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Poka-Yoke: A Misunderstood Concept

Shigeo Shingo introduced the concept of poka-yoke in 1961, when he was an industrial engineer at Toyota Motor Corporation. The initial term was baka-yoke, which means ‘fool-proofing’. In 1963, a worker at Arakawa Body Company refused to use 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’.

Poka-yokes are mechanisms used to mistake-proof an entire process. Ideally, poka-yokes ensure that proper conditions exist before actually executing a process step, preventing defects from occurring in the first place. Where this is not possible, poka-yokes perform a detective function, eliminating defects in the process as early as possible.

Many people think of poka-yokes as limit switches, optical inspection systems, guide pins, or automatic shutoffs that should be implemented by the engineering department. This is a very narrow view of poka-yoke. These mechanisms can be electrical, mechanical, procedural, visual, human, or any other form that prevents incorrect execution of a process step. Poka-yokes can also be implemented in areas other than production such as sales, order entry, purchasing, or product development where the cost of mistakes is much higher than on the shop floor. The reality is that defect prevention, or defect detection and removal, has widespread applications in most organisations.

The Centre for Excellence in Operations (CEO) has developed a poka-yoke framework to help its clients understand, the various classifications and applications of mistake-proofing mechanisms. Given below is a brief overview of the framework:

Prevention-Based Poka-Yokes

Prevention-based mechanisms sense an abnormality that is about to happen, and then signal the occurrence or halt processing, depending on the severity, frequency or downstream consequences. There are two approaches for prevention-based poka-yokes:

  • Control Method:This method senses a problem and stops a line or process, so that corrective action can take place immediately, thus avoiding serial defect generation. An example of this, is an assembly operation wherein, if one of the components is found to be missing before the actual assembly step takes place, then the process shuts down automatically. Another example is an incomplete sales order, which cannot be released for production until a true manufacturable configuration is defined.
  • Warning Method: This method signals the occurrence of a deviation or trend of deviations through an escalating series of buzzers, lights or other warning devices. However, unlike the control method, the warning method does not shut down the process on every occurrence. This method is used when a bandwidth of acceptance exists, for a process. An example of this is pressurising a vessel or a filling operation, in which the results need not be, exactly the same. Although the process continues to run, the poka-yoke signals the operator to remove a defect from the line, or make necessary adjustments to keep the process within control.

Detection-Based Poka-Yokes

In many situations, it is not possible or economically feasible to prevent defects, particularly where the capital cost of the poka-yoke mechanism, far exceeds the cost of prevention. For these situations, defects are detected early in the process, preventing them from flowing to downstream processes and multiplying the cost of non-conformance. The three categories of detection-based poka-yokes are as follows:

  • Contact Method: This method detects any deviation in shape, dimensional characteristics or other specific defects, through mechanisms that are kept in direct contact with the part. A subset of this category is the non-contact method, which performs the same function through devices such as photoelectric cells. An example of this, might include a chute that detects and removes upside-down or reversed parts, or an in-line gauge that removes dimensional defects and reroutes them to a defect lockbox.
  • Fixed Value Method: This method is used in operations, in which a set of steps is sequentially performed. The fixed value method employs automatic counters or optical devices and controls the number of moves, rate and length of movement as well as other critical operating parameters. In this case, mechanisms are usually built into progressive stamping, welding, Systems Manufacturing Technology (SMT), and automatic insertion equipment. Sometimes this is referred to as odd part out method, in which parts left over after assembly signal a defect. Fixed value also includes critical condition detection (pressure, temperature, current, etc.) through electronic monitoring devices.
  • Motion Step Method: This method ensures that a process or operator does not mistakenly perform a step that is not part of the normal process. An example of this is colour coding of electronic components on drawings and totes to prevent using mixed or incorrect parts. Another example is a visual to assist customer service representatives, in providing the right literature sets for various products.

The best poka-yoke in the world is a robust design. Many of the needs for poka-yokes are attributable to poor designs and/or unrepeatable processes. The second-best poka-yoke in the world is education and awareness. The automotive industry is a leader in this area, with the use of its Advanced Product Quality Planning (APQP) guidelines and supplier development programmes. Companies such as Motorola, Allied Signal and General Electric are leaders in this area, because they invested in their Six Sigma Black Belt programmes, and have taken them upstream into the new product development process.

Building a Strong Foundation

The poka-yoke philosophy requires a strong foundation in total quality management. First, organisations must learn to be customer focused. As Tom Peters once said, “The customer comes first, second, third, fourth, fifth, and there ain’t no sixth! “ Second, organisations must promote quality ownership at the source, and they need to ensure proper investment in their people, which enables them to be truly empowered. Third, a clear distinction needs to be made between good versus bad quality. Fourth, organisations must embrace the PSP philosophy: Pre-, Self-, and Post-Inspection at the source. Last, poka-yokes require real-time feedback and corrective action. These are the building blocks of an effective poka-yoke effort.

Mistakes happen in organisations for many reasons, but almost all of them can be prevented, if people make the effort to identify when problems happen, define root causes, and then take the proper corrective actions. The objective is to prevent, or at least, detect and weed out defects, as early as possible in the process. The use of simple poka-yoke mechanisms and other safeguards can also prevent mistakes from becoming catastrophic events.

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