Using AutoConstrain to Add Constraints Automatically

Start by opening a sample drawing and adding a few geometric constraints. The sample drawing is composed of two parallel lines connected by two arcs, as shown in Figure 16.3. These are just lines and arcs and are not polylines.

Here are the steps:

1. Open the Parametric01.dwg file from the sample files for this chapter (see Figure 16.3).
2. Select the Parametric tab, and click the AutoConstrain tool in the Geometric panel, or type Autoconstrain.
3. Use a selection window to select all the objects in the drawing, and press↵.

You've just used the AutoConstrain command to add geometric constraints to all the objects in the drawing. You can see a set of icons that indicate the constraints that have been applied to the objects (see Figure 16.4). The AutoConstrain command makes a “best guess” at applying constraints.

Notice that the constraint icons in the drawing match those you see in the Geometric panel. If you hover your mouse pointer over an icon, you'll see a tool tip that shows the name of the constraint.

The Tangent constraints that you see at the ends of the lines keep the arcs and the lines tangent to each other whenever the arcs are edited. The Parallel constraint keeps the two lines parallel, and the Horizontal constraint keeps the lines horizontal.

There is one constraint that doesn't show an icon, but you see a clue to its existence by the small blue squares where the arcs join the lines:

1. Place your cursor on one of the blue squares. A new icon appears below the tangent icon along with a description of the arc. The arc description may hide the new icon, so look carefully or move the cursor away and try again. The new icon appears immediately, whereas the arc description takes a second to appear.

   

2. Hover your cursor over the icon that has just appeared. You see the tool tip name appear for the Coincident constraint.

The Coincident constraint makes sure that the endpoints of the lines and arcs stay connected, as you'll see in the next few exercises.

Editing a Drawing Containing Constraints

Now try editing the drawing to see how these constraints work:

1. Click the arc on the left side of the drawing (see the top of Figure 16.5).

   

2. Click the grip at the bottom of the arc and move it downward to increase the radius of the arc. The objects move in unison to maintain their geometric constraints (see the bottom of Figure 16.5).
3. Click again to accept the change in the arc radius.
4. Click the Undo tool in the Quick Access toolbar to undo your change.

In this exercise, you saw how the Tangent, Parallel, Horizontal, and Coincident constraints worked to keep the objects together while you changed the size of one object. Next, you'll see what happens if you remove a constraint.

REMOVING A CONSTRAINT

The AutoConstrain tool applied quite a few geometric constraints to the drawing. Suppose that you want to remove a constraint to allow for more flexibility in the drawing. In the next exercise, you'll remove the Parallel constraint and then try editing the drawing to see the results:

1. Click the Parallel constraint icon that is just above the lower line.

   

2. Right‐click and select Delete. Notice that the Parallel icon disappears from both the top and bottom line.
3. Click the line at the bottom of the drawing.
4. Click the grip at the left end of the line, drag it downward, and then click. This time, the left end of the line moves downward. The lines are no longer parallel, but they remain tangent to both arcs and their endpoints remain connected (see Figure 16.6).

Notice that the top line remains horizontal as you edit the arc. The top line still has the Horizontal constraint. Next try removing the Horizontal constraint:

1. Click the Undo tool in the Quick Access toolbar to return to the previous shape.
2. Right‐click the Horizontal constraint icon above the top line and select Delete.

   

3. Select the arc on the left side again as you did earlier.
4. Hover over the bottom grip of the arc, and select Stretch from the multifunction grip menu.
5. Move the grip downward. This time both the lines change their orientation, as shown in Figure 16.7.

Notice that lines and arcs remain connected and tangent to each other. This is because the Coincident and Tangent constraints are still in effect.

ADDING A CONSTRAINT

You've seen how the AutoConstrain tool applies a set of constraints to a set of objects. You can also add constraints “manually” to fine‐tune the way that objects behave. In the next exercise, you'll add a circle to the drawing and then add a few specific constraints that you'll select from the Geometric panel:

1. Click the Undo tool in the Quick Access toolbar to change the drawing back to its original shape.
2. From the Home tab, click the Circle tool in the Draw panel.
3. Click a location roughly above and to the left of the drawing, as shown in Figure 16.8. You don't need to be exact because you will use a geometric constraint to move the circle into an exact location.

   

4. Type 0.25↵ for the radius of the circle.
5. Select the Parametric tab, and click the Concentric tool in the Geometric panel.
6. Click the arc at the left side of the drawing, and then click the circle that you just added. The circle moves to a location that is concentric to the arc, as shown in Figure 16.9.

In this exercise, you used the geometric constraint as an editing tool to move an object into an exact location. The Concentric constraint will also keep the circle inside the arc no matter where the arc moves.

Using Other Geometric Constraints

You've seen how several of the geometric constraints work. For the most part, each constraint is fairly easy to understand. The Tangent constraint keeps objects tangent to each other. The Coincident constraint keeps the location of objects together, such as endpoints or midpoints of lines and arcs. The Parallel constraint keeps objects parallel.

You have many more geometric constraints at your disposal. Table 16.1 gives you a concise listing of the constraints and their purposes. Note that, with the exception of Fix and Symmetric, all of the constraints affect pairs of objects.

The behavior of the geometric constraints might sound simple, but you may find that they can act in unexpected ways. Within the limited space of this book, we can't give exercise examples for every geometric constraint, so we encourage you to experiment with them on your own. And have some fun with them!

Using Constraints in the Drawing Process

Earlier you saw how the Concentric constraint allowed you to move a circle into a concentric location to an arc. You can use other geometric constraints in a similar way. For example, you can move a line into a collinear position with another line using the Collinear constraint. Or you can move a line into an orientation that's tangent to a pair of arcs or circles, as shown in Figure 16.10. The top image shows the separate line and circles, and the bottom image shows the objects after applying the Tangent constraint. Note that although the line is tangent to the two circles, its length and orientation do not change.

Adding a Dimensional Constraint

In this first dimensional constraint exercise, you'll add a horizontal dimension to the drawing that you've been working on already. The drawing already has some geometric constraints with which you are familiar, so you can see how the dimensional constraints interact with the geometric constraints.

Start by adding a dimensional constraint between the two arcs:

1. In the Parametric tab, click the Aligned tool in the Dimensional panel.
2. Shift+right‐click, and select the Center osnap option from the context menu.
3. Place the cursor on the arc on the left side of the drawing so that the Center osnap marker appears for the arc (see Figure 16.11) and click to select the center. Notice that the arc is highlighted as you select the center.

   

4. Shift+right‐click, and select the Center osnap option as you did in step 2.
5. Click the arc on the right side of the drawing (see Figure 16.12).

   

6. Click a location above the drawing, as shown in Figure 16.12.
7. At the Dimension text = prompt, press ↵ to accept the current value.

The dimensional constraint appears above the drawing and shows a value of d1 = 6.0000. The d1 is the name for that particular dimensional constraint. Each dimensional constraint is assigned a unique name, which is useful later when you want to make changes. You can select from a number of dimensional constraints in the Parametric tab's Dimensional panel. Table 16.2 gives you a list of the dimensional constraint options.

Now let's see how you can use the dimensional constraint that you just added.

Editing a Dimensional Constraint

A dimensional constraint is linked to objects in your drawing so that when you modify a dimension value, the objects to which the constraint is linked change. To see how this works, try editing the part by changing the dimension:

1. Double‐click the dimension value of the dimensional constraint (see Figure 16.13).

   

2. Type 4.5↵. The part shortens to the dimension that you entered. Press ↵ to clear the command.

Next, add a dimension to the arc on the left side:

1. Click the Aligned tool from the Dimensional panel.
2. Select the top endpoint of the arc on the left side (top of Figure 16.14). Do this by first hovering over the arc near the endpoint. When you see the endpoint marker, click the mouse. If the constraint icons are in the way, you can zoom into the drawing to see the marker.

   

3. Select the bottom endpoint of the arc in the same way.
4. Click a point to the left of the arc to place the dimension (bottom of Figure 16.14).
5. At the Dimension text = prompt, press ↵ to accept the current value.

Notice that the new dimensional constraint has been given the name d2. Now try changing the size of the arc using the dimensional constraint:

1. Double‐click the dimension value of the d2 dimensional constraint (see Figure 16.15).

   

2. Enter 2↵. The part adjusts to the new dimension.

As you can see from this example, you can control the dimensions of your drawing by changing the dimensional constraint's value. This is a much faster way of making accurate changes to your drawing. Imagine what you would have to do to make these same changes if you didn't have the geometric and dimensional constraints available.

Adding a Formula Parameter

Now let's add a formula that will “connect” the value of the circle diameter to the diameter of the arc:

1. Click the Parameters Manager tool in the Manage panel. The Parameters Manager appears (see Figure 16.17). Notice that the Parameters Manager gives a list of all the dimensional constraints that exist in the drawing.

   

2. Click the Creates A New User Parameter tool at the top of the Parameters Manager. A new category appears in the list called User Parameters, and you see a user1 parameter appear.

   

3. Double‐click in the Expression column for the user1 parameter (see Figure 16.18).

   

4. With the user1 expression value highlighted, type d2 * 0.5↵. You will see your entry replace the expression value (see Figure 16.18). This expression is saying, “Give the user1 variable the value of half the d2 constraint.” Remember that the d2 constraint is the one given to the arc.
5. Now double‐click the Expression column for the dia1 parameter (see Figure 16.19). This is the parameter for the circle's diameter.

   

6. Type user1↵ to change the dia1 expression from 0.5 to user1. This tells AutoCAD to use the expression in the user1 parameter in place of the fixed 0.5 diameter value.

You've set up your circle to follow any changes to the width of the part. Now any changes to the d2 dimensional constraint will affect the circle.

Testing the Formula

Try the following to see the parameters in action:

1. In the Parameters Manager, double‐click the Expression column for the d2 parameter.
2. Change the value from 1 to 2 and observe the effect on the part (see Figure 16.20).

The part changes in width, and the circle also enlarges to maintain its proportion to the width of the part. Here you see that you can apply a formula to a constraint so that it is “linked” to another constraint's value. In this case, you set the circle's diameter to follow the width constraint of the part.

In addition, you can adjust the constraint value from the Parameters Manager. You could also have changed the width of the part as before by double‐clicking the dimensional constraint for the arc.

Using Other Formulas

In the previous exercise, you used a simple formula that multiplied a parameter, d2, by a fixed value. You used the asterisk to indicate multiplication. You could have used the minus sign in the formula if, for example, you wanted the circle to be an exact distance from the arc. Instead of d2 * 0.5, you could use d2 – 0.125, which would keep the circle 0.125 from the overall width of the part.

You can also choose from a fairly large list of formulas. If you double‐click an expression in the Parameters Manager and then right‐click, you can select Expressions from the context menu that appears. You can then select from a list of expressions for your parameter (see Figure 16.21).

As you can see from the list, you have quite a few expressions from which to choose. We won't try to describe each expression, but you should recognize most of them from your high school math class.