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You can place your dimensions in paper space.

This was introduced as the second-generation method in an attempt to overcome the scaling issue. You can reach from paper space through a layout viewport and snap onto the model space objects. Dimensions remain associative and will usually change when the model space objects change.

•Advantage: You don’t need to set a dimension scale factor to suit the drawing scale. The dimension scale factor is always 1:1 because paper space layouts are usually plotted full-size, and the viewport scale sets the model space scale factor.

•Disadvantages: The dimensions aren’t available when editing in model space, so you’re flying blind. In addition, the associative connections between the paper space dimensions and the model space objects are very fragile and are easily broken. There is a repair command, but it’s a nuisance to have to use it.

You can place dimensions in model space by using annotative dimensions.

This is the newest, latest, greatest, and best way of placing dimensions. Annotative dimensions automatically size themselves to suit the drawing scale that you choose. We discuss annotative text in Chapter 13, annotative hatching in Chapter 15, and annotative dimensions later in this chapter.

•Advantages: Dimensions are available in model space while you’re drawing and editing, you don’t need to mess with dimension scale factors, and applying dimensions to drawing details that are at a different scale from the main drawing becomes trivial.

•Disadvantage: Annotative dimensions have been slow to catch on because people are stuck in the older ways (“That’s how we’ve always done it!”) and because they don’t understand a couple of subtleties about them that we reveal in Chapter 13. If you’re the first in your office to adopt annotative dimensions, then everyone else will hail you as the Great AutoCAD Guru and will be pestering you to help them learn how to use them.

The Latest Styles in Dimensioning

Near the beginning of this chapter, we show a quick exercise on placing dimensions. At the end of it, you perhaps said to yourself, “Yes, that’s cute, but we don’t dimension to four places of decimal, and normal text in our drawings uses a different font, and we need to dual-dimension in imperial and metric units, and some of our dimensions need to show manufacturing tolerances. . . .”

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Chapter 14: Entering New Dimensions 307

Now that we’ve progressed this far in dimensioning, you’re probably still asking yourself the same question. Note that talking to yourself isn’t a problem, and arguing with yourself is getting borderline, but losing those arguments is a definite sign of trouble.

In this section, we discuss how to set those factors that control the format and appearance of dimensions, but we start with a brief anatomy lesson. AutoCAD uses the names shown in Figure 14-3 and described in the following list to refer to the parts of each dimension:

Dimension line

Arrowhead Dimension text

Figure 14-3: The parts of a 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; you can add manufacturing tolerances; and you can show dual dimensioning in alternate units.

Dimension lines: In linear dimensions, the dimension lines indicate the true distance between points. 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

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308 Part III: If Drawings Could Talk

line simply points at or through the center of the object being dimensioned.(See Figure 14-2 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 14-3, 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.)

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 — refer to Figure 14-3 for examples. AutoCAD makes

dimensions look tidier by assigning fixed gap sizes and projection lengths for the extension lines, and if you need to dimension to circles or center lines, 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 ever print.

Because of the Defpoints layer’s nonprinting property, experienced users have been using it for years as a place to put sketches, guidelines, and important notes (for example, “Don’t forget to buy bread on the way home!”). We don’t endorse this practice because a convoluted relationship exists between the Defpoints layer and Layer0. Namely, it can be very hard to tell what’s

on which layer, and that can make the drawing harder to edit. If you want a “scratch” layer for those important notes, create one named Scratch and set it to NoPlot in the Layer Properties Manager palette. (For more about layers and the Layer Properties Manager, see Chapter 6.)

In the early days, AutoCAD controlled the appearance of dimensions by using about 20 dimensioning system variables. For example, to switch dimension text from showing decimal values to showing fractions, the setting of DIMLUNIT would be changed from 2 to 5. Similarly, DIMDEC sets the number of places of decimal, or the smallest fraction (would you believe 1/256 of an inch?). We discuss system variables in Chapter 26.

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The problem was that every time you wanted to switch back and forth between decimals and fractions, you had to change these two variables. To set a system variable, or SysVar, you enter its name, followed by a suitable value as determined from Help. The only reason we mention this is because you’re bound to run into older drawings that were done this way.

The good news is that AutoCAD later added named dimension styles. You can now set up a decimal style, a fractional style, a style with specific manufacturing tolerances, and so on. All it takes is a couple of quick mouse clicks to switch between them.

Some people will try to warn you that AutoCAD dimensioning is a big, complicated, difficult subject. Don’t be alarmed, however. The basic principles are actually quite simple. The problem is that every industry has its own dimensioning conventions, habits, and quirks. As usual, AutoCAD tries to support them all — and, in so doing, makes things a bit cumbersome for everyone.

The good news is that you should have to adjust things to suit your specific industry or company only once, and then all dimensions will suit the specified standard. The really good news is that it usually takes only a bit of finetuning of the default settings to cover most of your dimensioning needs.

Dimension styles are saved within the current drawing. The really, really good news is that you can save this drawing as a template file (we cover templates in Chapter 4) so that all new drawings created from this template will have the dimension styles predefined. The really, really, really good news is that you’re working in an office where someone has already set up suitable dimension styles. If this is the case, you can skim over the next section a little more quickly.

You add dimensions to a drawing after you’ve drawn at least some of the geometry; otherwise, you won’t have much to dimension! (Well, duh.) Your dimensioning and overall drafting efficiency improve if you add dimensions in batches rather than draw a line, place a dimension, draw another line, place another dimension. . . .

Creating and managing dimension styles

A dimension style (or DimStyle for short) is a collection of drawing settings called dimension variables (or DimVars for short), which are a special class of the system variables that we describe in Chapter 26.

If you do need to create your own dimension styles or you want to tweak existing ones, use the DIMSTYLE command. You can invoke it by clicking the small diagonal arrow in the lower-right corner of the Dimensions panel of the Annotate tab of the Ribbon. This brings up the Dimension Style Manager dialog box, as shown in Figure 14-4.

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310 Part III: If Drawings Could Talk

Figure 14-4: Yet another manager, this one for dimension styles.

Every drawing comes with a default dimension style named Standard (for imperial [feet-and-inches] drawings) or ISO-25 (for metric drawings) and a matching Annotative style. We cover Annotative dimensions later in this chapter. Although you can use and modify the Standard or ISO-25 style, we suggest that you leave them as-is and create your own dimension style(s) for the settings that are appropriate to your work. This approach ensures that you can use the default style as a reference. More important, it avoids a

potential naming conflict that can change the way your dimensions look if the current drawing gets inserted into another drawing. (Chapter 17 describes this potential conflict.)

When you install AutoCAD, it checks with Windows to see what country was selected when Windows was installed, and then sets its measurement system accordingly. If you’re in the United States or one of the few other (backward) countries that still use imperial units, the default dimension style is Standard; most of the rest of the world defaults to ISO-25. Canada, on the other hand, is nominally metric, but most people still use imperial. The system variable MEASUREINIT controls the default action. If set to 0 (zero), AutoCAD uses imperial units; if set to 1, metric units are used. Among other things, this setting also affects text styles (see Chapter 13), hatching (see Chapter 15), and noncontinuous line types (see Chapters 4 and 6).

Starting a new drawing from an ISO template (for example, acadiso.dwt) forces everything to metric, and starting a new drawing from a non-ISO template (for example, acad.dwt) forces everything to imperial, regardless of the setting of MEASUREINIT.

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