298 Part III: If Drawings Could Talk

Original dimension

Figure 14-1: Changing objects automatically updates dimensions.

How Do You Measure Up?

Start off with a quick exercise to introduce AutoCAD’s dimensioning functionality:

1.Start a new drawing, using the acad.dwt template file.

This puts AutoCAD into imperial mode, even if your installation is metric. This saves us lazy writers from having to duplicate everything for metric users.

2.Use the LINE command to draw a non-orthogonal line — that is, a line segment that’s not horizontal nor vertical.

Make the line about 6 units long, at an angle of about 30 degrees upward to the right.

3.Set a layer that’s appropriate for dimensions as current.

Okay, you started from a blank template, so it won’t have any specific layers, but we included this step as a gentle reminder. As we discuss in Chapter 6, correct procedure is normally to have dedicated layers for visible edges, hidden edges, text, dimensions, section lines, hatching, and so on.

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Chapter 14: Entering New Dimensions 299

4.Start the DIMLINEAR command by clicking the Dimension button on the left side of the Annotate tab’s Dimensions panel, or type DLI and then press Enter.

If the Dimension button isn’t displayed, click the down arrow at the bottom of whichever button is showing and then select Dimension.

AutoCAD prompts with

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 menu that appears. (See Chapter 7 for more about object snaps.)

AutoCAD prompts you:

Specify second extension line origin:

You must use object snaps when applying dimensions in order to make later editing work properly.

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 left-to-right direction — if you move the crosshairs above or below the line. It draws a vertical dimension — the length of the displacement in the up-and-down direction — if you move the crosshairs to the left or right of the line.

AutoCAD prompts you:

Specify dimension line location or

[Mtext/Text/Angle/Horizontal/Vertical/Rotated]:

7.Move the mouse to generate the type of dimension you want — horizontal or vertical — and then click wherever you want to place the dimension line.

AutoCAD draws the dimension.

When you’re specifying the dimension line location, you usually don’t want to object-snap to existing objects. Rather, you want the dimension line and text to sit in a relatively empty part of the drawing rather than have it bump into existing objects. If necessary, temporarily turn off running object snap (for example, click the OSNAP button on the status bar) to avoid snapping the dimension line to an existing object.

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300 Part III: If Drawings Could Talk

If you want to be able to align subsequent dimension lines easily, turn on Snap Mode and set a suitable snap spacing — more easily done than said! — before you pick the point that determines the location of the dimension line. See Chapter 4 for more information about Snap Mode. The DIMSPACE command can also be used to select several existing dimensions and then automatically space them equally.

8.Repeat Steps 4 through7 to create another linear dimension of the opposite orientation (vertical or horizontal).

9.Click the line to select it.

10.Click one of the grips at an end of the line and drag it around.

Watch in amazement as the dimensions automatically update, live and in real time, to reflect the current values as you move the mouse.

Your second reaction, after the shock and awe wear off, will probably be to say (hopefully not out loud if other people are within earshot), “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. . . .”

Why dimensions in CAD?

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