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DEFINITION OF ALLOWANCES AND TOLERANCES TERMS

ACTUAL FIT: The actual fit between two mating parts is the relation existing between them with respect to the amount of clearance or interference which is present when they are assembled. (Fits are of three general types; clearance, transition, and interference.)
ACTUAL SIZE: An actual size is a measured size.
ALLOWANCE: An allowance is an intentional difference between the maximum material limits of mating parts. (See definition of Fit.) It is a minimum clearance (positive allowance) or maximum interference (negative allowance) between mating parts.
BASIC SIZE: The basic size is that size from which the limits of size are derived by the application of allowances and tolerances.
BILATERAL TOLERANCE: A bilateral tolerance is a tolerance in which variation is permitted in both directions from the design size.
CLEARANCE FIT: A clearance fit is one having limits of size so specified that a clearance always results when mating parts are assembled.
DESIGN SIZE: The design size is that size from which the limits of size are derived by the application of tolerances. When there is no allowance, the design size is the same as the basic size.
DIMENSION: A dimension is a geometrical characteristic such as diameter, length, angle, or center distance.
FIT: Fit is the general term used to signify the range of tightness which may result from the application of a specific combination of allowances and tolerances in the design of mating parts
INTERFERENCE FIT: An interference fit is one having limits of size so specified that an clearance or interference fit may result when mating parts are assembled.
LIMITS OF SIZE: The limits of size are the applicable maximum and minimum sizes.
MAXIMUM MATERIAL LIMITS: A maximum material limit is the maximum limit of size of an external dimension or the minimum limit of size of an internal dimension.
MINIMUM MATERIAL LIMIT: A minimum material limit is the minimum limit of size of an external dimension or the maximum limit of size of an internal dimension.
NOMINAL SIZE: The nominal size is the designation which is used for the purpose of general identification.
SIZE: Size is a designation of magnitude. When a value is assigned to a dimension it is referred to as the size of that dimension. (It is recognized that the words "dimension" and "size" are both used at times to convey the meaning of magnitude).
TOLERANCE: A tolerance is the total permissible variation of a size. The tolerance is the difference between the limits of size.
TOLERANCE LIMIT: A tolerance limit is the variation, positive or negative, by which a size is permitted to depart from the design size.
TRANSITION FIT: A transition fit is one having limits of size so specified that either a clearance or an interference may result when mating parts are assembled.
UNILATERAL TOLERANCE: A unilateral tolerance is a tolerance in which variation is permitted only in one direction from the design size.

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Last modified: October 02, 2001