AutoCAD offers an easy and intuitive way to draw circles, and it also offers . . .other ways. The easy way is to define the center point of the circle and then to specify the radius (the default option) or diameter. You can also define a circle by choosing one of the following options of the command (for those other ways):

Figure 9-1 illustrates these six different ways of drawing circles. Whether these additional methods are useful or not depends on the kinds of drawings that you make and how geometry is defined in your industry. Get familiar with the default center point/radius method and then try the other methods to see whether they may be helpful to you. If you find yourself going around in circles, you can always draw them the default way and move them into position with other geometry.

Figure 9-1. Pi R squared; circles R round.

Follow these steps to use the CIRCLE command:

Arcs in AutoCAD are, quite simply, pieces of circles. As with circles, AutoCAD offers you an easy way to define arcs. Just specify three points on-screen to define the arc, easy as one-two-three. These points tell AutoCAD where to start the arc, how much to curve it, and where to end it.

Sounds pretty easy, right? So where's the problem? The trouble is that you nearly always have to specify arcs more exactly than is possible by using this method. AutoCAD helps you specify such arcs, too, but the procedure ain't easy.

You can start your arc by specifying the center of the arc or the start point. If you choose the Center option, AutoCAD prompts you for the center point first and the start point second. AutoCAD defines arcs counterclockwise, so pick a start point in a clockwise direction from the endpoint. After you specify the center and start point, AutoCAD presents several options you can choose, including the following:

If you specify the start point as the first option, you can choose among the following three command options as well:

Tip

To get a feel for how these permutations can be strung together to create different arc-drawing methods, press the down arrow beside the Arc tool button on the Ribbon's Draw panel, or choose Draw

Figure 9-2. A deluge of Arc options, with the results of using some of those options.

The following example shows how you draw an arc with the default start point/second point/endpoint method:

In case you've forgotten your ninth-grade math, an ellipse is like a squished circle. Mathematically, an ellipse is defined by a major (long) axis and a minor (short) axis. These axes determine the ellipse's length, width, and degree of curvature. An elliptical arc is an arc cut from an ellipse.

The AutoCAD ELLIPSE command provides a straightforward way of drawing an ellipse: You specify the two endpoints of one of its axes and then specify an endpoint on the other axis. But like the ARC command, the ELLIPSE command offers a bunch of other options:

The following command line example creates an ellipse by using the default endpoints of the axes method. Figure 9-3 shows an ellipse and an elliptical arc.

Command: ELLIPSE
Specify axis endpoint of ellipse or [Arc/Center]: pick or
          type the first endpoint of one axis
Specify other endpoint of axis: pick or type the other
          endpoint of one axis
Specify distance to other axis or [Rotation]: pick or type
          the endpoint of the other axis

Figure 9-3. An ellipse and elliptical arc.

Most people use CAD programs for precision drawing tasks: straight lines, carefully defined curves, precisely specified points, and so on. AutoCAD is not the program to free your inner artist — unless your inner artist is Mondrian. Nonetheless, even meticulously created CAD drawings sometimes need freeform curves. The AutoCAD spline object is just the thing for the job.

You can use AutoCAD splines in two ways:

Drawing splines is straightforward, if you ignore the advanced options. The following procedure draws a freeform curve with the SPLINE command:

Figure 9-4 shows some examples of splines.

Figure 9-4. A slew of splines.

A donut in AutoCAD is another special type of polyline object that you create with (what else?) the DONUT command. (The rectangles and regular polygons I show you in Chapter 8 are also polyline objects.) Creating a donut is a simple way to define a single object that consists of two concentric circles with the space between them filled.

When you start the DONUT command, AutoCAD prompts you for the inside diameter and the outside diameter — the size of the hole and the size of the donut — as measured across their widest points. After you've entered these values, AutoCAD prompts you for the center point of the donut. But one donut is rarely enough, so AutoCAD keeps prompting you for additional center points until you press Enter (the AutoCAD equivalent of saying, "no, really, I'm full now!").

The following example draws a regulation-size donut, with a 1.5-inch hole and 3.5-inch outside diameter.

Command: DONUT
Specify inside diameter of donut <0.5000>: 1.5
Specify outside diameter of donut <1.0000>: 3.5
Specify center of donut or <exit>: pick or type the center
          point of one or more donuts

Figure 9-5. Donuts, plain and jelly-filled.

It's customary in many industries to submit a set of drawings at different project milestones or stages of completion and then submit them again later with revisions — corrections, clarifications, and requested changes. Usually, the recipients like to locate changed stuff easily, and a common drafting convention in many industries is to call attention to revised items by drawing freeform clouds around them. The REVCLOUD command makes quick work of drawing such clouds.

Drawing revision clouds is easy, after you understand that you click only once in the drawing area. That one click defines the starting point for the cloud's perimeter. After that, you simply move the crosshairs around, and the cloud takes shape. When you return to near the point that you clicked in the beginning, AutoCAD automatically closes the cloud.

The following command line example shows you how to draw a revision cloud. Figure 9-6 shows what revision clouds look like.

Command: REVCLOUD
Minimum arc length: 0.5000  Maximum arc length: 0.5000
          Style: Normal
Specify start point or [Arc length/Object/Style] <Object>:
           pick a point along the perimeter of your future
           cloud
Guide crosshairs along cloud path... sweep the crosshairs
           around to define the cloud's perimeter

You don't need to click again. Simply move the crosshairs around without clicking. AutoCAD draws the next arc segment of the cloud when your crosshairs reach the minimum arc length distance from the end of the previous arc segment.

Continue moving the crosshairs around until you return to the point where you clicked first.

Figure 9-6. Cloud cover.