Machinery's Handbook, 31st Edition
LIMITS AND FITS
647
Actual Size: An actual size is a measured size. Limits of Size: The limits of size are the applicable maximum and minimum sizes. Maximum Material Limit: A maximum material limit is that limit of size that provides the maximum amount of material for the part. Normally it 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 that limit of size that provides the minimum amount of material for the part. Normally it is the minimum limit of size of an external dimension or the maximum limit of size of an internal dimension. * Tolerance Limit: A tolerance limit is the variation, positive or negative, by which a size is permitted to depart from the design size. Unilateral Tolerance: A unilateral tolerance is a tolerance in which variation is permit ted in only one direction from the design size. Bilateral Tolerance: A bilateral tolerance is a tolerance in which variation is permitted in both directions from the design size. Unilateral Tolerance System: A design plan that uses only unilateral tolerances is known as a Unilateral Tolerance System. Bilateral Tolerance System: A design plan that uses only bilateral tolerances is known as a Bilateral Tolerance System. Fits.— Fit: Fit is the general term used to signify the range of tightness that may result from the application of a specific combination of allowances and tolerances in the design of mating parts. Actual Fit: The actual fit between two mating parts is the relation existing between them with respect to the amount of clearance or interference that is present when they are assembled. (Fits are of three general types: clearance, transition, and interference.) Clearance Fit: A clearance fit is one having limits of size so specified that a clearance always results when mating parts are assembled. Interference Fit: An interference fit is one having limits of size so specified that an inter ference always results when mating parts are assembled. Transition Fit: A transition fit is one having limits of size so specified that either a clear ance or an interference may result when mating parts are assembled. Basic Hole System: A basic hole system is a system of fits in which the design size of the hole is the basic size and the allowance, if any, is applied to the shaft. Basic Shaft System: A basic shaft system is a system of fits in which the design size of the shaft is the basic size and the allowance, if any, is applied to the hole. Preferred Basic Sizes.— In specifying fits, the basic size of mating parts shall be chosen from the decimal series or the fractional series in Table 4. Preferred Series for Tolerances and Allowances.— All fundamental tolerances and allowances of all shafts and holes have been taken from the series given in Table 5. Standard Tolerances.— The series of standard tolerances shown in Table 6 are so ar- ranged that for any one grade they represent approximately similar production difficulties throughout the range of sizes. This table provides a suitable range from which appropriate tolerances for holes and shafts can be selected and enables standard gages to be used. The tolerances shown in Table 6 have been used in the succeeding tables for different classes of fits. Table 7 graphically illustrates the range of tolerance grades that various machining pro cesses may produce under normal conditions. * An example of exceptions: an exterior corner radius where the maximum radius is the minimum mate rial limit and the minimum radius is the maximum material limit.
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