Introduction
In
quality management, avoiding mistakes is a general aim with the highest priority. In order to
sustain a defect free operation in a mass-production environment, the need of preventing defects has increased significantly. Engineers started to design various methods for upfront failure prevention, but these
were single methods and operations.
It was Shigeo Shingo, who implemented the general principle of error proofing into the TPS (Toyota Production System) in the 1960's. First it was just called "fool-proofing"
("baka-yoke") methodology, but later it was renamed to "mistake proofing" ("poka-yoke"). The concept was based on common sense, however it was not implemented in any production in a systematic way.
The intention of the principles was simple: design and implement fail-safe mechanism into your processes, and equipment, to eliminate the possibility of failures.
Source: qMindset.com
Key Features
The need for implementing the "poka-yoke" philosophy was the highest in case of manual operations, where the operator could commit the mistake. As soon as the process was
redesigned according to "poka-yoke", the method was fail-safe, and the operator could not make the mistake, because the system itself inhibited it, giving no chance for failures.
The meaning of "poka-yoke" is coming from the words poka (inadvertent / unintended error) and yoke (to avoid). In the English speaking industry it is referred to various
names:
- Error-proofing.
- Mistake-proofing.
- Failure-prevention.
- Fail-safe concept.
The following figure shows a typical example for the easy but perfect "Poka-Yoke" solution, similar to the widely used electrical cables connectors. The
ears of part 1 and part 2 are designed differently, so that the groove of part 3 can only accept part 1. In this case the bar and the groove serve as a "physical code", to impede the attachment of part 2 and
part 3.
An example for poka-yoke (Source: qMindset.com)
The general philosophy was improved further by different industrial sectors, integrating sensors, computer aided solutions, and even artificial intelligence.
"Poka-Yoke" methods have three major levels, depending on their effectiveness:
- Level 1 - Warning: this is the simplest level, it only gives warning about the failure, so it is reactive.
- Level 2 – Shutdown: this level is better than the previous one, as it detects the failure, and automatically shuts down the operation (doesn’t let the wrong part to be forwarded to the next process step).
- Level 3 – Prevention or auto-correction: the system itself doesn’t let the failure to happen, or it automatically starts a repair loop for correction (based on an in-process test result).
Examples for "poka-yoke":
- Two sub-assemblies can only be attached in one direction, thanks to a physical code. In addition, only those part numbers can be attached, that fit together.
- Ethernet plug and USB stick.
- When you release the switch lever of the lawnmower, its engine stops immediately (safety reasons).
- Warning sound of seat-belt. Although it does not force you to joint your seat-belt, it is continuously getting on your nerves with an annoying sound.
- Safety light gates in a manufacturing environment prevent the damage of fingers or limbs.
Source: qMindset.com
Hints
Design error-proof processes every time, as it brings major benefits:
- Preventing errors decrease your internal scrap.
- Preventing or automatically detecting failures will decrease your external failure rates.
- It decreases cost (through the Cost of Poor Quality), and increases prestige.
Fail-safe solutions are not only usable for manufacturing processes, but also during design. If you implement error-proofing methods in your design and development phase,
the system can warn you (or completely prevent) design failures. Example: if your CAD software has an automatic tolerance and variation analysis, you will know upfront all of the potential sizing failures
that are coming from design itself.
Many times, "poka-yoke" solutions are good not only for quality purposes, but for safety reasons also. A typical example for that in the manufacturing environment is the
safety light curtain that prevents damage.
Source: qMindset.com
