Types of dimensioning

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Units of Measure

For uniformity, all dimensions in this Standard are given in SI units. However, the unit of measure selected should be in accordance with the policy of the user.

SI (Metric) Linear Units

The SI linear unit commonly used on engineering drawings is the millimeter.

U.S. Customary Linear Units

The U.S. Customary linear unit commonly used on engineering drawings is the decimal inch.

Identification of Linear Units

On drawings where all dimensions are in millimeters or all dimensions are in inches, individual identification of linear units is not required. However, the drawing shall contain a note stating “UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN MILLIMETERS (or IN INCHES, as applicable).”

Combination SI (Metric) and U.S. Customary Linear Units

Where some inch dimensions are shown on a millimeterdimensioned drawing, the abbreviation IN shall follow the inch values. Where some millimeter dimensions are shown on an inch-dimensioned drawing, the symbol mm shall follow the millimeter values.

Angular Units

Angular dimensions are expressed in both degrees and decimal parts of a degree or in degrees, minutes, and seconds. These latter dimensions are expressed by the following symbols: (a) degrees: ° (b) minutes: ' (c) seconds: " Where degrees are indicated alone, the numerical value shall be followed by the symbol. Where only minutes or seconds are specified, the number of minutes or seconds shall be preceded by 0° or 0°0', as applicable. Where decimal degrees less than one are specified, a zero shall precede the decimal value. See Fig. 1-3.

Decimal dimensioning shall be used on drawings except where certain commercial commodities are identified by standardized nominal size designations, such as pipe and lumber sizes.

Millimeter Dimensioning

The following shall be observed where specifying millimeter dimensions on drawings: (a) Where the dimension is less than one millimeter, a zero precedes the decimal point. See Fig. 1-4. (b) Where the dimension is a whole number, neither the decimal point nor a zero is shown. See Fig. 1-4. (c) Where the dimension exceeds a whole number by a decimal fraction of one millimeter, the last digit to the right of the decimal point is not followed by a zero. See Fig. 1-4. NOTE: This practice differs for tolerances expressed bilaterally or as limits. See paras. 2.3.1(b) and (c). (d) Neither commas nor spaces shall be used to separate digits into groups in specifying millimeter dimensions on drawings.

Decimal Inch Dimensioning

It uses a symbolic language rather than words on engineering drawings that explicitly describes nominal geometry and its allowable variation. The GD&T system has a strong mathematical base.

Decimal Points

It uses a symbolic language rather than words on engineering drawings that explicitly describes nominal geometry and its allowable variation. The GDecimal points must be uniform, dense, and large enough to be clearly visible and meet the reproduction requirements of ASME Y14.2M. Decimal points are placed in line with the bottom of the associated digits.D&T system has a strong mathematical base.

Conversion and Rounding of Linear Units

For information on conversion and rounding of U.S. Customary linear units, see IEEE/ASTM SI 10.

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