How Does Poka-yoke Work?

Poka yoke is the Japanese term for error reduction. Error reduction of this type is typically accomplished through design decisions that engineer out the ability of the assembly worker to make errors. These errors may take the form of equipment setup errors, incorrect part assembly, incorrect usage of parts, and many others. By incorporating error-proofing into Design for Assembly analysis, product redesign may incorporate both time improvement and non-conformance reduction.




These errors may cause inadvertent damage to the product such as in the case of O-ring seal damage found in the Varian Ion Implant Systems particle accelerator vacuum chamber. In this situation, alignment difficulties in assembling the sealing system resulted in the assembly being unable to hold a vacuum as required. Since the system could not hold vacuum, the accelerator was unable to function. After some investigation, it was discovered that alignment difficulties were causing an O-ring to be damaged. A minor redesign occurred that incorporated the use of positioning pins. This allowed for the system to be assembled with a reduced chance of misalignment. As a result, future mistakes of this nature were prevented.

Poka yoke can improve quality control of the end product. As quality control programs generally cannot sample every product due to manpower constraints, preventing error on certain parts will minimize the overall inspection requirements for the end product. If, through rigorous application of poka yoke improvements, a part may improve conformance rate to approximately 100%, the quality control inspections normally scheduled for that part may instead be shifted to a part more likely to have defects. As a result, more defects will be caught by the quality control department.

Though much of the focus is on error prevention, there is also focus on error detection as well through automated inspection. This can take the form of incorporation of optical sensors to detect when a part has exceeded a certain tolerance. It may also take the form of air blowers that knock defective parts off of an assembly line before they reach the assembler. As poka yoke prevents errors from reaching the end product, quality of the product is improved.

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