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POKA - YOKEThis chapter introduces the concept of preventing mistakes by design.

After completing this chapter, you will be able to:

• Understand the “Poka Yoke” principle.

• Understand the importance of Change Acceleration Process in mistake proofing

• Recognize how mistake proofing is applied

• Identify opportunities for mistake proofing

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Japanese manufacturing engineer Shigeo Shingo developed the idea into a formidable tool for achieving zero defects and eventually eliminating quality control inspections

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Poka-yoke = Mistake Proofing From the Japanese words:

yokeru - To Avoidpoka - Inadvertent Errors /

Unintentional Error

The idea is to respect the intelligence of workers by taking over repetitive tasks that depend on vigilance or memory.

Promotes Creativity and Value-Added Activities

Reduces Ego problem among employees.

This is based on the principle that the defects can be prevented by controlling the performance of a process so that it cannot produce the defects even when a mistake is committed by humans.

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Operator’s Carelessness

Negligence

Inattentiveness

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Monotonous

Mechanical

Long tense working environment

If we bracket the mistake as “carelessness” people

get demoralised and discouraged

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Free a person’s mind from maintaining repetitive

vigil.

Person without fear of making mistake can

constructively do more value added activities

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Solutions

• Repair or redo ?

• 100% inspection ?

• Mistake-proofing?

• Which solution you prefer?

Shigeo Shingo Zero ‘Quality control’

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THE FOUR ELEMENTS OF ZQCTHE FOUR ELEMENTS OF ZQC

1.It uses source inspection1.It uses source inspectionto catch errors before they become defectsto catch errors before they become defects

2.It uses 100% inspection2.It uses 100% inspection

3.It provides immediate feedback3.It provides immediate feedback

4.Because smart people do make mistakes ZQC uses 4.Because smart people do make mistakes ZQC uses Poka -Yoke devices to carry out the checking functionPoka -Yoke devices to carry out the checking function

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ERRORS

NO

ERRORS

Errors Cause Defects

Six SigmaProcess

Six Sigma Results

Six SigmaProcess

Results Less ThanSix Sigma

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ERRORS -- Causes of DefectsAN ERROR

When any of the conditions necessary for successful transaction or processing are wrong or absent.

The Ten Most Common Errors(Causes of Defects)

1.) Processing Omissions

2.) Processing Errors

3.) Error in Setting Up the Equipment

4.) Missing Parts

5.) Inclusion of Wrong Part / Item

6.) Wrong Piece of Equipment

7.) Operations Errors

8.) Adjustment, Measurement, or Dimensional Errors

9.) Error in Equipment Maintenance

10.) Handling Error

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Common Types of Human Error 1) Forgetfulness (Not Concentrating)

2) Errors Due to Misunderstanding (Jump to Conclusions)

3) Errors in Identification (View Incorrectly...Too Far Away)

4) Errors Made by Untrained Workers

5) Willful Errors (Ignore Rules)

6) Inadvertent Errors (Distraction, Fatigue)

7) Errors Due to Slowness (Delay in Judgment)

8) Errors Due to Lack of Standards (Written & Visual)

9) Surprise Errors (Machine Not Capable, Malfunctions)

10) Intentional Errors (Sabotage - Least Common)

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Human Errors

How Does Our Organization Approach Human Error?

NEGATIVE

• Defects Are Inevitable

• People Make Mistakes

• Place Blame

• Detect at Final Inspection

• Sampling Inspection

• Some Reach the Customer

POSITIVE

• Defects Can Be Eliminated

• Create the Environment

• Ask “Why?”

• Apply Mistake-Proofing Device for 100% Inspection

• Others?

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•Why did the machine stop?

Because the fuse blew due to overload.•Why was there an overload?

Because the bearing lubrication was inadequate.•Why was the bearing lubrication inadequate?

The lubrication pump was not functioning properly.•Why was wasn’t the lubrication pump working right?

Because the pump axle was worn out.•Why was it worn out?

Because the sludge got in?

Normal tendency is to replace the fuse. But the real

solution is attaching a strainer to the lubricating pump.

Ask why five times!

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Five best Poka-yoke? (Factory)

Guide pins of different sizes

Error detection and alarms

Limit switches

Counters

Checklists

78493

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Example - Processing errors Description of Process: A chassis was placed in a jig for matching

Problem: Chassis set backwards in jig

Solution: Additional guide pin taking advantage of asymmetry

Key Improvement: Jig modified to guarantee correct positioning

Before Improvement:It was possible to insert the chassis inthe jig backwards. Correct operationdepended on the workers vigilance.

After Improvement:A guide pin was added, keyed to anasymmetrical feature to the chassis.This completely eliminates the danger of backwards processing.

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Interlocking

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CONTACT METHOD

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Fixed value method

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Example - Omitted Processing Description of Process: A number of holes are drilled in each workpiece

Problem: Incorrect number of holes drilled

Solution: An automatic counter to keep track of the number of holes drilled

Key Improvement: Tool modified to guarantee correct processing count

Before Improvement:The operator counted the holes as they were drilled. However, the operator sometimes made errors, and the products with the wrong number of holes were produced.

After Improvement:A counter was mounted on the drill press to detect each hole as it is drilled. Along with this, a limit switch was mounted on the jig to detect when a part was removed before the proper number of holes was drilled.

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Motion step method

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Sensors that detect changes in physical condition

1.Pressure2.Temperature3.Electrical current

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Energy sensing device

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Others

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Common Mistake Proofing DevicesUsing wisdom and ingenuity to create devices that allow

you to do your job 100% defect free 100% of the time.

HOME• Automated Shut-Offs on

Electric Coffee Pots

• Ground Fault Circuit Breakers for Bathroom or Outside Electric Circuits

Retail• Tamper-Proof Packaging• Bar Coding at Checkout

OFFICE• Spell Check in Word

Processing Software• Questioning “Do you want to

delete?” After Depressing the “Delete” Button on Your Computer

FACTORY• Dual Palm Buttons and Other

Guards on Machinery• Bar Coding

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TransactionalProblem: Incorrect or missing information on customer problem reportSolutions:

Modify the computerized data entry process such that missing or wrong entries are flagged and must be corrected prior to acceptance of information into the system.

Fill in redundant fields automatically.

Use of pre-filled in data, based on some previously keyed entry.

Problem: Status of a truck for leasing. (has it been checked out and safe to use)

Solution: Implement a check sheet that must be filled in, dated, and signed as each portion of the truck preparation is completed.

Suggestions for other situations: • Colored stickers or date stamp on the unit.• Bar coding.• Color coded forms.• Uniquely shaped punches for each process step.• Implement a ‘traveler’ that goes with unit and is filled out and signed at each step in the process before continuing on.

Common Mistake Proofing Devices…Contd.

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Potassium Chloride in Intra Venous Bags - premixed to avoid fatal errors

Surgical Trays have indentations, preventing surgeon from leaving surgical instrument in patient.

Pre-filled syringes in hospitals to avoid mistakes in dosage.

To prevent “treatment” errors, banks require tellers to record the customer’s eye color on a checklist as they start transaction.

Hotels wrap paper strips around towels to reduce unnecessary replacements.

Service Fail Safe Methods

Common Mistake Proofing Devices…Contd.

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1. Prevention or Prediction:A poka-yoke designed in such a way that it helps the

operator to recognize the defect before it is about to occur.

2. Detection :A poka-yoke system that help the operator to know

immediately on the occurrence of a defect. Here the detction is after the defect has occurred. Even though this is not as good as prevention the principle of “earlier the better” is used.

Basic Classification of Poka-yoke

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Basic functions

States of Defect Function

Defects

About to occur(Prediction)

Occurred(Detection)

Shut down

Control

Warning

Poka-Yoke

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Now that we understand errors, we need a mistake-proofing model...

• First, recognize the need. If you start out at 0-2 sigma, expect process redesign and mistake-proofing. Do you need high technology knowledge when the defect in the process is obviously seen.

• Next, introduce Change Acceleration Process early. Mistake-proofing means controlling or changing human behavior. You need a shared vision and support to make change last.

• PM/CE/CNX/SOP helps you to select the process changes.

• Mistake-proofing helps to keep the process running and prevents compromise on the SOPs

Now, we need a road map...

Poka Yoke Methodology

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Inspection

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Shingo’s View of Inspection

1. Judgemental Inspection

2. Informative Inspection

3. Source Inspection

Three basic approaches to the inspection of products

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Judgment inspection

InformativeInspection

Eliminates defects by

catching and fixing their

causes

Source Inspection

Discovers defects but

does not reduce them

Reduces defects by informing the

process after they happen

1. Inspection that discovers defects: judgment inspection.

2. Inspection that reduces defects: informative inspection.

3. Inspection that eliminates defects: source inspection.Many companies use the first two kinds of inspection in their quality control programs; thus these can be considered astraditional types of inspection. The third approach – source inspection – actually eliminates defects.

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In a judgment inspection, a person or a machine

compares the product with a standard, discovers items

that don’t conform, and rejects them as defects. It may not reduce the number of defects since it discovers defects only after they have been made. It does not prevent the defects from occurring.

Judgment inspection generally happens at the end of the process or after several operations are completed. This means that there are built-in delays in the time it takes to discover a defect. In the meantime, the process creates more defective products and sometimes this information never gets back to the place where the problem began.

Judgment inspection discover defects

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Informative inspections overcomes the second problem of judgment inspection by giving feedback to the process that produces the defects. The focus is on alerting the defect-producing process (either the operator/assembler or the machine) about the problem as quickly as possible so that the problem can be corrected.

There are three ways to perform an informative inspection:

1.Statistical quality control (SQC)

2.Successive check of each product

3.Self-check of each product

Informative inspections help reduce defects, but they don’t prevent them entirely.

Informative inspection reduces defects

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Statistical Quality Control (SQC)The SQC approach to inspection is to check samples of the product after processing to determine whether they are acceptable. When the samples shows a problem, information is fed back to the process so that the problem can be corrected. Usually a control chart is made to track the results of this checking over time.

SQC is better than a mere judgment inspection because it gives information back to the process. However, since SQC relies on samples rather than checking every unit, it does not not ensure that 100 percent of the products are good. And, although information is recorded about the units that are checked, feedback and corrective action are often slow

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Self-Checks

Feedback and defect correction can happen even quicker when operators or assemblers themselves do a self-check. In a self-check, the operators or assembler checks his or her own work for defects. Self-checks give quicker feedback than successive checks; however, they can’t catch all the defects.

Successive ChecksOne way to improve on SQC inspections is to use successive checks. In successive check, people in the next process inspect each unit that is passed to them. If they find a defect, they tell the previous person right away so that it can be corrected before too many more defects occur.

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Successive checks

Step 1PROCESS

Step 1PROCESS

Step 2 PROCESS

Step 2 PROCESS

Step 3 PROCESS

Step 3 PROCESS

Feedback Inspect Feedback Inspect

Self checks

Inspect InspectInspect

Step 1 Step 2 Step 3 Process Process Process

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Source Inspection Keeps Errors From Becoming Defects

It is one of the four basic elements of the ZQC system. Source inspection differs from judgment inspection and informative inspection in a big way: It catches errors – and gives feedback about them – before processing, so the errors don’t turn into defects. This is the integration of Check and DO stages.

Feedback and corrective

action

Discovery of mistakes before

processing A Process with Zero Defects

Source Inspection

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Red Flag

A condition in the process A condition in the process which commonly provokes which commonly provokes

errorserrors

Red Flag Error Defect

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Red Flag Conditions

1) Frequent Adjustments

2) Constant Equipment Changes

3) Dimensionality / Specification /

Critical Condition

4) Many Parts / Mixed Parts

5) Multiple Steps

6) Lack of, OR Ineffective,

Standards

Some Red Flag Conditions Are:

7) Rapid Repetition

8) Volumea. Sudden changeb. Quantity push vs Quality

9) Environmental Conditions:a. Material / Process Handlingb. Housekeepingc. Foreign Matterd. Poor Lightinge. Other

10) Other?

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What could possibly go wrong here ?

What is the f requency of def ect ?

How severe is the def ect ?

What is the probability of detecting the def ect ?

How can we use Poka-Yoke to prevent the def ect ?

F M E A Questions

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Designing for maintainability

1.Design for minimum maint.skills2.Design for minimum tools3.Design for minimum or no adjustment4.Minimize interconnections5.Provide keys in modules to prevent wrong mating6.Design for safety

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Analyzing PerformanceGeneral Characteristics of DPU

Given the same degree of design margin and process control, defects per unit are directly proportional to the number of opportunities for creating defects within the product.

For equipment, defects per unit is directly proportional to parts count.

For software, defects per unit is directly proportional to lines of code.

For semiconductors, defects per unit is directly proportional to die

area (wafer processing related) and physical complexity (number of

layers, mask sets, bonds, etc).

Given the same product manufactured in more than one plant, the product built in the plant having the best process controls will have the lowest defect per unit level, and the product built in the plant having the worst process controls will have the highest.

Discrete data

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Rolled-Throughput Yield

YIELD DECREASES WHEN COMPLEXITY INCREASESYIELD DECREASES WHEN COMPLEXITY INCREASES

DIEBONDING

WIRE BONDING

PROCESSMAP

Process/ProductProcess/Product

100

P(d) = 6.2 10-3

Yield = (1-p)400

Yield = 8,3 %400

COMPLEXITYCOMPLEXITYProcess Steps

or CTQ’s

QUALITYQUALITY

P(d) = 3.4 10-6

Yield = (1-p)400

Yield = 99.8 %

P(d) = 6.2 10-3

Yield = (1-p)100

Yield = 53 %

P(d) = 3.4 10-6

Yield = (1-p)100

Yield = 99.96%

DICING MOULDING TESTING

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Why Six Sigma?Yields thru Multiple Steps/Parts/Processes

Zst (distribution shifted 1.5)

# of parts, steps, or

processes3 4 5 6

1 93.32% 99.38% 99.9767% 99.99966%

5 70.77% 96.93% 99.88% 99.9983%

10 50.09% 93.96% 99.77% 99.997%

20 25.09% 88.29% 99.54% 99.993%

50 3.15% 73.24% 98.84% 99.983%

100 53.64% 97.70% 99.966%

200 28.77% 95.45% 99.932%

500 4.44% 89.02% 99.830%

1000 0.20% 79.24% 99.660%

2000 62.79% 99.322%

10000 9.76% 96.656%

Yields thru Multiple Steps/Parts/ProcessesZst

(distribution shifted 1.5)

# of parts, steps, or

processes3 4 5 6

1 93.32% 99.38% 99.9767% 99.99966%

5 70.77% 96.93% 99.88% 99.9983%

10 50.09% 93.96% 99.77% 99.997%

20 25.09% 88.29% 99.54% 99.993%

50 3.15% 73.24% 98.84% 99.983%

100 53.64% 97.70% 99.966%

200 28.77% 95.45% 99.932%

500 4.44% 89.02% 99.830%

1000 0.20% 79.24% 99.660%

2000 62.79% 99.322%

10000 9.76% 96.656%

ReduceParts/Steps

ImproveSigma perPart/Step

Rolled-Throughput Yield

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Enhance Product Quality by ReducingParts Count

•Fewer processing steps•Fewer adjustments•Fewer mating points•Fewer tolerance stack-up problems•Fewer operator frustrations•Fewer material control problems•Fewer assembly fixtures

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Reducing parts count

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Reduce the number of partsAssembly Cost Reduction: 100%

Mold message onto surface•Eliminate parts by integration•Reduce assembly time

Used with permission from Boothroyd DeWhurst, Inc.

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Eliminated Parts Are Never . . .

•Maintained in document control•Ordered •Tracked by planning and scheduling or MRP•Received •Inspected•Dispositioned when discrepant •Held in inventory

and they never arrive late from the vendor!

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Eliminate Parts by Integration

Eliminate Welding Operation

Used with permission from Boothroyd Dewhurst, Inc.

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Product Design for AssemblyDrive Toward Simple Mechanical Processes

Avoid: • Welding or brazing • Resistance/friction welding • Hand soldering • Certain adhesives

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Advantages of Poka-Yoke

• Simple – No formal training programs required.Simple – No formal training programs required.• Inexpensive – No high cost investment required.Inexpensive – No high cost investment required.• Gives Prompt Feedback.Gives Prompt Feedback.• Eliminates many inspection operations.Eliminates many inspection operations.• Reduces operator dependence.Reduces operator dependence.• Promotes Creativity and Value-Adding Activities.Promotes Creativity and Value-Adding Activities.• Results in Defect-Free Work.Results in Defect-Free Work.• Requires immediate action when problems arise.Requires immediate action when problems arise.• Provides 100% inspection internal to the Provides 100% inspection internal to the

operation.operation.

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GOOD Detects error before it continues to the next operation

BETTER Allows for detection while error is being made.

BEST Makes it impossible for errors to occur.

SUPERIOR Makes it impossible for errors to occur with productivity gain.

Levels of Mistake-Proof Processes