You can use the Edge option to draw a polygon by specifying one side, instead of the center and radius of an imaginary inscribed or circumscribed circle. The imaginary circle method is much more common.
4. Specify the center point by clicking a point or typing coordinates.
The command line prompts you to specify whether the polygon will be inscribed in or circumscribed about an imaginary circle whose radius you will specify in the following step:
Enter an option [Inscribed in circle/Circumscribed about circle] <I>:
5. Type I or C and press Enter.
The command line prompts you to specify the radius of imaginary circle:
Specify radius of circle:
6. Specify the radius by typing a distance or clicking a point.
AutoCAD draws the polygon.
If you type a distance or you click a point with Ortho turned on, the polygon will be aligned orthogonally, as shown in Figure 5-5. If you click a point with Ortho turned off, the polygon most likely won’t be aligned orthogonally.
Figure 5-5: A polygonal party.
Figure 5-5 shows the results of drawing plenty of polygons — a practice known as “polygony,” and which, as far as I know, remains legal in most states.
Rectangles and polygons in AutoCAD are really just polylines that you specify in a way that’s appropriate to the shape you’re creating. You’ll notice this when you grip edit a rectangle or polygon and move one of the vertexes: Only the selected vertex moves. AutoCAD doesn’t make the entire rectangle or polygon larger or smaller. (See Chapter 6 for information about grip editing.)
(Throwing) Curves
Although straight line segments predominate in many CAD drawings, even the most humdrum, rectilinear design is likely to have a few curves. And if you’re drawing car bodies or Gaudi buildings, your drawings will be almost nothing but curves! This section shows you how to use the following AutoCAD curve-drawing commands:
? Circle: Draws circles (you were expecting rectangles, maybe?)
? Arc: Draws circular arcs — arcs cut from circles, not from ellipses, parabolas, or some other complicated curve
? ELlipse: Draws ellipses and elliptical arcs
? SPLine: Draws smoothly flowing curves of a variety of shapes
? DOnut: Draws filled-in annular rings and circles
? REVCLOUD: Draws free-form “clouds,” the most common application of which is to indicate revised areas in the drawing
The following sections describe each command.
AutoCAD offers an easy way to draw circles, and it also offers…
? 3P (3-Point): Specify any three points on the circumference.
? 2P (2-Point): Specify the endpoints of a diameter of the circle.
? Ttr (Tangent-Tangent-Radius): Specify two lines or other objects that are tangent to the circle, and then specify its radius.
Whether these additional circle-drawing methods are useful or superfluous 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.
1. Set object properties to the layer and other properties that you want applied to the circle that you’ll draw.
2. Click the Circle button on the Draw toolbar.
AutoCAD starts the Circle command and prompts you at the command line to specify the center point of the circle:
Specify center point for circle or [3P/2P/Ttr (tan tan radius)]:
The prompt shows the methods other than “center point plus radius” that you can use to draw circles in AutoCAD. (No, “tan tan radius” is not a mathematician’s dance.) Look up “CIRCLE command” in the online help if you think you may have a use for these less common circle-drawing methods.
3. Specify the center point by clicking a point or typing coordinates.
Use one of the precision techniques described in Chapter 4 if you’re doing real drafting. Object snap, snap, and typing coordinates all work well for specifying the center point.
The command line then prompts you to specify the circle’s radius:
Specify radius of circle or [Diameter]:
Type D and press Enter if you prefer to enter the diameter rather than the radius and you’ve forgotten your twos tables — or, more seriously, if the diameter is easier to specify with the cursor or type exactly than the radius is.
4. Specify the radius by typing a distance or clicking a point.
AutoCAD draws the circle, as shown in Figure 5-6.
Figure 5-6: Pi R square(d); circles are round.
Sounds pretty easy, right? So where’s the problem? The trouble is that you often must 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 end point. After you specify the center and start point, AutoCAD presents several options you can choose, including the following:
? Angle: This option specifies the included angle that the arc sweeps out. A 180-degree angle, for example, is a semicircle.
? Length of chord: This option specifies the length of an imaginary straight line connecting the endpoints of the arc. Most people use this option seldom or never.
? Endpoint: This option specifies where the arc ends. It’s the default option and is often the easiest to use.
If you specify the start point as the first option, you can choose among the following three command line options as well:
? Center: This option prompts you for the arc’s center point and then finishes with the three options listed above.
? End: This option specifies the endpoint of the arc. You then need to define the angle the arc covers, its direction, its radius, or its center point.
