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Not a problem. AutoCAD controls the look of dimensions by means of dimension styles, just as it controls the look of text with text styles, and tables with table styles. In fact, AutoCAD also uses text styles to control the appearance of the text in dimensions. We cover dimension styles in more detail later in this chapter, but suffice it to say that AutoCAD has about 80 variables that can be used to warp dimensions into just about any perversion that your industry or company can imagine.

Meanwhile, look at the other basic types of dimension that AutoCAD can apply, as shown in Figure 14-2.

DIMDIAMETER(DDI)

DIMARC(DAR)

Figure 14-2: Examples of dimensioning commands.

A Field Guide to Dimensions

AutoCAD provides several types of dimensions and commands for drawing them; most commands are shown in Figure 14-2. These commands are found on the Dimensions panel of the Annotate tab of the Ribbon. If you can’t find the button you want, it’s probably hidden in the drop-down list under the larger button on the left side of this panel. This panel remembers the last button used, so at any given time, it may be displaying any one of the following buttons:

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302 Part III: If Drawings Could Talk

Linear: DIMLINear. Indicates 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: DIMALIgned. 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.

You don’t always have to pick points when placing dimensions. Watch the Command window; it may ask you to

Specify first extension line origin or <select object>:

If you press Enter instead of picking the first point, AutoCAD will ask you to select an object. And when you do, it automatically selects each end of the object for you.

Angle: DIMANGle. Indicates 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.

Radial: DIMDIAmeter, DIMRADius. This has nothing to do with AM or FM electromagnetic radiation. 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; refer to Figure 14-2. If you position the text outside the curve, AutoCAD (by default) draws a little cross at the center of the circle or arc. 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).

You can also use DIMLINear to dimension a circle or arc. Trying to pick the endpoints of a circle can keep you busy for several hours, but if you press Enter at the first prompt and then select an arc or circle, it will apply a linear dimension across the diameter.

Arc Lenth: DIMARC. Measures the length of an arc as though it were stretched out straight, and then applies a curved dimension line concentric with the arc.

Jog: DIMJOGGED. No, this has nothing to do with dimensioning your morning exercise run. Standard drafting practice when dimensioning arcs is to create a dimension line that starts at the center of the arc and proceeds outward to the arc itself. A problem arises, however, when dimensioning arcs with a very large radius. The center may be at an inconvenient location

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where the dimension line would interfere with other geometry — or worse yet, may be well beyond the edge of the final drawing. DIMJOGGED neatly solves this problem for you. It works almost exactly like DIMARC except that it asks you to specify two extra points. After you select the arc (or circle, strangely enough), it then asks you to specify a center override point and then for a location for the jog itself. The resulting dimension line consists of three segments, starting at the center override point: The first and last point to the real center, and the middle segment connects the ends of the other two to put a jog in the line.

Ordinate: DIMORDinate applies X or Y coordinate dimensions from an origin point. Certain types of mechanical parts can have a great many holes and other features. The drawing would get impossibly cluttered if you were to apply two traditional dimensions to show the X and Y locations of each detail. Common practice in this situation is to use ordinate dimensioning. Each ordinate dimension appears simply as a single extension line and a number showing the X or Y distance from the origin of the drawing or from the origin of the current User Coordinate System (UCS) if there is one. We cover UCSs in Chapter 7. Normal practice is to specify the lower-left corner of the part to be the origin. The X and Y values are usually aligned neatly in a row and column along the edge of the part. There are no dimension lines or arrowheads.

These basic dimension commands are also found in the upper-right corner of the Annotation panel of the Home tab on the Ribbon.

Leaders used to be placed by using a command from the dimensioning set, but they are now much more powerful and versatile, and so are considered to be text annotations. We cover them in Chapter 13.

The lazy drafter jumps over to the quick dimension commands

Three dimensioning commands can help you place multiple dimensions very quickly. They are found on the right side of the Dimensions panel of the Annotate tab of the Ribbon.

Continue: DIMCONTinue. Having placed one linear dimension, you can now carry on to place a series of end-to-end dimensions. AutoCAD automatically picks the end of the previous dimension as the start of the next one, so you need to pick the end of only the next one.

Base: DIMBASEline. This works much like DIMCONTinue except that AutoCAD selects the start of the previous one as the start of the next one. You thus end up with a series of dimensions, all measuring from a common starting point.

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Quick Dimensions: QDIM. Interestingly, this is one of the very few dimensioning commands that doesn’t start with DIMxxx. When you invoke it, it invites you to select objects. The best bet usually is to use one or more Window selections (pick left to right), each of which completely surrounds several objects. Now, when you press Enter to continue, AutoCAD automatically applies DIMLINear dimensions wherever it can (and you might be surprised at some of those places!). It can place dimensions in several formats, including continue, baseline, staggered, ordinate, and several others. For convenience in repeat operations, it remembers the last mode you used.

If you want to be superefficient, memorize the three-letter keyboard aliases for the dimension commands you use the most. The aliases for the most common commands are shown in Figure 14-2.

The AutoCAD dimensioning commands prompt you with useful information at the command window or dynamic-input prompts. Read the prompts during every step of the command, especially when you’re trying a dimensioning command for the first time. When all else fails. . . .

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.

The associative dimensions we’re talking about here first appeared in AutoCAD 2002. Before that, AutoCAD had a more primitive kind of dimensioning. Dimensions were single objects, and they did update if you stretched an object if you were very careful to include the dimension itself in the crossing selection for the STRETCH command. Here’s where things can get a bit confusing: AutoCAD used to call these old-style, single-object dimensions associative but now calls them non-associative. And what used to be called non-associative dimensions before AutoCAD 2002 are now called exploded dimensions — but

if you Explode a dimension, you get four lines, two arrowheads, and a piece of text. Sometimes you can’t tell the players even with a program. For more

information about how to determine which kind of dimension AutoCAD draws, see the “Controlling and editing dimension associativity” section, later in

this chapter. We mention the old style here only because you will probably encounter it in old drawings.

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