Mistake-Proofing by Design

Mistake-Proofing a product’s design and its manufacturing process is a key element of design for manufacturability / assembly (DFM/A). Mistake proofing is also a key element of improving product quality and reliability and an element of the design for six sigma (DFSS) concept. 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 more and more as a product or item flows through a process. Early work on poke-yoke by Japanese authorities like 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 how the design of the product to avoid mistakes in production. Often the benefits of mistake-proofing not only help with production of the product, but can also contribute to correct user operation and maintenance of the product, and servicing of the product.

The concept of Mistake-Proofing involves:

• Controls or features in the product or process to prevent or mitigate the occurrence of errors and/or;
• Requires simple, inexpensive inspection (error detection) at the end of each successive operation to discover and correct defects at the source

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

1. Elimination seeks to eliminate 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 substitutes a more reliable process to improve consistency.
   

   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 engineers the product or process so that it is impossible to make a mistake at all.
   

   Examples : Limit switches to assure a part correctly placed or fixtured before process is performed; 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 employs techniques and combining steps to make work easier to perform.
   

   Examples: visual controls including color coding, marking or labeling parts to facilitate correct assembly; exaggerated asymmetry to facilitate correct orientation of parts; a staging tray that provides a visual control that all parts were assembled, locating features on parts.

5. Detection involves identifying an error before further processing occurs so that the user can quickly correct the problem.
   

   Examples : sensors in the production process to identify when parts are incorrectly assembled, built-in self-test (BIST) capabilities in products.

6. Mitigation seeks to minimize 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; extra design margin or redundancy in products to compensate for the effects of errors.

Ideally, mistake-proofing should b 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).

Mistake-proofing opportunities can be prioritized by performing design and process failure modes and effects analysis (FMEA). FMEA is supported with our Product Development Toolkit. Alternately, a mistake-proofing technique(s) can be developed for every process step in a manufacturing or service process.

Examples of the principles of mistake-proofing are shown below.

1. Elimination

Air baffles needed to be attached to cover to direct airflow over hot components. Adhesive could come loose if not properly applied. Baffle also prone to damage during assembly.	Alternative design has baffle function stamped into sheet metal cover eliminating troublesome assembly step.

 

2. Replacement

Alignment of the adhesive overlay on the faceplate was critical over a series of port holes. If mis-aligned, the overlay would need to be stripped off, the surface cleaned, and a new overlay applied.	The new model replaced this process by eliminating the adhesive overlay and using a more reliable silk screen process.

 

3. Prevention

This fixture is an example of process mistake-proofing. Two-door and four-door models are produced on this line. The fixture is built so that it is impossible to put a four-door part in place when the fixture is set-up for a two-door model and vice-versa. Notice the limit switch in center-right which helps insure proper orientation. If correct part is turned backward or upside down, it will not fit.

 

This is example of product mistake-proofing, that is, designing in part features to mistake-proof the assembly by only allowing assembly one way, the correct way.

 

4. Facilitation

Board orientation on bottom row 180° different from top row. Color coded board insertion screws and panel color-coding facilitate correct board orientation for insertion.

5. Detection

A conveyor carries the product under a pivoting flag. A correctly assembled product passes under the flag. An incorrectly assembled product tips the flag, and a sensor detects the flag movement.