Dual dimensioning is the use of dimensions in both the U. S. Customary (inch) and SI (metric) systems of measurement on drawings. This dimensioning technique is being used by many manufacturers during the transition period from the U. S. Customary (inch) system of measurement to the metric system. Manufacturers who market their products to international consumers also use dual dimensioning.

Dual-dimensioned prints are more costly to prepare; however, they permit greater flexibility in their appli­cation. Through the use of dual-dimensioned prints, parts that are to be manufactured to metric dimensions may be produced by using machines that are graduated in the inch system of measurement. Also, machines that are designed to manufacture metric-dimensioned parts may be used to produce parts that are specified in inches.

Dual dimensions are equivalent to one another to a degree of accuracy that will permit functional inter­changeability of parts, regardless of the system of measurement used in producing them. The degree of accuracy between inch and metric dimensions is determined by the number of decimal places to which each dimension is rounded off. This same principle applies to tolerance dimensions.

The metric unit of measurement most commonly used on dual-dimensioned prints is the millimeter. How­ever, the meter is also used in cases where dimensions are so large that it is impractical to show or perform the measurement in millimeters. For example, 18000 millimeters is best shown as 18 meters. Once a metric unit

FIGURE 24.1 ■ Variations of the positioning method, utilizing a horizontal line, for dual dimensioning.

FIGURE 24.2 ■ Variations of the positioning method, utilizing a slash line, for dual dimensioning.

OR INCH

.

FIGURE 24.4 ■ Application of positioning method for dual dimensioning of a drawing using horizontal and slash lines.

is selected, it is used consistently throughout the drawing. The accompanying U. S. Customary unit of mea­surement may be given as a common or decimal fraction, but the decimal fraction is preferred. The application of common and decimal fractions is shown in Figure 24.4.

There are two standard methods used to distinguish the U. S. Customary (inch) unit from the SI (metric) unit when dual dimensions are applied. One method is known as the positioning method. When this method is used, the inch and metric dimensions are positioned in specific locations with respect to one another. The second method is known as the bracket method. In this method, either the inch or metric dimension is enclosed in brackets. Either method may be used on a drawing, but not both.

When the positioning method is used, the units are separated either by a horizontal line (usually a dimen­sion line), Figure 24.1; or a slash line, Figure 24.2. Note the optional locations for the dual dimensions.

The positioning arrangement selected for dual dimensioning is indicated by a note on the drawing, Figure 24.3.

FIGURE 24.5 ■ Symbols for designating angle of projection.

Refer to Figure 24.4 for a review of the application of the positioning method for dual dimensioning. Note the use of both the horizontal line and the slash line techniques for separating dimensions. The method selected will depend on which is most convenient for presenting a particular dimension.

In reviewing Figure 24.4, note that the symbol 0 is used to indicate a diameter rather than the abbreviation “DIA.” However, it is standard practice to use “DIA” in a note. Note in the lower right-hand corner of the drawing that a symbol is shown to indicate the angle of projection. This is standard practice for dual-dimensioned drawings.

The angle of projection is indicated on a dual­dimensioned drawing when it is to have interna­tional applications.

Countries that use the metric system nor­mally prepare drawings using first-angle projec­tion. Drawings prepared in countries where the U. S. Customary system is used, show the views arranged in third-angle projection. To avoid con­fusion in the interpretation of dual-dimensioned drawings, the angle of projection symbol is shown,

Figure 24.5. Observe that the views used in the symbol for first-angle and third-angle projection are the same, except that they are arranged differently.

In the bracket method for dual dimensioning, either the inch or metric dimension is enclosed in brackets. The bracketed unit may be located above or below, or to the right or left of the equivalent unit, Figure 24.6. Observe that one or both of the units is centered on the dimension line.

A note is shown on the print to indicate which unit is enclosed in brackets, Figure 24.7.

Refer to Figure 24.8 for a review of the application of the bracket method for dual dimensioning. Note that all of the dimensions are shown in decimal fractions. However, where common fractions are normally used for nominal sizes, such as thread and pipe sizes, the dimensions are shown as common fractions.

Dimensioning a drawing with decimal fractions has become standard practice. It is generally preferred for new drawings because it is easier to convert to metric dimensions, if necessary.

FIGURE 24.8 ■ Application of bracket method for dual dimensioning in a drawing.

Drawings dimensioned in common fractions are generally those that were not dual dimensioned originally. As the need arose, equivalent millimeter dimensions were added to the common fractions.

An alternative to the positioning and bracket methods for dual dimensioning is to provide a conversion chart on the print. Only one unit of measurement or a series of letters is shown on the print to represent the dimensions. A chart located on the print gives the equivalent dimension for the dimension or letter shown. In some cases, it is more practical to attach the conversion chart to the print as a separate document. Such charts can be computer-generated, thus simplifying the dual-dimensioning process.

Examples of methods used for displaying conversion tables are shown in Figure 24.9 and Figure 24.10. Note that dimensions shown in conversion charts are arranged in order of their value, usually from the small­est dimension to the largest. The reverse order may also be used.

In some cases, although conversions are necessary, they are not shown on the print. A note specifying the conversion factor to be used is included on the print. A typical example is:

Note: Convert inch dimensions to millimeter equivalents using the factor 25.4 (inch X 25.4 = millimeter equivalent), or inch X 22/7 = millimeter equivalent as a fraction.

Another method of converting dimensions is the direct reading of dual-dimensioned machine dials, Figure 24.11. This method eliminates the need for dual-dimensioned prints or conversion charts.

Note in Figure 24.9 and Figure 24.10 that a separate conversion chart for the fillet weld sizes is given. As shown on the chart, the millimeter dimension for the fillet size is rounded off to a whole number and the inch conversion is shown as a decimal fraction with a zero preceding the decimal point. These are standard practices applied to fillet conversion charts found on dual-dimensioned drawings.

Additional standards and practices applicable to dual-dimensioned drawings are:

■ General tolerances applicable throughout the drawings are indicated by a note and in dual dimensions.

Example: Unspecified tolerances inch^ ±.03/0.8 ^mm

TABLE OF INCH/MILLIMETER EQUIVALENTS
INCH	.03	.375	со о о о о о + ! о о Lf) ■Qi	.750	1.000	1.000	0 1.125
mm	0.76	9.53	0 '2.70:° °0в0	19.05	25.40	25.40	0 28.58
INCH	1.250	1.750	0 2.000	3.000		0 4.375		6.000
mm	31.75	44.45	0 50.80	76.20		0 110.13		152.40

FIGURE 24.10 ■ Application of conversion table for dual dimensioning by use of letters.

■ Plus and minus tolerances are shown with the same number of decimal places.

+0.15

25.4

Example:

-0.10

+0.15

0.1

not 25.4

■ Angle dimensions indicated in degrees are common to the inch and metric systems, and, therefore, are not normally shown as dual dimensioned (in radians).

■ Welding symbols are not usually dual dimensioned. Ei­ther the U. S. Customary unit or metric unit of measure­ment is applied, Figure 24.12. When an equivalent unit is required, a conversion table is included. Figure 24.10 and Figure 24.12(c) and (d) illustrate dual dimensioning followed by some companies. The same practice applies to testing symbols presented in Unit 25.

■ If a metric dimension is a whole number (254), a deci­mal point and a 0 are not required.

Example: 254 not 254.0

FIGURE 24.12 ■ Dimensioning practices used for welding symbols.

■ If a conversion is needed and not shown on the print, the inch dimension may be multiplied by 25.4 or 22/7 to obtain the millimeter equivalent.

■ To convert a millimeter dimension to inches, divide it by 25.4 or 22/7.

■ In converting an inch value to a millimeter value, it is standard practice to show the millimeter value to one less decimal value than the original inch value.

Example: .375" = 9.53 mm

This rule does not always apply to dimensions shown as common fractions. A millimeter conversion of a frac­tional dimension to two decimal places may exaggerate the precision that is required. Therefore, the number

of decimal places shown will vary with the accuracy intended.

Example: 3з8" may be shown as a conversion to 9.53 mm, 9.5 mm, or 9 mm.

■ If a common fraction is to be converted to millimeters and the required degree of accuracy is unknown, an acceptable rule is to show the millimeter equivalent to two decimal places.

■ When a metric dimension is less than a full unit, a 0 precedes the decimal point.

Example: 0.39 mm not.39 mm