Poka-yoke is a mechanism that prevents an error from occurring, or from passing to the next step, in production or assembly processes. It is also known as error proofing or mistake proofing.
Prevention or detection?
This distinction is critical and is often confused in practice.
Prevention makes the occurrence of the error physically or systemically impossible: the part is designed so it can only be fitted the right way round; if a screw is missing the cycle does not complete.
Detection catches an error that has already occurred: a sensor detects the missing part and raises an alarm.
Prevention is always more economical, but it may not be possible in every case.
The verification obligation
This is the point most often overlooked. A poka-yoke mechanism can be disabled without anyone noticing: the sensor gets dirty, the setting drifts, it is bypassed during maintenance and never switched back on.
For this reason, the operation of the mechanisms is expected to be verified periodically, and this verification is expected to be defined in the control plan. It is one of the most frequently asked topics in audits.
Where are the opportunities found?
The most productive source is the PFMEA: high-risk failure modes show the priority points where a mechanism should be designed.
Mechanism design, setting up the verification programme and the FMEA link are addressed hands-on in the CQI-18 error proofing training. For the risk side, see the FMEA training.