The Poka-Yoke That Caught the Missing Screw: Detecting, Not Relying on the Operator
An assembly station where the operator installed 4 screws. Sometimes he missed one. The product shipped. It came back. We added a screw-present sensor. The machine won’t release the part until all 4 screws are detected. This is about poka-yoke in assembly, not about the operator being lazy.
The original process
The operator tightened 4 screws with a torque wrench. He pulled the part out. The machine didn’t check. If he missed a screw (distracted, interrupted), the part went to the next station. The torque wrench clicked on the 3 he did tighten. The missing screw was invisible until the field failure.
The poka-yoke methods
| Method | How it works |
|---|---|
| Count the torque clicks | The torque wrench has a counter. The machine waits for 4 clicks before releasing. |
| Vision check | A camera looks for the screw heads. Missing screw = no release. |
| Pressure foot switch | The operator stands on a mat. If he leaves the station mid-process, the machine locks. |
| Sequence check | The operator must follow a sequence (screw 1, 2, 3, 4). Out of sequence = fault. |
| Weight check | The part on the fixture is weighed. 4 screws = correct weight. 3 screws = light. |
What we used
1. Torque wrench with encoder. We used a torque wrench that communicates with the PLC. Each screw that’s correctly torqued sends a signal. The PLC counts. It waits for 4 signals. If only 3 arrive, the machine locks. The operator can’t remove the part until all 4 are torqued.
2. Vision check (backup). After the torque sequence, a camera looks at the 4 screw positions. It verifies 4 screw heads are present. This catches a screw that wasn’t torqued (or was over-torqued and stripped). The vision check is the final gate.
3. Sequence enforcement. The operator must torque the screws in order (1, 2, 3, 4). The PLC enforces the order. If he starts with screw 3, the machine faults. This prevents skipping around and missing one.
The result
The missing-screw rate dropped from 1 in 50 (2%) to zero. The machine physically won’t release the part until all 4 screws are verified. The operator doesn’t have to remember — the system enforces it. The field returns stopped.
The cost
The system (torque wrench with encoder, vision camera, PLC logic) cost $2,000. The field returns (shipping, warranty, customer dissatisfaction) were $5,000 per incident. It paid for itself in the first return it prevented.
The poka-yoke rule: Don’t rely on the operator to remember. Detect the missing screw with a torque counter, vision, or weight. The machine won’t release the part until all fasteners are present. A poka-yoke that catches the error at the station is cheaper than a recall. The missing screw wasn’t a laziness problem — it was a detection gap.
