In precision mechanics, fit refers to the degree of 'looseness' with which an shaft is inserted into an orifice.
This coupling is related to the tolerance or allowance of both parts dimensions. The shaft and the orifice must be of a similar diameter, otherwise there will not be a correct adjustment. With this in mind, measurements have been internationally standarised according to ISO regulation to ensure the interchangeability of items and their mass production.
Tolerance values are designated with a capital letter in the case of orificies and lower case letters in the case of shafts. The lower the value the higher the machining costs, as a greater precision is required.
Maximum and minimum clearance
The maximum clearance of a fit is the difference between the upper bound of the orifice diameter and the lower bound of the shaft diameter.
Maximum clearance = Maximum orifice diameter - Minimum shaft diameterThe minimum clearance meanwhile is the difference between the lower bound of the orifice diameter and the upper bound of the shaft diameter.
Minimum clearance = Minimum orifice diameter - Maximum shaft diameterThe maximum clearance in a loose or sliding fit is always greater than zero; on the other hand, in a tight fit both the maximum and minimum clearance are negative.