Poka yoke: when defects stop depending on the operator staying alert
If your quality control depends on nobody getting distracted for eight hours, you do not have control. You have hope.
Poka yoke is a mistake proofing device or design that prevents a defect from happening, or makes it obvious the instant it does. The term is Japanese and means proofing against inadvertent errors. Shigeo Shingo developed it inside the Toyota Production System from one simple idea: human error is inevitable, defects are not.
The three levels, from least to most reliable
- Warning
- signals that something is wrong: an alarm, a light. Depends on someone reacting.
- Control
- stops the process when the wrong condition is detected. Does not depend on reaction.
- Prevention
- makes the error physically impossible. This is the level to reach when the defect is critical.
- 01 Locate the exact point where the defect is born Not where it is detected. A defect caught at final inspection was born three stations earlier.
- 02 Ask what makes the error possible, not who made it If two similar parts fit the same fixture, the error was designed in.
- 03 Look for the physical solution before the electronic one A pin, a stop or a guide that allows only one orientation costs less and fails less than a sensor.
- 04 Test it with the bad part on purpose Validating a poka yoke means trying to push the defect through and proving it cannot pass. If you did not try, you did not validate.
- 05 Define what happens when the device itself fails A disconnected poka yoke that nobody notices is worse than none, because it creates false confidence.
- Solving with training and a sign what a two dollar stop block would solve.
- Placing the device at the end of the line, where the defect has already been paid for.
- Not documenting the device, then losing it at the next model changeover.
- Installing it and never auditing that it still works.
The workshop ends with proposed devices for real cases from your operation, each with its level defined and its validation test designed. My rule is simple: if the proposed solution starts with "train the operator to", it is not a poka yoke yet.
This lives inside Problem solving and continuous improvement
Is poka yoke the same as inspection?
No. Inspection catches the defect after it exists. Poka yoke prevents it or makes it obvious in the instant. One costs you scrap, the other avoids it.
How much does one cost to implement?
The best ones I have seen cost less than an hour of downtime was worth. Most are mechanical: a pin, a guide, a stop, a geometry change in the tooling.
Does it apply in the office or only on the plant floor?
It applies anywhere there is a repetitive process. A required field in a form or a locked cell in a spreadsheet is poka yoke under another name.
What is holding your operation back today?
Tell me the concrete problem. I will tell you whether it is mine to solve or someone else’s, and what I would do first.