When a consultant shows a factory how a poka yoke (mistake-proofing) system can get rid of frequent mistakes committed by operators, they look at the consultant like he/she is a genius.
However, anybody with a dose of creativity and an understanding of the main types of poka yokes can come up with good ideas. Here are the 6 main types of mistake proofing techniques...
The 6 Commonly Implemented Poka Yoke (Mistake Proofing Techniques)
1. A change in a die design
In some operations (for example in stamping, drilling…), a die is necessary and can be placed incorrectly. That error can result in product quality issues and in damage done to the die.
How to avoid this? By making it impossible to place the die only in one way. This can be realized in many ways. An approach is to have guide pins of different sizes, so that they can only “fit” in one direction. There are many other approaches.
2. A change in a fixture design
This is relatively similar to point 1, with a key difference: the part being worked on can be placed the wrong way into a fixture, with resulting quality issues.
The fixture can be modified in order to make it impossible to place the part incorrectly. (Sometimes the design of the part itself has to be changed, for the same effect.)
3. Sensors that prevent processing under certain conditions
The most common sensors are listed below:
- Limit switch – convenient when a part is in contact with a tool/fixture.
- Proximity sensor – a good solution when a part is/might be at a certain distance.
- Infrared sensor – appropriate for checking presence from a distance.
There are many scenarios in which a sensor can detect an issue. We show 3 mistake-proofing examples that use sensors in our free e-book.
Enjoying reading about mistake proofing? Explore the topic in even more detail by reading the eBook I created on the subject:
4. A vision system
In simple terms, a vision system captures images, analyzes them, and triggers an action in pre-determined cases. It does not require contact with the product.
For example, it might detect that a part is poorly positioned, that a component (or labeling element) is missing, that a step was done before another, etc. As a response, it might sound an alarm, or it might make it impossible to proceed (often by stopping a piece of equipment) until a positive change is made.
This approach appeared in the 1980s in a simple form and has kept improving since them. It doesn’t appear in the Poka Yoke red book that seems to include examples dating back to the 1960s.
I have seen vision systems that use consumer electronics (cheap webcams if the environment is not very dusty) and require relatively little programming. With the advances of artificial intelligence / machine learning, vision systems will get better and better. Remember, a Tesla car can pretty much self-drive based on cameras alone!
5. A checklist
By any standard, a checklist is one of the weakest mistake proofing techniques. It does help a lot when no other approach listed above is possible and when operators are trained and careful – think pilots in a plane.
The more a checklist’s elements are integrated into the work content, the better. Think of color codes where a checklist step matches a certain tool. Or a form to fill out that contains the steps in the right order.
6. Creative solutions to avoid or detect errors for close to zero investment
Poka yokes are a science but also an art. Think this way and you will see many opportunities that will take different shapes depending on the application.
For example, let’s say some parts move on a conveyor. A few of them have a defect and are taller. The “obvious” countermeasure is a sensor that detects that abnormality. But a better approach is to place a stick that will block the way to all defective parts and push them into a red container on the side of the conveyor. It is faster and cheaper to implement, it is easier to maintain, and it immediately acts on its findings!
We show 2 creative applications in our free e-book, but a researcher could probably list hundreds of them.
There are many other types of devices and systems that reduce human errors without requiring a heavy investment. Some of them actually add a step to the process (for example, kitting components together before assembling them usually results in fewer missing parts and in faster assembly) and typically don’t qualify as a method of mistake proofing. Others cost virtually nothing and cut defects by 80%.
In many industries, this tool is the best way to reduce defects from about 3% to a tenth of that. That’s a major component of ‘jidoka’, which is one of the 2 pillars of the Toyota Production System.
Do you have any questions about mistake proofing, or examples of where it has worked for you to share with our community?
In this blog post I have covered some mistake proofing examples relatively briefly, but if mistake proofing is something that you find useful we have a lot more information on the topic for you!
You can explore fourteen examples in far more detail by getting your free copy of the eBook below: