Poka Yoke 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.
Pokayoke 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.
Prevent Defects with Poka Yoke
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; but can cost us our business through dissatisfied customers.
Pokayoke seeks to prevent these mistakes 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, however sometimes I fail to see the red underline!
Error proofing by using poka yoke devices is a 100% sure way to prevent those mistakes becoming defects that reach the customer.
Shigeo Shingo Pokayoke
The definitive work on mistake proofing by the master himself Shigeo Shingo. If you want to learn all there is to know about poka yoke this is the book.
Learning About Pokayoke
In my mind there is only one book that you will ever need on the subject if you really want to learn about Jidoka and Poka Yoke in particular and that is this book by the master himself. I have a copy and it is a valuable book to read through and will give you many ideas as to just how many changes you can make to prevent rejects.
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 to 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 Fs in the Paragraph
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!
100% Inspection effectiveness
How many "f"s did you find in the paragraph?
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 Pokayoke 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
Pokayoke 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 pokayoke 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 Pokayoke
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 Poka Yoke
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.
Poka Yoke Video Examples
Mistake Profing Video
Poka Yoke Examples
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 instances 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 Pokayoke devices that you may not notice are your automatic car not starting if you leave it in drive, the inability to put it into drive without depressing the brake first. 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.
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.)