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.
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.