Poka-yoke

 

Idiot proof

Poka-yoke is a Japanese term that means ‘fail-safing’ or ‘mistake-proofing.’ A poka-yoke is any mechanism in a lean manufacturing process that helps an equipment operator avoid (‘yokeru’) mistakes (‘poka’).

Its purpose is to eliminate product defects by preventing, correcting, or drawing attention to human errors as they occur. The concept was formalized, and the term adopted, by Shigeo Shingo as part of the Toyota Production System. It was originally described as ‘baka-yoke,’ but as this means ‘fool-proofing’ (or ‘idiot-proofing’) the name was changed to the milder poka-yoke. More broadly, the term can refer to any behavior-shaping constraint designed into a process to prevent incorrect operation by the user.

Similarly, a constraint that is part of the product (or service) design is considered DFM (design for manufacturability) or DFX (design for X). A good example of a poka-yoke design is the shape of a cell phone SIM card which can only be inserted in the cell phone in the correct way. Shingo argued that errors are inevitable in any manufacturing process, but that if appropriate poka-yokes are implemented, then mistakes can be caught quickly and prevented from resulting in defects. By eliminating defects at the source, the cost of mistakes within a company is reduced.

Poka-yoke can be implemented at any step of a manufacturing process where something can go wrong or an error can be made. For example, a jig that holds pieces for processing might be modified to only allow pieces to be held in the correct orientation, or a digital counter might track the number of spot welds on each piece to ensure that the worker executes the correct number of welds.

Shigeo Shingo recognized three types of poka-yoke for detecting and preventing errors in a mass production system: the contact method identifies product defects by testing the product’s shape, size, color, or other physical attributes; the fixed-value (or constant number) method alerts the operator if a certain number of movements are not made; and the motion-step (or sequence) method determines whether the prescribed steps of the process have been followed. Either the operator is alerted when a mistake is about to be made, or the poka-yoke device actually prevents the mistake from being made. In Shingo’s lexicon, the former implementation would be called a warning poka-yoke, while the latter would be referred to as a control poka-yoke.

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