Poka – yoke is a Japanese term for mistake proofing. It is a device used in manufacturing as part the Zero Quality Control method, helping to ensure that build processes are right first time. Shigeo Shingo, an industrial engineer at Toyota, was the pioneer of these methods during the 1960’s, and its success has ensured it is now used in many industries around the world.
The manufacture of complex products (automobiles, air-conditioners, medical equipment, etc.) requires a large number of individual build steps to create the final article. Any one of these steps can introduce a failure or product fault, either through human error or material defects. Detecting and fixing these errors can be time and financially expensive, especially if it only comes to light further down the production line, or when the customer has taken delivery. To overcome this, techniques have been developed to ensure that a build step can only be completed if it will be correct, or if that is not feasible, a simple quality check is carried out ensuring the build step was successful. If the quality check fails, then it is easier and cheaper to correct it at this stage than wait until later on. How this is done, will depend on the manufacturing process, but it is likely that a modification to the production machine or process will be needed. This modification is called a poka – yoke device.
The best way to visualise a poka – yoke device is to imagine a single build step in a production line. Suppose you need to drill a hole in a particular place, in a piece of metal. Nothing more difficult than that. But now ask yourself, what can go wrong? Then ask, how can that be avoided?
Well, lets look at this example. The first thing that can go wrong is that we drill the hole in the wrong position. There are many ways that the position of the hole can be pinpointed. You can measure and mark every piece by hand. Or there could be a mark already made on the surface that you line up by eye. Both methods allow mistakes to be made, especially in a production line environment. How can this error be avoided? Well, we can use a jig to position the metal piece in only one precise position in the drill stand, and have the drill only movable in a vertical direction. This will mean that if the work piece is located properly, the hole can only be drilled in the required position. In this case, the jig will be the poka – yoke device.
But we are not finished yet – what else can go wrong? Imagine the work piece is positioned correctly in our jig, but the drill was broken. Now, we have items with no hole. To overcome this, a poka – yoke device could be made that would detect if the drill was broken (this can be done with an electrical contact perhaps). If an error is detected, then the machine is stopped, and an alarm sounded, so human intervention can be made to correct the problem.
There are many other ways in which the process can fail; perhaps the wrong size drill is being used, or it is blunt. For each of these a poka – yoke device can be made to detect and prevent such errors.
The advantage of poka – yoke devices is that they are simple, cheap and easy to implement within a production line, and that errors are detected either before they occur, or before they become costly to correct.
