POKA YOKE

• Another contribution from Dr. Shigeo Shingo

• In Japanese POKA YOKE means “Mistake Proofing”

• Initially it was coined as ”Fool Proofing”. Later changed to Mistake Proofing

Poka yoke aims at:

1. Error free production

2. Defect free product (Zero Defect)

3. User misuse tolerant product

Mistake proofing

• The original “ Fool Proof” was changed to “Mistake proofing” because:

• Merely not noticing that an error is made or a machine is not functioning properly does not make a person stupid or foolish.

Sources of defects

• Human error• Processing errors• Omitted processing• Errors in setting up process• Missing parts.• Wrong parts Processing• Wrong work piece.• Adjustment error • Tools and jigs improperly prepared.

Different kinds of Human errors • Poka-Yoke dwells on the theme that to err is human and

errors can be corrected.

• Forgetfulness: • Errors due to misunderstanding• Errors in identification• Errors made by amateurs• Willful errors• Errors due to slowness• Errors due to lack of standards.• Surprise errors (unexpected m/c operation resulting in

panic situation)• Intentional errors/ sabotage

The concept of Poka-Yoke is a counter measure to a well known law called “MURPHY’S LAW”

What is Murphy’s Law?

If any thing has a chance to fail, it WILL fail, at the worst possible time, in the worst possible way.

The entire focus of Poka- Yoke is to avoid such a thing happening to your product or process or user.

The concept of Mistake proofing involves

• Controls or features in the product or process to prevent or mitigate the occurrence of defects

• Requires simple inexpensive inspection at the end of each successful operation to discover and correct defects at the source.

Zero Defect Quality (ZDQ)

Poka Yoke aims at Zero Defect Quality.

Three Components of Zero Defect quality are

• Point of origin Inspection.

• 100% inspection

• Immediate feed back.

Point of origin InspectionInspection is of 3 types

1. Standard / Judgment inspection. Pass or fail, good or bad, accepted or rejected etc.

2. Informative Inspection. To know the quality level of the process or product.To estimate the Average Outgoing Quality Level

(AOQL) etc.

These are traditional methods. Here defects are detected after they are made.

Point of Origin Inspection (contd.)

• Only Point of Origin inspection eliminates defects.

• Here focus is on prevention and not on detection.

• How this is Done?• By introducing checks and sensors all

along the process in as many places as possible.

Point of Origin Inspection (contd.)

• Corrective actions• If a defect is found, two types of corrective actions

are possible.• 1. Stop the process, check what has gone wrong,

take corrective action and then continue.

or• 2. Give an alarm. When stopping production

immediately is not possible, warning signals are given. The defective products are segregated elsewhere.

100% Inspection

• Poka Yoke recommends 100 % Inspection.

• In statistics there is a view that 100 % inspection is not 100 % reliable.

• This general rule is over ruled in poka yoke.

• Poka yoke does not rely on sampling.

• When every product or process is checked at every possible stage 100 %, then defectives can be identified and isolated.

• Corrective action must be taken immediately.

Six Mistake Proofing Principles

1. Elimination: To eliminate the possibility of error by redesigning the process or product so the task or part is not needed at all. Example: Use of clamps instead of nuts and bolts. (Scooty vs. Bajaj Scooter)

2. Replacement : Replace the process by a more reliable one.

Example: a) Use of Robotics in assembly line. b)Automatic dispensers- right material

at the right time at the right quantity.

3. Prevention: By design the product or process is made mistake-proof. Example 1: Use of jigs, fixtures and limit switches which allow fixing and assembling in only one way, the correct way.

4. Facilitation: By using techniques that facilitate doing the assembly jobs correctly.Example: visual controls including colour coding,

By keeping samples or models in front of the assembler.

5. Detection: Error detection before further processing so that defects are identified and corrected early in assembly.

Example: Providing sensors at every stage of the process. Simple built in self checks at every stage

6. Mitigation: Seeks to minimise the effects of errors.

Example: Providing a trip or fuse for over load or short circuit.

Types of sensors used in Poka Yoke

Contact type sensors

Limit Switches

Micro Switches

Touch Switches.

Non Contact type Sensors

Inductive Sensors

Capacitive Sensors

Photo electric sensors

Metal detectors

Most frequently used sensing Parametersin Poka yoke

• Pressure Force

• Weight Vibration

• Shape Gas

• Presence or Absence Counter

• Colour sensing Position

• Vacuum Flow

Ideally, Poka Yoke should be done at the design stage of a product or process or service.

This is what is insisted by DFSS of Six sigma strategy.

Failure Mode Effects Analysis (FMEA)

• Failure Mode Effects Analysis is a study of the product or process with an objective to analyse what happens if there is a wrong operation or a wrong usage?

• What are the consequences of a failure?.

• After such an analysis the corrections and improvements can be made by using Poka-Yoke principles.

Is Poka Yoke applicable only to industries and Processes?

No. It is applicable to services sectors also.

Let us examine some Poke-Yoke examples that we come across in our day to day life.

What is the Poka-yoke you do on a cheque?

What are the poka-yoke the bank does while issuing their demand draft?

What is the poka –yoke done on a sim card of any mobile phone?

What Poka-yoke warning you get when you close a word or excell programme?

What poka –yoke warning you get when you delete a file from your computer?

What is the poke-yoke that is done on a washing machine?

What poka-yoke is done while filling air to your tyre?

What poka-yoke you observe in lifts of flats?

What Poka Yoke APSRTC was using to mitigate rash driving of their buses?

What Poka Yoke the municipal workers use while cleaning the roads in the night?

Some general points• Poka Yoke and SMED are not Stand alone

concepts like quality circles, six sigma, TPM etc.

• They mesh very well with 5S, Kaizen, quality circles, QIT, TPM and Six Sigma initiatives.

• Many quality circles apply these principles quite naturally in their case studies.

• In fact, QCFI expects qc circles to spell out the Poka Yoke they have carried out at the end of their case studies and marks are allotted for this under the heading “Follow up”.