November 1, 2004
By David McBride
Mistake proofing a product's design and its manufacturing process is a key element of design for manufacturability (DFM), and improving product quality and reliability. A difficult to assemble product is more likely to be assembled incorrectly.
The Japanese concept of Poka-Yoke (mistake-proofing) is oriented to finding and correcting problems as close to the source as possible because finding and correcting defects caused by errors costs increasingly more as a product or item flows through a process. Early work on poka-yoke by Japanese authorities like Shigeo Shingo focused on mistake-proofing the process after a product has been designed and is in production. As time has passed, more emphasis has been placed on designing the product such that mistakes are prevented in production. Often the benefits of mistake proofing not only help with production of the product but can also contribute to correct user operation, maintenance, and servicing of the product.
The concept of Mistake-Proofing involves:
Simple and inexpensive inspection or error detection, at the end of each successive operation to determine and correct defects at the source.
- Controls or features in the product or process to prevent or diminish the occurrence of errors.
There are six mistake-proofing principles or methods. These are listed in order of priority in fundamentally addressing mistakes:
Elimination: This method eliminates the possibility of error by redesigning the product or process so that the task or part is no longer necessary.
Example: product simplification or part consolidation that avoids a part defect or assembly error in the first place.
Replacement: To improve reliability, simply substitute an unpredictable process with a more reliable process.
Examples: Use of robotics or automation that prevents a manual assembly error, automatic dispensers or applicators to insure the correct amount of a material such as an adhesive is applied.
Prevention: Design engineers should design the product or process so that it is impossible to make a mistake at all.
Examples: Part features that only allow assembly the correct way, unique connectors to avoid misconnecting wire harnesses or cables, part symmetry that avoids incorrect insertion.
Facilitation: Utilizing specific methods and grouping steps will make the assembly process easier to perform.
Examples: Visual controls that include color-coding, marking or labeling of parts. A staging bin that provides a visual control that all parts were assembled. Locating features on parts.
Detection: Errors are detected before they move to the next processing step so that the user can quickly correct the problem.
Example: Sensors in the production process to identify when parts are incorrectly assembled.
Mitigation: The principle of attempting to decrease the effects of errors.
Examples: Fuses to prevent overloading circuits resulting from shorts; products designed with low-cost, simple rework procedures when an error is discovered.
Mistake-proofing opportunities can be prioritized by performing design and process failure modes and effects analysis (FMEA). Alternately, mistake-proofing techniques can be developed for every process step in a manufacturing or service process. Ideally, mistake proofing should be considered during the development of a new product to maximize opportunities to mistake-proof through design of the product and the process (elimination, replacement, prevention, and facilitation). Once the product is designed and the process is selected, mistake-proofing opportunities are more limited (prevention, facilitation, detection and mitigation).
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