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Poka-Yoke: A Misunderstood Concept
Shigeo Shingo introduced the concept of poka-yoke
in 1961, when he was an industrial engineer at Toyota Motor
Corporation. The initial term was baka-yoke, which means
‘fool-proofing’. In 1963, a worker at Arakawa Body Company
refused to use baka-yoke mechanisms in her work area, because
of the term’s dishonourable and offensive connotation. Hence,
the term was changed to poka-yoke, which means ‘mistake-proofing’.
Poka-yokes are mechanisms used to mistake-proof
an entire process. Ideally, poka-yokes ensure that proper
conditions exist before actually executing a process step,
preventing defects from occurring in the first place. Where
this is not possible, poka-yokes perform a detective function,
eliminating defects in the process as early as possible.
Many people think of poka-yokes as limit switches,
optical inspection systems, guide pins, or automatic shutoffs
that should be implemented by the engineering department.
This is a very narrow view of poka-yoke. These mechanisms
can be electrical, mechanical, procedural, visual, human,
or any other form that prevents incorrect execution of a
process step. Poka-yokes can also be implemented in areas
other than production such as sales, order entry, purchasing,
or product development where the cost of mistakes is much
higher than on the shop floor. The reality is that defect
prevention, or defect detection and removal, has widespread
applications in most organisations.
The Centre for Excellence in Operations (CEO)
has developed a poka-yoke framework to help its clients
understand, the various classifications and applications
of mistake-proofing mechanisms. Given below is a brief overview
of the framework:
Prevention-Based
Poka-Yokes
Prevention-based mechanisms sense an abnormality
that is about to happen, and then signal the occurrence
or halt processing, depending on the severity, frequency
or downstream consequences. There are two approaches for
prevention-based poka-yokes:
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Control Method:This method senses a problem
and stops a line or process, so that corrective action
can take place immediately, thus avoiding serial defect
generation. An example of this, is an assembly operation
wherein, if one of the components is found to be missing
before the actual assembly step takes place, then the
process shuts down automatically. Another example is an
incomplete sales order, which cannot be released for production
until a true manufacturable configuration is defined.
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Warning Method: This method signals the
occurrence of a deviation or trend of deviations through
an escalating series of buzzers, lights or other warning
devices. However, unlike the control method, the warning
method does not shut down the process on every occurrence.
This method is used when a bandwidth of acceptance exists,
for a process. An example of this is pressurising a vessel
or a filling operation, in which the results need not
be, exactly the same. Although the process continues to
run, the poka-yoke signals the operator to remove a defect
from the line, or make necessary adjustments to keep the
process within control.
Detection-Based
Poka-Yokes
In many situations, it is not possible or
economically feasible to prevent defects, particularly where
the capital cost of the poka-yoke mechanism, far exceeds
the cost of prevention. For these situations, defects are
detected early in the process, preventing them from flowing
to downstream processes and multiplying the cost of non-conformance.
The three categories of detection-based poka-yokes are as
follows:
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Contact Method: This method detects
any deviation in shape, dimensional characteristics or
other specific defects, through mechanisms that are kept
in direct contact with the part. A subset of this category
is the non-contact method, which performs the same function
through devices such as photoelectric cells. An example
of this, might include a chute that detects and removes
upside-down or reversed parts, or an in-line gauge that
removes dimensional defects and reroutes them to a defect
lockbox.
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Fixed Value Method: This method
is used in operations, in which a set of steps is sequentially
performed. The fixed value method employs automatic counters
or optical devices and controls the number of moves, rate
and length of movement as well as other critical operating
parameters. In this case, mechanisms are usually built
into progressive stamping, welding, Systems Manufacturing
Technology (SMT), and automatic insertion equipment. Sometimes
this is referred to as odd part out method, in which parts
left over after assembly signal a defect. Fixed value
also includes critical condition detection (pressure,
temperature, current, etc.) through electronic monitoring
devices.
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Motion Step Method: This method
ensures that a process or operator does not mistakenly
perform a step that is not part of the normal process.
An example of this is colour coding of electronic components
on drawings and totes to prevent using mixed or incorrect
parts. Another example is a visual to assist customer
service representatives, in providing the right literature
sets for various products.
The best poka-yoke in the world is a robust
design. Many of the needs for poka-yokes are attributable
to poor designs and/or unrepeatable processes. The second-best
poka-yoke in the world is education and awareness. The automotive
industry is a leader in this area, with the use of its Advanced
Product Quality Planning (APQP) guidelines and supplier
development programmes. Companies such as Motorola, Allied
Signal and General Electric are leaders in this area, because
they invested in their Six Sigma Black Belt programmes,
and have taken them upstream into the new product development
process.
Building a Strong
Foundation
The poka-yoke philosophy requires a strong
foundation in total quality management. First, organisations
must learn to be customer focused. As Tom Peters once said,
“The customer comes first, second, third, fourth, fifth,
and there ain’t no sixth! “ Second, organisations must promote
quality ownership at the source, and they need to ensure
proper investment in their people, which enables them to
be truly empowered. Third, a clear distinction needs to
be made between good versus bad quality. Fourth, organisations
must embrace the PSP philosophy: Pre-, Self-, and Post-Inspection
at the source. Last, poka-yokes require real-time feedback
and corrective action. These are the building blocks of
an effective poka-yoke effort.
Mistakes happen in organisations for many
reasons, but almost all of them can be prevented, if people
make the effort to identify when problems happen, define
root causes, and then take the proper corrective actions.
The objective is to prevent, or at least, detect and weed
out defects, as early as possible in the process. The use
of simple poka-yoke mechanisms and other safeguards can
also prevent mistakes from becoming catastrophic events.
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