Dimension Lines

Dimension lines are solid lines similar in weight to extension lines that are used to indicate length. They are drawn from one extension to the next, representing the distance between the extension lines. Once the external shape and internal features of a part are represented by a combination of lines, further information is provided by dimensions. Fractional, decimal, and metric dimensions are used on drawings to give size descriptions. Each of these three systems of dimensioning is used throughout the text.

Dimension lines are fine lines that are often broken at the dimension and ending with arrowheads, dots, or a small diagonal line (Figures 3.6, 3.7A and B). The tips or points of these arrowheads indicate the exact distance, referred to by a numerical dimension placed either at a break in the line or directly above the lines near their center.

Figure 3.5B Center lines indicate the centers of circles, arcs, and symmetrical objects.

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Figure 3.5B Center lines indicate the centers of circles, arcs, and symmetrical objects.

Different types of arrowheads are used in dimensioning. The point or tip of the arrowhead touches the extension line. The size of the arrow is determined by the thickness of the dimension line and the size of the drawing. Closed and open arrowheads are the two shapes generally used. The closed arrowhead is preferred. The extension line usually projects 1/16 inch beyond a dimension line. Any additional length to the extension line is of no value in dimensioning.

When dimensions can be added together to come up with one overall dimension, they are known as chain dimensions. Chain dimensions are usually expressed in a single line whenever possible (Figure 3.7B).

Figure 3.6 Extension lines are typically used for dimensioning.

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