422 Part IV: Advancing with AutoCAD

By the time they get their coats on, you meet them at the door and hand them a print of the revised drawing. The client invites you to join them for lunch.

This chapter shows you how to make this scene come true.

Maintaining Design Intent

Parametric (rule-based) drawing is by far the best way of enforcing design intent in 2D drafting. Design intent in AutoCAD (or any other engineering software) means that when drawings are edited — “this part” made wider, “that hole” made larger — all the attached or related objects behave in a predictable way that honors the designer’s intentions when she created the drawing in the first place.

Before AutoCAD 2010, there was simply no way of maintaining the design concepts that went into an AutoCAD drawing. You could use AutoCAD drawing and editing commands to draw accurate, precise plans, sections, and details, but as far as AutoCAD was concerned, they were just a bunch of lines and circles.

Take, for example, that base-plate drawing example in Chapter 3. Maybe the engineer has had a second look and determined that those 1 1/2" (38mm for the metric crowd) bolts aren’t quite up to the job — they need to be changed to 1 3/4" (44mm). To revise the drawing by using AutoCAD in the traditional way, you draw a new, larger circle for the bolt and erase the old one. Now the nut is too small, and so is the hole in the plate. (Maybe you can’t see it, but you know and we know it’s there.) There’s a whole lot of editing required to fix this drawing.

In AutoCAD, you can add some intelligence to those lines and circles by applying parametric constraints to them. Instead of trying to explain what this means, the easiest way is to show you.

Unfortunately, the following exercise won’t work in AutoCAD LT.

1.Start a new drawing.

2.Draw a circle and a line anywhere in the drawing.

Exact sizes and alignments aren’t critical. You’ll see why in a moment, and we’ll explain even more a little later.

Apply a tangent constraint between the circle and the line.

Parametric functions are found on the Parametric tab (what a surprise!) of the Ribbon. Click the Tangent button, and then select the circle and the line (sequence doesn’t matter) and observe how they move until

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Chapter 19: Call the Parametrics! 423

they are tangent to each other, and also how a little gray icon — a constraint bar — appears to indicate the type of constraint. Note that “tangent” doesn’t necessarily mean that they have to touch.

3.Grip edit the line and the circle.

We cover grip editing in Chapter 10. You can move the circle or line, you can change the diameter of the circle, you can move either end of the line — but no matter what you do, the other object obediently remains tangent. But wait! There’s more!

4.Apply a diameter dimension to the circle.

Don’t use the normal DIMDIAmeter command from Chapter 14, but use the Diameter button in the upper-right corner of the Dimensional panel of the Parametric tab of the Ribbon. When you select the circle as prompted, AutoCAD places a gray, funny-looking dimension that reads something like dia1=5.3716. It has the same background color as the MTEXT edit box (see Chapter 13); in fact, that’s exactly what it’s inviting you to do. Enter a suitable value (say, 7.5), and the circle resizes itself to suit the dimension value you entered, and the line adjusts itself to remain tangent!

5.Apply an Aligned dimension to the line.

Again, be sure to use the Aligned button from the Dimension panel on the Parametric tab, and select each end of the line in turn (or press Enter and then select the line). This time, when it places the dimension, enter =dia1*2 (including the = sign, no spaces). The line automatically resizes itself to be twice as long as the diameter of the circle.

6.Change things.

You can still grip edit the position of the line and circle, but you can’t grip edit the diameter or the line length, nor can you STRETCH the line length any more. Now double-click the diameter dimension and change its value. Surprise! The circle changes size, and the line changes its length so it’s still twice as long as the new diameter of the circle! Now that is one smart drawing. Imagine the effect that this can have on your productivity in creating and editing drawings.

7.Close your mouth.

You’re drooling all over the keyboard.

Defining terms

The word parameter is derived from two Greek roots. Para means to work with, to assist, to work alongside (think of paramedic ambulance attendants; you were right when you thought “ambulance” when we mentioned “parametrics” earlier), and metros means measure (and hence meter, and metric). For

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424 Part IV: Advancing with AutoCAD

the purposes of making drawings in AutoCAD, think of a parameter as a rule that works alongside AutoCAD objects — and therefore think of parametric drawing as rule-based drawing.

AutoCAD’s parametric rules — officially called constraints — fall into two categories:

Geometric constraints: Based on object types and relationships between objects; different object types have different potential constraints. In the earlier example in the preceding step list, we show you how to apply a Tangent constraint between the line and the circle — and as you moved or changed one, the other followed along.

Dimensional constraints: Based on dimensioned distances on or between objects or points on objects. In the earlier example, you see how to apply a dimensional constraint to the circle. Changing the dimension value changes the circle size. Better yet, you can establish a relationship between the circle and the line so that changing the circle diameter also changes the length of the line.

Both AutoCAD and AutoCAD LT support constraints, but as is usually the case, the feature is limited in LT. If you use the full version, you can create and modify geometric and dimensional constraints, as we describe in the steps in this chapter. If you’re using LT, you can’t create either type of restraint, but you can work with existing constraints in drawings that were created in AutoCAD itself. Figure 19-1 shows the difference between the AutoCAD and AutoCAD LT Parametric tabs.

If you’re using AutoCAD LT, or if you just want to check out some ready-made parametric possibilities, you can download the sample drawing datasets from

either www.autodesk.com/autocad-samples or www.autodesk.com/ autocadlt-samples. The parametric samples are architectural_

example-imperial.dwg, civil_example-imperial.dwg, and mechanical_example-imperial.dwg.

Figure 19-1: Parametric tabs in AutoCAD (above) and AutoCAD LT (below).

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