Tolerance and Dimensioning Terms

Transition fit

A fit between mating parts having limits of size so prescribed as to partially or wholly overlap, so that either a clearance or interference may result in assembly.

Interference fit

A fit between mating parts having limits of size so prescribed that interference always results in assembly.

Unilateral tolerance

A tolerance in which variation is permitted in only one direction from the specified dimension

Limit dimensions

A tolerancing method showing the maximum and minimum size values. The maximum dimension is placed above the minimum dimension. When expressed in a single line, the lower limit precedes the upper limit.

Allowance

Allowance is the minimum clearance or maximum interference intended between the maximum material condition (MMC) of mating parts. Example: Smallest hole with the largest shaft. The tightest permissible fit.

Design Size

the basic size and the allowance is applied to the shaft.

Dual Dimensioning

used to show both metric and decimal inch dimensioning on the same drawing.

Position Method

millimeter value is placed above (or below) the inch value or separated by a dash.

Bilateral-Unequal Tolerance

Bilateral tolerance that has an unequal number of tolerances on each side

Bilateral-Equal Tolerance

Bilateral tolerance that has the same number of tolerances on each side

Tolerance dimensioning

Expressing tolerances on a drawing

Plus-or-minus dimensions

For this method, the basic dimension is followed by a plus-or-minus expression.

Interchangeable

Parts that are made to easily fit mating parts without additional machining at the time of assembly.

Basic hole system

In a basic hole system, the design size of the hole is the basic size and the allowance is applied to the shaft. The basic hole system can be used to keep the costs of tooling down since standard tools such as a ream or broach can be used for machining. Notice that the lower limit of the hole is the basic size 1.5000.

Basic shaft system

In a basic shaft system, the design size of the shaft is the basic size and the allowance is applied to the hole. The basic shaft system should only be used when a standard size shaft is needed. Notice that the upper limit of the shaft is the basic size 1.5000.

Single-Line Limit Dimension

In a single-line note the lower limit precedes the upper limit and they are separated by a dash.

Tabular Dimensioning

Tabular dimensioning is used when a series of parts consists of the same features or geometry but vary in dimension. Letters are used in place of dimension values, and the values are then placed in a table. Most standard parts are dimensioned this way in catalogs, the machinery handbook, and in the back of most textbooks.

Actual size

The actual size is the measured size.

Basic size

The basic size is the size to which allowances and tolerances are added to get the limits of size.

Nominal size

The nominal size is a designation used for general identification. Typically this is a fraction when working in inches.

Bracket Method

millimeter value is enclosed in square brackets. A note should be placed on the drawing such as: DIMENSIONS IN [ ] ARE MILLIMETERS

Tolerance

The total amount a single dimension can vary. The difference between the upper and lower limits.

Locational Clearance Fits

These fits are intended for parts that are normally stationary but can be freely assembled or disassembled.

Locational Interference Fits

These fits are intended where accuracy of location is of prime importance, and for parts requiring rigidity and alignment with no special requirement for bore pressure.

Running and Sliding Fits

These fits provide a similar running performance, with suitable lubrication allowance, throughout the range of sizes.

Force and Shrink Fits

These types of interference fits are normally characterized by maintenance of constant bore pressures throughout the range of sizes.

Rectangular Coordinate Dimensioning

This type of dimensioning system is specifically used when computer-controlled production machines are used to manufacture parts. Notice in the examples that all dimensions are referenced from an origin or 0,0 location. The designer should consult with personnel in manufacturing to ensure that the origin is located in an appropriate position.

Clearance Fit

fit between mating parts having limits of size so prescribed that a clearance always results in assembly.