Drawing Linear Dimensions


The most common type of dimension you'll be using is the linear dimension, an orthogonal dimension measuring the width and length of an object. AutoCAD provides three dimensioning tools for this purpose: Linear, Continue, and Baseline. These tools are readily accessible from the Dimension drop-down list.

Placing Horizontal and Vertical Dimensions

Linear dimensions are those that are aligned either vertically or horizontally. They constitute the bulk of dimensions in most projects. The following steps describe how to apply linear dimensions.

Using osnaps is crucial in producing accurate results in all dimensioning tasks.

  1. Choose Dimension è Linear from the drop-down menu, or enter dli at the command prompt.

  2. At the Specify first extension line origin or <select object>: prompt, Shift+right-click, and use an osnap to select the exact location on an object, such as an endpoint or an intersection of two lines on the drawing you're dimensioning (see Figure 10.2).

    The prompt in step 2 gives you the option of pressing to select an object. If you do this, you're prompted to select the object you want to dimension, rather than the actual distance to be dimensioned.

  3. At the Specify second extension line origin: prompt, use an osnap to select the other end of the object you're dimensioning, as shown in Figure 10.2.

  4. In the next prompt, Specify dimension line location or [Mtext/Text/Angle/Horizontal/Vertical/Rotated]:, you'll see a temporary dimension at the cursor location. Position the temporary dimension where you want the dimension to appear, and then click.

image from book
Figure 10.2: Using a linear dimension

If you prefer to be more precise about the dimension line location, in step 4 you can enter a relative distance from the last point selected.

Continuing a Dimension

You'll often want to enter a group of dimensions strung together in a line. You can use the Continue option in the Dimension menu to do this. This option assumes that you've already placed one linear or aligned dimension and are ready to continue with a string of dimensions from the last dimension (see "Dimensioning Nonorthogonal Objects" later in this chapter for information about aligned dimensions).

  1. Choose Dimension è Continue, or enter dco.

  2. At the Specify a second extension line origin or [Undo/Select] <Select>: prompt, select the next location you want to dimension (see the left image in Figure 10.3). You can continue to add more dimensions until you press .

image from book
Figure 10.3: A continued string of dimensions

If you find that you've selected the wrong location for a continued dimension, click the Undo tool, or enter u.

The Continue Dimension option continues from the last dimension you added to the drawing. The last drawn extension line is used as the first extension line for the continued dimension. If you need to continue a string of dimensions from a dimension other than the last one you placed in the drawing, press at the Specify a second extension line origin or[Undo/Select]: prompt in step 2 of the previous example. When you see the Select continued dimension: prompt, click the extension line from which you want to continue. You can then proceed to add dimensions.

Drawing Dimensions from a Common Base Extension Line

Frequently, you need to dimension from a single datum point, as shown in Figure 10.4. This means you need several dimensions starting from the same location. To accommodate this, AutoCAD provides the Baseline option.

image from book
Figure 10.4: A baseline dimension

As with the Continue option, Baseline assumes you have already placed at least one other linear or aligned dimension in the drawing.

Since you usually select exact locations on your drawing as you dimension, you might want to turn on running osnaps to avoid the extra step of selecting osnaps from the Osnap shortcut menu.

  1. Choose Dimension è Baseline or enter dba to start a baseline dimension.

  2. At the Specify a second extension line origin or [Undo/Select] <Select>: prompt, use an osnap to select another feature you want to dimension in your drawing.

  3. Press twice to exit the Baseline Dimension command.

The Baseline Dimension option works in a similar way as the Continue Dimension option does, except that Baseline Dimension allows you to use the first extension line of the previous dimension as the base for a second dimension.

The distance between the two parallel dimension lines is controlled by the Baseline Spacing setting in the Lines tab of the New Dimension Style and Modify Dimension Style dialog boxes.

Just as with the Continue Dimension option, the Baseline Dimension option continues from the last dimension you added to the drawing by default. If you need to add more baseline dimensions from a dimension other than the last one you placed in the drawing, press at the Specify a second extension line origin or [Undo/Select] <Select>: prompt in step 2 of the previous example. Then click the extension line from which you want to continue. You can then proceed to add dimensions.

Adding a String of Dimensions with a Single Operation

AutoCAD provides a method for creating a string of dimensions using a single operation while in the Model tab. The Qdim command lets you select a set of objects instead of having to select points. The following exercise demonstrates how the Qdim command works.

To use Qdim, choose Dimension è Quick Dimension, or enter Qdim. Next, place a crossing selection window (start your window to the left of the selection area) around the area you want to dimension, and then press . You'll see a string of dimensions at the cursor. Click to place the dimension string in the drawing.

The Qdim command can be a time-saver when you want to quickly dimension a wall in an architectural drawing, for example, or any set of contiguous lines in a drawing. It might not work in all situations, but if the object you're dimensioning is fairly simple, it can be all you need.

image from book

This example uses a simple crossing window to select the wall. For more complex shapes, try using a crossing polygon selection window (see "Selecting Objects" in Chapter 5 for more about the Crossing Polygons selection).




Introducing AutoCAD 2008
Introducing AutoCAD 2008
ISBN: 0470121505
EAN: 2147483647
Year: 2007
Pages: 147
Authors: George Omura

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