Abstract :
Human
errors are usually inadvertent. Poke Yoke devices help to avoid defects, even
when inadvertent errors are made.
Examples of
'attention-free' Poke Yoke solutions:
-A jig that prevents a
part from being misoriented during loading
-Non-symmetrical screw
hole locations that would prevent a plate from being screwed down incorrectly
-Electrical plugs that
can only be inserted into the correct outlets
-Notches on boards that
only allow correct insertion into edge connectors
-A flip-type cover over
a button that will prevent the button from being accidentally pressed
Three
levels of Poka-Yoke:
-Elimination of spills,
leaks, losses at the source or prevention of a mistake from being committed
-Detection of a loss or
mistake as it occurs, allowing correction before it becomes a problem
-Detection of a loss or
mistake after it has occurred, just in time before it blows up into a major issue
(least effective).
Although
many techniques have been developed to prevent or control mistakes, most of
these techniques are relatively ineffective. Effective mistake proofing cannot
be developed without a sound understanding of the true characteristics of
mistakes. A mistake occurs when a required action is not performed or is
performed incorrectly, a prohibited action is executed, or information
essential for an action is not available or is misinterpreted.
There are a
number of characteristics associated with poka yoke techniques that makes it
more effective than other techniques. This includes:
-Mistake proofing
requires 100% inspection. It is impossible to detect and control rare random
events with sampling inspection. Since, 100% traditional inspection is too
expensive and not 100% effective in detecting nonconforming product,
mistake-proofing methods based on poka-yoke are essential and the only
practical solution.
-Mistake proofing must
be inexpensive. Because mistakes are rare events and many different types of
mistakes must be controlled, companies cannot afford to spend large sums of
money on each mistake-proofing device.
-Many mistake-proofing
devices are needed. Toyota has an average of 12 mistake-proofing devices at
each workstation.
-Outcome intervention is
best. The best mistake proofing physically prevents errors or detects when a
mistake is about to occur or has occurred. Thus, these techniques intervene to
block undesired outcomes rather than controlling casual factors.
-Prevention is better
than detection. Preventing mistakes is better than detecting mistakes, which is
better than detecting defects. If a mistake is not detected until a defect is
generated, rework will be required or the hardware must be scrapped. Thus,
where possible, it is always better to detect or control the mistake before a
defect is generated. Similarly, there will be less wasted effort if mistakes
are prevented rather than detected.
-Control, Shutdown, or
Warn. Because resources may be wasted if a process is shutdown, it is better to
control mistakes. If a process is shutdown, however, the problem must be
addressed to proceed. Hence, shutdown provides a more positive control of
mistakes than warnings, which can be ignored.
-The Most Important
Quality Initiative. Only mistake-proofing effectively controls mistakes. Significant
quality initiatives other than mistake proofing have marginal impact on the
customer perception of quality since mistakes are the dominant source of
customer problems.
Poka-Yoke:
A Misunderstood Concept
Poka-Yoke
is a Japanese methodology for mistake proofing to avoid non-conformities from
entering into processes. It allows defect detection and elimination at the
source. It can also be used as a continuous improvement tool.
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:
Control Method: This
method senses a problem and stops a line or process, so that corrective action
can take place immediately. Thereby it avoids 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, the process
shuts down automatically. Another example is an incomplete sales order, which
cannot be released for production until a true manufacturability configuration
is defined.
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. It is used when a bandwidth of acceptance exists, for a process. An
example could be 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 under control.
Detection-Based
Poka-Yokes:
In many
situations, it is not possible or economically feasible to prevent defects.
This is particularly so where the capital cost of the Poka-Yoke mechanism,
significantly exceeds the cost of prevention. In 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:
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. Fixed Value Method:
This method is used in operations, where a set of steps is sequentially performed.
The fixed value method
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