Design Mistake Proof

So far in our DFM Basics series we've explored explored how to minimize material costs by minimizing part count and by standardization. We then explored ease of assembly and optimizing component fabrication by design. This next chapter in our DFM journey focuses on the quality aspects of our design. Here we answer the classic DFM challenge put forth by Toyota: How to design with zero defects.

Quality plays a key role in design for manufacturability. A poor quality design can directly impact manufacturability through poor yield and rework. Quality issues also indirectly impact overhead cost, especially in cases where quality issues arise after the product has shipped.

What is mistake proofing?

One generation after the launch of Ford's legendary Model T, a second generation of manufacturing innovators were pioneering a new wave of manufacturing methodologies in Japan. The Toyota manufacturing team closely studied Ford's manufacturing methods, however different circumstances called for innovative new manufacturing techniques. Due to the smaller Japanese market, it was impossible for Toyota to utilize the purchasing power that Ford had for the Model T. This team would instead need to find a way to keep minimal inventory and make the most out of each car to be assembled.

From this revolution, Toyota introduced the highly influential lean methodologies that allowed for manufacturing efficiency with minimal inventory. They also introduced the idea of mistake-proofing and zero defect production. This would allow Toyota to launch new products by means of an extremely efficient production system, while making the most out of each car through means of zero defect production quality.

Toyota's methodology of Poka-yoke (mistake proofing) applies to all aspects of manufacturing. The fundamentals of mistake proofing were eventually categorized into 6 principals listed as follows, from most to least effective:

1. Elimination: Redesign such that the root cause of the mistake no longer exists.
2. Replacement: Substitutes a more reliable design or process over the error prone design/process.
3. Prevention: Modifies the design or process to prevent a mistake.
4. Facilitation:  Modifying the design or process to reduce errors.
5. Detection:  Implementing controls to identify mistakes before further processing.
6. Mitigation:  Minimizing the effects of a mistake after they occur.

Tips when designing for mistake proofing

In looking where DFM meets mistake proofing, we endow all DFM engineers with the following challenges and mantras:

1. Aim high on the scale of mistake proofing principles

Preferably, DFM pushes us to move all mistake proofing efforts to category 1: Elimination or category 2: Replacement. Elimination is very similar to the classic DFM challenge of minimizing parts. Likewise, replacement is synonymous with design for standardization. Hence, one can see how DFM and mistake proofing principles have a compounding effect.

2. Prioritize the design

Mistake proofing principles apply to both product design and manufacturing processes.  DFM should focus on design only. That's not to say that mistake proofing manufacturing processes is not important. There is a time and place for these efforts after the product is launched and the design is frozen. On the other hand, mistake proofing the design has only a narrow time frame at the onset of a project where meaningful changes can be made. Hence, during this crucial time, all efforts should be focused on mistake proofing the design. Mistake proofing manufacturing processes can come later.

3. Detection and Mitigation are marks of shame 😣

Detection and mitigation are not DFM. Not to say that these efforts are bad, as they indeed serve a crucial purpose in manufacturing.  That said, they are marks of shame for the DFM purists who could not find a way to design these issues out. Both detection and mitigation  add more parts and complication to a process or design. The DFM aspirational challenge is to figure out how to design out mistakes so that detection and mitigation are no longer needed.

Next Up:

Design for Manufacturability | Design Robustly

Challenging ourselves in how creatively and elegantly we can design out a manufacturability problems.

DFM4.0Andrew Akagawa

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