Using the CIRCLE Command

Another basic AutoCAD shape is the circle. Circles are used to represent holes, wheels, shafts, columns, trees, and so on. Several methods exist for drawing circles, and unlike in manual drafting, constructing circles in AutoCAD is quick and accurate. Circles have centers, diameters, radii, and tangent and quadrant points. By providing a combination of these parameters, you can draw any circle and place it anywhere you want.

In the following exercise, you construct a few basic circles as you add holes and sleeves to the fixture base.

Exercise 7.5 Adding Holes and Sleeves by Drawing Circles

1.	Continue working with the drawing from the previous exercise. Refer to Figure 7.9 throughout this exercise. Figure 7.9. Drawing and placing circles.
2.	Start the CIRCLE command by typing C and pressing Enter or by selecting the Circle tool from the Draw toolbar.
3.	At the Specify center point for circle: or [3P/2P/Ttr (tan tan radius)]: prompt, specify the point at by entering 156,88 . Move the cursor and watch the radius and circle follow as you drag. The coordinate display in the lower left of the AutoCAD window should show changing X,Y coordinates. If it doesn’t, click on the display pane until it does. Then click twice in the coordinate display to change to a polar coordinate display. While you’re watching the polar coordinate display, slowly move the cursor and pick when the display reads 4.00@,0.00. AutoCAD draws a circle passing through .
4.	Press Enter or the spacebar to restart the CIRCLE command. At the Circle prompt, type @ and press Enter. When entered at any AutoCAD “point” prompt, the @ symbol automatically enters the last point entered—in this case, the center point of the circle you just drew.
5.	At the Specify radius of circle or [Diameter]: prompt, enter D to specify a diameter and then enter 20 . This specifies the diameter and draws the circle.
6.	Next, from the Draw menu, choose Circle and then 3-Points. At the Specify first point on circle: prompt, enter the point 104,88 .
7.	At the Specify second point on circle: prompt, enter the point 112,96 . At the Specify third point on circle: prompt, enter the point 120,88 . AutoCAD draws the circle using the three points on the circumference that you picked.
8.	Now press Enter to restart the CIRCLE command with the Specify center point for circle: or [3P/2P/Ttr (tan tan radius)]: default prompt. Enter 112,88 for the center point.
9.	To experiment with the effects of ORTHO mode, turn off ORTHO mode by pressing F8. Then move the cursor around while watching the polar coordinate display. Pick when a radius of 20 displays. AutoCAD draws the circle with a radius of 20 units. The baseplate should now resemble Figure 7.9. You will continue with this drawing in the next section.

CIRCLE Command Options

The CIRCLE command provides you with several options for controlling the sequence in which you create circles. In addition to the default center point/radius mode, you can create a circle by specifying three points on the circumference or by selecting two objects (lines, circles, or arcs) to which the circle is to be tangent and then specifying a radius.

When working with the CIRCLE command, you can use any of the following options:

• Center point. Type or pick the center point, and you are prompted for a radius or diameter. Radius is the default option. You override the radius default by entering D . When using either the radius or diameter option, you can enter a value or pick two points to show the distance.

• 3P (3 Points). Use this option to specify the circumference by entering or picking three points that will lie on the circumference.

• 2P (2 Points). Use this option to specify two diameter points on the circumference.

• TTR (Tangent Tangent Radius). Use this option to select two lines, arcs, or circles (any combination) that form tangents to the circle. Then specify the circle’s radius.

In the following exercise, you practice using the TTR option of the CIRCLE command.

Exercise 7.6 Practicing Circles with the TTR Option

1.	Continuing in the drawing from the preceding exercise, make sure object snap is on (press F3 if necessary and check the command line to see the Osnap status).
2.	Start the CIRCLE command by selecting the Circle tool from the Draw toolbar. Respond to the Specify center point for circle: or [3P/2P/Ttr (tan tan radius)]: prompt by entering T to indicate the tangent, tangent, radius option.
3.	Respond to the Specify point on object for first tangent of circle: prompt by resting the cursor anywhere on the circumference of the circle at (see Figure 7.10). Note the appearance of the tangent Osnap symbol and the label Deferred Tangent on the circle’s circumference. Pick to establish the tangent object. Figure 7.10. Placing circles using the tangent, tangent, radius method.
4.	At the Specify point on object for second tangent of circle: prompt, rest the cursor anywhere on the line . Note the appearance of the tangent Osnap symbol and the label Deferred Tangent on the line. Pick to establish the second tangent object.
5.	At the Specify radius of circle: prompt, enter 20 .
6.	AutoCAD draws the only circle that meets the requirements of being tangent to both the circle at and the line at and having a radius of 20 units (see Figure 7.11). Type U and press Enter to delete the circle. Figure 7.11. Drawing circles with the TTR option.
7.	Repeat steps 2 through 5, but at step 5 type 50 and press Enter. Once again, AutoCAD draws the only possible circle that is tangent to both the circle at and the line at and has a radius of 50 units (see Figure 7.11).
8.	Issue the UNDO command again to delete the last circle you drew. (It is not needed to complete the fixture.)
9.	Leave the drawing open for the following exercise.