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Application of Dimensions

Dimensions are applied by means of dimension lines, extension lines, or a leader from a dimension, note, or specification directed to the appropriate feature. Some of the standard rules to be followed when you are drawing DIMEN-SION LINES are as follows:

1. The breaking of dimension lines for insertion of numerals, as shown in figure 10-4,

Figure 10-4.-Applications of dimensions and dimension lines: A. Breaking dimension lines for insertion of numerals; B. Grouping lines for uniform appearance; C. Proper spacing of dimension lines from object.

view A, is the preferred method of drawing dimension lines in many forms of drafting. However, for construction drawings, it is permissible, and in fact customary, to draw dimension lines from one extension line to another without breaking them. The numerals are then placed above the dimension line and parallel to the direction of measurement. This method is easier and saves considerable time.

2. Dimension lines are to be aligned if practical and grouped for uniform appearance, as shown in figure 10-4, view B. The space between the first dimension line and the object line should be not less than 10 mm, minimum; the space between succeeding parallel dimension lines should be not less than 6 mm, minimum, as shown in figure 10-4, view C. Where there are several parallel dimension lines, you may stagger the numerals for easier reading.

When using U.S. standards, you should ensure that the minimum space between the first dimension line and the object line is 3/8 in., and the succeeding parallel dimension lines are spaced at least 1/4 in. apart.

3. An angle is to be dimensioned with an arc drawn so that its center is at the apex of the angle and the arrowheads terminate at the extension of the two sides, as shown in figure 10-3.

4. Crossing dimension lines should be avoided insofar as possible. If crossing them is unavoidable, dimension lines are to be unbroken. Figure 3-23, chapter 3, shows the characteristics of dimension lines.

As explained in chapter 3, extension lines (also called projection lines) are used to indicate the extension of a surface or point to a location outside the outline of the object (or view). They are usually drawn perpendicular to dimension lines. Where space is limited, you may draw extension lines at an oblique angle. Figure 10-5, view A, clearly shows this application. You should also minimize the crossing of extension lines over one another and over dimension lines by placing the shortest dimension line closest to the outline of the object, as shown in figure 10-5, view B. Where extension lines cross arrowheads or dimension lines close to arrowheads (fig. 10-5, view C), a break in the extension line is advisable. For examples in the proper use of extension lines, refer to chapter 3, figures 3-30 and 3-31. LEADERS (or leader lines), also explained in chapter 3, direct dimensions, notes, or symbols to the intended place on the drawing.

Figure 10-5.-Applications of extension lines: A. Where space is limited; B. Minimizing crossing of extension lines; C. Where extension lines break.


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