Poka-Yoke the Path to Zero Defects

The author of this article, Craig Thompson, holds certifications as a Certified Quality Manager (CQM), Certified Quality Engineer (CQE), and Certified Quality Auditor (CQA) and is an RAB/QSA-certified AS9100/ISO 9000 Lead Aerospace Auditor (previously AIEA). Mr. Thompson has extensive experience working in the Aerospace, Oil & Gas, Automotive, and Manufacturing Industries. Craig has also developed supplier process flow maps for OEM manufacturers of complex machines for both aerospace and automotive.

In any type of manufacturing environment, the biggest problem encountered is mistakes made by human errors that cause disruption to the flow of products between receiving and shipping. This article details how you can implement a Poka-Yoke system by briefly explaining what Poka-Yoke is, methods used in a Poka-Yoke system, how and when to use Poka-Yoke concepts, and everyday examples of Poka-Yoke. As a problem solving expert and a previous Director of Quality, the utilization of Poka-Yoke was one of the best tools in my management toolbox. The use of error proofing, if implemented correctly, can lead to zero defects, less rework and scrap costs, less fire fighting, and enhanced process improvement within your facility.

The best way to explain what Poka-Yoke is, may be to describe its beginning. Originally the concept was part of the Toyota Production System adopted by Shigeo Shingo and was called Baka-Yoke, meaning fool-proofing or idiot-proofing. However, as with most things, to be politically correct it was changed to Poke-Yoke meaning mistake or error proofing which is a milder term with a broader scope. The purpose of Poka-Yoke is to eliminate defects by preventing and correcting any human error before they can occur. This is done by implementing techniques that enable operators to avoid missing a part or feature, installing a part backwards, and applying reliable methods for detecting errors in station.

Most people, even the most seasoned manufacturing personnel, go about their daily lives without even noticing that there are a multitude of Poka-Yoke designs all around us. Some are for convenience, while some are for safety reasons. For example, one that is often overlooked are the built-in ground fault circuit breakers in the outlets of kitchens or bathrooms. Another one can be found at the gas station’s gas pump, the sizes of the nozzles are different to reduce the risk of putting diesel in regular gas cars. One example that has surely saved us all, is the auto save feature built into computer software like word and excel. The list goes on, elevator doors have sensors so that if there is anything in between them they won’t shut. USB drives and cords can only be inserted in one direction. And lastly, my personal favorite, and one I have used many times while writing this article, is spell check.

Poka-Yoke is a big part of the 5S system, in particular the Standardize phase. The method of implementing a Poka-Yoke system consists of three primary methods for the detection of mistakes.

1. Contact Method – Identifies errors in products based on shape, size, and position utilizing a type of sensor with an alarm.
2. Counting Method – Allows the operator to easily track how often an action or several similar features are completed.
3. Motion/Sequence Method – Identifies specific motions required for each step of the process function and if it was performed.

Each method could either be used as a control or a warning system. However, each control system utilizes a different process for preventing the error. Along with the detection and prevention methods you need to understand that there are 6 primary potential cause categories for any defect. They are: Manpower, Method, Materials, Mother-Nature (the environment), Management (information) and Machines. You may notice that these are the same categories as a fishbone diagram utilized in problem solving, this is not by coincidence.

In my experience the best way to develop a robust Poke-Yoke system within your QMS is to follow 6 simple rules when implementing error proofing into the continuous improvement activities:

1. Try to spend little if any additional budget dollars.
2. Remember that the simpler the concept, the better when dealing with Poka-Yoke designs.
3. One of the most important, is to get the operators of the process involved, they are the most knowledgeable about the process steps.
4. Do not to confuse extra gauging as an actual Poka-Yoke system, they are only a detection not a prevention.
5. The hardest rule to follow, is to look at the ideas and make a choice. It is better to decide on a concept and if it is wrong, learn from it. The longer you go without a decision, the worse the problem gets.
6. The final rule is, don’t let the mistake proofing improvements be an option for the employees. You developed them, use them.

When to use Poka-Yoke is a hard question that seems to come up often. My suggestion based on experience is easy, use error proofing often and as economically as possible. But if you are looking for a more technical answer you can consider the following. There is the proactive approach which requires Poka-Yoke usage prior to any inspection point, meaning the closer to the point of possible error, the better. If done correctly the Poka-Yoke effort will be 100% effective eliminating an error before a defective part is completed. The other approach is reactive, however still as close to the error point as it can be. This method is usually the easiest to implement, because most facilities are already performing these activities. Product checks immediately after the process, or operator check before the next operation step. The reactive approach is not 100% effective; yet it will eliminate defects being passed on to the next process.

When I first started my career journey in Quality Management Poka-Yoke was still relatively new in the United States. However right away I understood it’s potential to save the company money, reduce my stress as a manager, and improve our production processes. As with anything else there will be trial and error efforts in developing the right Poka-Yoke concepts, especially where each facility has different difficulties in producing defect free products. I learned from the mistakes, gained more knowledge and became adept at identifying ways to prevent human and equipment faults. There is no magic formula to establishing a robust Poka-Yoke system. However, I do know that implementing it results in reliable and repeatable processes and usually the simpler the idea, the better it works.