Joining Objects

The whole idea of using AutoCAD is to draw precisely. Not surprisingly, you end up joining objects in exacting ways. One of the most common tasks is joining the endpoints of objects so that they meet exactly end to end.

AutoCAD provides several tools that join the endpoints of objects, particularly arcs, lines, and polylines. In this instance, joining means shortening or extending a line, an arc, or a polyline to meet another object, without disturbing the object's orientation.

Joining End to End with Intermediate Arcs

The Fillet command is one of the more frequently used commands because it is great at doing one thing: it joins lines end to end no matter where the endpoints of those lines are. Optionally, Fillet adds an intermediate arc between the joined lines to form a bullnose, or rounded corner, as shown in Figure 5.1.

Figure 5.1: Fillet will join the two lines at the left from end to end (center) or join them with an intermediate arc (right).

Fillet also joins two parallel lines with an arc. The following steps describe Fillet's default behavior:

1. From the 2D Draw control panel fly-out toolbar, choose Fillet.

2. At the following prompt, click one line:

    Current settings: Mode = TRIM, Radius = 0.0000 Select first object or [Polyline/Radius/Trim/Multiple]: 

3. At the Select second object or shift-select to apply corner: prompt, select the other line. The lines will join end to end without disturbing their orientation.

Fillet joins lines, arcs, and polylines. If you join a polyline with a line or an arc, the line or arc becomes part of the polyline.

Rounding Corners

Fillet also lets you round corners by adding an intermediate arc. You might notice several options in step 2, including a Radius option. If you enter R↲ in step 2, you will see the following prompt:

 Specify fillet radius <0.0000>: 

You can enter a radius value and then proceed to select two objects. Instead of joining the objects end to end, AutoCAD adds an arc between the objects.

The Fillet command continues to add arcs until you change the Radius option back to 0.

If you use Fillet to join two parallel lines, AutoCAD automatically uses an arc to join the two lines.

You can also round the corners of a polyline, as shown in Figure 5.2, by using the Radius option in conjunction with the Polyline option.

Figure 5.2: The polyline on the right is the result of using the Fillet command on the polyline on the left.

Start the Fillet command, enter R↲ and a radius, enter P↲, and select the polyline. The vertices of the polyline change into arcs of the specified radius. If you indicate a radius that is too large for the line segments, AutoCAD will report that the radius is too large and will cancel the operation. This happens when the line segments are shorter than the radius.

Rounding without Trimming

If you want to add an arc that is tangent to two objects but you don't want the objects trimmed to join the arc, you can change the Trim setting of the Fillet command. Figure 5.3 shows the effect of Fillet's No Trim option. (Note that the Chamfer command has a similar option that works the same way. See the section "Joining with a Chamfer" later in this chapter.)

Figure 5.3: Fillet with the No Trim setting turned on

To change the way Fillet trims objects, start the Fillet command, and then enter T↲. You'll see the following prompt:

 Enter Trim mode option [Trim/No trim] <Trim>: 

Enter N↲.

The No Trim option becomes the default method that Fillet uses to join lines until you set the option back to Trim.

Controlling Fillet's Behavior

Some additional options are available with the Fillet command that control how Fillet behaves. The following list describes them:

• Polyline Applies the Fillet command to all the corners of a polyline composed of straight-line segments. (See Figure 5.2 earlier in this chapter.)

• Radius Lets you set the radius of the fillet.

• Trim Lets you specify whether lines are trimmed when they are filleted. By default, lines are trimmed. You can specify No Trim to leave the lines as they are. Once Trim/No Trim is set, it becomes the default until it is changed.

• Multiple Lets you select multiple sets of lines to fillet.

Joining with a Chamfer

Another common editing operation is joining two lines with another intermediate line to form a chamfer, as shown in Figure 5.4. The Chamfer command works much like the Fillet command, with some different options.

Figure 5.4: Chamfer joins the two lines at the left to form the chamfered corner shown at the right.

To join lines with a chamfer, do the following:

1. In the 2D Draw control panel's Modify fly-out toolbar, click the Chamfer tool, choose Modify è Chamfer, or enter cha↲ at the command prompt. You will see the following message:

    (TRIM mode) Current chamfer Dist1 = 0.0000, Dist2 = 0.0000 

   This tells you the current settings for the chamfer distances.

2. At the Select first line or [Undo/Polyline/Distance/Angle/Trim/Method/Multiple]: prompt, enter d↲ to specify a Chamfer distance.

3. At the Specify first chamfer distance <1.0000>: prompt, enter a distance value. The value you enter determines the distance from the intersection of the two lines being chamfered to the beginning of the chamfer, as shown in Figure 5.5.

4. At the Specify second chamfer distance <2.0000>: prompt, enter a value for the other chamfer distance. Press ↲ if you want the first and second chamfer distances to be equal.

5. At the Select first line or [Polyline/Distance/Angle/Trim/mEthod/Multiple]: prompt, select one line.

6. At the Select second line: prompt, select the other line. The two lines are joined with a chamfer, as shown in Figure 5.6.

Figure 5.5: The Chamfer distance shown by arrows. The first Chamfer distance is determined by the line that is selected first (at the Select first line: prompt).

Figure 5.6: The polyline on the right is the result of using the Chamfer command on the polyline on the left.

Once you've set the distances in steps 3 and 4, AutoCAD remembers them for the current drawing. The next time you use chamfer, you won't have to set the distances. You can immediately select two lines as soon as you invoke the Chamfer command, skipping steps 2, 3, and 4.

Chamfer works on lines and polylines with straight-line segments only. You cannot use the Chamfer command on an arc to a line, for example.

You might notice other options in the previous steps. The following list describes what they do. Remember that to use an option, enter its capitalized letter at the prompt. For example, to use the Trim option in step 5 of the previous example, enter T↲. For the mEthod option, enter E↲.

Polyline Applies the chamfer to all the corners of a polyline composed of straight-line segments (shown earlier in Figure 5.6).

Distance Lets you specify the distance of the chamfer from the intersection of two lines. The distance can be equal for each line, or lines can have different values. (Also see "mEthod" later in this list.)

Angle Lets you specify an angle for the chamfer. You are first asked for a distance value and then asked for the angle (see Figure 5.7). (Also see "mEthod" later in this list.)

Figure 5.7: Chamfer's Angle option lets you specify the chamfer based on a distance and an angle. The angle is relative to the first line selected at the Select first line: prompt.

Trim Lets you specify whether lines are trimmed when they are chamfered. By default, lines are trimmed. You can specify No Trim to leave the lines as they are (see Figure 5.8). Once you have set Trim/No Trim, it becomes the default until it is changed. This option affects both Chamfer and Fillet.

Figure 5.8: A chamfer with the default Trim option appears on the left, and the No Trim option appears with the fillet on the right.

mEthod Lets you choose whether the Chamfer command uses the Distance or Angle setting by default. (See the "Distance" and "Angle" entries in this list.)

Multiple Lets you select multiple sets of lines to chamfer.

Extending or Trimming Lines to Other Objects

One fairly common operation in AutoCAD is to extend a line to meet another object or to trim a line or other object back to meet another object. For example, you can quickly trim two rectangles with a circle to form a wrench shape, as shown in Figure 5.9. Or you can extend a pair of lines to meet another line to form a wall extension, as shown in Figure 5.10.

Figure 5.9: Use the Trim command and select the locations indicated by the dots at left to quickly draw a new shape at right.

Figure 5.10: Extending the vertical lines shown at left to form an extension to a wall at right

Trim and Extend work on most AutoCAD objects with the exception of hatch patterns, text, splines, and regions. You can explode hatches and regions to their constituent parts to be trimmed or extended. You can use Trim or Extend on blocks and xrefs by first using the Refedit command. (See Chapter 8 for more about blocks and xrefs.)

Trimming Objects

You can use the Trim command to both trim and extend objects. The following steps show how you might use Trim to truncate a circle, as shown in Figure 5.11.

1. In the 2D Draw control panel's Modify fly-out toolbar, click the Trim tool, choose Modify è Trim from the menu bar, or enter tr↲ at the Command prompt. You'll see the following message:

    Current settings: Projection=UCS, Edge=None 

2. At the following prompt, select the object or set of objects you want to trim to, and then press ↲:

    Select cutting edges ... Select objects or <select all>: 

   In this example, select the circle and line shown in Figure 5.11.

3. At the Select object to trim or shift-select to extend or [Fence/Crossing/Project/Edge/eRase/Undo]: prompt, select the portion of the object you want to trim. In the circle example, select the locations indicated by the dots in the left image in Figure 5.11.

4. When you are finished, press ↲.

Figure 5.11: Trimming a circle and line at the left to form a truncated circle at the right

Extending Objects

To extend an object, start the Trim command as in the first two steps of the previous example, but instead of clicking the ends you want to trim, Shift+click to select the objects you want to extend, as shown in Figure 5.12.

Figure 5.12: Shift+click to select the lines shown on the left to get the results on the right.

To extend a series of lines to another object, as shown in Figure 5.13, use the Fence Selection option.

Figure 5.13: Extending a set of lines to a curve using the Fence option. Hold down the Shift key while placing the fence.

Another option is to use the Extend tool in the Modify menu or the 2D Draw control panel. Extend works just like Trim, but by default it assumes you want to extend an object to another object rather than trim objects.

Using Trim and Extend Options

You can control the way the Trim and Extend commands work through their options. Six options are available in both commands: Fence, Crossing, Project, Edge, eRase, and Undo. The following list describes their functions:

• Fence This allows you to select objects using the Fence selection option (see "Selecting Objects" earlier in this chapter).

• Crossing This allows you to select objects using a crossing window (see "Selecting Objects" earlier in this chapter).

• Project If you are working in 3D, the effects of the Trim or Extend command depend on your viewpoint. Project lets you control whether Trim or Extend are determined by the UCS or the current view. (See Chapter 6 for more about the UCS.)

• Edge You might want to extend or trim to an object that does not actually cross the object's path, as shown in Figure 5.14. You can use the Edge option to allow the Trim command to work whether or not objects actually cross.

• eRase This lets you erase an object while in the middle of a trim or extend operation.

• Undo This lets you undo a trim or extend operation.

Figure 5.14: Even though the lines do not cross, as shown in the left image, you can turn on the Edge option to trim to the edge defined by the dotted line in the middle image.