Chapter 19: Call the Parametrics! 435

Figure 19-7: Auto-constrainable geometric relations in the Constraint Settings dialog box.

Understanding Dimensional Constraints

The normal practice in AutoCAD is to create some geometry — of course, using all the precision techniques we discuss in Chapter 7 — and then apply dimensions as we describe in Chapter 14. Assuming that you’re using fully associative dimensions, you can then edit the geometry and watch the dimensions update automatically. The length of the line or the radius of

the circle are in control, and those dimensions are called driven dimensions because they change when the object geometry changes.

Dimensional constraints, unlike regular AutoCAD dimensions, are driving dimensions, which means that when you change the value of a dynamic dimension on a line, the line changes to match. In other words, the dimension is driving the length of the line, not the other way around.

There are only eight dimensional-constraint options, but they cover all the bases. Table 19-2 lists them and describes their purposes.

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436 Part IV: Advancing with AutoCAD

Practice a little constraint

The objects that you add to your drawing from the Dimensional panel are not the same as the dimension objects you add from the Annotate tab. Dimensional constraints are driving dimensions, meaning that when you change the value of one of these dimensions, the geometry changes.

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Chapter 19: Call the Parametrics! 437

A lot is happening behind the scenes as you apply parametric constraints. You can get a great sense of how these constraints work at keeping your drawing objects in order by trying the STRETCH command on objects after you apply a constraint to them.

You can find the files we use in this sequence of steps at this book’s companion website. Go to www.dummies.com/go/autocad2013fd and download afd19.zip. The drawing named afd19a.dwg contains the unconstrained geometry, and drawing afd19b.dwg contains the end product.

The following steps present a very simple example of dimensional constraints:

1.Start a new drawing and make the Ribbon’s Parametric tab current.

Turn on some appropriate precision drawing aids on the status bar, such as Snap, Ortho, and Osnap.

2.Draw some reasonably precise geometry by using some of those precision techniques we describe in earlier chapters.

In the following example, we use the RECTANG and CIRCLE commands to draw the geometry you see in Figure 19-8. The rectangle is 10 units square, and the 2.5-unit–radius circle is deliberately drawn away from the middle of the square.

Figure 19-8: Simple geometry badly in need of constraining.

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438 Part IV: Advancing with AutoCAD

3.On the Dimensional panel of the Parametric tab, click the top part of the Linear split button.

A linear dimension icon appears beside the pickbox, and AutoCAD prompts you to specify the first constraint point or to pick an object.

Just like the DIMLINEAR command, the Linear dimensional constraint tool is inferential: that is, which way you drag the crosshairs controls whether you get a horizontal or vertical dimension. Also, just like DIMLINEAR, you can press Enter at the command prompt and select an object to dimension.

4.Press Enter at the command prompt to confirm that you want to select an object and then select the bottom horizontal line segment.

If you see red markers at the midpoint and ends of the bottom line, you didn’t press Enter, and you’re in point-selection rather than objectselection mode.

AutoCAD generates a preview of a dimensional constraint and prompts you for a location.

5.Click to locate the dimension position.

AutoCAD draws a dimensional constraint with a highlighted text field displaying the dimension name (d1 in this example) and the value returned by AutoCAD. You could type a new value in the edit box, but for now, just press Enter to confirm the value and the dimension location (see Figure 19-9).

If your dimensional constraints disappear as soon as you place them, click the Show All button on the Parametric tab’s Dimensional panel (refer to Figure 19-1).

Dimensional constraints are not regular dimension objects — they’re not going to plot, so it doesn’t really matter where you put them or what they look like. (We show you how to turn dimensional constraints into properly styled, plottable dimensions in the next section.)

6.Repeat Steps 3 through 5 and add a dimensional constraint to the right vertical edge of the rectangle.

AutoCAD draws a second dimensional constraint, this one named d2.

As you mouse over the Linear button, you can see that unlike its Aligned neighbor, it’s split into two parts. You can force a linear dimensional constraint to be either horizontal or vertical (rather than dependent on the direction in which you drag your crosshairs) by clicking the bottom part of the Linear button and making your choice from the drop-down menu.

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