In the early 1960s, quality guru Shigeo Shingo combined the concepts of “successive,” “independent” and “source” inspections with Toyota’s in-house “fool proofing” techniques and devised the famous production philosophy “poka-yoke” (poka means mistake or inadvertent errors, while yoke means proofing. It is derived from the word “yokeru” meaning, to avoid).
The initial term was “baka-yoke,” which means “fool-proofing.” In 1963, a worker at Ara- kawa Body Company refused to use the “baka-yoke” mechanisms in her work area because of the term’s dishonourable and offensive connotation. Hence, the term was changed to “poka-yoke”, which means “mistake-proofing.”
It was designed as a tool to achieve and sustain “zero defects.” Poka-yoke refers to the absolute elimination of defects in the production desired goal, but in many instances essential for suppliers. In many cases, a single defect has been found to be the cause of outright rejection of the entire batch or 100 per cent manual reinspection of every component in the batch at the expense of the supplier.
It is simply a way to prevent the occurrence of defects, injuries or losses of any kind at the workplace. It makes use of sensors and instruments and devices that can identify disorders, abnormalities or mis-steps, without the workers having to be extra sensitive to the smallest details.
Poka-yoke helps people and processes work right the first time. It can be thought of as an extension of Failure Mode Effect Analysis. It can also be used to fine tune improvements and process designs from Six Sigma Define-Measure-Analyse-Improve-Control (DMAIC) projects. The use of simple poka-yoke ideas and methods in product and process design can eliminate both human and mechanical errors. Poka-yoke does not need to be costly. For instance, Toyota has an average of 12 mistake-proofing devices at each workstation and a goal of implementing each mistake-proofing device for under USD 150.
Poka-yoke works on five pillars as suggested by Shingo:
- Use source inspection. The application of control functions at the stages where errors, mistakes may get converted into defects or accidents. That is, use control/warning function at the origin of the defect.
- Always use 100 per cent source inspection at least for attributes. It is better than sampling inspection.
- Minimize the time to carry out corrective actions when abnormalities appear or try to speed up the feedback corrective actions.
- Set up mistake proofing devices (called poka-yokes), i.e. sensors, and transducers; limit switches, metal detectors according to products and process requirements.
- Respect workers and operators. They are human beings and not robots or programmable machines.
1. Step-by-step Process in Applying Poka-yoke
- Identify the operation or process based on a Pareto analysis.
- Analyse the five whys and understand the ways a process can fail.
- Decide on the right poka-yoke approach such as using a shut out type (preventing an error from being made) or an attention type (highlighting that an error has been made). Poka- yoke takes a more comprehensive approach and should not be deemed limit switches or automatic shutoffs. A poka-yoke can be electrical, mechanical, procedural, visual, human or any other form that prevents incorrect execution of a process step.
- Use the method of creating a checklist to ensure that all process steps have been completed appropriately.
- Try the method and see if it works.
- Train the operator, review performance and measure success.
2. Poka-yoke Devices
Poka-yoke is a simple device or method to prevent mistakes at their source. These devices are used either to prevent the special causes that result in defects or to inexpensively inspect each item that is produced to determine whether it is acceptable or defective. Some of the examples of poka-yoke devices8 are given below:
- A large steel press is automatically monitored for wear. If the thickness becomes less than a specified amount, an alarm sounds and action has to be taken to rectify the error.
- Packaging screws in groups of three to ensure that the assembler uses three screws. The package is a poka-yoke device.
- A simple electrical check is made to verify that nuts are properly welded to a sheet metal panel at General Motors.
- An airplane pilot may use a simple checklist to make sure everything is ready before flying his airplane.
Source: Poornima M. Charantimath (2017), Total Quality Management, Pearson; 3rd edition.