According to ASQ, “mistake proofing, or its Japanese equivalent poka-yoke, is the use of any automatic device or method that either makes it impossible for an error to occur or makes the error immediately obvious once it has occurred.”
Mistake proofing tools are numerous and varied but they provide low cost, effective defect prevention with relatively quick operator feedback. They are intended to stop mistakes from being made or make mistakes obvious at a glance. Mistake proofing devices are typically designed to prevent the special causes that result in defects or inefficiencies. According to Dr. Joseph M. Juran and other quality professionals, those occurrences happen about 15% of the time; therefore, they can be significant in driving an organization’s cost of poor quality.
Ideally, the mistake proofing device, not the person performing the work, should inspect the product so that any mistakes become instantly apparent. If desired, the operation is not able to be completed unless the defect-free condition is satisfied. For example, take the computer CD disk that won’t operate if inserted upside down. Another example involves a continuous flow conveyor system. A component with finished machine dimensions would have a tendency to “bump” into each other, causing costly repairs. A simple and inexpensive sensing device installed in less than 15 minutes solved the problem.
Mistake proofing should be one of the cornerstones of any quality management system, especially in manufacturing. When designing a manufacturing system, operator errors are routinely encountered, but those errors are rarely single, independent events. They are usually part of a larger sequence.
Typically, complex, technology based solutions that inspect quality into the process are often considered first, but, although they are usually the most expensive, they aren’t always the best. As an alternative, organizations should work to prevent errors and design the system to handle them when they do occur. This effort will lead organizations to design future processes or redesign current ones to include mistake proofing tools and techniques over traditional inspection and SPC tools.
A team environment is the best approach to implementing mistake proofing with members from all areas involved including machine operators, assemblers, fabricators, supervisors, manufacturing engineering, quality and product engineering. Unfortunately, most organizations do not use this approach. Instead, engineers and managers force-feed mistake proofing devices to the factory floor.
The result of this effort is usually not well received and ends up providing little to no process improvement. Only when everyone from the factory floor to at least mid management levels receive training on mistake proofing concepts and becomes directly involved in its implementation will the initiative be successful.
One of the students in my quality engineering training courses had a great success story. One of his company’s many sub-processes involves fabrication. One fabricated assembly has many components that couldn’t tolerate the high temperatures of the welding process and were either destroyed or degraded when no thermal controls were used.
In one example, a filter canister had to be welded to an undercarriage frame. Heat from the welding process was damaging the filter base, so the manufacturing engineer offered a solution to the operators. A new fixture was developed to hold the filter base. It was also intended to dissipate the heat before it reached the sensitive filter body. This method worked well if the filter base was placed correctly into the fixture, but the filter base’s shape made it very awkward to position properly.
Welders became discouraged because of the difficulty locating the part into the new fixture so they deviated from the procedures and attempted to locate the filter bases in their own way, sometimes very ineffectively.
Finally, a cross-functional team was formed to meet, review the process, and solve the problem. As a result, the team developed a two-function fixture, which held the filter base steady during welding and provided an automatic heatsink effect. The new fixture served as a clamp for welding stability, and its built-in heatsink capability dissipated heat away from the sensitive filter base.
When the fixture was used, it was impossible to weld the product without simultaneously dissipating the heat away from the filter base. The welders willingly accepted the fixture and the redesigned process change because it helped them without requiring any extra effort. Internal failures dropped to near zero and external customer complaints did drop to zero after the new process was implemented.
Most mistake proofing devices are simple and inexpensive. Adding a team approach to the process ensures everyone has a voice, which leads to greater success. Everyone benefits. The worker gets a better process, which tends to be less burdensome. The company benefits from better efficiency and effectiveness, typically leading to lower cost. The customer receives better quality and delivery. It’s hard to argue with these results.
You can reach Jim at [email protected].
