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Poka-Yoke

Author: Mr D Wilson

Source of document: Internet

Date: 28/12/95

Shigeo Shingo was an industrial engineer at Toyota who has been credited with creating and formalising Zero Quality Control (ZQC), an approach to quality management that relies heavily on the use of poka-yoke (pronounced POH-kah YOH-kay) devices. Poka-yoke is Japanese for mistake-proofing.

• A poka-yoke device is any mechanism that either prevents a mistake from being made or makes the mistake obvious at a glance. Defects arise because errors are made; the two have a cause-and-effect relationship. Yet errors will not turn into defects if feedback and action take place at the error stage.
• Suppose a worker must assemble a device that has two push-buttons. A spring must be put under each button. Sometimes a worker will forget to put the spring under the button and a defect occurs. A simple poka-yoke device to eliminate this problem was developed. The worker counts out two springs from a bin and places them in a small dish. After assembly is complete, if a spring remains in the dish, an error has occurred. The cost of this inspection (looking at the dish) is minimal, yet it effectively functions as a form of inspection. The preferred outcome is still to find the dish empty at the end of assembly and to avoid rework even when its cost is small.
• This example also demonstrates that poka-yoke performs well when corrective action involves trying to eliminate oversights and omissions. In such cases, poka-yoke devices are often an effective alternative to demands for greater worker diligence and exhortations to "be more careful".
• Shingo identified three different types of inspection:
• Judgment Inspection. This involves sorting the defects out of the acceptable product, sometimes referred to as inspecting in quality.
• Informative Inspection. This uses data gained from inspection to control the process and prevent defects (SPC or measurement). Both successive checks and self-checks in ZQC are also a type of informative inspection. Successive checks were Shingo's response to the insight that improvements are more rapid when quality feedback is more rapid.
• Source Inspection. This determines before the fact, whether the conditions necessary for high quality exist. With source inspection, poka-yoke devices ensure that proper operating conditions exist prior to actual production. Often these devices are also designed to prevent processes from being performed, until the necessary conditions are satisfied.
• General Motors had an operation which involved welding nuts into a sheet metal panel. When the panel is loaded by the operator, the weld nuts are fed automatically underneath the panel, the machine cycles, and the weld nuts are welded to the panel. If the equipment jams or misfeeds and there is no nut loaded, the machine will still cycle. Therefore, there is some probability of failure of the process. An error of this nature is sometimes not detected until the car is welded together. This results in a major repair or rework activity. To correct this problem, a wire was passed through a hole in the electrode holding the nut, insulating it away from the electrode so as it passes through it will only make contact with the weld nut. Since the weld nut is metal, it conducts electricity and with the nut present, current will flow through, allowing the machine to complete its cycle. If a nut is not present, there will be no current flow. The process is controlled so that the machine will actually remain idle unless there is a nut in place.
• Source inspection, self-checks, and successive checks are inspection techniques used to understand and manage the process more effectively. Each involves inspecting 100 % of the process output. In this sense, ZQC is a misnomer. These inspection techniques are intended to increase the speed with which quality feedback is received. Although every item is inspected, Shingo was emphatic that the purpose of the inspection is to improve the process and prevent defects, and therefore is not intended to sort out defects (although in some cases that may also be an outcome).
• Source inspection is intended to keep defects from occurring. Self-checks and successive checks provide feedback about the outcomes of the process. Self-checks and successive checks should be used when source inspection cannot be done or when the process is not yet well enough understood to develop source inspection techniques.
• Shingo believed that source inspection is the ideal method of quality control since quality feedback about conditions for quality production is obtained before the process step is performed.
  

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