122 Part II Drawing in Two Dimensions
Cross-
Reference
Figure 7-2: The completed air compressor.
Thanks to the U.S. Army Corps of Engineers at Vicksburg, Mississippi, for this drawing. They maintain a Web site of drawings at http://cadlib.wes.army.mil.
It may have occurred to you that this task would have been easier if you could simply have copied one circle to another location instead of creating each circle from scratch. I cover copying in Chapter 9.
Drawing Arcs
An arc is a portion of a circle. Therefore, to define an arc, you have to define not only a circle — for example, by specifying a center and a radius — but also the start and endpoints of the arc. The ARC command offers several methods for defining an arc. The method you pick depends on the information that you have about the arc that you want to draw.
Understanding arc options
Making sense of all of the arc options may seem overwhelming, but after you understand the parts of an arc, you can choose the options that suit your needs. Figure 7-3 shows the parts of an arc that you can use to draw an arc. Refer to this figure as you read through the arc options.
Figure 7-4 shows the flow of the arc options. When you start the ARC command, you have two options, Start Point and Center. Depending on how you start, more options become available.
You can also press Enter at the first arc prompt to draw a second arc starting from the endpoint of a previous arc, line, polyline, and so on. The new arc continues in the same direction as the end of the first arc. The only other prompt is the endpoint.
Chapter 7 Drawing Curves and Points 123
Endpoint 
Direction
Radius 
Center |
Length of chord |
Start point
Included angle
Figure 7-3: The parts of an arc.
Figure 7-4: The ARC command options.
Drawing arcs
To draw an arc, choose Arc from the Draw toolbar and follow the prompts. As with circles, object snaps are often helpful when drawing arcs.
When drawing an arc by using the Start, End, and Radius options, the three specifications actually define two possible arcs, one minor and one major. The ARC command draws the minor arc by default, in the counterclockwise direction. (A minor arc is less than half a circle.) If you enter a negative number for the radius, the command draws the major arc. The options requiring an angle also define two possible arcs, one drawn counterclockwise and one drawn clockwise. AutoCAD and AutoCAD LT draw the counterclockwise arc by default. If you type a negative number for the angle, the arc is drawn clockwise.
124 Part II Drawing in Two Dimensions
On the
CD-ROM
The drawing used in the following exercise on drawing arcs, ab07-b.dwg, is in the Drawings folder on the CD-ROM.
STEPS: Drawing Arcs
1.Open ab07-b.dwg from the CD-ROM.
2.Save the file as ab07-02.dwg in your AutoCAD Bible folder. ORTHO is on, and units are set to Fractional. Choose Tools Drafting Settings (clicking the Object Snap tab, if necessary) and set running object snaps for intersection, center, and endpoint. Make sure OSNAP is on. In this exercise, you draw part of the sealing plate shown in Figure 7-5.
3.For a change of pace, start the LINE command by choosing Line from the Command Tools tab of the tool palette. (Press Ctrl+3 to open the tool palette.) Start at 2,3 and use Direct Distance Entry to create a 7-unit horizontal line to the right. End the LINE command. (See Chapter 4 for a full explanation of how to use Direct Distance Entry.)
4.Draw another line starting at 5-1/2,1-5/8 and draw it 2-3/4 units long in the 90-degree direction. These two lines are center lines and would ordinarily appear in a different color and linetype than the object you’re drawing. (You can read about colors and linetypes in Chapter 11.)
5.Draw a circle with its center at the intersection of the two lines (use the Intersection object snap) and a radius of 11/16.
6.Use the Center object snap to draw another circle with the same center as the first circle and a radius of 1.
Figure 7-5: The dimensioned sealing plate for a valve.
Thanks to Jerry Bottenfield of Clow Valve Company in Oskaloosa, Iowa, for this drawing.
Chapter 7 Drawing Curves and Points 125
7.Draw a third circle, using the From object snap (Shift+right-click to open the object snap menu, and then choose From). For the base point, use the Center object snap and pick either of the first two circles that you drew. The offset is @-1-15/16,0 (this means 1-15/16 units to the left of the center of the first two circles). Its radius is 3⁄8.
8.Draw a fourth circle. Use the From object snap again. For the base point, use the Center object snap and pick either of the first two circles. The offset is @1-15/16,0. The radius is 3⁄8.
9.
Choose Arc from the Draw toolbar. Follow the prompts:
Specify start point of arc or [Center]: Choose the From object snap.
Base point: Use the Center object snap to pick the center of the
leftmost circle.
<Offset>: @-5/8,0
Specify second point of arc or [Center/End]: Right-click and choose Center. Use the Center object snap to pick the center of the leftmost circle.
Specify end point of arc or [Angle/chord Length]: Right-click and
choose Angle.
Specify included angle: 67.23
10.Start the LINE command. At the Specify first point: prompt, press Enter to con-
tinue the line in the same direction as the end of the arc. At the Length of line: prompt, type 1-13/16 . End the LINE command.
11.Choose Arc from the Draw toolbar. Follow the prompts:
Specify start point of arc or [Center]: Use the Endpoint object snap to pick the end of the line that you just drew.
Specify second point of arc or [Center/End]: Right-click and choose Center. Use the Center object snap and pick any point on one of the large central circles.
Specify end point of arc or [Angle/chord Length]: Use Endpoint object snap to pick the lower end of the vertical construction line.
12.Repeat the ARC command. Follow the prompts:
Specify start point of arc or [Center]: Right-click and choose Center. Use the Center object snap and pick any point on one of the large central circles.
Specify start point of arc: Use the Endpoint object snap to pick the endpoint of the arc that you just completed.
Specify end point of arc or [Angle/chord Length]: Right-click and
choose Angle.
Specify included angle: 22.77
13.Start the LINE command. At the Specify first point: prompt, press Enter to con-
tinue the line in the same direction as the end of the arc. At the Length of line: prompt, type 1-13/16 . End the LINE command.
14.Start the ARC command. Follow the prompts:
Specify start point of arc or [Center]: Use the Endpoint object snap to pick the endpoint of the line that you just drew.