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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 endpoint of the arc; as you move the crosshairs around, AutoCAD shows how the arc will look.
5.Specify the endpoint of the arc by clicking a point or typing coordinates.
AutoCAD draws the arc, as shown previously in Figure 9-2.
As you may recall, pressing Enter repeats the last command. What often throws new AutoCAD users is that Enter doesn’t repeat the options of the last command. If you draw an arc by using the Center, Start, End option, for example, pressing Enter isn’t going to repeat that method — it’s going to repeat the ARC command in its default form, and the three points you pick probably won’t give you the arc you meant to draw. Bottom line: Watch the command line!
Solar ellipses
In case you’ve forgotten your ninth-grade math, an ellipse is like a squished circle (please excuse the technical jargon). Mathematically, an ellipse is defined by a major (long) axis and a minor (short) axis. These axes determine the ellipse’s length, width, and degree of curvature. An elliptical arc is an arc cut from an ellipse.
The AutoCAD ELLIPSE command provides a straightforward way of drawing an ellipse: You specify the two endpoints of one of its axes and then specify an endpoint on the other axis. But like the ARC command, the ELLIPSE command offers a bunch of other options:
Arc: Generates an elliptical arc, not a full ellipse. You define an elliptical arc just as you do a full ellipse. The methods that we discuss in this section for creating an ellipse apply to either.
Center: This option requires that you define the center of the ellipse and then the endpoint of an axis. You can then either enter the distance of the other axis or specify that a rotation around the major axis defines the ellipse. If you choose the latter, you can enter (or drag the ellipse to) a specific rotation angle for the second axis that, in turn, completely defines the ellipse.
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184 Part II: Let There Be Lines
Rotation: With this option, you specify an angle which defines the curvature of the ellipse — small angles make fat ellipses (0 degrees creates a circle, in fact), and large angles make skinny ellipses. After dinner tonight, if you’ve eaten everything including the vegetables, pick up your dinner plate and hold it up vertically in front of you and look straight on at it. It will look like a circle. Now slowly rotate it about its vertical axis until you are looking edge-on. In between, you will have seen every possible ellipse as defined by the rotation angle.
The following command line example creates an ellipse by using the default endpoints of the axes method. Figure 9-3 shows an ellipse and an elliptical arc.
Command: ELLIPSE
Specify axis endpoint of ellipse or [Arc/Center]: pick or type the first endpoint of one axis
Specify other endpoint of axis: pick or type the other endpoint of one axis
Specify distance to other axis or [Rotation]: pick or type the endpoint of the other axis
You can create elliptical arcs (as opposed to the circular arcs that the AutoCAD ARC command draws) by using the Arc option of the ELLIPSE command; it can draw a pretty good representation of a snowball trajectory, neglecting air resistance. Alternatively, you can draw a full ellipse and use the TRIM or BREAK command to cut a piece out of it.
Figure 9-3: To make an omelet, sometimes you have to break a few ellipses.
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Chapter 9: Dangerous Curves Ahead 185
Splines: The sketchy, sinuous curves
Most people use CAD programs for precision drawing tasks: straight lines, carefully defined curves, precisely specified points, and so on. AutoCAD is not the program to free your inner artist — unless your inner artist is
Mondrian. Nonetheless, even meticulously created CAD drawings sometimes need free-form curves. The AutoCAD spline object is just the thing for the job.
You can use AutoCAD splines in two ways:
Wing it. Eyeball the location and shape of the curve and don’t worry too much about getting it just so. That’s the free-form, sketchy, not-too- precise approach that we describe in this section.
Specify their control points and curvature characteristics precisely.
Beneath their easygoing, informal exterior, AutoCAD splines are really highly precise, mathematically defined entities called NURBS curves (NonUniform Rational B-Spline curves). Mathematicians and some mechanical and industrial designers care a lot about the precise characteristics of the curves they work with. For those people, the AutoCAD SPLINE and SPLINEDIT commands include a number of advanced options. Look up spline curves in the AutoCAD online help if you need precision in your splines.
Drawing splines is straightforward, if you ignore the advanced options. The following procedure draws a free-form curve with the SPLINE command:
1.Set an appropriate layer as current.
2.Click the Spline Fit button on the Home tab’s Draw panel slideout, or type SPL and press Enter.
AutoCAD starts the SPLINE command and prompts you to specify the start point of the spline. The command line shows
Current settings: Method=Fit Knots=Chord
Specify first point or [Method/Knots/Object]:
3. Specify the start point by clicking a point or typing coordinates.
AutoCAD prompts you to specify additional points:
Enter next point or [start Tangency/toLerance]:
4. Specify additional points by clicking or typing coordinates.
After you pick the second point, press the down-arrow key to display additional options at the Dynamic Input tooltip. (Enable Dynamic Input at the status bar if you need to.) The command line shows
Enter next point or [end Tangency/toLerance/Undo/
Close]:
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186 Part II: Let There Be Lines
Because you’re drawing a free-form curve, you usually don’t need to use object snaps or other precision techniques when picking spline points.
5.Press Enter after you’ve chosen the endpoint of your spline.
AutoCAD draws the spline.
You can specify the start and end tangency of the spline to control the curvature of the start points and endpoints of the spline. If all you’re after is a swoopy free-form curve, just picking points as in this example works fine.
Figure 9-4 shows some examples of splines.
Figure 9-4: A slew of splines.
After you’ve drawn a spline, you can grip edit it to adjust its shape. See Chapter 10 for information about grip editing. If you need finer control over spline editing, look up the SPLINEDIT command in the AutoCAD online help.
Donuts: The circles with a difference
A donut in AutoCAD is another special type of polyline object that you create with (what else?) the DONUT command. (The rectangles and regular polygons we show you in Chapter 8 are also polyline objects.) Creating a donut is a simple way to define a single object that consists of two concentric circles with the space between them filled.
When you start the DONUT command, AutoCAD prompts you for the inside diameter and the outside diameter — the size of the hole and the size of the donut — as measured across their widest points. After you’ve entered these values, AutoCAD prompts you for the center point of the donut. But one donut is rarely enough, so AutoCAD keeps prompting you for additional center points until you press Enter (the AutoCAD equivalent of saying, “No, really, I’m full now!”).
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