Subscribe
PLANT

Poka yoke

You can’t predict human error on the production line but you can take steps to prevent it.


March 29, 2011
by Richard Kunst

You can’t predict human error on the production line but you can take steps to prevent it. That’s what a Matsushita manufacturing engineer named Shigeo Shingo had in mind when he came up with poka yoke quality management. Pronounced “poh-kah yoh-kay,” it comes from two Japanese words – “yokeru” which means, “to avoid,” and “poka,” which means “inadvertent errors.”

One of the many components of Shingo’s Zero Quality Control (ZQC) system to eliminate defective products is the main objective: achieving zero defects in the simplest and lowest-cost manner.

Since poka yoke is more of a concept than a procedure, its implementation is determined by what people think they can do to prevent errors, not by a set of step-by-step instructions.

Simple objects such as fixtures, jigs, gadgets, warning devices and paper systems are used to stop the machine and alert the operator if something is about to go wrong. These devices should be useable by all workers, simple to install, effective without continuous attention from the operator, low-cost and provide instantaneous feedback, prevention or correction. Many of Shingo’s devices cost less than $50!

Poka yoke is at its best when it prevents mistakes, not when it merely catches them. Since human errors usually stem from people who are distracted, tired, confused or de-motivated, a good poke yoke solution is one that requires no attention from the operator. Examples include:
• A jig that prevents a part from being disoriented during loading.
• Non-symmetrical screw-hole locations that prevent a plate from being screwed down incorrectly.
• Electrical plugs that inset only into the correct outlets.
• Notches on boards that only allow correct insertion into edge connectors.
• A flip-type cover over a button that prevents the button from being accidentally pressed.

There are three levels of poka yoke: elimination of spills, leaks, losses at the source or prevention of a mistake; detection of a loss or mistake as it occurs, allowing correction before it becomes a problem; and the least effective, detection of a loss or mistake after it has occurred, just in time before it blows up into a major issue.

Keeping count

Poka-yoke systems consist of three primary methods.

The contact method detects whether or not a sensing device makes contact with a part or object, such as limit switches that are pressed when cylinders are driven into a piston that holds the part in place. If a cylinder is missing, the part is not released to the next process.

The counting method is used when a fixed number of operations are required within a process, or when a product has a fixed number of parts attached to it. For example, a sensor counts the number of times a part is used or a process is completed and releases the part only when the right count is reached.

Another approach is to count the number of parts or components required to complete an operation in advance. If an operator finds parts left over, something has been omitted from the process.

Motion sequence uses sensors to determine if a motion or step in a process has occurred. If the step has not occurred or has occurred out of sequence, the sensor signals a timer or other device to stop the machine and signal the operator.

Common mistake-proofing devices include guide pins, blinking lights and alarms, limit switches, proximity switches, counters and checklists.
Anybody can and should practice poka yoke in the workplace. All it takes is common sense and the appropriate poka yoke device.

Of course, error proofing can be achieved by extensive automation and computerization, although expensive and complicated, and it may be an impractical solution for small operations. Besides, it defeats the original purpose of poka yoke, which is to reduce defects and mistakes in the simplest and lowest-cost manner possible.

Richard Kunst is president and CEO of Kunst Solutions Corp., which publishes the “Lean Thoughts” e-newsletter.

E-mail rkunst@kunstartofsolutions.com.