Introduction to poka yoke
A poka yoke (poh-ka yo-kay) is a device that automatically detects an abnormality and takes action in response.
- The literal meaning in English is “avoiding inadvertent errors”, but it’s better described as “error-proofing”, i.e., making a process immune to errors (which are unavoidable).
- Depending on what the poka yoke detects and what actions result, there are several types of poka yoke, with different purposes:
- Detection poka yoke (doesn’t prevent the defect but stops production and alerts the team for immediate investigation; the purpose is root cause analysis).
- Rejection poka yoke (detects the defect and confines the defective product either for rework or for scrap; the purpose is to prevent passing defects downstream).
- Prevention poka yoke (actually prevents a defect from occurring—our goal).
- The goal of zero defect is achieved when a prevention poka yoke is in place for every possible defect. Needless to say, that is a lot of poka yokes.
- The poka yoke is only a tool, which must be part of a broader quality management
What is Jidoka?
The concept of jidoka is confusing for beginners, partly because of how it developed in the history of Toyota.
- For purposes of implementation, we can reduce it to three principles:
- The goal is not to reduce quality defects to a low enough level—the goal is to prevent defects from ever occurring (Zero Defect).
- The only way to achieve this goal is to address the root cause(s) of the defect.
- The method of root cause identification is an immediate “freeze” of the system upon detection and investigation using tools like the “five times why” method.
- The control poka yoke is therefore the deliverable of this process, and the detection or rejection poka yokes are only intermediate steps.
- This is also called “built-in-quality”, because it integrates quality control into the process itself. Separate quality inspectors should in theory no longer be needed. In practice, because the list of potential defects is so long, there are still quality inspectors (humans or machines), but they check only those characteristics that don’t have “built-in quality” at every potential point of origin.
How Poka Yokes work
A poka yoke is a combination of one method per column.
|Where is it located?
||How does it detect the abnormality?
||What happens upon detection?
The poka yoke detects the precursor to a defect.
Example: detects if the part is placed upside down in the machine, before the cycle starts
Geometric or dimensional defects are detected by whether or not contact is made between the product and the sensing device.
Example: a limit switch is triggered if a part height exceeds the tolerance
It is not possible to produce the defect, either because of physical constraints or because an error (precursor to a defect) can be detected and stopped before it produces a defect.
Example: connector shape makes it impossible to connect the wrong cable or connect it upside down.
Detects the presence of a defect before the part leaves the workstation
Example: detects a dimensional defect before the part is ejected from the machine
Defects are detected by comparing a measured count or value with a standard
Example 1: 4 holes must be drilled in a part; a limit switch detects the motion of the drill and compares the count with a standard of 4. The counter is reset when the part is removed.
Example 2: A hole must have an ID of 7.0mm±0.2mm; a laser device measures the ID and determines if the hole is within tolerances.
It is possible to produce a defect, but the defective part is automatically prevented from reaching the next process step.
Example: dimensional gates on conveyor divert all over- and under-dimensioned parts to scrap bins
||Stop and Alert
Detects the presence of a defect after the part leaves the workstation.
Example: automatic scale placed on the conveyor detects missing component
Defects are detected by a variance in the sequence or motion in the process.
Example: operator must assemble components in a certain sequence. A sensor detects if the components are removed from their bins in that sequence.
It is possible to produce a defect, but this will stop production (e.g. stop the machine) and set off an alarm
Example: after a dimensional defect has been detected, the part holder is kept shut, the machine stops, and an acoustic alarm sounds
Some examples of how poka yokes works
If your vehicle is too high, you'll hit the hanging panel and avoid real damage on concrete roof of the garage.
The spacers must be between 9.5mm and 10.5mm thick. Parts coming out of the machine slide on a chute fitted with “bridges” that separate the good from the bad parts. Photoelectric laser sensors detect any part falling into the scrap bins and automatically stops the machine and sounds an alarm.
Immediate feedback-Value comparison-Stop and Alert
A barcode reader detects the type of components needed for each product, and activates the light and sensor for the correct bin. If the operator picks the correct part, the bin with the next needed lights up. If the wrong bin is picked, an alarm sounds and the conveyor stops.
Procedures for using poke yokes
Step 1: A Pareto Analysis helps determine where to start
- The first-level Pareto can identify at which process steps the most quality problems occur. The second-level Pareto drills down at the most critical process step and shows which defects are most frequent.
- By starting with the defects that have the biggest impact, you can ensure that management will provide the necessary resources.
- Alternatively, you can start with the defects that can be most easily eliminated. Perhaps an operator has approached you with an idea that is easy to implement and will do the job of a poka yoke. This is probably a better approach, but only because you are certain to get rapid results.
- To generate poka yoke ideas, it is critical to involve operators who actually work- in the selected process. Don’t try this with only engineers.
- In addition, you must collect actual examples of defective products. If you can capture videos of the defect-producing event(s), that it even better, but it is not necessary.
- Now that you have the preparation covered, you can kick off the poka yoke project. I do not recommend using a kaizen event format, because in my experience, the best ideas come after a few days of “digestion”.
- In the first meeting, present the goal and explain why this important for the business (e.g., “we’ve received three complaints for this defect type in the past month.” ) and go observe the actual process in action
Step 2: Provide a quick training on poka yoke but don't spend too much time on it
- The best learning will occur as the team tries to reach its goal.
- Now give to every team member one good part and one part with the defect (the parts must be taken at the point of occurrence and therefore are not finished parts unless the defect occurs at the very last process step).
- With the team, make a list of every difference between a good part and a bad part:
- The weight may be different
- One or more dimensions may be different
- The temperature may be different
- A feature (a hole, sticker, wire…) may be present or absent
- The color may be different
- The center of mass may be at a different place
- A function may exist or not
Why do we start with this list? Because every poka yoke must first be able to detect a defect. Each entry in this list has a corresponding detection method.
- A scale (mechanical or electronic) can detect through the weight difference
- A limit switch can detect a dimension
Step 3: Do the same thing, now from a process perspective.
What difference in sequence of steps produces the defect, as opposed to a good part? For example:
- Omitted a step
- Assembled components in wrong sequence
- Pressed the button twice
- Forgot to press a button
- Waited too long (or not enough) in between two steps, etc.
This list will help when exploring “motion comparison” methods.
- Quantity of ideas, not quality, is the goal (selection happens later)
- Use other people’s ideas to create new ones (modify, mix, extend…)
- Nobody can comment—just give your idea.
- Think out of the box. A crazy idea might be the seed of a great solution!
- Everyone creates a list individually
- Everyone stands in a circle, list in hand, and provides ONE idea from their list
- Go in round-robin fashion, always providing only one idea per person
- If someone has exhausted their list, they have time to continue thinking. If they can’t imagine a new idea, they say “I pass” Begin the brainstorming. The rules are:
- Facilitator writes all ideas, encourages team to utterly exhaust their ideas
Step 4: Set up the next meeting and describe the goal
- This first session should take between 2 and 3 hours. Rarely will you have a complete poka yoke idea in hand at this point. Send participants a copy of the list.
- Set the next meeting in a couple of days and describe the goal: “In meeting two, you will each (or in teams of two) present some poka yoke ideas”. This gives participants enough time to think this through, talk with each other, and try things out.
- In the second meeting, each person/team will present their ideas, and at this point, the rest of the team can provide feedback, suggestions, etc.
- At the end of this second meeting, a vote will be held to weed out the weakest ideas. It may happen that an idea is the clear winner and the next step is to implement it, but if more time is needed to come up with ideas, set another meeting.
- If after three meetings there is still no usable poka yoke idea, you may try for something easier, such as a Detection poka yoke (which detects a defect, and stops the process or sends an alarm).
- Follow the jidoka method: if at first, you don’t have a prevention poka yoke, use a detection or rejection poka yoke and continue learning about root causes.
- Publicize the poka yoke projects. People love the “brain teaser” aspect of coming up with a cheap and simple poka yoke device, and teams post their ongoing projects where others can read about them, they can also give additional ideas.
- Celebrate and communicate. Creating opportunities for success and truly celebrating the people involved is very important in creating a culture of continuous improvement.
- Quality is something that everybody can rally around. Because operators have the most intimate knowledge of the process from their work, they are the ones most likely to come up with the best ideas. If you truly celebrate every improvement coming from the shop floor, you can make more progress than through any PR program.
- Learn the “five times why” root cause analysis method, but most importantly, practice it relentlessly. It is harder than it seems, and practice is key