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.