In Metrology, tolerance means the limit or acceptable limits of the variations of a physical dimension, a physical property of a manufactured object, of a system, or other measured values such as temperature, humidity or time. In other words, it is the maximum allowable error in the measurement is specified in terms of some value.

Tolerance allows the operator to establish a measurement with a confidence interval, even in the presence of imperfections and variables due to influence quantities, without the measurement being compromised.

It is essential for the manufacturer to have an in-depth knowledge of the tolerances to manufacture parts economically but, at the same time, adhere to quality and reliability aspects. Precision is engineered selectively in a product depending on the functional requirements and its application. To achieve increased compatibility between mating parts to enable interchangeable assembly, the manufacturer needs to practice good tolerance principles.

The tolerance in mechanical devices is the deviation deemed acceptable between the dimensions established in the design phase of a component and its actual dimensions; for its proper operation in relation with another piece, i.e., in dimensional coupling.

It is not possible to produce any mechanical object with the exact desired size, called nominal size, because in the production cycle there are errors due to:

  • inaccuracy of machine tools (i.e., tool wear effect during processing);
  • possible assembly and equipment inaccuracies;
  • inaccuracies of measuring instruments used for dimensional control.

To ensure the correct functionality of a mechanical part and therefore to be able to consider it as accurate, it is adequate that its size lies within two limits, namely the tolerance, which define the dimensional variation allowed in the construction.

In the same way, in order to obtain a correct coupling between two pieces, a certain margin of error is necessary, said deviation, positive and negative for the nominal size of the pieces to be assembled, to determine the clearance or the required interference.