Why mistake-proofing rather than more inspection
People make occasional errors however well trained they are. Inspection catches some of them after the fact; poka-yoke changes the process so the error is impossible or is caught at the point it occurs, which is cheaper and more reliable than any inspector.
Types of poka-yoke
- Prevention devices make the error impossible: a part that cannot be loaded backwards, a fixture that will not close with a component missing, a form field that only accepts a valid value.
- Detection devices catch the error at once: a sensor that stops the machine when a part is missing, a scale that rejects an under-weight pack, a scanner that flags a wrong label.
Within those, three common mechanisms: contact (shape, size or colour detects the error), fixed value (a count or number of steps must be reached), and sequence (steps must happen in order).
Everyday examples
- Colour-coded hoses and connectors so lines cannot be crossed.
- Pick-to-light bins that flag the wrong pick.
- A changeover checklist that cannot be signed off until each step is confirmed.
- Shadow boards that show a missing tool at a glance.
- Software that will not close a corrective action until the verification step is recorded.
How to apply poka-yoke after a defect
- Find the root cause of the defect with 5 whys or a fishbone.
- Ask whether the process could be changed so that cause cannot occur.
- If not, ask how the error could be detected at the moment it happens.
- Prefer simple, physical devices over procedures that depend on memory.
- Verify the device works and update the standard work.
Poka-yoke and jidoka
Poka-yoke is one of the ways jidoka, built-in quality, is achieved: the process itself detects abnormality and stops, rather than passing defects along. Andon is the signal that follows.