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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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Designers often claim that the tolerances are determined by the function of the design. In reality, the required tolerances are determined by the design alternative selected. Functional performance is what customers always want. There are always design alternatives with some being more robust than others. In other words, some designs require tight tolerances while other design alternatives will function while allowing looser tolerances. All other things being equal the design professional should select the design that allows the largest tolerances while assuring that the functional requirements of the design are met.
In the turret lathe example, if the tolerance had been specified as ±0.03mm, which is ±6s and the mean of the process was not allowed to drift off center by more than 1.5s or 0.0075mm, the Cpk of the process as shown in Fig. 7 would be:
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Fig. 7 CNC Turret lathe process capability
This would be considered a 6 design. By converting Cpk to a 'z' score, probability tables may be used to determine that the likelihood of producing defects would be 3.4 parts per million (ppm) or less. Designers and design engineers must have a way to quickly evaluate the implications of the tolerances they assign.
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