Using the ARC Command

The ARC command is used to draw portions of circles known as arcs. Its several options make constructing an arc with a variety of known parameters such as center, start point, chord length, radius, and so on much easier than with traditional manual drafting methods.

In the following exercise, you will draw an arc by specifying three points on its circumference. After completing the arc, you will erase it and then draw two small arcs to round the corners of the fixture base. Use the coordinate display on the status bar and the Snap feature to identify points.

Exercise 7.3 Using ARC to Draw Circular Arcs

1.	Continue with the drawing from the preceding exercise. If necessary, turn on the SNAP mode by pressing the F9 key. Begin the ARC command by selecting the Arc tool from the Draw toolbar. Respond to the Specify start point of arc: or [Center]: prompt and specify the point at in Figure 7.5 by entering its coordinate: 156,48 . Figure 7.5. Drawing an arc using the 3-Points method.
2.	At the Specify second point of arc:or [Center/End]: prompt, specify the point at by typing 196,88 and pressing Enter. Note that the arc passes through as you move the cursor.
3.	At the Specify end point of arc: prompt, specify the point at by entering 156,128 . The ARC command draws the arc and terminates. Your drawing should resemble Figure 7.5.
4.	At the Command: prompt, issue the U command by typing U and pressing Enter. The arc is deleted. Before starting the next step, you may want to zoom in to enlarge the view. To do this, type Z and press Enter. Then type W and press Enter. Click and drag a window around the left side of the baseplate, and then click to zoom.
5.	Start the ARC command again by entering A . At the Specify start point of arc: or [Center]: prompt, enter the coordinate 56,128 (see in Figure 7.6). Figure 7.6. Creating rounded corners with the ARC command.
6.	Respond to the next prompt by typing CE and pressing Enter to select the Center option. Then, at the Specify center point of arc: prompt, specify the point at by entering 56,120 .
7.	Make sure that ORTHO is on (press F8 if necessary) and note that as you move the cursor, the arc snaps in 90-degree increments. Move the cursor to the left of and pick. The ARC command draws the arc; ORTHO forces it to 90 degrees.
8.	Restart the ARC command by pressing Enter. At the Specify start point of arc: or [Center]: prompt, enter the coordinate 48,56 to start the arc at .
9.	At the Specify second point of arc or [Center/End]: prompt, type CE and press Enter. This indicates that the next point you supply will be the center of the arc.
10.	At the Specify center point of arc: prompt, enter the relative rectangular coordinate @8,0 . This places the center of the arc at . With ORTHO on, the arc jumps to 90-degree increments as you move the cursor around.
11.	Disable ORTHO mode by pressing F8 and notice that the arc now drags with the cursor. Answer the Specify end point of arc or [Angle/chord Length]: prompt by entering A . This indicates that you will next supply the included angle for the arc.
12.	Turn ORTHO back on and move the cursor anywhere above the center point. Note that the arc now snaps to 90-degree points again. Pick any point directly above the center point near to complete the arc and end the ARC command.
13.	Leave the drawing open for a following exercise.

Whenever you draw an arc, you know either its center or its start point and can supply the other necessary information from existing geometry in the drawing. Figure 7.7 shows the Arc submenu from AutoCAD’s Draw menu. This submenu conveniently lists the various choices you can make, depending on the information supplied. Starting the ARC command by using this submenu provides a shortcut for bypassing the longer prompt choices provided at the Command: prompt.

The ARC Command Options

The ARC command requires three pieces of information to complete an arc, one of which must be either the center of the arc or its start point. The other required parameters can be supplied in various combinations. The following list explains these combinations:

• 3-Points. This method creates an arc that passes through three points you supply. The first point is considered the start point, the second point is the endpoint, and the third point is any other point between those two. This is the default method of constructing arcs.

• Start, Center. This method requires the arc’s starting and center points. The third piece of data can be the endpoint, an included angle, or the length of the chord. Counterclockwise arcs are drawn if the included angle is supplied as a positive angle; clockwise arcs are drawn if the angle is supplied as a negative angle. A positive chord length draws a minor arc (less than 180 degrees), and a negative chord length creates a major arc (greater than 180 degrees).

• Start, End. This method enables you to supply the start point and endpoint of the arc and then to specify how to draw the arc. You define the arc with an angle, direction, radius, or center point. When you supply a positive angle, AutoCAD draws a counterclockwise arc; when you supply a negative angle, it draws a clockwise arc. If you choose the radius option, AutoCAD always draws the arc counterclockwise. A negative radius forces a major arc, and a positive radius forces a minor arc.

• Center, Start. This method enables you to first identify the center of the arc and then the start point. Supplying either the angle, length of chord, or endpoint completes the arc. When you supply the length of chord, a negative length creates a major arc, and a positive length creates a minor arc. If you supply an angle, a negative angle draws the arc clockwise; a positive angle draws the arc counterclockwise.

• Continue. This method is the built-in default. You invoke this option by pressing Enter at the first arc prompt. It begins a new arc tangent to the last line or arc that was drawn.

The ARC command’s Continue option is often convenient for use when arcs are associated with line segments. The following exercise demonstrates this feature of the ARC command.

Exercise 7.4 Putting Lines and Arcs Together

1.	Continue from the drawing in the preceding exercise. Make sure ORTHO mode is turned on, and then take a look at Figure 7.8. It will be helpful to zoom in to provide a larger view of the baseplate features you have drawn so far. Figure 7.8. Use the Continue feature to draw slots.
2.	Start the LINE command by entering L . At the Specify first point: prompt, specify the point at by typing 64,56 and pressing Enter. Then at the Specify next point: or [Undo]: prompt, enter the relative coordinate @12<0 . See for reference.
3.	Choose the Arc tool from the Draw toolbar to cancel the LINE command and start the ARC command.
4.	At the Specify start point of arc: prompt, press Enter. This activates the Continue option of the ARC command and starts an arc tangent to the endpoint of the line you just drew. Then specify the point at by entering 76,76 .
5.	Restart the LINE command by selecting the Line tool from the Draw toolbar. At the Specify first point: prompt, press Enter. This activates the Continue option of the LINE command and starts the line tangent to the arc.
6.	At the Length of line: prompt, enter 12 . This draws a line to .
7.	Again, start the ARC command and press Enter. This again cancels the LINE command and starts an arc at the endpoint of the line.
8.	At the Specify endpoint of arc: prompt, enter the relative polar coordinate @20<270 . This completes the arc at .
9.	Your drawing should now resemble Figure 7.8. Leave the drawing open for the following exercise.

The drawing shows the fixture base with rounded corners on the left side. After the preceding exercise, you should be familiar with the continuing features of both the LINE and ARC commands. The “trick” is to disregard the first Specify first (or start) point of these two commands and press Enter instead. When you’re drawing an alternating series of line segments and arcs, this method can save you a significant amount of time selecting start points.