Production of exact size of a feature is impossible to achieve due to following reasons;
• Variations in properties of material being machined
• Inaccuracies of production machines
• Operator and environmental dependant uncertainties
For satisfactory function of a part, exact size of a feature is not necessarily required.
A part can fulfill the design intent functions when the deviation of feature size remains in established boundaries.
Maximum and minimum permissible sizes (established boundaries) within which the actual size of a feature lies are called limits of size.
Another function of establishment of the boundaries is to have interchangeable parts which are so nearly identical and fit into any assembly of the same type.
Limits, fits and tolerance calculators have been designed to calculate the limits and
fits according to ISO 286-1 (2010), ISO 286-2 (2010) ,ANSI B4.2 (1978), ANSI
B4.1-1967 (R2004) standards which are based on metric and inch units.