In engineering, tolerance refers to the permissible limit or limits of variation in a physical dimension, a measured value or physical property of a material, manufactured object, system, or service, or other measured quantities. Tolerances are assigned to parts for manufacturing purposes, as no machine can hold dimensions precisely to the nominal value, so there must be acceptable degrees of variation. The use of tolerances helps to ensure that the final product is readily usable, especially if it is a part of a larger assembly. Not setting a tolerance in a critical area may render the part unusable according to the design intent, as each fabrication method comes with a certain level of inaccuracy.
Tolerances can be expressed in several ways, including:
- Dimensional tolerance: This is related to, but different from fit in mechanical engineering, which is a designed-in clearance or interference between two parts.
- Bilateral tolerance: This is a type of tolerance where the allowable variation is specified as a plus or minus value from the nominal dimension.
- Unilateral tolerance: This is a type of tolerance where the allowable variation is specified as either a plus or minus value from the nominal dimension.
Tolerances are used on production drawings to control the parts, and they are particularly assigned to mating parts in an assembly to allow for interchangeable parts, thus permitting the replacement of individual parts.
