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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:
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
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| 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
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| 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
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| 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. |
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| 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
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| 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
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| 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. |
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