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 17.3. These are just lines and arcs and are not polylines.

You've just used the Autoconstrain command to add geometric constraints to all of 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 17.4). The Autoconstrain command makes a "best guess" at applying constraints.

Figure 17.4. The drawing with geometric constraints added

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.

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

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

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 Autoconstraint tool applied quite a few geometric constraints to the drawing. Suppose 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 arc at the left end of the drawing.

4. Click the arrowhead grip to the left of the arc, drag it to the left, and then click. This time, only the arc on the left side changes. The lines are no longer parallel but they remain tangent to both arcs and their endpoints remain connected (Figure 17.6).

Figure 17.6. Editing the arc with the parallel constraint removed

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 icon above the top line and select Delete.

   

3. Edit the arc on the left side again as you did in the previous exercises. This time the lines both change their orientation as shown in Figure 17.7.

Figure 17.7. Without the horizontal constraint, both lines change as the arc is edited.

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 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. 
2. 

3. Click a location roughly above and to the left of the drawing as shown in Figure 17.8. You don't need to be exact as you will use a geometric constraint to move the circle into an exact location.

   

   Figure 17.8. Place the circle roughly in the location shown here.

4. Type 0.25

   
5. 

6. Click the arc at the left side of the drawing, and then click the circle you just added. The circle moves to a location that is concentric to the arc as shown in Figure 17.9.

Figure 17.9. The circle is concentric to the arc on the left side.

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.

The Order Makes a Difference

When you add constraints, sometimes the order that you add them makes an important difference. In the concentric constraint example, you selected the arc first, then the circle. Had you selected the circle first, the arcs and lines would have moved to the circle. Instead, as you saw in the exercise, the circle moved to the inside of the arc.

You've seen firsthand 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.

There are many more geometric constraints you have at your disposal. Table 17.1 gives you a concise listing of the constraints and their purposes. Note that, with the exception of fixed and symmetric, all of the constraints affect pairs of objects.

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

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 a tangent orientation to a pair of arcs or circles as shown in Figure 17.10. The top image shows the separate line and circles and the bottom shows the objects after applying the tangent constraint. Note that while the line is tangent to the two circles, its length and orientation does not change.

Figure 17.10. You can connect two circles so that they are tangent to a line using the tangent constraint.

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

Start by adding a dimensional constraint between the two arcs:

The dimension 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.

Now try editing the part by changing the dimension:

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

Figure 17.14. Adding a dimensional constraint to the arc

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

Figure 17.15. Adjusting the arc dimension

As you can see from this example, the dimensional constraint lets you control the dimensions of your drawing by changing the 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.

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

Figure 17.19. Applying the user1 parameter to the dia1 parameter

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.

Try the following to see the parameters in action:

Figure 17.20. Change the d2 parameter and the part changes in size, including the circle.

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.

In the previous exercise, you used a simple formula that multiplied a parameter, d2, times 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, then right-click, you can select Expressions from the shortcut menu that appears. You can then select from a list of expressions for your parameter (Figure 17.21).

Figure 17.21. The expressions offered from the Parameters Manager

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