Mistake-proofing (Poka-Yoke)


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Mistake-proofing (Poka-Yoke) - short version

(1) The design and use of error prevention to achieve zero defects.

(2) Mistake proofing is making it impossible for errors to be passed to the next step in the process.



Mistake-proofing (Poka-Yoke) - long version

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.
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.

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.



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