The Toyota Production System formalized a lot of concepts and terms, including the term Poka-yoke. Translated from Japanese it means mistake-proofing or avoid (yokeru) mistakes (poka) and is any mechanism in Lean manufacturing that helps floor workers using equipment from making mistakes. Eliminating product defects through drawing attention to errors and preventing or correcting them as they occur.
The mechanisms will not accept a defect for the step in the process it’s about to undertake, it will prevent a defect from occurring – either via informing the workers or preventing it itself – and it will not allow a defect to be passed on to the next stage. Reducing the time spent on quality control and assuring that products are of high quality at each step of the manufacturing journey.
Types of Poka-yoke
Shigeo Shingo, an expert in manufacturing processes and the Toyota Production System, identified three types of poka-yoke within a mass production system. The three types are:
- The Contact Method – think of a connector, if the connectors inputs do not match the cables that will attach to it, then there is an error. This type will identify through shape, size, color and other physical attributes for errors. Products which can only be fitted in a particular way with another product would fall under this type.
- The Fixed-Value Method – a physical or visual method that identifies whether the correct measures or movements required as part of a process has been completed. Think of Crayola Crayons. Each box of crayons needs to have a set rainbow of colors, Crayola uses a light sensor to determine that the colors are correct and complete. If a color is missing, or there is duplicated colors then the machine stops.
- The Motion-Step Method – this type ensures that the required number of steps, and sequence of steps, to complete a process is fulfilled. The Trinity Industries Railcar division set up a layout jig that ensures workers do not miscalculate size, shape or location of components. It assures that each necessary component is installed in the perfect position and without missing anything.
Poka-yoke mechanisms are not just electronic devices applied to equipment but can be any number of mechanisms that can assist a worker in assuring every step is completed accurately and error-free. Motorola, for example, uses a clear keyboard template that allows it’s workers to compare what the keyboard should look like to what they have done. If there is an error they can quickly correct it before passing it on to the next step.
Benefits of Poka-yoke Implementation
Once implemented not only does poka-yoke solutions reduce the number of errors occurred during manufacture but they also offer additional benefits.
- Worker training time reduced
- Quality control operations reduced
- Repetitive operations reduced
- Improvement orientated approach promoted in the workplace
- Reduced number of defects and waste products
- Built-in quality control
Poka-yoke is most effective in environments where workers are still hands-on with steps within the process (those that make use of Jidoka are best), and the process from start to finish is a steady stream. If the journey is high speed, then the application of poka-yoke is harder to implement and other methods may be more effective.