Mistake Proofing

Mistake proofing devices, also called poka yokes, are the most effective way to improve quality. In a nutshell, a product (or process) is designed in which a mistake is impossible to make.

This problem prevention is targeted at specific errors. In order to mistake proof a process, one must have a thorough understanding of the ways the process can fail. Mistake proofing can be done proactively by considering likely quality problems, or it can be done reactively as countermeasures to actual quality issues. A single process step may have multiple mistake proofing devices built into it if there are many ways the process can fail.

At their core, most mistake proofing devices check just a few things:

• Orientation: Is the product oriented in the right direction?
• Counts: Is the correct quantity of parts installed?
• Size: It the product too large or too small?
• Configuration: Are the right components installed?
• Presence: Are all the proper parts, holes, etc. present and accounted for?

Mistake proofing devices may be physical barriers, switches, electronic eyes, or any other method that triggers a stoppage. To be clear, the stoppage is not necessarily a line stop. It may be a simple speed bump. An example would be asymmetrical parts. An operator would try to install them incorrectly, recognize they don’t fit, and then quickly flip over the part. The stoppage may be only a second or two.

The key to a solid mistake proofing device is that it can’t be forgotten or missed. For example, a gauge, commonly thought of as a mistake proofing device, is really not a poka yoke. It must be actively and correctly used, and if it is not, the product can still make it down the line.

Some people try to build anti-bypass methods into their mistake proofing. I generally don’t recommend that, as it takes a lot of extra effort, plus it is not the root cause of the problem. The real problem in those cases lies in leadership or ethics.

Finally, mistake proofing is not just for shop floor work. Office workers have a variety of mistake proofing devices in their jobs as well. Computer programs limit data fields or cords plug in only certain ways.