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152 Part II: Let There Be Lines
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; itâ€™s perfect for drawing those cannonball trajectories! Alternatively, you can draw a full ellipse and use the TRIM or BREAK command to cut a piece out of it.
An ellipse and elliptical
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 â€” unless your inner artist is Mondrian. Nonetheless, even meticulously created CAD drawings sometimes need freeform curves. The AutoCAD spline object is just the thing for the job.
You can use AutoCAD splines in two ways:
^ Eyeball the location and shape of the curve and donâ€™t worry too much about getting it just so. Thatâ€™s the free-form, sketchy, not-too-precise approach that we describe here.
^ 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 (SPL) commands include a number of advanced options. Look up â€śspline curvesâ€ť in the AutoCAD online help if you need precision in your splines.
Chapter 6: Where to Draw the Line 153
Drawing splines is straightforward, if you ignore the advanced options. The following procedure draws a free-form curve with the SPLINE command:
1. Set the desired layer current, and set other object properties that you want applied to the spline that youâ€™ll draw.
2. Click the Spline button on the Draw toolbar.
AutoCAD starts the SPLINE command and prompts you to specify the first endpoint of the spline. The command line shows:
Specify first point or [Object]:
3. Specify the start point by clicking a point or typing coordinates.
AutoCAD prompts you to specify additional points.
Specify next point:
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 cursor. The command line shows:
Specify next point or [Close/Fit tolerance] <start tangent>:
Because youâ€™re drawing a free-form curve, you usually donâ€™t need to use object snaps or other precision techniques when picking spline points.
5. Press Enter after youâ€™ve chosen the final endpoint of your spline.
AutoCAD prompts you to specify tangent lines for each end of the spline.
Specify start tangent: Specify end tangent:
The Specify start tangent and Specify end tangent prompts can control the curvature of the start and end points of the spline. In most cases, just pressing Enter at both prompts to accept the default tangents works fine.
6. Press Enter twice to accept the default tangent directions.
AutoCAD draws the spline.
Figure 6-9 shows some examples of splines.
After youâ€™ve drawn a spline, you can grip edit it to adjust its shape. See Chapter 7 for information about grip editing. If you need finer control over spline editing, look up the â€śSPLINEDITâ€ť command in the AutoCAD online help.
154 Part II: Let There Be Lines
A slew of splines.
Donuts: The circles with a difference
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 â€” the size of the hole and the size of the donut, as measured across their widest points. After youâ€™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, â€śno, really, Iâ€™m full now!â€ť).
The following example draws a regulation-size donut, with a 1.5-inch hole and 3.5-inch outside diameter. Figure 6-10 shows two kinds of donuts.
Specify inside diameter of donut <0.5000>: 1.5 Specify outside diameter of donut <1.0000>: 3.5 Specify center of donut or <exit>: pick or type the center point of one or more donuts Specify center of donut or <exit>:
You can use the DONUT command to create a filled circle â€” also known as a jelly-filled donut. Just specify an inside diameter of 0.
Revision clouds on the horizon
Itâ€™s customary in many industries to submit a set of drawings at a stage of completion and then submit them again later with revisions â€” corrections, clarifications, and requested changes. Often, the recipients like to locate
Chapter 6: Where to Draw the Line