Defining terms

The word parameter is derived from these two Greek roots:

• para: To work with, to assist, to work alongside. Think of paramedic ambulance attendants. You may have seen an “ambulance” when I mentioned parametrics earlier in this chapter.
• metros: To measure (hence meter and metric). For the purposes of making drawings in AutoCAD, a parameter is a rule that works alongside AutoCAD objects. You can think of parametric drawing as rule-based drawing.

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

• Geometric: Based on object types and relationships between objects; different object types have different potential constraints. In the example in the preceding step list, I show you how to apply a Tangent constraint between the line and the circle — as you move or change one, the other one follows along.
• Dimensional: 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 of AutoCAD, you can create and modify geometric and dimensional constraints, as I describe in the steps in this chapter. If you’re using LT, you can’t create either type of constraint, but you can work with existing constraints in drawings that were created in AutoCAD. 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.

Forget about drawing with precision!

“Are you serious? Throughout this book, you continually nag about the need for drawing with precision, and now you want me to forget about it?”

I’m 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 that I coined for a magazine article a few years back: NAD, or Napkin-Aided Design. It has been my observation that the vast majority of design projects start life as a sketch on a coffee-shop napkin.

A major strength of CAD is that drawings can (and should) be so precise that you can make use of information within them, such as areas. A major weakness of CAD is that you’re forced to be precise, even early in the design process, when circumstances often change. Yes, commands such as Move and Stretch make editing relatively simple, but relatively is a relative term, and easy to use is easy to say.

With parametrics, you can start out with a NAD sketch that shows the general idea of your design. Remember my parametric motto: Close enough is good enough. By the way, the motto 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 arrive at 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 constraints available in each of the two categories. I start with geometric constraints because I’ve found that it’s usually best to pin down objects (pun intended) before applying dimensional constraints so that the drawing objects behave in a predictable manner when dimension values are changed.

Applying a little more constraint

If you’re using 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, I show you how to get started with geometric constraints.

These steps may seem tedious, but I want you to become familiar with the basic properties of each type of constraint. Later, I show you a couple of shortcuts that speed things up.

You can find the files I use in this sequence of steps at this book’s companion website: Go to www.dummies.com/go/autocadfd18 and download afd19.zip. The drawing named afd19c.dwg contains the unconstrained geometry, and drawing afd19d.dwg contains the end product.

Follow these steps to use geometric constraints:

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

   For production drawings, you will want to make sure you use precision drafting aids, but in this example, you gain a better sense of parametrics with an extremely imprecise drawing.

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

   Draw it to look similar to Figure 19-2 if you want to follow these steps closely.

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

   As I mention earlier, applying constraints is easier if at least one point on the geometry is fixed in space. In many cases, consider applying the Fix constraint first — it constrains a point to a single location in the drawing area. This topic has nothing to do with tomcats.

   In this example, I show you how to lock one endpoint of the polyline into position.

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

   A blue padlock icon displays beside the pickbox, and the red pick point marker appears when you hover over a point that you can constrain (see Figure 19-3). In this example, I clicked an endpoint on the polyline. The padlock icon appears in the constraint bar, indicating that the endpoint of the polyline segment is fixed in place.

   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 the cursor over an object, a marker appears over relevant points. In the case of lines or polyline segments, they’re at the endpoints and the 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.

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 align with the drawing’s X-axis.

   In this example, we picked the bottom segment of the polyline. The segment realigns horizontally from the endpoint nearest to the spot 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’re 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 at 90 degrees to the horizontal segment, and a constraint bar showing a vertical constraint appears. With one line segment horizontally constrained and another vertically constrained, you’re halfway to a geometrically precise rectangle.

8. Because rectangles have parallel and perpendicular sides, 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 to it.

   Because a vertical constraint is already 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 prioritize your design intent as you consider which constraints to apply and when to apply them. As a general rule, start with the most important ones, and drill down to the least important. As I remind you earlier in this chapter, applying a fix constraint early in the game can prevent your geometry from 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 the Parallel button, so use Perpendicular on the final segment.

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 extending upward from one end of a horizontal line.” A line at an angle to the X-axis can still be perpendicular to another, and the lines don’t have to touch.

   Some constraints work on one object (horizontal, vertical), and 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.

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

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

    It’s a perfect square! Now you have to constrain those circles.

    The design intent in this example is to make the circles concentric and to locate their centers in the exact center of the rectangle. First, the easy part: Make 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 fast. A new constraint bar appears in their vicinity. Move one circle by clicking it, and watch the other tag along.

    As I suggest, concentric is the easy constraint. I’d love to be able to use the Mid Between 2 Points object snap and simply 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 to maintain the design intent.

12. Draw a line between diagonally opposite corners 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).

    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 your construction geometry (or plot it), turn off the construction geometry layer, or freeze it. 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 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).

Using inferred constraints

Parametrics were a new feature in AutoCAD 2010 and were considerably enhanced in AutoCAD 2011 by the addition of inferred constraints. A status bar button (to the left of the Snap button) toggles Infer Constraints mode off and on. When it’s on, you don’t have to specifically apply many of the geometric constraints.

To produce the best results, go at least partway back to drawing with precision. For example, a coincident constraint is applied pretty much any time you use an object snap, and horizontal and vertical constraints are placed if polar or ortho modes are on.

Inferred constraints also work when editing existing geometry.

Inferred Constraints mode is a powerful component of parametric drafting in AutoCAD, but I recommend that you first familiarize yourself with manually applying constraints, as described in this chapter.

You AutoConstrain yourself!

The geometric constraints (refer to Table 19-1) are meant to be applied one at a time. That level of flexibility is helpful, but it can be pretty darned time-consuming to go around a complex object, trying to figure out what the best constraint option would be and then applying it.

The Auto Constrain button on the Parametric tab’s Geometric panel can apply a whack of constraints to a selection of objects with a single click. Before you execute that click, however, take a look at the Auto Constrain settings. Click the dialog box launcher (the little angled arrow at the right end of the Geometric panel label) to open the Constraint Settings dialog box and make the AutoConstrain tab active (see Figure 19-7). Choose the constraints you want to apply automatically, and their order in the list, so that the ones that should take precedence have a higher priority. Your best bet initially is to stick with the defaults until you become more familiar with how Auto Constrain works.

Auto Constrain works only with geometric constraints, not with dimensional constraints.

AutoCAD doesn’t let you overconstrain an object. For example, if a horizontal constraint appears on one line, and a parallel constraint appears between it and a second line, AutoCAD objects if you try to add a horizontal constraint to the second line.

Practice a little constraint

The objects you add to a drawing from the Dimensional panel aren’t the same as the dimension objects you add from the Annotate tab. Dimensional constraints are driving dimensions — when you change the value of one of these dimensions, the geometry changes.

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

You can find the files I use in these steps at this book’s companion website: Go to www.dummies.com/go/autocadfd18 and download afd19.zip. The drawing named afd19a.dwg contains the unconstrained geometry, and the drawing named afd19b.dwg contains the end product.

These steps present a simple example of dimensional constraints:

1. Start a new drawing and make the Ribbon’s Parametric tab current.
2. Turn on the appropriate precision drawing aids on the status bar, such as Snap, Ortho, and Osnap.

3. Draw some reasonably precise geometry by applying the precision techniques I describe in earlier chapters.

   In the following example, I use the RECtang and Circle commands to draw the geometry you see in Figure 19-8. The rectangle is ten units square, and the 2.5-unit-radius circle is deliberately drawn away from the middle of the square.

4. 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 either specify the first constraint point or pick an object.

   Like the DimLInear command, the Linear dimensional constraint tool is inferential — which way you drag the cursor controls whether you create a horizontal or vertical dimension. Also, as with DimLInear, you can press Enter at the command line and select an object to dimension.

   Hover the mouse over the Linear button to see (unlike its neighbor, Aligned) that the button is split into two parts. To force a linear dimensional constraint to be either horizontal or vertical (rather than dependent on the direction you drag the cursor), click the bottom part of the Linear button and choose from the drop-down menu.

5. Press Enter at the command line 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 endpoints of the bottom line, you didn’t press Enter — and AutoCAD is in Point Selection mode rather than Object Selection mode.

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

6. 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.

7. Press Enter to confirm the value and the dimension location (see Figure 19-9).

   Best practice is usually to accept the default value for all dimensional constraints as they are applied and then edit them later to the exact values that are desired. This avoids problems such trying to make the width of a slot greater than the width of the piece that contains the slot, which can turn your drawing inside out.

   If the 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).

   Because dimensional constraints aren’t regular dimension objects, you can’t plot them, so it doesn’t matter where you put them or what they look like. I show you in the next section how to turn dimensional constraints into properly styled, plotable dimensions.

8. Repeat Steps 4–6 and add a dimensional constraint to the rightmost vertical edge of the rectangle.

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

Making your drawing even smarter

If your drawing were a traditional mechanical drawing that followed the rules of drafting, the placement of a horizontal and vertical dimension in the example of the previous section would be enough. Whoever reads your drawing understands that if sides are parallel and perpendicular, a dimension on one side applies to the opposite side as well. But in this chapter, I talk about intelligent drawings that respect design intent — not dumb collections of lines and circles, even if they do follow the rules of drafting.

If you stretch a rectangle in various ways (from the upper-left corner, for example), you can see that only the bottom and right sides are constrained to 10 units in length. You can add two more linear dimensional constraints to the unconstrained sides, but then you’d have to remember to edit both dimensions. Rather than constrain both sides to be 10 units long, you maintain design intent by making both sides equal in length.

You can make the two sides equal by using either dimensional or geometric constraints. You should usually apply most, or all, of the geometric constraints to the drawing before adding dimensional constraints. This way, the objects behave in a more predictable manner when you change the dimension values.

Follow these steps to apply three geometric constraints:

1. On the Geometric panel of the Parametric tab, click Fix (the padlock icon) and then click the lower-left corner of the rectangle.

2. On the Geometric panel, click Horizontal and click the bottom side of the rectangle. Then click Vertical and click the left side.

   Icons appear near the drawing geometry, showing that those three geometric constraints are active.

3. On the Dimensional panel of the Parametric tab, click Linear and add a dimensional constraint to the top horizontal line.

4. Click to locate the dimension, type d1, and then press Enter when AutoCAD prompts you for the dimension text.

   Rather than display numeric values, like the first two linear constraints, this new dimensional constraint displays fx: d3=d1.

   The main part of this expression sets the d3 dimension to equal the value of the d1 dimension. The fx: reminds you that other variables in other dimensions are being referenced.

5. Repeat Steps 3 and 4, and this time add a constraint to the vertical line at the left. Then enter d2 as the dimension text.

   All four dynamic dimensional constraints display their names and a value or expression for each one.

Dimensional constraints have names as well as values. They can also include expressions or formulas. You can set the default appearance of dynamic dimensional constraints by clicking the dialog box launcher (the little arrow at the right end of the Dimensional panel label) to open the Constraint Settings dialog box, with the Dimensional tab active (see Figure 19-10).

The options are as follows:

• Name: The first linear dimension is named d1, the second d2, and so on. You use the dimension names in expressions.
• Value: The numeric value that you enter into the dimensional constraint (or that AutoCAD enters if you don’t override it).
• Name and Expression: The dimension name shown as equal to an expression. The expression can be a value, as in this example, or it can be a formula that includes other dimension names.

Using the Parameters Manager

Both AutoCAD and AutoCAD LT include the Parameters Manager palette, accessible from the Manage panels of the Ribbon’s Parametric tab. You can use the Parameters Manager to give all those dimensional constraints more sensible names than d1 and d2, but you can (even more usefully) enter expressions instead of plain numeric values, as I explain in these steps:

1. Click Parameters Manager in the Manage panel.

   The Parameters Manager palette appears, showing a list of the dimensional constraints applied in the drawing (see Figure 19-11).

   In Figure 19-11, the Expression column shows the numeric values that I specified for d1 and d2 and the expressions I entered for d3 and d4. The read-only Value column shows the calculated value. You can’t change a value in the Value column (it’s grayed out to remind you). You can only edit the cells in the Expression column.

2. In the d1 row, click in the Expression field to highlight the current value (10, in this example). Then click again and type a new value. For example, type 13 and press Enter.

   The rectangle resizes in the drawing editor, and because the d3 constraint on the top side was made equal to the d1 constraint on the bottom side, both sides change equally.

   Next, you use an equation as an expression.

3. In the d2 row, click in the Expression field to highlight the current value, and then type an expression. For example, type d1*0.75 and then press Enter.

   The read-only Value column and the drawing geometry show that the new d1 distance of 13 has been multiplied by 0.75 and is now 9.75 (see Figure 19-12).

4. Close the Parameters Manager.

   Constrain the circle so that its center is always at dead center in the rectangle, no matter how the rectangle’s size changes.

5. Apply a horizontal dimensional constraint from the upper-left corner of the rectangle to the center of the circle. Locate the dimension, type d3/2, and then press Enter.

   Because the rectangle is now dimensionally constrained on all four sides, it doesn’t matter which corner you start from. Note that you don’t have to type the whole expression: d5=d3/2. AutoCAD knows what you mean!

6. Repeat Step 5, and this time add a vertical constraint from one corner to the center of the circle. Locate the dimension, and then type d4/2.

   Figure 19-13 shows the object geometry with all constraints added in this section. Who knew that drafting could be such fun?

If your drawing becomes overrun with parameters, you can add parameter filters in the Parameters Manager palette. Right-click any parameter and choose Show Filter Tree, or click the double right arrow at the top left of the palette to open the Filters pane. Click the funnel icon on the toolbar to create a new filter group, and then simply drag and drop parameters into the group. Figure 19-14 shows the two constraints (refer to Steps 5 and 6 of the preceding step list) dragged into a new group filter.

Dimensions or constraints? Have it both ways!

After all the hard work of adding dimensional constraints to a drawing, it would be a downright shame to have to go back and apply regular dimensions, wouldn’t it? Well, you don’t have to. You can make dimensional constraints look and behave like regular dimensions. You can go the other way, too, and make your regular dimensions act like dimensional constraints.

Dimensional constraints are available in two flavors:

• Dynamic: In this default form, a dynamic constraint is gray, with a padlock icon next to it in the drawing area. You can make the constraint appear and disappear by clicking Show All in the Dimensional panel. Dynamic constraints don’t plot, and they’re always legible because they resize as you zoom in and out of the drawing.
• Annotational: This form is controlled as an object property, so you have to set it in the Properties palette. Though annotational constraints plot, they don’t resize as you zoom in and out, and they don’t disappear as you toggle the Show All button on and off. Annotational constraints conform to dimension style settings.

The dimension name format of annotational constraints can be set to Name, Value, or Name and Expression, just like dynamic constraints. Refer to Figure 19-10 for another look at the Constraint Settings dialog box. If you want to plot a drawing with annotational constraints, reset the format so that it doesn’t show the dimension name or the expression.

Here’s how to turn dynamic dimensional constraints into annotational constraints:

1. Open a drawing that contains some geometry with dimensional constraints.

   You can also start a new drawing, draw some simple geometry, and add a dimensional constraint or two.

2. Select a dynamic constraint, right-click, and choose Properties.

   The Properties palette opens with the object properties of the selected dimensional constraint listed in table form.

3. Click in the Constraint Form field, and in the drop-down list, change Dynamic to Annotational (see Figure 19-15).

   The dynamic constraint becomes annotational and takes on the appearance of the current dimension style. If you change the dimension style in the Properties palette, the annotational constraint updates to the new dimension style format.

   You can go the other way, too — from regular dimension objects to dimensional constraints to convert an existing dumb drawing to an intelligent parametric one.

4. Add a linear, radius, diameter, aligned, or angular dimension (that is, a regular dimension, not a dimensional parameter) to your drawing geometry.

   Nearly every type of dimension object has a parametric analog; the exceptions are arc length, jogged radius, jogged linear, and ordinate dimensions.

5. Click Convert in the Dimensional panel, select a dimension type (refer to Step 4), and press Enter.

   The Dimensional Constraint text box displays as soon as you click an associative dimension, and the dimension becomes a dynamic constraint as soon as you press Enter.

The only clue that a dimension is an annotational constraint rather than a regular associative dimension is the padlock icon that appears next to the dimension value. You can turn off the display of the padlock from the Constraint Settings dialog box, but I recommend that you leave it on. It doesn’t plot anyway, and you might decide to delete the dimension without realizing that it’s controlling your object geometry.

Annotational dimensions can also be annotative so that they size themselves automatically to the drawing scale. I cover annotative dimensions in Chapter 14. Annotative and annotational dimensions are also associative, so you can have annotative annotational associative dimensions. Try saying that quickly after a few drinks.

In this chapter, I present the two varieties of parametric constraints separately, but you get the most mileage from this feature when you incorporate both geometric and dimensional constraints with other precision techniques (described in Chapter 8). In fact, if you start a drawing by using Snap, Ortho, Osnap, and other precision techniques before you add dimensional and parametric constraints, you’ll be well on your way to creating a library of intelligent drawings that maintain your design intent.

Geometric and dimensional constraints can be equivalent. For example, a 90-degree-angle dimensional constraint is equal to a perpendicular geometric constraint, and AutoCAD doesn’t let you apply both.

All examples in this chapter involve a single drawing view. Geometric constraints apply “to infinity,” so you can synchronize objects from one orthographic view with objects in another. For example, coincident and collinear constraints can link objects in the front view with their equivalents in the top and right-side views, and liberal use of the equal constraint means that you should never have to measure or dimension anything twice, you should never need more than minimal construction geometry, any changes in one view reflect through to the others, and your views always remain orthogonally aligned.