1/32 Equipment Efficiency: Quality and Poka-Yoke (Mistake- Proof, 防錯法 ) Operations Analysis and Improvement 2010 Fall Dr. Tai-Yue Wang Industrial and Information.

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Equipment Efficiency:Quality and Poka-Yoke (Mistake-

Proof, 防錯法 )Operations Analysis and Improvement

2010 Fall

Dr. Tai-Yue WangIndustrial and Information Management Department

National Cheng Kung University

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Presentation

Shigeo Shingo developed a system to improve inspection tasks with the goal of guaranteeing 100% quality for manufactured parts. Leading the charge toward a defect-free

process.

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Presentation This chapter will explain inspection

processes based on mistake-proving (called Poka-Yoke). Complements Statistical Process Control

(SPC). Poka-Yokes are visual and physical tools

that should be utilized in conjunction with inspection. This concept will also be presented in this

chapter.

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Introduction

Poka-Yoke is part of the Just-in-time philosophy and key number 11.

Poka-Yoke reduces the defect rate that is part of the OEE rate.

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Inspection and Statistical Quality Control

Every production process generates defective products. The objective for all quality control systems is

to reduce the number of defects produced. Types of defects.

Isolated (a crack appeared on a product). Sequence (repetitive) defects.

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Inspection and Statistical Quality Control

Product inspection is performed in order to avoid defective products from reaching the customer.

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A Taxonomy of Inspection

Total or partial. 100% of the products or extrapolating a sample

study. Statistical or not statistical.

Based on statistical methods or not. Quantitative or qualitative.

Number of elements or appearance of the product.

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A Taxonomy of Inspection Based on measures or based on functional trials.

Numerical values or without measures. Sensory or physical.

Through human senses or by means of devices. Subjective or objective.

Evaluated by a person based on appearance or evaluated based on a specific measurable feature.

Internal or external. Within the process or as an independent activity.

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Inspection and Statistical Quality Control

Inspection should be able to identify defects. Develop a corrective action to eliminate the errors

or more specifically the cause of the defects. Otherwise, inspection process will become a pointless

exercise.

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Inspection and Statistical Quality Control

Many years ago, it was believed that the only way of insuring the quality of all manufactured products was to inspect all of the parts - 100% inspection. 100% inspection does not result in defect free

products to the customer.

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Inspection and Statistical Quality Control

To avoid this problem and reduce inspection cost, it is possible to duplicate the inspections. How many control stages would be necessary

to guarantee the quality of the entire manufactured lot?

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Inspection and Statistical Quality Control

Using this procedure, it is not possible to avoid product defects in an efficient way. Statistical quality control techniques (SQC)

seem to be the right alternative. Some percentage of defects is always accepted.

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Inspection and Statistical Quality Control

A high defect production rate (high defect level) can be deadly for a company well being. It is very important to eliminate or capture all

defective parts before they reach the market. Shingo utilizes the term SQC.

SQC is a term that today has become obsolete. SQC has been replace by SPC.

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From SQC to Zero defects

Shingo introduced new ways of carrying out inspection processes that were based on SQC. The inspection process evolution should be

similar to the historical evolution of these methods.

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From SQC to Zero defects SQC techniques are based on two principles.

Evaluate (statistical samplings). Inform (feedback analysis until reaching the

cause/process that created the defect). SQC implementation has two main problems.

Do not guarantee the quality of all products. Because 100% of them are not inspected.

Feedback and corrective actions are slow or in many cases do not even exist.

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From SQC to Zero defects

Therefore, an inspection process, which is based on control charts, does not reduce the factory defective rate. It only detects defects.

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From SQC to Zero defects

Shingo proposed two methods to avoid these problems. 100% inspection of the product utilizing Poka-Yoke

devices. Accelerating feedback by self-checking

Where production workers check their parts and by successive check systems.

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Poka-Yoke

The Poka-Yoke concept was created by Shingo. “fool-proof devices” -> Poka-Yoke “mistake-proving”

To consider an inspection device as a Poka-Yoke. Ingenious, simple and cheap.

Several Poka-Yoke devices can be found in our daily life activities. Recording protection window on floppy disks

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Poka-Yoke

Poka-Yoke systems can be used in a company in order to mistake proof activities. Automatic part feeders.

Poka-Yokes can reduce undesired workload components. It is possible to separate parts with different

specifications.

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Poka-Yoke

Poka-Yoke devices such as Go-no Go devices are utilized in order to avoid inspections based on trials.

It is not always possible to design a Poka-Yoke to carry out 100% of the inspection.

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Self-check and successive check systems

Self-check procedure. The worker who produces the part is the same

worker that carries out the inspection. This system is the most efficient one.

The worker obtains immediate feedback. Criticizing one’s own performance may not be totally

objective.

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Self-check and successive check systems

Successive check. Next worker will typically conduct the inspection

task. Can reduce the defect rate by as much as one fifth of the

initial value in about one month. It is necessary to fix 2 or 3 check points. In the beginning of this checking procedure, the defect rate will

increase.

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Self-check and successive check systems

Sensory based inspections (scratches, painting quality). It is advantageous to place samples next to the

checking point to show acceptable limits.

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Source inspection

These techniques reduce the defect rate. Do not eliminate the error(s).

Causal relationship between errors and defects. If the error source is eliminated, errors will never become

defects.

Error Defect Information

Feedback

Error Information

Feedback

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Source inspection

Source inspection eliminated all the errors except the unnoticed errors. These unnoticed errors can be detected by an

efficient use of Poka-Yoke devices. Types of source inspection

Vertical -> Before the process. Horizontal -> Inside the same process

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Poka-Yoke design methodology

Three suggested methods. Weight, the dimensions or the shape.

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Poka-Yoke design methodology

The use of meters/counters, spare pieces method or a fixed sequence in the assembly process

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Poka-Yoke design methodology

Use technology to design the Poka-Yoke devices. Contact mechanisms.

Limit switches. Mechanisms without contact.

Sensors. Meters/counters.

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Poka-Yoke examples Improper parts (extra material) can break or

otherwise adversely affect the tool of the following process. The introduction of a mechanical blade stop can

redirect the parts. It is possible to avoid the line from stopping by

using a “size limit” Poka Yoke.

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Poka-Yoke examples

Some templates (previously designed) eliminate the errors when adjusting the parameters before a process. By designing separate face covers for the

proper product. The template patterns indicate the proper

settings and values that should be utilized.

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Poka-Yoke examples

A simple slot in the conveyor line can avoid an incorrect product. Without the utilization of a Poka-Yoke, filling material

can be wasted and a major spillage can occur. With the Poka-Yoke the line will not stop and the

improperly oriented parts will fall to collector box

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Summary

The chapter had the primary objective to present one of the many quality related improvement tools: The Poka-Yoke (mistake-proving devices). The Lean manufacturing philosophy considers this tool as one of the pillars to improve the overall equipment efficiency. The chapter has presented several examples of this kind of devices showing that in order for a device to be considered as a Poka-Yoke it must be ingenious, simple and cheap.