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Quality Assurance by D. H. Stamatis

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Poka-Yoke
Overview
The term poka-yoke was applied by Shigeo Shingo in the 1960s to indus-
trial processes designed to prevent human errors (Robertson 1997). Shingo
redesigned a process in which factory workers would often forget to insert
the required spring under one of the switch buttons while assembling a
small switch. In the redesigned process, the worker would perform the task
in two steps—rst preparing the two required springs and placing them
in a placeholder, then inserting the springs from the placeholder into the
switch. When a spring remained in the placeholder, the workers knew that
they had forgotten to insert it and could correct the mistake effortlessly
(Shingo 1987).
Shingo distinguished between the concepts of inevitable human mistakes
and defects in the production. Defects occur when the mistakes are allowed
to reach the customer. The aim of poka-yoke is to design the process so that
mistakes can be detected and corrected immediately, eliminating defects at
the source.
Historical Perspective
Poka-yoke can be implemented at any step of a manufacturing process where
something can go wrong or an error can be made (Quality Portal 2013). For
example, a jig that holds pieces for processing might be modied 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 exe-
cutes the correct number of welds (Shimbun 1988).
Shingo recognized three types of poka-yoke for detecting and prevent-
ing errors in a mass production system (Shingo and Dillon 1989, Quality
Portal, 2013). They are
200 Quality Assurance
1. The contact method, which identies product defects by testing the
products shape, size, color, or other physical attributes
2. The xed-value (or constant number) method, which alerts the oper-
ator if a certain number of movements are not made
3. The motion-step (or sequence) method, which determines whether
the prescribed steps of the process have been followed
One can see that from the three options, 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 and Dillon, 1989).
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 be prevented from resulting in defects. By eliminating
defects at the source, the cost of mistakes within a company is reduced. This
is because defects are a form of waste. Waste is not value-added and there-
fore, if present, productivity and protability are decreased. A methodic
approach to build up poka-yoke countermeasures has been proposed by the
applied problem-solving (APS) methodology (Fantin 2014), which consists of
a three-step analysis of the risks to be managed:
1. Identication of the need
2. Identication of possible mistakes
3. Management of mistakes before satisfying the need
This approach can be used to emphasize the technical aspect of nding
effective solutions during brainstorming sessions.
Strategy of Mistake Proofing
The strategy of mistake proong is to establish a team approach to mistake-
proof systems that will focus on both internal and external customer concerns.
This will include quality indicators such as on line inspection and probe
studies. Appendix III provides several items of interest in the area of signals,
forms, and rationale of defects and mistakes. A strategy of mistake proong
involves the following:
Concentrating on the things that can be changed rather than on
the things that are perceived as having to be changed to improve
process performance

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