Lean Manufacturing & Six Sigma : Zero Defects (無欠陥) – Part 3 of 3 Using Poka-Yoke (ポカヨケ) System

This is the third part of the article Zero Defects, and I have chosen the theme of pokayoke system, focused on the use of devices that are required to build a Pokayoke.

In my experience as a consultant, I have seen some Lean Engineers who lack practical experience in Pokayokes (they focus on philosophy but never built for themselves a system); so I hope that this post will provide information to help some people who need it, in next project.

Poka-yoke System Functions

A poka-yoke system processes two functions : It can carry out 100 percent inspection and, if abnormalities occur, it can carry out immediate feedback and action. The effects of poka-yoke methods in reducing defects will differ depending on the inspection systems with which they are combined: source inspections, self-checks, or successive checks.

Types of Poka-yoke

Poka-yoke systems fall in to regulatory function categories, depending on their purposes, and setting function categories, according to techniques they use.

Poka-yoke Regulatory Functions

Two regulatory functions are performed by poka-yoke systems.

1. Control methods

These are methods that, when abnormalities occur, shut down machines or lock clamps to halt operations, thereby preventing the occurrence of serial defects. Such methods have a more powerful regulatory function than do those of the “warning” type discussed below, and maximum efficacy in achieving zero defects is obtained by the use of these control type systems.

2. Warning methods

These methods call abnormalities to workers’ attention by activating a buzzer or a light. Since defect will continue to occur if workers do not notice these signals, this approach provides a less powerful regulatory function than control methods.

In cases where workers’ attention is captured by means of light, blinking lights can attract attention more powerfully than steady ones. Ultimately, this method is effective only when workers take notice, and the passive aspect of light signals makes it necessary to regulate placement, intensity and colour, etc.

Poka-yoke Setting Functions

The setting functions of poka-yoke systems can be divided in to three categories.

Setting Function Types and Examples

1. Contact methods

Methods in which sensing devices detect abnormalities in product shape or dimensions by whether or not contact is made between the products and the sensing devises are called contact methods.

2. Fixed-value methods

With these methods, abnormalities are detected by checking for the specified number of motions in cases where operations must be repeated a predetermined number of times.

3. Motion-step methods

There are methods in which abnormalities are detected by checking for errors in standard motions in cases where operations must be carried out with pre determined motions. These extremely effective methods have a wide range of application, and the possibility of their use should by all means be examined when poka-yoke setting functions are considered.