Chapter 19: Call the Parametrics! 425

Forget about drawing with precision!

“Are you serious? All through this book you continually nag about the need for drawing with precision!”

We’re sure that you’re familiar with (or at least have come across) computer acronyms and abbreviations, such as RAM, ROM, DVD, USB — and, of course, CAD. Okay, here’s a new one we coined for a magazine article a few years back: NAD. It has been our observation that the vast majority of design projects start life as a sketch on a coffee shop napkin: hence, NAD or Napkin-

Aided Design.

A big strength of CAD is that drawings can (and should) be very precise and so you can make use of information within them, such as areas. A big weakness of CAD is that you are forced to be very precise, even early in the design process when things are likely to change. Yes, commands like MOVE and STRETCH make editing relatively simple, but “relatively” is a relative term. “Easy to use” is easy to say.

With parametrics, you can start out with a NAD sketch that shows the general idea of our design. Remember our parametric motto: “Close enough is good enough” (by the way, this also applies to horseshoes, hand grenades, and dancing). Then apply geometric and dimensional constraints and formulas to control the behavior of the drawing, and fine-tune the values until you get your final design.

Constrain yourself

You can apply dimensional or geometric constraints to new geometry as you create it, or you can assign constraints to existing geometry in old drawings. You can even convert existing associative dimensions to dynamic dimensional constraints. The following sections describe the available constraints in each of the two categories. We start with geometric constraints because we’ve found that it’s usually best to pin objects down (pun intended) before applying dimensional constraints so that the drawing objects will behave in a predictable manner when dimension values are changed.

Understanding Geometric Constraints

Adding geometric constraints to object geometry is quite simple because it’s much like using normal object snaps. In fact, in several cases, AutoCAD’s

geometric constraints have equivalent object snaps. There are tangent, parallel, concentric (center), and perpendicular object snaps and tangent, parallel, concentric, and perpendicular geometric constraints. The difference between

www.it-ebooks.info

426 Part IV: Advancing with AutoCAD

these relations as object snaps and as geometric constraints is that constraints are persistent.

For example, in the example earlier in this chapter, you can see how the line always stays tangent to the circle, no matter what. The objects remember how to get along with their siblings. Now if we could just figure out a way to make that work with our kids.

Twelve geometric constraints are at your disposal that you can apply to your drawing objects, and the easiest way to apply them is by clicking buttons on the Geometric panel of the Ribbon’s Parametric tab. Table 19-1 presents a list of the geometric constraints and explains what they do.

www.it-ebooks.info

Text objects can also be geometrically constrained. You can apply horizontal or vertical constraints to text, and you can make text parallel, perpendicular, or collinear with lines. Anyone who’s had to label utility lines on civil engineering drawings in earlier releases so that the text aligns with the linework (who . . . moi??) will be very happy with this feature!

Applying a little more constraint

If you’re using the full version of AutoCAD (not AutoCAD LT), you can constrain drawing objects geometrically by clicking buttons on the Geometric panel of the Parametric tab. In the following steps, we show you how to get started with geometric constraints.

This procedure may seem a little tedious, but we want you to become familiar with the basic properties of each type of constraint. Later, we show you a couple of shortcuts that greatly speed things up.

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 afd19c.dwg contains the unconstrained geometry, and drawing afd19d.dwg contains the end product.

www.it-ebooks.info

428 Part IV: Advancing with AutoCAD

1.Start a new drawing. Make the Ribbon’s Parametric tab current and turn off Snap, Ortho, Polar, Osnap, and Otrack at the status bar.

For real drafting, you probably want to use these precision aids, but for this example, you get a better sense of parametrics with a really imprecise drawing.

2.Draw some linework by using the PLINE command and then add a couple of circles.

Draw something similar to Figure 19-2 if you want to follow closely.

Figure 19-2: Random shapes that badly need constraining.

Every drawing is going to be tackled differently, so think ahead and figure out the most efficient way to apply the constraints you need so that your design intent is maintained. In this example, you want to end up with two concentric circles in the middle of a square.

As we mention earlier, you’ll find it a lot easier to apply constraints if at least one point on the geometry is fixed in space. In many cases, one of the first constraints you should think about applying is Fix, the one that constrains a point to one location in the drawing area. This has nothing to do with tomcats.

In this example, you’re going to lock one of the endpoints of the polyline into position.

www.it-ebooks.info

Chapter 19: Call the Parametrics! 429

3.Click Fix in the Geometric panel and then click a point you want to fix in space.

A blue padlock icon is displayed beside the pickbox, and the red pick point marker appears when you’re over a point you can constrain (see Figure 19-3). In this example, we click one of the endpoints on the polyline. A padlock icon appears in the constraint bar, indicating that the endpoint of the polyline segment is fixed in place.

Figure 19-3: Locking down an object in drawing space.

With at least one point on the geometry fixed in place, you can start constraining the geometry by closing the gap in the linework.

4.Click Coincident in the Parametric tab’s Geometric panel.

You use a coincident constraint to make two points coincide. A blue coincident icon appears near the pickbox, and as you move your crosshairs over an object, a marker appears over relevant points — in the case of lines or polyline segments, at the endpoints and midpoint.

Use appropriate drawing commands based on your design intent. We use a polyline in this example because the ends of the segments are already coincident-constrained by being part of a single polyline object. If you had drawn this shape with lines, you’d have to apply individual coincident constraints to each corner.

www.it-ebooks.info

430 Part IV: Advancing with AutoCAD

5.Click an endpoint on the first polyline segment that you want to connect and then click an endpoint on the second segment.

The endpoint of the second polyline segment jumps to the endpoint of the first line, and a small blue square — the marker for coincident

constraints­ — appears at the intersection. (If you don’t see the little blue square, click Show All in the Geometric panel.)

6.Apply some orthographic parameters so that the linework starts to look a little more like a rectangle. Click Horizontal in the Parametric tab’s Geometric panel and then click a line or polyline segment that you want to be aligned with the drawing’s X axis.

In this example, we picked the bottom segment of the polyline. The segment realigns itself horizontally from the endpoint nearest to where you picked the line (unless a Fix constraint is added first), and a horizontal constraint marker — a constraint bar — appears near the object. A single item is still called the constraint bar because if you add additional constraints to the object, they get added to the existing one, like a toolbar.

7.Click Vertical to align a line or polyline segment with the drawing’s Y axis.

In this example, we pick the left vertical-ish polyline segment. The segment realigns itself at 90 degrees to the horizontal segment, and a constraint bar showing a vertical constraint appears. With one horizontally and one vertically constrained line segment, you’re halfway to a geometrically precise rectangle!

8.Because rectangles have parallel and perpendicular sides, next you apply those constraints to your linework. Click Parallel in the

Parametric tab’s Geometric panel, click the vertically constrained line segment, and then click the line segment opposite.

Because there’s already a vertical constraint applied to one segment, it doesn’t matter which line you pick first. If neither line had an existing constraint, the second line you picked would become parallel to the first line.

Always keep your design intent in mind as you think about which constraints to apply and when to apply them. As a general rule, start with the most important and drill down to the least important. And, as we remind you earlier in the chapter, applying a fix constraint early in the game can prevent your geometry rearranging itself in ways you don’t expect.

To make the final side of the almost-rectangle orthogonal with the other three, you could use another parallel constraint and click the bottom line. However, you don’t want to wear out our Parallel button, so use Perpendicular on the final segment.

www.it-ebooks.info

Chapter 19: Call the Parametrics! 431

9.Click Perpendicular on the Geometric panel, click either vertical side, and then click the non-orthogonal side.

Again, because three of the four sides are already constrained to horizontal and vertical, it doesn’t matter which segment you pick first.

“Perpendicular” means “at right angles to.” It isn’t restricted to “a vertical line going upward from one end of a horizontal line.” A line at an angle to the X axis can still be perpendicular to another.

Some constraints work on one object (horizontal, vertical), while others work on two objects (parallel, tangent, perpendicular). If you roll the cursor over a two-object constraint, its soul mate lights up.

To delete a constraint, move the mouse pointer over the constraint marker to display the constraints bar (see Figure 19-4), right-click, and choose Delete from the menu that appears.

Figure 19-4: Constraining to orthogonal.

From four non-orthogonal, not-even-closed line segments, applying a handful of constraints yields a perfect rectangle. However, you really want a square! Here’s where the Equal constraint comes in.

www.it-ebooks.info

432 Part IV: Advancing with AutoCAD

10.Click Equal on the Geometric panel, click the bottom side, and then click either of the vertical sides.

A perfect square! Now you want to constrain those circles.

The design intent in this example is to have the circles to be concentric and to locate their centers in the exact center of the rectangle. First the easy part: Making the circles concentric.

11.Click Concentric in the Geometric panel, click one circle, and then click the other.

The two circles are concentrically constrained (try saying that ten times in a hurry!), and a new constraint bar appears in their vicinity. Try moving one circle by clicking it and then watch the other tag along.

As we suggest, concentric was the easy constraint. We’d love to be able to use the Mid Between 2 Points object snap and just click two diagonally opposite corners to locate the circles dead center in the rectangle. However, because you can constrain only objects or points on objects, you have to add some construction geometry in order to maintain the design intent.

12.Draw a line between diagonally opposite corners by using Endpoint object snaps, and then apply coincident constraints between the endpoints of this line and the corners of the rectangle (see Figure 19-5).

Figure 19-5: Adding some construction geometry.

www.it-ebooks.info

Chapter 19: Call the Parametrics! 433

Draw construction geometry on a separate layer and set that layer to NoPlot in the Layer Properties Manager. If you don’t want to even see

it — let alone, not plot it — you can turn off your construction geometry layer or even freeze it — and the geometric constraints will still work.

13.Click Coincident in the Geometric panel. Click either circle so that the parametric marker appears in the center, move the pickbox over the construction line, and click when the parametric marker is over the midpoint of the line.

You’re done! You can test your design intent by using the STRETCH command on the corner of the rectangle that doesn’t display a little square, blue coincident icon. As you drag the corner, you should see the two circles moving, too, always maintaining their position in the middle of the rectangle (see Figure 19-6).

Figure 19-6: Full-on geometric constraints.

Parametrics was a new feature in AutoCAD 2010, and it got a considerable enhancement in AutoCAD 2011 with the addition of inferred constraints. A status bar button (to the left of the Snap button) toggles Infer Constraints mode off and on. When Infer Constraints mode is enabled, you don’t have to specifically apply many of the geometric constraints.

www.it-ebooks.info