Discovering New Dimensions

Before digging into the techniques that you use to create dimension styles and the dimensions themselves, here's some AutoCAD dimensioning terminology that you need to understand. If you're already familiar with CAD dimensioning lingo, just skim this section and look at the figures in it. Otherwise, read on.

Anatomy of a dimension

AutoCAD uses the names shown in Figure 14-2 and described in the following list to refer to the parts of each dimension:

• Dimension text: The numeric value that indicates the true distance or angle between points or lines. Dimension text can also include other information in addition to or instead of the number. For example, you can add a suffix, such as TYP., to indicate that a dimension is typical of several similar configurations. AutoCAD's default settings for dimension styles center the dimension text vertically and horizontally on the dimension lines, as shown in Figure 14-2, but you can change those settings to make the text appear in a different location — sitting over an unbroken dimension line, as shown previously in Figure 14-1, for example. See the section “Adjusting style settings,” later in this chapter, for instructions.

  

  Figure 14-2: The parts of a dimension.

• Dimension lines: In linear dimensions, the dimension lines indicate the true distance between points. Linear dimension lines can be horizontal, vertical, rotated at a specific angle, or aligned with the object being dimensioned. Angular dimensions have curved dimension lines with the center of the curve at the vertex point of the objects being dimensioned. For radius and diameter dimensions, the dimension line simply points at the object being dimensioned (see Figure 14-3 for examples of these dimension types).
• Arrowheads: The dimension's arrowheads appear at the end or ends of the dimension lines and emphasize the extent of the dimensioned length. AutoCAD's default arrowhead style is the closed, filled type shown in Figure 14-2, but you can choose other symbols, such as dots or tick marks, to indicate the ends of the dimension lines. (Don't get ticked off, but AutoCAD calls the line ending an arrowhead even when, as in the case of a tick mark, it doesn't look like an arrow.)

  

  Figure 14-3: Common types of dimensions.

• Extension lines: These extend outward from the definition points (also known as defpoints) that you select (usually by snapping to points on an object) to the dimension lines. By drafting convention, a small gap usually exists between the defpoints and the beginning of the extension lines. Also by convention, the extension lines usually extend just past the dimension lines — see Figures 14-2 and 14-3 for examples. You can make a set of dimensions look tidier by assigning fixed lengths for the extension lines. And if you need to dimension to circles or centerlines, you can assign dash-dot linetypes to either or both extension lines.
• Defpoints: When you create any kind of dimension, AutoCAD places one or more definition points (universally known as defpoints) on a special layer named (what else?) Defpoints, which the program creates when a dimension command is issued for the first time. These points are usually invisible because the objects being dimensioned are on top of them, but you can see where they sit by selecting a dimension to turn on its grips. The grips on the objects being dimensioned are on the dimension's definition points. Because you wouldn't want these points to appear when you plot your drawings, the Defpoints layer has a special property: Nothing created on the Defpoints layer will print.

A field guide to dimensions

AutoCAD provides several types of dimensions and commands for drawing them. Figure 14-3, earlier in this chapter, shows the most common types, and the following list describes them:

• Linear dimensions: Indicate the linear extent of an object or the linear distance between objects. Most linear dimensions are either horizontal or vertical, but you can draw dimensions that are rotated to other angles, too.
• Aligned dimensions: Similar to linear dimensions, but the dimension line tilts to the same angle as a line drawn through the origin points of its extension lines.
• Radial dimensions: A radius dimension shows the radius of a circle or an arc, and a diameter dimension calls out the diameter of a circle or an arc. You can position the dimension text inside or outside the curve, as shown in Figure 14-3. If you position the text outside the curve, AutoCAD (by default) draws a little cross at the center of the circle or arc. As shown in Figures 14-2 and 14-3, AutoCAD automatically adds the diameter and radius symbols to the appropriate dimension type.

  If you're wondering when you should use which, because radius and diameter dimensions seem to do the same thing, the convention in most drafting disciplines is to use diameter dimensions for whole circles (for example, a hole) and radius dimensions for part circles or arcs (for example, a fillet).

• Angular dimensions: Indicate the angular measurement between two lines, the two endpoints of an arc, or two points on a circle. The dimension line appears as an arc that indicates the sweep of the measured angle.

Other types of dimensions and dimension-like annotations you can add to AutoCAD objects include arc length and ordinate dimensions, tolerances, inspection dimensions, and center marks. For more information about other, less frequently used kinds of dimensions, click Help to open the Exchange window's Help tab, then choose User's GuideAnnotate DrawingsDimensions and Tolerances.

Dimension associativity

By default, AutoCAD groups all the parts of each dimension — the extension lines, dimension lines, arrowheads, and text — into a special associative dimension object. Associative means two things:

• The different parts of the dimension function as a single object. When you click any part of the dimension, AutoCAD selects all its parts.
• The dimension is connected to the points on the object that you specified when you drew the dimension. If you change the size of the object (for example, stretch a line), the dimension updates appropriately — the lines and arrows move, and the text changes to reflect the line's new size.

Finding your dimension tools

Both the Annotation panel on the Ribbon's Home tab and the Dimensions panel on the Annotate tab provide access to AutoCAD's most frequently used basic dimensioning commands. (If you're an AutoCAD Classic workspace user, you'll find them on the classic Dimension menu and Dimension toolbar.)

Here's how to turn on the Dimension toolbar in a Ribbon-based workspace:

1. Click the View tab on the Ribbon.
2. On the Windows panel, click Toolbars.

   AutoCAD displays a menu of loaded customization files. If your system does not have the Express Tools installed, or if you're using AutoCAD LT, you may see only “AutoCAD” (or “AutoCAD LT”) on this menu. If you're working for a large company, you may well see additional custom menus on this list.

3. Click AutoCAD or AutoCAD LT to display a list of classic toolbars.
4. Choose Dimension from the menu.

   AutoCAD closes the Toolbars menu and the classic Dimension toolbar appears either floating or docked on an edge of the drawing area.