Next to lines, circles and arcs are the easiest objects to draw. But like everything else in AutoCAD, they offer a wide range of options to allow you to draw them in nearly any situation.
Click the Circle tool in the 2D Draw control panel, click the location for the center, and then click a point to indicate a radius or enter a radius value. You can use osnaps to select points on objects to determine the center and radius. You can also specify the diameter instead of the radius by typing D↲ after selecting a center point.
If you want to draw a circle based on two points, you can do so by using the 2P option. Here's how it works:
Click the Circle tool in the 2D Draw control panel, or enter C↲ at the command prompt.
Enter 2P↲. You can also right-click and select 2P from the shortcut menu.
At the Specify first end point of circle's diameter: prompt, click the first point.
At the Specify second end point of circle's diameter: prompt, click the second point. A circle appears using the two selected points to determine the circle's diameter.
You can also choose Draw è Circle è 2 Points, which automatically issues the 2P option in step 2. Use osnaps to select points on objects in steps 3 and 4.
If you know you want a circle to pass through three points but you don't know the center point or radius, you can use the 3P option. This lets you select three points to indicate the location of the circle.
Click the Circle tool in the 2D Draw control panel.
Enter 3P↲, or right-click and select 3P from the shortcut menu.
Select three points through which you want the circle to pass.
You can also choose Draw è Circle è 3 Points, which automatically issues the 3P option in step 2. Use osnaps to select points on objects in step 3. One use of the 3P option in conjunction with osnaps is to draw a circle tangent to three other circles or arcs. Choose Draw è Circle è 3 Points, and then use the Tan osnap to select a circle. Repeat using the Tan osnap to select two more circles (see Figure 3.2). AutoCAD draws the new circle tangent to the selected circles. This is how the Draw è Circle è Tan, Tan, Tan option works.
Figure 3.2: Drawing a circle tangent to three other circles
You can also draw a circle tangent to two objects. You must also specify a radius:
Click the Circle tool in the 2D Draw control panel.
Enter ttr↲, or right-click and select Ttr (tan tan radius) from the shortcut menu.
Select two objects to which the circle is to be tangent, and then enter a radius value (see Figure 3.3).
Optionally, you can choose Draw è Circle è Tan, Tan, Radius.
Figure 3.3: Drawing a circle tangent to two objects
AutoCAD does its best to draw a circle to your specifications, but if it's impossible, don't expect miracles.
The default method for drawing an arc is the following:
Click the Arc tool in the 2D Draw control panel, or enter A↲.
Select the start point.
Select a second point through which the arc is to pass.
Select the endpoint.
If this default method for drawing arcs does not fill your needs, you can use several other AutoCAD methods. Choose Draw è Arc to display a list of 11 ways to draw an arc. The first option, 3 Points, is the default just described. Figure 3.4 shows you how the other options work.
Figure 3.4: The methods for drawing an arc. The numbers indicate the order of point selection.
The Continue option, shown at the bottom of the menu, lets you continue an arc from the last line or arc drawn. You might notice that there isn't a Continue option in the Arc command prompt. To continue an arc, choose Draw è Arc è Continue or start the Arc command, and then press ↲ instead of selecting a point or entering an option. The arc begins from the last line or arc that was drawn.
You can draw an arc by first drawing a circle and then crossing the circle with two lines or other objects. Use the Trim command to trim the arc back to the crossing objects. This method is useful for constructing an arc that is connected to another object. See Chapter 4 for more about the Trim command.