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Error-Proofing Enhances Quality


In all manufacturing operations, the goal should be zero defects

By Subramaniam Manivannan
Quality Coach/Assessor—PTO Quality
Mfg. Process/Product Support Dept.
Ford Motor Co.
Dearborn, MI

W. Edwards Deming observed: "Quality comes not from inspection, but from improvement of the process." It's a point that's too often forgotten. Rather than looking for defects after the fact, the true goal of manufacturing engineers and managers should be to install processes that yield zero defects.

What tools can we use as we strive to achieve this goal? Well, we can begin with the great Shigeo Shingo's invention of Poka-Yoke (pronounced POH-kah YOH-kay). Shingo introduced the concept of Poka-Yoke in 1961, when he was employed as an industrial engineer at Toyota Motor Corp. The initial term was Baka-Yoke, which means foolproofing. In 1963, a worker at Arakawa Body Company refused to use baka-yoke mechanisms in her work area, because of the term's dishonorable and offensive connotation. Hence, the term was changed to Poka-Yoke, which means Error Proofing or Mistake-Proofing.

Today, we use Shingo's idea in two ways: Mistake-proofing is the term applied to prevent mistakes from occurring in the manufacturing process, to eliminate the error from further processing, to warn that the error has occurred etc. We use errorproofing in design to prevent assembly errors. Examples include adding design features upside-down; backwards or reversed assembly; using snap-together features to eliminate fasteners (thus eliminating missing fasteners or incorrect, high/low torque etc.). However, most people use the terms mistake-proofing and errorproofing interchangeably.

In a modern lean production system, Poka-Yoke is a process improvement designed to prevent a specific defect from occurring. It prevents personal injury, promotes job safety, eliminates faulty products, and prevents machine damage.

Poka-Yoke mechanisms mistake-proof an entire process. Ideally, Poka-Yoke will ensure that proper conditions exist before actually executing a process step, and so prevent defects from occurring in the first place. Where this is not possible, Poka-Yoke performs a detective function, eliminating defects in the process as early as possible.

Many people think of Poka-Yoke techniques as the application of limit switches, optical inspection systems, guide pins, or automatic shutoffs that should be implemented by the engineering department. This is a very narrow view of Poka-Yoke. Poka-Yoke mechanisms can be electrical, mechanical, procedural, visual, human, or any other form that prevents incorrect execution of a process step. Poka-Yoke can also be implemented in areas other than production, such as sales, order entry, purchasing, or product development, where the cost of mistakes is actually much higher than the cost of mistakes that occur on the shop floor. The reality is that defect prevention, or defect detection and removal, has widespread application in most organizations.

Error proofing falls into certain specific categories:

  • Physical: Error proofing involves installing components like fixtures or sensors to eliminate conditions that may lead to an error.
  • Operational: Error proofing involves making modifications or installing devices that reinforce the correct procedure sequence.
  • Philosophical: Error proofing involves identifying situations that cause defects and doing something about it—empowerment of the workforce, for example.

Approaches to error proofing include prevention, which seeks to prevent errors from creating defects, and detection, which detects defects and immediately initiates corrective action to prevent multiple defects from occurring.

And what is a defect, you say? Try this definition on for size: A defect is the result of any deviation from product specifications that may lead to customer dissatisfaction.

Two points cause a deviation to be classified as a defect.

  • The product has deviated from manufacturing or design specifications.
  • The product does not meet internal and/or external customer expectations.

On the floor, an error is any deviation from the specified manufacturing process. There can be an error without a defect, but there cannot be a defect without an error.

Error proofing, as understood and practiced today, is an outgrowth of the quality movement, specifically the zero defects initiative. A team-based plantfloor improvement strategy, it focuses on production processes and operations. Error proofing aims to prevent errors and deviations from standards of all kinds that can impact quality, safety, manufacturing costs, and customer satisfaction.

Another tool useful in error proofing is Failure Mode Effects Analysis (FMEA).

A design-oriented FMEA is an analytical technique used by product or process designers as a means to ensure that, to the extent possible, potential failure modes and their associated causes have been considered and addressed. The design must be improved based on the results of the FMEA study. All the potential design issues and possible error proofing should be identified during the development of Design Failure Mode Effects Analysis (DFMEA) and integrated into design.

All the manufacturing/process issues are prioritized to help identify opportunities for the greatest impact upon the customer and return on investment. The most common tool used to identify/prioritize the issues is Process Failure Mode and Effects Analysis (PFMEA). The Process FMEA method is used in a cross-functional team approach to answer all process-related questions, and to quantify the results in the form of a Risk Priority Number (RPN). The PFMEA tool helps the team to ask the key questions necessary to identify and implement the proper error-proofing techniques to improve processes.

Error-proofing techniques include Design for Manufacturability (DFM), a set of techniques leading to designs that cannot be incorrectly manufactured or assembled. DFM can also be used to simplify the design, and therefore reduce its production cost. A Poka-Yoke system will use setup devices or inspection techniques that ensure setup is done correctly—that is, produces 100% good parts from the first piece on. The design stage remains the best opportunity to impact quality and cost.

Let's summarize: Why implement an error-proofing system?

  • Competitive advantage: In a global market the cost of quality is part of a company's competitive advantage. It costs far less to prevent defects from occurring in the first place than to catch them later through inspection, and then find that you must rework or repair them.
  • Knowledgeable workers: When every employee understands the principles of error proofing, work teams can see more easily how defects are generated, and can then act effectively to eliminate them. They can participate in the design and improvement of parts-processing and assembly operations to prevent defects from occurring. These methods can also be employed in the company's offices to eliminate errors in paper processes and administration.
  • Predictability: If our machines (manual or robotic) include error-proofing devices, then we are assured that the end product will be defect-free. This outcome eliminates inspection and rework operations, as well as scrap, all of which increase manufacturing costs.
  • Reduced variation: Error-proofing devices also ensure that all sub-assemblies and completed assemblies are exactly the same. There will be little chance of part-to-part variation if machines are designed or modified to prevent errors and their resulting defects.

Human error is natural. But sometimes, when errors can be traced back to the operator's interaction with the process, there is a tendency to blame the operator.

We encourage the operator to try harder NOT to make mistakes, but the root cause of the error is usually failure on the part of people who design machinery, layouts, or operating procedures to account for the possibility of human errors or omissions. Error proofing can correct this situation.

There are important facts to realize about human error. It's important to incorporate error proofing into the work environment. Understanding human limits is essential. These limits include:

  • Vision: People vary in ability to distinguish details, colors, or adjust vision to lighting.
  • Hearing: Individual upper and lower thresholds of hearing change when background noise is added.
  • Repetition Ability: Muscular efficiency and mental tracking decrease as rate of repetition increases.

Six sigma and error-proofing are related. In a DMAIC (Define-Measure-Analyze-Improve-Control) project, error-proofing is usually performed in the Control phase to prevent a specific defect from occurring. It's next to impossible to reach six sigma and lean implementation without applying error-proofing concepts.


This article was first published in the November 2006 edition of Manufacturing Engineering magazine. 

Published Date : 11/1/2006

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