For manufacturers to properly follow lean manufacturing principles, errors and defects must be properly defined, corrected, and ultimately prevented — efficiently and quickly. That means accounting for every person and every element involved in the manufacturing process.
To achieve this, the manufacturing process must be treated as a dynamic process robust enough to respond when things do not go as planned. You need to know the current state of the product through the manufacturing process — its location on the assembly line, its weight, its movement, and so on. All these goals stem from a broad philosophy called poka-yoke, first put forward and championed by Toyota engineers as part of the Toyota Production System developed in the mid-20th century.
Defining poka-yoke
Essentially, poka-yoke translates to mistake-proofing, and a poka-yoke device is anything that prevents an error from occurring in the manufacturing process or is used to make defects easily detectable so they can be sorted or corrected.
The scope of poka-yoke
A poka-yoke device can be put in place at any step of the production process — both on the production floor and during the final design phase of a product (e.g., designing an error-proof element to prevent misuse by the end user). The three-prong electrical outlet is often cited as an example.
The paragraphs below describe the types of poka-yoke methods commonly used in a manufacturing plant.
- Contact method
- Fixed-value method
- Motion-step method
Poka-yoke and the contact method:
A poka-yoke device in the contact-method category is used to inform any person or machine involved in producing a product of the product's current state by detecting physical contact or contact with an energy source (e.g., a photoelectric beam).
Poka-yoke: common examples of error-prevention means
- Limit switch: Activated when contact is made. This can be used to confirm a hole is drilled to the required depth.
- Electrode: When conductive parts are used, their presence can be detected with an electrode that closes a circuit on contact.
- Guide blocks and pins: Used to ensure a part is correctly aligned by including custom-made pins or rails in the design of a particular workstation to constrain the part to an obvious location.
Poka-yoke: common examples of error-detection means:
- Weight sensors: A fast and easy way to determine whether parts are missing from a product.
- Dimension sensors: Implemented through physical inspection or a built-in feature (i.e., a mesh that only lets items of a certain size pass).
- Parts bins: Used to determine whether the exact number of parts has been included in a particular workstation design.
The examples above are only a few illustrations of how errors can be prevented and detected using existing technologies and devices that rely on physical contact as the detection mode. The method is universally accepted in a manufacturing context, but how it is implemented can vary significantly depending on the process and materials involved.
Fixed-value method:
The fixed-value method applies to error sources that arise when a certain fixed value is not respected. It may simply be the number of screws in a part, the torque to which a bolt is limited, or the number of times a product is labeled. Regardless of the kind of manufacturing, there are inevitably steps in the process that require a particular fixed value to be respected to meet quality standards.
The following examples show how these types of errors can be avoided.
Common examples of error-prevention means:
- Counters: simple devices that measure occurrences, by contact or by motion.
- Logic controllers: if a certain number of operations are not completed at a workstation, a controller can stop the conveyor to prevent the error from propagating.
- Parts dispensers: at a workstation, a dispenser can release the required quantity of parts into a bin so the bin is empty at the end of each cycle, ensuring the correct number of parts has been used.
Common examples of error-detection means:
- Parts bins
- Lights and alarms: A more general way to detect errors — a light or alarm can be triggered when a fixed value is not reached.
Motion-step method:
The final major category of poka-yoke devices concerns the prevention and detection of errors caused by an improper sequence of events. Such an error can be flagged at any point in the manufacturing process using means and techniques like those described below:
Common examples of error-prevention means:
- Timed gates: Used to track a process to ensure timing is respected.
- Numbering scheme: Parts are assembled in the prescribed assembly order.
- Ordered instructions: ensuring instructions at a workstation are set in a fixed order.
- Successive checks and controls: ensuring that several people verify the product throughout the manufacturing process to surface information as early as possible.
Conclusion:
Poka-yoke mechanisms will only succeed if you first answer the following questions:
- What is the cause of the errors?
- Why do errors occur?
- Is there an easy way to detect when an error occurs?
- In the event of an error, how can we prevent it from getting worse and isolate it?
Once these questions have been addressed and answered, it will be easier to determine which poka-yoke mechanism is the most appropriate to put in place.
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References:
- Tutorial, www.mistakeproofing.com/tutorial.html.
- Dolcemascolo, Darren.Mistake Proofing Poka Yoke Lean Overview, emsstrategies.com/dd110104article1.html.
- "POKA-YOKE."Reference for Business, www.referenceforbusiness.com/management/Or-Pr/Poka-Yoke.html.
- "Poka Yoke."Lean Manufacturing Tools, leanmanufacturingtools.org/494/poka-yoke/.
- "Toyota Production System."Wikipedia, Wikimedia Foundation, 16 Mar. 2018, en.wikipedia.org/wiki/Toyota_Production_System.
