POKA YOKE: Zero Defects through Mistake-Proofing
Defects contribute greatly to the cost of production because they lead to rework or scrapping of the manufactured product.
By using poka-yoke or mistake-proofing, the inherent cost of defects will be greatly reduced.
As a kaizen tool, poke-yoke aims at preventing process or product errors from occurring in the first place so as reduce the need for reworking of defective parts.
Importance of a zero-defects culture
At the very core of the Kaizen lean philosophy is the elimination of waste which is defined as any activity that consumes resources but does not add value to a product.
In Kaizen, there are generally 7 broad categories of waste that a lean thinking organization aims at eliminating or reducing. These are transportation, inventory, motion, waiting, over-production, over-processing and defects.
The reduction or elimination of defects is important to any business because it reduces costs while at the same time increasing customer satisfaction levels. Defect elimination leads to reduced rework, scrap, warranty and inspection costs.
These costs tend to increase the further down the value stream the errors are found. For example, the cost of an error captured at the raw materials receiving section is far less than the cost of an error caught at the point of sale.
Benefits of Poka-Yoke
Poka-yoke is a very important tool to achieve a defect free production environment. The benefits of achieving a zero-defect environment in the organization are:
Quality processes resulting in quality products. It is very hard for a process that is not of high quality to result in quality products or services. All processes must be capable of achieving value for the customer and this is possible through elimination of waste.
Effective teams who work in a coordinated manner so as to deliver value to the customer. Such teams look at the system as a whole and know their what roles they have to play so as to achieve the overall organization goals
Problem solving culture where there is no blaming when problems occur, but a concerted effort to resolve the real issues. This culture allows constant learning to take place within an organization because there is no finger-pointing when a problem occurs.
Solving the root cause guarantees that it will not recur in the future. This is achieved by conducting a root cause analysis to get to the real reason why a problem occurred.
First time quality is an important principle because it ensures that every effort is made to achieve quality within process. This is in contrast to a culture that continuously pushes out parts irrespective of the quality because there will be a reworking if bad parts are produced
Waste elimination is at the heart of lean thinking because product quality improves and delivery times and costs are reduced. Waste in lean thinking is defined as an action or activity that consumes resources but does not add value to the customer
Continuous improvement of the solutions. This incremental improvement takes a cyclical pattern of problem identification, problem analysis, solution implementation and improvement.
The origin and meaning of poka-yoke
Developed by Shigeo Shingo over 30 years ago, Poka Yoke is a technique that guarantees process and product quality and negates the need for 100% final inspection.
Shigeo Shingo is considered the father of the quality movement for his seminal work on the development of the popular lean manufacturing tools of Just-In-Time, which consequently gave rise to the Toyota Production System.
His influence in his home land of Japan and the Western industrial thinking makes him the most respected contributor to the quality movement in the world.
Dr Shigeo Shingo developed the ‘zero defects’ approach to manufacturing by using poka-yoke which improved the product quality in many industries in Japan.
Shigeo Shingo also introduced the Single Minute Exchange of Die (SMED) concept which tremendously reduced the time required to change-over from one product type to the next in a manufacturing firm.
If the processes that are used to manufacture products are so reliable that there are no defects, there will be no use for final inspection. Capturing defects after they have already occurred means that the company has already incurred production costs and any rework will just add further costs to the products.
The poka-yoke system developed by Toyota experts is an approach that focuses on controlling the process conditions so that defects do not occur. The main tenet of this system is that no process should produce, accept or pass defective parts to the next process.
In-process inspection is more effective than final inspection because every stage in the manufacturing process checks for, and prevents errors before passing to the subsequent process.
By carrying out successive self-checks within the process, the ultimate end product will have little or no errors which will negate the need for 100 % final inspection before delivery to the customer.
Types of poka-yoke
There are three ways in which you can mistake-proof a process using poka-yoke:
- Contact method where a sensor determines the presence of a part in a process and if the part is missing, it prevents the process from continuing. The sensors could be physical sensors or energy sensors such infrared.
- Fixed-value method is a form of poka-yoke that uses a sensor to determine whether a process is complete by the number of parts that have been used or the number of process steps taken. If the right number of steps or parts have not been used, this indicates an error and the process is stopped. This method is also known as the counting method of mistake proofing.
- Motion-step method uses a sensor to determine whether all the prerequisite process steps have been undertaken. If a step in the process has been missed, a signal is sent to the subsequent process to stop. This forces a correction of the problem before the process can continue and is a very effective error-proofing technique.
As can be seen from the methods highlighted above, the main advantage of mistake-proofing in process is that the final product will be of high quality as the mistakes were corrected long before they could reach the final customer. This reduces the cost of rework and inspection at the point of dispatch.