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Mistake proofing, applying Poka Yoke principles
What is Poka Yoke?
Poka yoke means to mistake proof. It is derived from the Japanese words yokeru (avoid) and poka (mistakes). Poka yoke is often misspelled poke yoke and pokayoke A poka yoke device can be placed into two categories. The first is an informative inspection or warning poka yoke which identifies a mistake before it becomes a defect. It requires the operator to interpret the poka yoke and make corrective actions. The second is a source inspection or preventative poka yoke which prevents the mistake at the source. This type of poka yoke will not allow the operator to make a mistake. Poka yoke principles are widely used in lean manufacturing environments and in DFMA systems. They are also a large part of Shigeo Shingo's zero quality control method and included in the Toyota Production System. The goal when applying poka yoke principles should be to reduce mistakes and eliminate defects. Poka yokes are generally not very expensive, but create a great value by increasing quality. For this reason they are an invaluable asset that should be practiced when at all possible.
Informative inspections, warning poka yoke
Informative inspections are a form of poka yoke in which the the operator is alerted that a condition exists that may lead to a defect. The operator then rectifies the conditions that could cause mistakes before a defective part is made. They are presented in many different ways. From a buzzer or light that lets you know that the air pressure being supplied to a tool is below a critical level to pre-counted hardware that lets you know when when a bolt or other hardware has been left out of an assembly. This type of poka yoke should be used when a preventative poka yoke is not a viable option.
Source inspection, preventative poka yoke
Source inspections are a form of poka yoke in which a mistake is avoided by correcting the source of the potential mistake. They have also been called preventative poka yokes. They present there selves in various ways. From electrical connections that can only be plugged in one way and a microwave that cannot be turned on while the door is open to a machine tool that requires two hand to be on two separate buttons preventing operator from injuring there self. This is the preferred type of poka yoke since it effectively eliminates the mistakes.
Application of poka yoke principles
The secret to applying a poka yoke is to first identify a condition that is resulting in a mistake. A great way to identify this condition is through a root cause analysis. Lets use the following hypothetical situation to demonstrate how to implement a poka yoke after the root cause is identified. In manufacturing a wood frame the operator uses a pneumatic nail gun to fasten a frame together. The operator has had problems with nails not being fully set. After careful observation it was determined that two conditions have contributed to the defects. The first is that since the air compressor is used to power many tools throughout the shop, occasionally the air pressure drops below a critical level and there is not enough force to correctly set the nail. This problem can be rectified by the use of a warning poka yoke. An alarm is applied to the airline to warn the operator of a low pressure situation. The operator then refrains from nailing the frame when this condition exists thereby avoiding the mistake. The second condition leading to improperly set nails is the position of nail gun. Sometimes the nail gun is out of position and not perpendicular to the wood frame causing the nails to go in at an angle keeeping them from being set properly. A preventative poka yoke could be applied by creating a fixture the holds the nail gun in place and perpendicular to the wood frame preventing the mistake.
It is easy to see how these low cost poka yoke provide value by eliminating defects that would lead to rework or scrapped parts. Poka yokes should be an integral part of any lean manufacturing system.
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