What is Natural Tolerance?

The natural tolerance in manufacturing systems is defined as 6σ where σ is the standard deviation of the observed data.

Being 6 times of standard deviation would mean that natural tolerance includes 99.73% of observed variability in dimensions. Take a look at the graph below.

Each band has a width of 1 standard deviation. The nominal dimension lies at the crest of the wave, and the parts touching the horizontal lines would be the max and min allowable dimensions.

Imagining a specification of 20±0.5mm, the crest would be at 20mm  and the parts touching the horizontal lines would be at 20.5mm and 19.5mm.

And with σ being the standard deviation, 6σ would include most (99.73%) of the variability.

This variability is called Natural Tolerance.

A plot of normal distribution (or bell-shaped curve) where each band has a width of 1 standard deviation.



Where is Natural Tolerance used?

Natural tolerance is used in the statistical analysis of dimensional variability. Most notably in Cpk calculations. They form a very important part of Process Capability Analysis and have direct impact on part quality.

Click here to read more about Process Capability Tools Cp and Cpk

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