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Concepts and Cp/Cpk |
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Excerpt of Process
Capable Tolerancing (PCT) article written by Don Day,
Tec-Ease Inc, USA, Martin Raines and Ken Swift,
CapraTechnology Ltd, UK
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Cp is a measure of the precision of a given process. It compares the allowable deviation stated on the drawing to the process variation.
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Imagine sighting in a target rifle. The marksman will usually aim at the intersection of two lines and fire off several shots. The majority of shots will often be clustered in a group. By comparing the dispersion of the majority of shots to the size of the target, the capability of the target shooting process may be determined. In the example shown in Fig. 1 on the left, if the goal is to always shoot off the head of a match, this process is not at all capable. However, if the goal is to always hit within the target on the right, it appears that the process is very capable.
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Fig. 1 Capability of the shooting process
An index that can be used to describe the capability of a process is Cp: C for capability and p for process.
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Fig. 2 Targets and
dispersion
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The symbol s, sigma, represents the standard deviation of a population or "A statistic used as a measure of the dispersion or variation in a distribution, equal to the square root of the arithmetic mean of the squares of the deviations from the arithmetic mean." The standard deviation is easy to understand by looking at how it is determined in an actual case study.
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