Poka-yoke vs inspection: stop defects instead of finding them

Inspection finds, poka-yoke prevents

Inspection catches a defect after the part is made; the material, the machine time and often a whole batch are already lost. Poka-yoke — error-proofing — makes the mistake impossible, or stops the process the moment it happens.

Three kinds of poka-yoke

• Contact. The part's shape decides: a fixture pin that only fits the correct orientation, a nest that rejects the wrong variant.
• Fixed value. A count must be reached: every one of six bolts torqued before the station releases the part.
• Motion step. Steps must happen in order: the press won't cycle until the sensor confirms the part is seated.

Where go/no-go gauges fit

A go/no-go gauge checks one critical dimension in seconds with no reading to interpret: the GO end must enter, the NO-GO end must not. It suits bores, shafts, threads and slots checked at the machine. Pair it with a rule — the next part isn't started until the gauge check is done — and it becomes part of the process, not an afterthought.

Choosing the device

1. Find where the defect starts, not where it is found.
2. Ask whether the mistake can be made physically impossible (contact type).
3. If not, detect it with a sensor and stop the machine through a PLC interlock.
4. If a sensor can't see it, use a gauge, a vision check or a leak test at the station.

Keep it working

Every poka-yoke device needs a daily check that it still trips, a record in the control plan, and a rule that nobody bypasses it without approval.