Tolerance Dimensioning

Interchangeable ManufacturingTolerancingNominal SizeBasic Size Actual SizeAllowance

By B. Cowan &R. Cash

Interchangeable manufacturing allows parts made in widely separated localities to be brought together for assembly. That the parts all fit together properly is an essential element of mass production. Without interchangeable manufacturing, modern industry could not exist, and without effective size control by the engineer, interchangeable manufacturing could not be achieved. Dimensions should be given to as large atolerance as possible without interfering with the function of the part to reduce production cost. Manufacturing to close tolerance is expensive.

Mass Production

Basic Dimension

Toleranced Dimension

Unilateral Tolerance

Bilateral Tolerance

Unilateral Tolerance allowsvariation in only one direction.From basic size.

Bilateral Tolerance allow variationin either direction from basic size.

Other Methods OfPositioning andIndicating TolerancesBilateral andUnilateral

NOTE: All dimensions Plus or Minus 1/16 Inch

Specify Tolerance By Note

Tolerance

Tolerance

Glossary Of Terms

Mating Parts are parts that fit together within a degreeof Accuracy.

Tolerance is the difference between the limits prescribed for a single part.

Allowance is the tightest fit between two mating parts.

Nominal Size is an approximate size that is usuallyexpressed with common fractions.

Basic Size is the exact theoretical size from whichlimits are derived. The basic size expressed as adecimal

Tolerance is the difference betweenthe limits prescribed for a single part.The Tolerance for the Shaft is .002

Shaft

Hole

Allowance is the tightest fit betweentwo mating parts. The smallest holeand the largest shaft. The Allowancefor the Shaft and Hole is .003

2 1/2 Inch Shaft

Nominal Size

Basic Size

Type of fits between mating parts whenassembled.

Clearance Fit is a fit that gives a clearance between two assembledmating parts.

Interference Fit is a fit that results in an interference between the twoassembled parts. The Shaft is larger than the hole.

Transition Fit can result in either an interference fit or a clearance fit.The shaft can be either larger or smaller than the hole.

Line Fit can result in a contact of surfaces or a clearance fit betweenshaft and hole.

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American National Standard of Limits and Fits

RC Running or Sliding Clearance Fits - Intended to provide asimilar running performance with suitable lubrication allowance.Piston in a car.

LC Locational Clearance Fits - Intended to determine only thelocation of the mating parts.

FN Force Fits or shrink fits constitute a special type ofinterference fit, normally characterized by maintenance of constant bore pressure throughout the range of sizes.

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Example: Specify a RC3Fit for the 1 1/4 Inch Shaftand Hole

See Page a25 in the Appendix

Nominal Size Range Hole Shaft

1.19 - 1.97 +1.0 -1.0 -0 -1.6

Basic Size 1.2500

Nominal Size Range Hole Shaft

1.19 - 1.97 +.0010 -.0010 -0 -.0016

Multiply by .001

Nominal Size Range Hole Shaft

1.19 - 1.97 +1.0 -1.0 -0 -1.6

Nominal Size Range Hole Shaft

1.19 - 1.97 +.0010 -.0010 -0 -.0016

Add or Subtract From Basic SizeTo Calculate Tolerance.

Nominal Size Range Hole Shaft

1.19 - 1.97 +.0010 -.0010 -0 -.0016

Hole1.25 + .0010 = 1.25101.25 - 0 = 1.2500

Shaft1.25 - .0010 = 1.24901.25 - .0016 = 1.2484

Hole1.25 + .0010 = 1.25101.25 - 0 = 1.2500

Shaft1.25 - .0010 = 1.24901.25 - .0016 = 1.2484