Using mistake-proofing in product design

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 toward 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:

• Controls or features in the product or process to prevent or diminish the occurrence of errors.
• Simple and inexpensive inspection or error detection at the end of each successive operation to determine and correct defects at the source.

There are six mistake-proofing principles or methods. These are listed in order of priority in fundamentally addressing mistakes:

1. 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.

2. 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.

3. 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.

4. 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.

5. Detection: Errors are detected before they move to the next processing step so that the user can quickly correct the problem.
   
   Examples: Sensors in the production process to identify when parts are incorrectly assembled. 

6. 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).

About the author:David McBride is the co-founder of EMS Consulting Group (http://www.emsstrategies.com), a Carlsbad, Calif.-based engineering and management consulting firm. David has a bachelor of science degree in mechanical engineering from Ohio State University. He has a successful track record in the development and implementation of FMEA and Design for Manufacturability programs at several organizations and has greatly reduced manufacturing costs through the utilization of lean manufacturing, kaizen events and manufacturing system analysis. He has also been highly successful at developing and executing new product introduction processes, and staffing and capital equipment plans. For more information, call 866-559-5598 or visit www.emsstrategies.com.