Page 218 - The Toyota Way Fieldbook

194 THE TOYOTA WAY FIELDBOOK

TRAP
Don’t Go Overboard with Poka Yoke
Unfortunately, too much of a good thing can be bad. We’ve seen
a trend toward higher and higher levels of mistake proofing. In
many cases the devices are developed by engineers, and the
actual workers have no input. The devices become extremely
sophisticated and add layers of complexity. We have seen opera-
tions that take 15 seconds to perform the actual work, but an
additional 25 seconds or more to engage the mistake-proofing
device! In one example, a molded part has a few clips and slide
rails added, then is placed in a mistake-proofing device. The part
is locked in the fixture and then cycled to test for application of
the correct parts. After the cycle is completed, it’s unclamped
and removed from the fixture. The checking takes nearly twice as
long as the actual work! The sad part is that after this elaborate
testing process it’s possible for the parts to fall out, causing a
defective part to reach the customer anyway! In addition, the
sophistication of the devices creates problems with understand-
ing how to operate them, reset after errors, and so forth. Try to
find simple, effective mistake-proofing methods.

(jidoka devices fit this category). The device may detect a broken tool or signal
the operator that a component is missing. The tool is already broken and some
parts may be damaged, but the problem is detected and corrected quickly.
In any case, it’s important to prevent any defective items (or mistakes) from
affecting the customer. This is true even if an effective error-proofing device
is used. There must always be protective “gates” to ensure that the customer is
never compromised. As mentioned earlier, each employee acts as a “gate” by
self-inspecting and inspecting key areas of others’ work. More inspection points
(gates) reduce the possibility of escape.
This list of possible causes of errors or omissions may not be all-inclusive,
but it covers the primary causes:
1. Deviation from defined work method (work must be standardized before
attempting poka yoke).
a. Omitted steps
b. Steps out of sequence
2. Missing parts (or components of the work)
3. Improper part (watch out for interchangeable parts)
4. Incorrect setup (wrong tools or settings)