I saw a drawing with "ST" for strategic tolerancing. How does this differ from regular tolerancing?
I've seen ST for statistical tolerances. This means that a Cpk threshold is associated with the dimension to prove that the production process can hold close to the the nominal value of the dimension.
what is Cpk?
Matt Calzone wrote: what is Cpk?
Matt Calzone wrote:
how does this differ from a typical tolerance?
It just means that there is a statistical control on the distribution of values or measurements with respect to the mean dimension. It indicates the shape of the normal distribution curve (or bell curve) between the upper and lower tolerance limits. This material has already been linked a few times by Paul so I won't duplicate his info. Brush up on six-sigma principles and things should hopefully make a bit more sense as to what this statistical requirement means.
In overly broad terms, this makes sure that your measurements of the dimension aren't just random and all over the place while being between your upper and lower limits. It shows that there is control over the process that produces the part with respect to the given dimension if there is a normal distribution curve produced by the measurements of the completed parts.
I don't know if this is correct, but I've only seen ST for datums that reference a specific place that the part is supposed to be fixed on to be turn/milled.
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