What is poka-yoke, with an example?
Poka-yoke is mistake-proofing: a simple device or method that stops a mistake from happening, or catches it at once. A pin in a fixture that lets a part sit only one way is an example. It works every time, without anyone having to remember.
Every plant has the same story. A wrong insert is picked from a mixed tray. A shaft is loaded the wrong way round. One bolt is missing from a cover. The operator is told to be careful. It happens again next month.
People make mistakes every day, however skilled they are. The answer is not to shout. It is to change the process so that the mistake cannot reach the customer.
What is the difference between a mistake and a defect?
A mistake is a slip: the wrong part picked, a step missed, a wrong setting. A defect is the result: a bad part that leaves the machine.
Between the two there is a chance to act. A good poka-yoke stops the mistake, or catches it so quickly that no defect is made.
| The old way | The poka-yoke way |
|---|---|
| Blame the person | Change the process |
| "Be more careful" | Ask how the process allowed it |
| More inspection at the end | Add a pin, a tray, a counter or a sensor |
| Same mistake next month | The mistake cannot happen again |
What are simple poka-yoke examples?
These are examples. Your parts and numbers will differ.
- Locating pin in a fixture. A shaft must be loaded with a hole facing a certain way. A pin in the fixture enters that hole. If the shaft is loaded wrongly, it will not sit or clamp. A few rejections a week stop, and the cost is one pin from the tool room.
- Kit tray with fixed pockets. A cover needs eight bolts. The assembler takes bolts only from a tray with eight pockets. A bolt left in the tray means one was not fitted. The count checks itself.
- Separate fixtures for left and right parts. Two brackets look alike. Two fixtures, in two colours, each with a stop block, accept only the correct hand.
- Weighing each box. Each box should hold ten pieces. The scale shows green only between two weight limits. Red means open the box and count.
- Colour code for look-alike material. Sheet material of four similar grades gets a different colour mark for each grade. A wrong grade is seen from a few steps away.
None of these needs a big budget.
What are the three methods?
Almost every poka-yoke uses one of three methods.
- Contact. Shape, size or position tells right from wrong. Examples: a pin, a stop block, a switch that confirms the part is seated.
- Fixed value. The right number. Examples: the kit tray, a counter on a torque tool, weighing a box.
- Motion step. The right order. Examples: the drill will not start until the clamp is closed, the next step is locked until the last one is confirmed.
How strong is your solution?
Not all fixes are equal. Rate yours on this ladder, from strongest to weakest.
| Level | What it means |
|---|---|
| 5 | Designed out. The mistake is impossible, for example a symmetrical part. |
| 4 | Cannot happen. The part will not fit or the machine will not start. |
| 3 | Caught and stopped. The machine stops at once. |
| 2 | Caught and warned. A light or a buzzer. |
| 1 | Found later, by final inspection or by the customer. |
"Be careful" is not on the ladder at all. A warning is weaker than a control, because people get used to alarms. Aim for levels 4 and 5 where you can.
If a mistake has happened twice, the fault is in the process. Fix the process, not the person.
How do you make a poka-yoke in seven steps?
- Pick the problem from rejection slips, complaints and near-misses. Start with the mistake that happens most or costs most.
- Go and see where it happens. Ask the operator how it can go wrong.
- Find the root cause. Ask why until you reach it.
- Choose prevent or detect, and control or warning. Aim for prevent and control.
- Choose the method: contact, fixed value or motion step.
- Try it cheaply first with cardboard, a wooden block or tape. Then make it properly.
- Train, check daily and measure. Is the mistake gone? Record it.
A good poka-yoke works on every part, every time, not on a sample. It is placed where the mistake happens, not at the end of the line. It does not depend on anyone remembering.
What goes wrong with poka-yoke?
A poka-yoke that is not checked slowly stops working, and nobody knows. A pin wears. A sensor gets dirty. Someone bends a stop block to make loading easier.
So test each one at the start of every shift with a known bad part, painted red and kept at the machine. Keep a list of every poka-yoke: what it stops, who checks it and how often.
How to start in your plant
- Take last month's rejection slips and find the mistake that happens most.
- Stand at the machine and watch it happen.
- Ask the operator for ideas. He usually has one.
- Try a cardboard or wooden version for a week.
- Make it properly, and write it on the Kaizen report with the result.
Key points
- Poka-yoke stops a mistake or catches it at once, so it never becomes a defect.
- Do not rely on "be careful." Change the process.
- Use contact, fixed value or motion step methods, and aim for prevention with control.
- Most poka-yokes are simple and cost very little.
- Test each one every shift, or it will quietly stop working.
