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Poka Yoke Mistake Proofing

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I am a trainer and consultant in Lean Manufacturing, Six Sigma, quality management, and business management.

Mistake proofing

Mistake proofing

What Is Poka Yoke?

Poka yoke literally means mistake proofing; it is about preventing the creation of defects. Usually, it is a simple and inexpensive device or modification to the product itself to prevent there from being any possibility of defects being made.

Poka yoke was refined by Shigeo Shingo of Toyota, who published the definitive work on this subject and introduced it to us in the west.

Poka yoke devices can come in many shapes or forms, but all either physically prevent us from doing something wrong or warn us that a mistake has been made in some way, such as the tone sounding in your car when you forget to switch off your lights (although some more modern cars now switch the lights off automatically, which is far better than an alarm that may be overlooked.)

Poka Yoke is part of Jidoka, one of the main pillars of the Toyota production system (TPS) and hence lean manufacturing. Along with Autonomation, it is a major part of any total productive maintenance (TPM) implementation to ensure that machines can only produce correct parts and highlight any abnormalities.

Mistakes vs Defects

Everyone makes mistakes. The secret is to prevent those mistakes from moving onto the customer as defects. Defects not only cost us significant amounts of cash; they can cost us our business through dissatisfied customers.

Poka yoke seeks to prevent these mistakes from becoming defects by either preventing or highlighting the mistakes in the first instance. For instance, the spell checker on my computer will either auto-correct the word that I have typed or it will underline it in red, even though sometimes I fail to see the red underline!

Error proofing by using poka yoke devices is a 100% sure way to prevent those mistakes from becoming defects that reach the customer.

Shigeo Shingo Pokayoke

Effectiveness of Human 100% Inspection

Many companies rely on human inspectors (quality control) to inspect product to ensure that it meets specifications, but how effective is this? Most people tend to agree that 100% inspection is only about 80% effective, although I am at a loss to find concrete research on this. In my experience, I would say that this totally depends on what is being inspected and how.

To demonstrate the effectiveness of human inspection, use this well-used paragraph from the American Society for Quality; read through this paragraph just once and count the number of “f”s that you find.

How many F's are in the paragraph? How effective is inspection?

How many F's are in the paragraph? How effective is inspection?

How Many F's Can You Find?

Use the quiz below to record your answer and have a look at the results to see how you compare to other people – please don’t cheat, as it will spoil the test!

Effectiveness of 100% Inspection

As you will see, there is variation in the answers. Few people will manage to find all of the “f”s within the paragraph, so how can you expect your operators to find defects before they get to the customers?

This is why Poka yoke is such an effective tool. If you can design your products and process to eliminate the possibility of them being wrong, then there is no need for inspection.

Types of Poka Yoke

Poka yoke devices can operate in a number of different ways and are usually broken down into three different types; Contact, Fixed Value, and Motion Step. Each can then be either a Full Control or a Warning type poka yoke.

Contact Poka Yoke

Contact type poka yoke devices are those that actually make contact with the product or service, this could be fixtures that are shaped to only accept the correct shape components or sensors that detect if the component is there.

These Poka Yoke devices should prevent the process from operating if the wrong components are used by using sensors to ensure clamps are closed and locked etc.

Fixed Value

These are Poka Yoke devices that make it obvious if a part is missing, for instance an egg tray type container for the supply of parts for your process – missing parts would immediately be visible. These can often be combined with contact type pokayoke devices to confirm that all parts are present.

Motion Step

This a poka yoke device that ensures that the correct number of steps have been taken in the process; for instance, if five nuts have to be tightened to a specific torque, the nut runner would be programmed to reach the required torque the correct number of times before releasing the product to the next step of the operation.

Examples of Devices Used

The most useful types of devices are:

Guides or pins to ensure the correct orientation and assembly of parts (Designed so they can only go together in one way – like a three-pin electrical plug into its socket)

Limit switches that detect if parts are present, either on the location of the part or on the clamping system.

Fixtures that prevent the use of defective parts from previous operations, for instance, pins that have to fit into holes drilled in the previous operation or locations that only accept parts machined to the correct specification (oversized parts will not enter and undersized will fall through.)

Counting systems that ensure the correct steps are taken before allowing the release of the part, as in the nut runner example above.

Some everyday examples of poka yoke devices that you may not notice are your automatic car not starting if you leave it in drive, and the inability to put it into drive without depressing the brake first. Also, the shape of your USB connector which can only be inserted one way up in your computer, the battery location in your mobile phone, or the slot for the sim card.

Do You Have Questions?

If you have any questions about using poka yoke or about lean manufacturing in general, please leave a comment below (and the answer by the way is 48 F's.)

How many F's: There are 48 in the paragraph.

How many F's: There are 48 in the paragraph.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


Tony (author) from At the Gemba on May 25, 2014:

Well done Sig, most people just cannot do 100% inspection reliably and for extended periods of time which is why Poka Yoke devices are so important. If you look at the poll above you can see that the people that have tried to count have come up with all sorts of results.

Sig on May 24, 2014:

I counted 48; however, my day job is performing 100 percent inspection on parts, and I appear to be competent in the ability to perform this function.

Tony (author) from At the Gemba on October 24, 2011:

Thanks cags, just shows the need for Pokayoke devices for things to be done correctly!

Raymond D Choiniere from USA on October 24, 2011:

Hey LeanMan, pretty cool hub. I took the quiz, however, I actually punched the wrong number into the quiz. So you will have to make a manual change. I accidentally punched into 42-44, but actually meant to click on 45-47. Sorry about that. :/

Tony (author) from At the Gemba on October 23, 2011:

Thanks for reading and trying the inspection quiz, it will be interesting to see how many people can get anywhere near the real answer.

It truly shows the need for pokayoke.

Kelly Umphenour from St. Louis, MO on October 23, 2011:

I was very close - you said to read it once but I read it backwards but only once. It is a proofreading trick for misspellings the old fashioned way:). Saw this on the Inn:)