Lean Tools: Mistake Proofing (Poka-Yoke)

At one time or another we have all put something together upside down or backwards and still have it fit. But then, it probably didn’t work as advertised when we turned it on or used it. Can someone do something the wrong way and still pass it on to the next step in your process?

Poka-Yoke is the method of applying Mistake Proofing techniques to eliminate the very possibility of many errors occurring. Workers, engineers, and managers all must work together to write procedures and design devices to prevent errors from occurring at their source of origin. Remember: the best, most efficient, most cost-effective, and safest place to catch errors is always at their source of origin.

Inspections that discover errors but do not provide feedback after completion and have no chance of reducing defects (creating only wasted effort) are called judgment inspections. Inspections that provide data and information about where and when errors occur, and can be of assistance in preventing future errors, are called informative inspections. The inspections that identify, fix, and/or prevent errors from happening at the point at which they could or do happen are called source inspections. The key issue here is that Source Inspections prevent errors from being passed on to the next step in the process and provide preventative and corrective action data. Source Inspections are also known as In-Process Inspections within some organizations.

Source Inspections check 100% of the processes or products that pass through one point of the operation or process. Source Inspection devices can be switches, jigs, or fixtures that halt the operation or stop the process if materials are provided upside-down or backwards, if the wrong type or quantity of materials are provided, or if the machine or product is handled incorrectly. These Source Inspections should be a critical part of a comprehensive Zero Defect program.

Often, bells, sirens, or lights are used as signals that a Source Inspection has discovered a defect or error. These warning or control systems notify an operator of a problem and signal for the machine to be shut off, or accompany an automated shutdown of the equipment. These warning or control systems are usually attached to a sensing device that uses contact or motion methods to determine a problem exists. Contact methods include passive devices such as guide pins, specially shaped or standardized fixtures and jigs that will accept only one size or not operate if installed incorrectly. Limit switches and touch or antenna switches identify passage of a desired characteristic or object or the presence of an unwanted characteristic or condition. Energy-sensing devices and physical-contact devices can be used to sense motion, pressure, or absence of an object.

While many different devices are available for all sorts of conditions to be monitored, one primary use is constant—they are used to detect errors or defects and prevent them from reaching the next operation or ultimately reaching the customer.

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