Dimensioning

Technical DrawingStuarts Draft High School

Introduction

When adding dimensions to drawings, you should follow certain practices. Over time, these methods have been found to be the best way to do things. Following are the common “rules” you need to follow when dimensioning.

Dimension lines should be spaced about 6 mm (about 1/4 in.) apart from each other and about 10 mm (about 3/8 in.) from the view.

If the aligned system is used, dimensions must read in line with the dimension line and from the lower and/or right-hand side of the sheet.

If the unidirectional system is used, all dimensions must read from the bottom of the sheet

Consider this…

On machine drawings, dimensions should be given in millimeters or in decimal inches, even if the drawings are of large objects such as airplanes or automobiles. Values are given to at least two digits, except when the value is 0. Single numbers from 1 to 9 use a decimal marker followed by a 0, for example, 3.0.

Notice that when all the dimensions are in millimeters or inches, the symbol marks are generally omitted. A note can be added to the drawing such as "All dimensions are in millimeters."

Special Note

Very large areas on architectural and structural drawings may be dimensioned in meters. In that case, whole numbers stand for millimeters and decimalized dimensions stand for meters. However, this is true only on architectural and structural drawings. Customary measure is also used. Any measurement over 12 in. is given in feet and inches.

Drawing Units

Sheet-metal drawings are usually dimensioned in millimeters or inches. Furniture and cabinet drawings are usually dimensioned in millimeters or inches.

More Drawing Units

When using customary measure, feet and inches are shown as 7'-3. Where the dimension is in even feet, it is written 7'-0.

Remember…

Dimensions should be positioned clearly and drawn with a light line weight so that the object’s view drawing stands out. The same dimension is not repeated on different views of the drawing.

Dimensions that are not needed should not be given. Figure A shows unnecessary dimensions. They have been omitted in Figure B. This is how the object should be dimensioned.

Overall dimensions should be placed outside the smaller dimensions. When the overall dimension is given, one of the smaller distances should not be dimensioned (see Fig. B) unless it is needed for reference.

On circular end parts, the center-to-center dimension is given instead of an overall dimension.

When you have to put a dimension within a sectioned area, leave a clear space for the number(s).

American National Standard practice is to avoid placing dimensions in the shaded area (see figure to the left) when the aligned system is used.

Where to put these things…

Dimensions should be given from centerlines, finished surfaces, or datums where needed.

Never, Never, Never…

Never use a centerline or a line of the drawing as a dimension line. Never have a dimension line that extends from a line of a view. Never place a dimension where it is crossed by a line.

Always…

Always give the diameter of a circle, not the radius. The symbol “” is used before the dimension, unless it is obviously a diameter. The radius of an arc should always be given with the abbreviation “R” placed after the dimension.

Coming Down the Home Stretch…

In general, dimensions should not be placed inside the view outlines. . Extension lines should not cross each other or cross dimension lines if this can be done without making the drawing more complicated. Do not dimension to hidden lines.

Last,but not least…

Remember… there are no hard-and-fast rules or practices that are not subject to change under the special conditions or needs of a particular industry. However, when there is a variation of any rule, there must be a good reason to justify it.