The Poka-Yoke That Caught the Missing Screw
We had a manual station (article 199). The operator forgot a screw (on 1 in 100). The bad part shipped (the customer complained). The problem: no poka-yoke (the error wasn’t caught). We added a poka-yoke: a sensor (a photo-eye, or a pressure sensor) that checks the screw is in. If the screw is missing, the station won’t release the part (a gate locks). The operator is forced (he must put the screw in) before the part moves. The missing-screw error dropped to zero. The mistake was not poka-yoking (relying on the operator to remember).
Poka-yoke (error-proofing) prevents the error. This article covers the types.
The Poka-Yoke
Poka-yoke (Japanese: “mistake-proofing”) makes the error impossible (or catches it). Types:
- Contact (sensor): A sensor detects the part (or the screw). If it’s missing, the machine stops (or locks).
- Fixed-value (count): Count the parts (e.g., 4 screws). If the count is wrong (3, or 5), the machine stops.
- Motion-step (sequence): The steps must be in order (e.g., clamp, then press). If the order is wrong, the machine won’t proceed.
Step 1: The Contact (Sensor)
A sensor (a proximity, article 108) detects the part (or the screw). If the part is missing, the sensor doesn’t trigger (the machine stops). The operator can’t proceed (the gate locks). The error is caught (not shipped).
The poka-yoke rule: Make the error impossible (or caught). The station that shipped a missing-screw part had no poka-yoke. Add a sensor (contact). The operator can’t forget (the machine stops). Zero defects (not “less errors”).
Step 2: The Fixed-Value (Count)
For a count (e.g., 4 screws), use a counter (the sensor counts the screws). If the count is 3 (or 5), the machine stops. The operator must put in the right count. No missing (or extra) parts.
Step 3: The Sequence (Motion-Step)
For a sequence (clamp, then press), the machine enforces the order. The operator can’t press before the clamp (the machine won’t proceed). The sequence is enforced (no wrong order).
Step 4: The Andon (Article 98)
When the poka-yoke catches an error, the station alarms (an andon light, article 98). The operator (or the team leader) is alerted. The error is fixed (on the station, not shipped).
| Type | Example |
|---|---|
| Contact (sensor) | Proximity detects the screw |
| Fixed-value (count) | Count 4 screws (not 3, or 5) |
| Motion-step (sequence) | Clamp before press |
| Andon (alarm) | Alert on error (article 98) |
A Poka-Yoke Checklist
- What is the likely error? (Missing screw?)
- Is there a sensor? (Contact? Article 108?)
- Is the count right? (Fixed-value?)
- Is the sequence enforced? (Motion-step?)
- Does the machine stop? (On error?)
- Does the andon alarm? (Article 98?)
- Can the operator bypass? (No?)
- Is the error caught? (Not shipped?)
- Is the quality zero? (Defects?)
- Is the operator trained? (Use it?)
The Bottom Line
Poka-yoke makes the error impossible (or caught). The station that shipped a missing-screw part had no poka-yoke. Add a sensor (contact). Enforce the count and sequence. The station that had zero defects wasn’t the best operator — it was poka-yoked.
