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Variation is the naturally
occurring dispersion in a process which can be controlled, but not
eliminated. Tool wear, vibration, and temperature fluctuations are a few of
the factors that may contribute to variation. Variations are typically
described statistically and traditionally controlled using Statistical
Process Control (SPC) or Statistical Quality Control (SQC).
Controlling variation with settings
World class leaders in quality are eliminating Statistical Quality
Control from the factory floor by controlling variation with "settings"
rather than "adjustments." The setting control methods developed by Shigeo
Shingo are generally simpler, less expensive, and more effective than
adjustment methods.
Traditional adjustments
A simple example is useful in clarifying the difference in settings and
adjustments. To compensate for tool wear on a milling machine, the milling
head could be advanced as illustrated below. This action represents an
adjustment. When the tool is changed, or the fixture is changed to process a
different type of part, the relationship between the milling head and the
bed of the milling machine has changed as a result of these adjustments.
Consequently, the operator must spend a lot of time adjusting the fixtures
and the tools to reestablish the proper relationship between the milling
head, the tool tip, and the fixture.
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