AutoCAD 2005 for Dummies

(For example, ? inch is common in the architectural industry.) In any case, make sure you pick a height that’s not too small to read on your smallest check plot.

  AutoCAD 2005’s new background mask feature, described in Chapter 9, works for dimension text, too. To turn on masking, choose either Background or a specific color from the Fill Color drop-down list. When you do, AutoCAD hides the portions of any objects that lie underneath dimension text. (To ensure that dimension text lies on top of other objects, use the DRaworder or TEXTTOFRONT command — see Chapter 9 for more information.

Getting Fit

The Fit tab includes a bunch of confusing options that control when and where AutoCAD shoves the dimension text if it doesn’t quite fit between the dimension lines. The default settings leave AutoCAD in “maximum attempt at being helpful mode” — that is, AutoCAD moves the text, dimension lines, and arrows around automatically so that things don’t overlap. If these guesses seem less than satisfactory to you, try the modified settings shown in Figure 10-7: Select the Over the Dimension Line, without a Leader radio box under Text Placement and the Always Draw Dim Line between Ext Lines check box under Fine Tuning. (You can always move the text yourself by grip editing it, as I describe later in this chapter.)

Most important, the Fit tab includes the Use Overall Scale Of setting, as noted in Figure 10-8. This setting acts as a global scaling factor for all the other length-related dimension settings. Always set Use Overall Scale Of to the drawing scale factor of the current drawing.

Figure 10-8: Keep Fit and don’t forget the Scale.

  If your drawing includes areas of different scales, you can create multiple dimension styles, one for each scale. Alternatively, you can set the Use Overall Scale Of setting to 1.0 and draw dimensions in a paper space layout, rather than in model space. See the “Trans-spatial dimensioning” section, later in this chapter, for more information.

  The Use Overall Scale Of setting corresponds to the DIMSCALE system variable, and you’ll hear AutoCAD drafters refer to it as such. AutoCAD accepts zero as a special DIMSCALE setting for dimensioning in paper space layouts. Look up the DIMSCALE system variable in the AutoCAD online help system for more information about additional dimension scale options.

Using Primary Units

The Primary Units tab gives you incredibly — or maybe overly — detailed control over how AutoCAD formats the characters in the dimension text string. You usually want to set the Unit format and Precision and maybe specify a suffix for unit-less numbers, such as mm for millimeters. You may also change the Zero Suppression settings, depending on whether you want dimension text to read 0.5000, .5000, or 0.5. (“Zero Suppression!” also makes a great rallying cry for organizing your fellow AutoCAD drafters.)

Other style settings

If your work requires that you show dimensions in two different units (such as inches and millimeters), use the Alternate Units tab to turn on and control alternate units. If your work requires listing construction tolerances (for example, 3.5 mm +/–0.01), use the Tolerances tab to configure the tolerance format that you want.

  The New/Modify Dimension Style dialog box Tolerance tab settings are for adding manufacturing tolerances (for example, +0.2 or -0.1) to the text of ordinary dimensions — the kind of dimensions I cover in this chapter. AutoCAD also includes a separate TOLERANCES command that draws special symbols called geometric tolerances. If you need these symbols, you probably know it; if you’ve never heard of them, just ignore them. Look up “Geometric Tolerance dialog box” on the Index tab in the AutoCAD online help system for more information.

Drawing Dimensions

After you’ve copied or created a suitable dimension style, you’re ready to dimension. Fortunately, adding dimensions to a drawing with existing dimension styles is usually pretty straightforward.

When you want to dimension something in AutoCAD, you can either select the object, such as a line or polyline segment, or select points on that object, such as the endpoints of the line or polyline segment. If you select an object, AutoCAD finds the most obvious points on it to dimension, such as the endpoints of a line. If you choose to select individual points instead, use object snaps (see Chapter 4). The points that you pick — or that AutoCAD finds for you — are called the origins of the dimension’s extension lines. When you change the size of the object (for example, by stretching it), AutoCAD automatically moves the dimension’s origin points and updates the dimension text to show the new length.

  If you don’t use object snaps or another AutoCAD precision technique to choose dimension points, the dimension text probably won’t reflect the precise measurement of the object. This lack of precision can cause serious problems. When in doubt, osnap to it!

  When you set up a new drawing, make sure that you change the Use Overall Scale Of setting on the Fit tab in the New/Modify Dimension Style dialog box so that it matches the drawing scale factor. Before you draw any dimensions in a drawing that you didn’t set up, check this setting to make sure it’s correct.

  The AutoCAD dimension drawing commands prompt you with useful information at the command line. Read the command line prompts during every step of the command, especially when you’re trying a dimensioning command for the first time.

Lining up some linear dimensions

Linear dimensions are the most common type of dimensions, and horizontal and vertical are the most common of those. The following example demonstrates all the important techniques for creating horizontal and vertical linear dimensions, as well as aligned dimensions (which are similar to linear dimensions): 

1. Use the LINE command to draw a nonorthogonal line — that is, a line segment that’s not horizontal or vertical.

An angle of about 30 degrees works well for this example.

If you want to apply dimensioning to an object other than a line, use these steps as a general guideline, filling in the appropriate commands and data as applicable to your drawing. 

2. Set a layer that’s appropriate for dimensions current.

See Chapter 4 for details. 

3. Set a dimension style that’s appropriate for your needs current.

Choose an existing dimension style from the Dim Style Control drop-down list on the Styles toolbar, or create a new style by using the procedure in the section, “Creating and managing dimension styles,” earlier in this chapter.

4. Choose Dimension>Linear or click the Linear Dimension button on the Dimension toolbar.

AutoCAD prompts you:

Specify first extension line origin or <select object>:

5. To specify the origin of the first extension line, snap to the lower-left endpoint of the line by using endpoint object snap.

If you don’t have endpoint as one of your current running object snaps, specify a single endpoint object snap by holding down the Shift key, right-clicking, and choosing Endpoint from the cursor menu. (See Chapter 4 for more about object snaps.)

AutoCAD prompts you:

Specify second extension line origin:

6. To specify the origin of the second extension line, snap to the other endpoint of the line by using endpoint object snap again.

AutoCAD draws a horizontal dimension — the length of the displacement in the