I O N - C H EC KS C O N D I T I O N S AT T H E S O U RC E
• 1 0 0 % I N S P EC T I O N - S I M P L E P O K A - YOK E D E V I C ES I N P L AC E TO C H EC K E V E RY
TIME
T H E P R O CES S O C C U RS
• I M M E D I AT E A C T I O N - O P E R AT I O NS S TO P A N D C O R REC T I V E A C T I O N I S TA K E N
WHEN A
D E F EC T O C C U RS
• T E A M S B R A I N S TOR M A N D I M P L E M E N T N E W D E V I C ES W H E R E V E R T H E R E I S
AN
O P P OR T U N I T Y F O R E R R OR

Quality can only be assured when there is always

immediate feedback at the source of the defect.

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Basic Function of Poka-Yoke
A DEFECT EXISTS IN ONE OF TWO STATES:
1. it is about to occur
2. it has already occurred

THREE BASIC POKA-YOKE FUNCTIONS TO USE AGAINST

DEFECTS:
1. shut down a machine or operation
2. control a process or operation
3 . WA R N T H AT A D E F EC T H A S O C C UR R E D

Recognizing that a defect is about to occur is called PREDICTION.

Recognizing that a defect has happened is called DETECTION.

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 Poka-Yoke Culture
FOC U S ON B U I LDI NG QUALITY I NTO T H E P ROCES S.
• R E ME MB E R TO C O N S I D E R T H E S O U R CES O F VA R I AT I O N , E N G AG E G B A N D B B
S U P P O RT.
• M A K E I T I M P O S S I B L E TO C R EAT E A D E F EC T.

B E LIEVE T H AT I N ADVERTENT E R RORS C A N B E E LI MINATED.

• engage the people in the process.
• 10 heads are better than one.

I MPLEMENT T H E B EST I D EAS A F T ER YOU F I ND T H E ROOT C AU SE

• C H A S E T H R O U G H T H E F I V E ‘ W H Y ’ S ’, T H E N … .
B R AINSTORM TO GE T TO T H E B EST S OLUTI ON TO AT TACK ROOT C AU SE.
No more excuses! Build a culture that finds a way to do it right the
first time and does not tolerate errors!

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Contrasting Cultures
N EG ATIVE A P PROACH POSITIVE APPROACH
M I S TA K ES A R E I N E V I TA BL E Mistakes Can Be Eliminated
• people make mistakes • create the ‘right’ environment
• focus on the process deficiency
• PLACE BLAME • ask “why?”, then “how?”
• detect at final inspection
• sampling inspection
• learn from the opportunity
• errors are inevitable
• apply mistake-proofing methods
• train everyone to challenge
• NO TIME, NO errors.
MONEY • show them it’s possible; share
• no support examples

What is the culture in your workplace?

What is your approach?

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 Top 10 Errors that Cause Defects
1 . PROCES SI NG O M I SS IO NS ( A S T EP WA S F ORGOT T EN)

2 . P R O C E S S I N G E R R O R S ( S O M E T H I N G W A S D O N E I N C O R R E C T LY )

3. ERROR IN SETTING UP THE WORK PIECE

4 . A S S E M B LY O M I S S I O N S ( A PA R T W A S F O R G O T T E N )

5 . A W R O N G PA R T / I T E M W A S I N C L U D E D

6. WRONG WORK PIECE

7 . O P E R AT I O N S E R R O R S ( I N C O M P L E T E I N F O R M AT I O N , P R O C E D U R E S N O T F O L L O W E D )

8 . A D J U S T M E N T, M E A S U R E M E N T, D I M E N S I O N A L E R R O R S

9. EQUIPMENT MAINTENANCE ERRORS

1 0 . E R R O R S I N P R E PA R AT I O N O F T O O L S , F I X T U R E S , B L A D E S , E T C

ERROR: WHEN ANY CONDITION NECESSARY FOR SUCCESSFUL PROCESSING

I S W R O N G O R A B S E N T.

What is the culture in your workplace?

What is your approach?

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Other Defect Causes
– Forgetfulness
• Language Barriers- (i.e. English as a second language)
• Misunderstanding instruction or procedure
• Errors in identification (e.g.: part and/or placement recognition)
• Errors made through lack of experience
• Pace (too fast or too slow)
• Lack of standards, expectations, procedure, or instructions
• Incomplete information
• Incorrect or incomplete processing (parts, dwell times, etc)
• Out of tolerance tools, fixtures, and jigs
– adjustment or placement errors

What is the culture in your workplace?

What is your approach?

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Common Defect Categories
– leaks
• mis-wired • mismatched items
• loose parts • misaligned parts
• noise • wrong dimensions
• ‘inconsistent’ test
• damaged parts / results
product • incorrect incoming
• reversed parts material
• foreign material • wrong material
• missing parts / information

What is the culture in your workplace?

What is your approach?

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Examples of Common Devices
• guide / reference / interference rod or pin
• T E M P L AT E
• limit switch /micro-switch
• C O U NT E R
• odd-part-out method
• sequence restriction
• critical condition indicator
• detect delivery chute
• S TO P P E R / G AT E
• SENSOR

Remember, the intent is to identify an abnormal condition and

prevent further action until the abnormality is removed.

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 Mistake Proofing Device Example
A C R I T I C A L C O N D I T I O N D E T EC TO R I S A D E V I C E T H AT D E T EC T S T WO
TYPE OF CONDITIONS:
1 ) T H E P R ES E NC E O R A B S E NC E O F A S P E C I F I C , V I S I B L E P R E - S E T
Q UA NT I T Y S U C H A S T H E C O R R EC T N U MB E R O F PA R T S , C O R R EC T
W E I G H T, H E I G H T, V O LU ME O R D E P T H
2 ) F LU C T UAT I O N S I N A N O N - V I S I B L E C O N D I T I O N S U C H A S P R ES S U R E ,
T E M P E R AT U R E , C U R R E N T A N D N O N - V I S I B L E F LU I D S ( A I R ) .

Here’s how it works: a critical manufacturing condition (pressure,

current, temperature, time, etc.) is measured -and work cannot
proceed if the value is not within a pre-determined range as
indicated on the detector.

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Critical Condition Device Example
• F LU I D E L E M E N T D E T EC TO RS : D E T EC T C H A N G ES I N A I R S T R EA M S
O C C AS I O N E D BY T H E P L AC E M E N T O R R E MOVA L O F O B J EC T S ( U S E F U L
I N D E T EC T I N G B R O K E N D R I L L B I T S ) .
• P R ES S U R E C H A N G E D E T EC TO RS : D E T EC T I N T E R R U P T I O NS I N F LOW
T H R OU G H P R E S S U R E G AU G ES O R P R ES S U R E - S E N S I T I V E S W I TC H ES .
• T E M P E R AT U R E C H A N G E D E T EC TORS : D E T EC T T E M P E R AT U R E C H A NG E
A N D VA R I AT I O N T H R O U G H T H E R MO M E T E RS , T H E R MOS TAT S ,
T H E R M I STO RS , T H E R MO COU P L ES , E TC .
• C U R R E N T F LU C T UAT I O N D E T EC TORS : D E T EC T V I A M E T E R R E L AY S
O C C U R R E N C E O F E L EC T R I C C U R R E N T S O R S E CON DA RY C U R R E N T S
( I M P O RTA N T A P P L I C AT I O N S I N S P OT W E L D I N G ) .

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Poka-Yoke Examples

Control (prevention):
The disc is only able to be inserted in one orientation.

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Poka-Yoke Examples

Shutdown/Warning (Detection):
The compact disc will only work in one orientation.

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Poka-Yoke Examples

Shutdown (Prevention): Newer mowers are required to

have a safety bar on the handle that must be pulled back in
order to start the engine. If you let go of the safety bar, the
mower blade stops in 3 seconds or less.

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 KISS Guide to Mistake-proofing
N OTE: THI S GUI DE AS S UM ES A R EAC TI VE EF FO RT (A PRO BLEM ALR EADY
EXISTS)
• Mistake-proofing should be also be used in a proactive mode.
• Defects can be prevented by mistake-proofing products and processes

AT T H E D E S I G N S TA G E .

S T E P 1 : D E S C R I B E T H E D E F E C T ( O R P OT E N T I A L D E F E C T ) .
STEP 2: DETERMINE WHERE THE DEFECT I S DISCOVERED OR WHERE I T I S
MADE.
S T E P 3 : D E TA I L T H E S E Q U E N C E O F E V E N T S I N T H E D O C U M E N T E D S TA N D A R D .
S T E P 4 : O B S E R V E T H E P R O C E S S A N D D E TA I L T H E S T E P S T H AT D I F F E R F R O M
T H E S TA N D A R D.
S T E P 5 : I D E N T I F Y C O N T R I B U T I N G C O N D I T I O N S ( TO O L S , T R A I N I N G , E T C ) .
Refer to step 4 observations; ask the 5 ‘whys’ to determine root cause.

S T E P 6 : I D E N T I F Y T H E M I S TA K E - P R O O F D E V I C E N E C E S S A R Y TO P R E V E N T T H E
D E F E C T.
S H O W T H E T E A M T H E P O K A - YO K E H I N T S A N D E X A M P L E S O F P O K A - YO K E
DEVICES.
S T E P 7 : A D D T H E D E V I C E TO T H E P R O C E S S - N O W !

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Combining Lean And Variation Reduction

• START WITH AN FMEA. THIS ORGANIZED APPROACH

WILL IDENTIFY POTENTIAL PROBLEMS THAT CAN BE
ADDRESSED WITH MISTAKE PROOFING ACTIVITY.

• PROCESS MAPS CAN ALSO BE BENEFICIAL.

UNDERSTANDING THE INPUTS
AND OUTPUTS CAN HELP IDENTIFY SPECIFIC SOLUTIONS.

Mistake proofing is an excellent opportunity to

combine the tools of variation reduction and lean.

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 Poka-Yoke Hints
T H I NK A B O U T H O W TO V E R I F Y I T E M S BY T H E I R C H A R AC T E R I ST I C S
• weight, dimension, shape

T H I NK A B O U T P R O C ES S S EQ U E N C E
• can’t perform next step if prior step not done
• process flags if steps are omitted

D E T EC T D E V I AT I O N S F R O M F I X E D VA LU ES
• counters, scales, odd part out, critical condition detection; pressure, temperature,
current, time

When brainstorming, review hints and examples to get people thinking.

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Additional Hints for Defect Prevention
DON’T MAKE SURPLUS PRODUCTS
• make only what the customer needs, when it is needed
• surplus inventory makes poor quality harder to find
• in a high inventory environment, the sense of urgency is not there

E L I M I N AT E , S I M P L I F Y, O R C O M B I N E P R O D U C T I O N O P E R AT I O N S
• identify and eliminate waste; drive 5S
• follow standard operations & utilize visual aids

O N C E P R O C E S S E D , U S E I M M E D I AT E LY
• adopt a flow process
• implement immediate feedback and action cycles

I N V O LV E E V E R Y O N E I N D E F E C T P R E V E N T I O N
• form action teams - team brainstorming

M A K E D A I LY I M P R O V E M E N T S A N D I N V E N T M I S TA K E - P R O O F I N G D E V I C E S

A Lean environment promotes defect prevention.

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Engineering Examples
A P P R O V E D PA R T S D ATA B A S E W H I C H O N LY A L L O W S A P P R O V E D PA R T S T O B E O N T H E B O M -
C O N T R O L ( P R E V E N TAT I V E )

• C H EC KLI STS ( I N FO M I S S IN G S O D R AWI N G D O E S N OT R E L EAS E ) S H UT DOWN ( D E T EC T ION )

• C A D S Y S T E M S T H AT D E T E C T I N T E R FA C E I S S U E S WA R N I N G
(DETECTION)

A N D T H E N T H E R E ’ S T H E D E S I G N I T S E L F. ( A L I G N M E N T P I N S , K E Y E D C O N N E C T O R S , PA R T
S H A P E S , PA R T C O U N T R E D U C T I O N … )

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Service and Administrative Examples
A P P R O V E D PA R T S D ATA B A S E W H I C H O N LY A L L O W S A P P R O V E D PA R T S T O B E O N T H E B O M -
C O N T R O L ( P R E V E N TAT I V E )

• C H E C K L I S T S F O R R E Q U I R E M E N T S G AT H E R I N G D U R I N G S A L E P R O C E S S S H U T D O W N
(DETECTION)

• REDUCE COMPLEXITY OF FORMS ; UNNEEDED FIELDS.

• O N L I N E F O R M S W I T H R E Q U I R E D F I E L D S - C A N ’ T C O M P L E T E TA S K W I T H O U T C O M P L E T E
I N F O I N C O R R E C T F O R M AT

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Supply Chain Example
M R P P R O V I D E S I N F O R E L AT E D TO S U P P LY V S . D E M A N D
WA R N I N G ( D E T E C T I O N )

• H I G H P R I C E D P O ’ S C A N N OT R E L E A S E W I T H O U T A P P R O VA L
CONTROL (PREVENTION)

• APPROVED VENDOR LISTS LIMIT SUPPLIERS

CONTROL (PREVENTION)

• C A N N OT C O M P L E T E T R A N S A C T I O N W I T H M I S S I N G F I E L D S
SHUTDOWN (PREVENTION)

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Summary

E V E RY I N SPEC TION ME T HOD H A S W EAKNES SES .

• PREVENTION OF DEFECTS IS BETTER THAN DETECTION.

• P O K A - YO K E D E V I C E S C A N B E A P P L I E D W I T H I N A N Y P R O C E S S .

• FOCUS ON THE CRITICAL FUNCTIONS AND ACTIONS.

• S O U R C E S O F D E F E C T S C A N B E C H A L L E N G E D A N D E L I M I N AT E D.

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All About “Poka-Yoke”
(Mistake Proofing)

CONTINUOUS IMPROVEMENT TRAINING

DEEPJYOTI SERVICES