Defining Arcs in AutoCAD 2008 - dummies

Defining Arcs in AutoCAD 2008

By David Byrnes

AutoCAD 2008 offers you an easy way to define arcs: Just specify three points on-screen. These points tell AutoCAD where to start the arc, how much to curve it, and where to end it. The trouble is that you must nearly always specify arcs more exactly than is possible by using this method. AutoCAD helps you specify such arcs, too, but the procedure isn’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:

  • 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 seldom or never use this option.
  • 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 previously.
  • 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.
  • Second point: This is the default option. The second point you choose is not the endpoint; instead, it’s a point on the arc that, along with the start and end points, defines how much the arc curves. After you enter the second point, you must enter an endpoint to complete the arc.

To get a feel for how these permutations can be strung together to create different arc-drawing methods, choose Draw –> Arc and look at the impressive submenu that unfurls.

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

1. Set the desired layer current and set other object properties that you want applied to the arc that you’ll draw.

2. Click the Arc button on the Draw toolbar.

AutoCAD starts the ARC command and prompts you to specify the first endpoint of the arc. The command line shows

Specify start point of arc or [Center]:

3. Specify the start point by clicking a point or typing coordinates.

AutoCAD prompts you to specify a second point on the arc.

4. Specify a second point on the arc by clicking a point or typing coordinates.

The second point lies somewhere along the curve of the arc. AutoCAD determines the exact curvature of the arc after you choose the final endpoint in the following step. To align the second point with an existing object, use an object snap mode.

AutoCAD prompts you to specify the other endpoint of the arc; as you move the crosshairs around, AutoCAD shows how the arc will look.

5. Specify the other endpoint of the arc by clicking a point or typing coordinates.

AutoCAD draws the arc.

Pressing Enter repeats the last command. What often throws new AutoCAD users is that Enter does not repeat the options of the last command. If you go through the command prompts or the Draw menu to draw an arc using the Center, Start, End option, for example, pressing Enter is not going to repeat that method — it’s going to repeat the ARC command in its default form, and the three points you pick will probably not give you the arc you meant to draw. Luckily though, AutoCAD has you covered. If you right-click instead of pressing Enter to repeat the ARC command, the first option is Repeat Start, Center, End.