Error Proof the Pokayoke to Build in Quality

In order to have a just in time production system function properly you need to work towards zero defects and build in quality to every process. Otherwise the lack of extra buffer inventory will shut down the material flow when a defect is produced. One of the ways to build in quality is to check quality at every process. Another is pokayoke, a Japanese word meaning “error proofing” or “mistake proofing”.
Pokayoke are typically two types. The first is to error proof human errors and the second is to error proof machine errors. For human errors, pokayoke are implemented based on an analysis of the opportunities for people to make errors in judgment, errors in checking their work after completion, errors in detection or errors in the human motions themselves. For machines, pokayoke are implemented based on a similar analysis of failure factors by root cause.
Pokayoke can apply to not only quality improvement (preventing errors that result in defects) but also for safety improvement (preventing errors that result in harm to people). For instance, physical covers and boards or warning signals or lights can be pokayoke devices when they provide a clear signal that an action or area is unsafe.
It’s very important to pokayoke the pokayoke. You must error proof the pokayoke, just as you error proof the process. At a recent visit to stamping facility on a supplier development Lean assessment, we saw a visual board representing the welding line posted in the factory. It was full of green and red buttons marked ‘pokayoke’.
“What do the colors mean?” we asked.
“The green pokayoke are functioning. The red ones are not.”
In this case the pokayoke devices of various types received rough treatment and often broke down and needed repair. About a third were red.
As in all aspects of a Lean transformation, with pokayoke implementation you are never really done. If a pokayoke is a mechanical error proofing device, there will be design limitations, and it will someday fail. If it is an electromechanical device or if it is software driven, there will be flaws that cause malfunction. Of course there will also be deterioration. Pokayoke devices will require maintenance be people who understand the original design intent of the error proofing device.
The existence of the Pokayoke devices need to be included in the Standard Work Instruction Sheets (develop these first if you don’t have them, it’s silly to error proof a non-standard process). When there is a change to the way the work is done, this may render the pokayoke useless so consider whether the pokayoke is requires redesign. The Lean manager’s job never ends. You have to error proof the pokayoke, then pokayoke that error proof, and so on and on and on…