Poka Yoke – Three Effective Methods to Reduce Rework and Defects

Poka Yoke Definition & Meaning

“Poka-Yoke” a Japanese term coined by Shiego Shingo in the 1960’s meaning “mistake-proofing”. It is a highly effective quality control tool. It stems from recognising that making mistakes is part of being human. A Poka-yoke removes or greatly reduces the opportunities for us to make mistakes. They can make a process run more smoothly, reduce defect rates, decrease takt time and even improve quality of life. The term was initially applied in the manufacturing world but today examples can be seen in all sectors and in many aspects of our lives. A can improve the yield and efficiency of many processes and above all reduce defect rates.

A second advantage of implementing a poka-yoke is better moral and improved atmosphere among employees and managers. This is achieved by removing a “blame culture”. In the working environment telling an employee to do better or penalising them for mistakes is often unsuccessful in reducing error rates. Poka yoke theory recognised that it is the process which is flawed and not the people. Companies should do everything they can to ensure the success of their employees. Small design features and visual cues can do just that. They can have a profound effect on safety, efficiency and ensuring a product or service is right first time. Giving those who work in the process the opportunity come up with clever solutions to greatly reduce the error rate or eliminate it altogether is incredibly rewarding for everyone involved.

 

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Poka-yoke distinguishes between defects and mistakes.  A defect is an error in a process that continues through until it reaches the customer (at great cost to the company). A mistake on the other hand, can be detected and corrected immediately.  Poka-yoke seeks to eliminate defects at the source, by removing the possibility of a mistake occurring or alerting the operator immediately.

There are three recognized types of poka-yoke for detecting and preventing errors in a process:

  1. The contact method identifies defects using a part’s shape, size, colour, or other physical attributes. The contact method can also include limit switches and proximity sensors. For example, if we want to avoid placing a part backwards or upside down in a fixture. This part should have an asymmetrical design with a receptacle only allowing the part to fit in the correct orientation. Alternatively, a pin can protrude from the part which will only allow the part to fit when correctly oriented providing a contact method poka-yoke.plug-1821536_1280
  2. The constant number (or fixed-value) method alerts the operator if a certain number of movements are not made.  Counters could also be a fixed number of parts must be used at each step. Similar to building Ikea furniture, if when you’re finished there is a screw left over, you know you have done something wrong (although in that case it’s probably too late). In manufacturing or service processes the operator is provided with a bin containing the exact number of parts to complete the stage of the task. A part left over or one missing will highlight a defect at this stage. The part will not be allow it to go to the next stage, where it would increase the associated cost of the defect.

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  1. The sequence (or motion step) method aims to ensure the prescribed steps of the process have been followed before starting the next stage. Not allowing the mistake to be made in the first place. This could include part boxes only opening in sequence. Colour coding and tagging are other examples of sequence poka-yokes, these are then inspected to ensure that critical prior steps were performed.

A Poka-yoke should ask did we complete something with the right thing the right number of times in the right order.  Ideally, the poka-yoke prevents the error altogether by the nature of how it is designed. If a poka-yoke cannot eliminate the opportunity for error completely then it must warn immediately if the error occurs through inspection or in the form of an alarm or warning light.

Poka-Yokes in Everyday life

  1. (Sequence method): An everyday example of a Poka-yoke can be seen at an ATM. The order of this transaction occurs is a result of trial and error. Before the user entered their card completed the screen interaction and the cash was supplied, then their card was returned. As the consumer had received what they came for (cash) a large number of cards were being left at the teller machine. The order has been changed so the card is returned to you before the cash, thus eliminating the likelihood of walking away without the card and the cash. There can also be an alarm incorporated in the ATM that chimes until the card and the cash have been removed.
  2. (Contact method): At the fuel station there are different size/shaped fuel pump and diesel and petrol cars have different fuel inlet. The diesel pump will not fit into an unleaded petrol inlet. Saving the user from a headache a few kilometres down the road.refuel-331992_1280
  3. (Sequence method): Airplane restroom lights only come on when the door lock is engaged. This keeps customers from failing to lock the door.
  4. (Contact method) A final example that combines the contact and warning method is the clearance bar that is seen above any drive-thru or car park.  If a vehicle is too tall, then contact will be made with the bar warning the driver that their vehicle will not fit through.

Poka-Yokes in Manufacturing

  1. (Contact method): If a machine required an operator to only press a button with their right hand, their left hand could still accidentally be injured by the machine. Many machines have been redesigned to only operate if you pressed one button with your left and a second button with your right hand, ensuring that both hands are clear of the machinery.
  2. (Constant number method) When assembling a part with some nuts and bolts, the exact number of nuts and bolts required is available to the worker.  If any are left over after the worker has assembled the part, a mistake is detected right away because somewhere a bolt is missing from the part.

Poka-Yoke in Services Environments

Service industries have an added complication when implementing a poka-yoke system. As they must consider both the server and the employee causing an error. The possibility that service errors can be created by both the customer and the server, service poka-yokes are grouped into two categories: fail-saving the server and fail-saving the customer.

Additionally, service organizations have multiple method of communicating with the customer (in person, online, through the phone). Each interface should be considered when implementing an poka-yoke system.

  1. (sequence method) An online mail-order company has designed its boxes and packing material to avoid mistakes. The inner flaps of the box bottom have a large brightly coloured warning to “Stop! Open the other side.”
  2. (sequence method) Computer systems can be extremely useful in a process where a customer must book a service, or order an item. Where there is a data entry omission by the customer, such as leaving out the city when filling in their address. They cannot move to the next stage and a warning message will appear at the top and often the box omitted will be highlighted.
  3. In healthcare many instances of poka-yokes using colour coding and tagging can be seen. An example is shown below, the outlet providing oxygen is green and the attached equipment is the same colour green, while the medical air in yellow. These will also have different connecting recepticals.

Poka-Yoke Stumbling Blocks

There can be cases where a lot of time and effort is spent on poka-yoke only to see it ultimately abandoned to return to the original methods. This typically happens because the poka-yoke chosen had the following pitfalls:

  1. It was too onerous for the operator in the process.
  2. It was so strict that the machine efficiency was reduced.
  3. It was unreliable, allowing some defects through. These defects may have stopped production too often.

It’s important to ensure that poka-yoke is reliable, easily diagnosed, and not burdensome if you expect it to achieve its goal of increasing productivity and reducing defects.  Another oversight when implementing a poka-yoke is understand human limits. Examples include:

  1. Vision. People have varying ability to distinguish details and colours.
  2. Hearing. The upper and lower thresholds of an individual’s hearing can vary and can also change when background noise is added.
  3. Performance of repetitive tasks. Mental tracking and muscular efficiency decrease as rates of repetition increase.
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