# Drawing Curves

## Drawing Curves

Not everything you draw will consist of straight lines and perfect circles. For those more complex curves, AutoCAD offers some additional objects. Ellipses and elliptical arcs are two objects that are self-explanatory. Splines are specially designed to draw smooth curves that are mathematically accurate.

Polylines are the all-purpose lines that let you draw anything from a closed irregular polygon to contour lines on a topographical map. If you think you'll need to draw any of these types of objects, check out the following sections.

### Drawing Ellipses

When drawing an ellipse, remember that it has a major axis and a minor axis, as shown in Figure 3.5. You'll see prompts that ask you to select an axis. It doesn't matter which axis you select first; it can be either the major axis or the minor axis.

The default method for drawing an ellipse is to select two points defining one axis and then select a third point defining the other axis. Here are the steps:

1. Click the Ellipse tool in the 2D Draw control panel, or enter el . You can also choose Draw Ellipse Axis, End.

2. At the Specify axis endpoint of ellipse or [Arc/Center]: prompt, click the first point defining one end of an axis of the ellipse, as shown in Figure 3.5.

3. At the Specify other endpoint of axis: prompt, click another point for the axis, as shown earlier in Figure 3.5. You'll see an ellipse that is "fixed" at the two points you've selected. As you move the cursor, the ellipse changes shape to follow the cursor.

4. At the Specify distance to other axis or [Rotation]: prompt, click another point to complete the ellipse.

You might notice the Rotation option in the prompt in step 4. Selecting this option lets you simulate the way a circle looks on the side of an isometric or other 3D view. If you enter R in step 4 instead of clicking a point, you can enter a value indicating the angle from which you are viewing the circle.

You might use osnaps to determine the location of the points on the ellipse.

If you need to place the center of an ellipse at a specific location, you can use the Center option of the Ellipse command:

1. Click the Ellipse tool in the 2D Draw control panel, or enter el .

2. Enter C (or right-click, and choose Center from the shortcut menu), and then select a point for the center of the ellipse (see Figure 3.6). A rubber-banding line emanates from the point you select.

3. At the Specify endpoint of axis: prompt, select a point to define one axis of the ellipse.

4. At the Specify distance to other axis or [Rotation]: prompt, select a point to define the other axis of the ellipse.

### Drawing Elliptical Arcs

You draw elliptical arcs using the Ellipse command. In the menu bar options and 2D Draw control panel, you'll find an Ellipse Arc tool. Drawing an elliptical arc is the same as drawing an ellipse with the addition of prompts that ask you for beginning and end angles defining the arc. Here's how it works:

1. Click the Ellipse Arc tool in the 2D Draw control panel, or choose Draw Ellipse Arc.

2. At the Specify axis endpoint of elliptical arc or [Arc]: prompt, click the first point defining one end of an axis of the ellipse, as shown in Figure 3.7.

3. At the Specify other endpoint of axis: prompt, click another point for the axis. Once this is done, you'll see an ellipse that is "fixed" at the two points you've selected. As you move the cursor, the ellipse changes shape to follow the cursor.

4. At the Specify distance to other axis or [Rotation]: prompt, click another point to indicate the overall ellipse shape that defines the arc.

5. At the Specify start angle or [Parameter]: prompt, notice that a rubber-banding line emanates from the center of the ellipse. This helps you visualize the beginning of the arc. Click a point to determine its beginning.

6. At the Specify end angle or [Parameter/Included angle]: prompt, notice that an elliptical arc now appears from the location you selected in the previous step. As you move the cursor, the arc expands or contracts depending on the cursor location. Click a point to fix the arc in position.

### Drawing Smooth Curves

Whether you're a naval architect drawing a ship's hull or a civil engineer drawing map contours , you'll eventually need some way to draw smooth curves. AutoCAD provides two commands that do just that. Splines let you draw curves that conform to NURBS. Polylines are a general-purpose linetype that can simulate a smooth curve.

If you need accurate curves, use the Spline command to generate NURBS curves (see Figure 3.8).

To draw a spline, do the following:

1. Click the Spline tool in the 2D Draw control panel. To find the Spline tool, click and drag the fly-out arrow to the far, upper right of the 2D Draw control panel to open the fly-out toolbar, and then select the Spline tool. You can also choose Draw Spline from the menu bar.

2. At the Specify first point or [Object]: prompt, select a point to start the spline.

3. At the Specify next : prompt, select another point.

4. At the Specify next point or [Close/Fit tolerance] <start tangent>: prompt, continue to select points. As you select points, a curve appears that passes through each point. When you complete your curve, press .

5. At the Specify start tangent: prompt, you'll see a rubber-banding line emanating from the spline's starting point. This lets you adjust the tangent angle of the beginning of the spline. Press to accept the default angle, or indicate a direction with the rubber-banding line and click. You can always change it later if you want.

6. At the Specify end tangent: prompt, you'll see a rubber-banding line emanating from the spline's endpoint. Here, you can adjust the tangent angle of the end of the spline. Press to accept the default angle, or indicate a direction with the rubber-banding line and click. You can always change the tangent angle later if you want.

By default, AutoCAD draws the spline through the points you select. You can change the Fit Tolerance option as you draw so that the points you select indicate a direction for the curve rather than a point along the curve. In step 4 of the previous exercise, you'll see two options in the command prompt Close And Fit Tolerance. If you enter F , you can enter a value to indicate the amount of "pull" the selected points have on the curve. A value of 0 "pulls" the curve through the point, and a value greater than 0 draws the curve toward the point but not through it, as shown in Figure 3.9.

The Fit Tolerance value affects all the points along the spline, not just the currently selected point. You can modify the fit tolerance of individual points using the Splinedit command described in Chapter 5.

### Drawing Curves with Polylines

If you don't need the most accurate spline curves, you might want to draw your curves using the Polyline command. Polylines are the most versatile object type in AutoCAD because they can be shaped and duplicated quickly. You can quickly find the area enclosed by a polyline, and you can "explode" a polyline to smaller components when the need arises, which is something you cannot do to a spline.

To use a polyline to draw a curve, do the following:

1. Click the Polyline tool in the 2D Draw control panel, choose Draw Polyline, or enter pl at the command prompt.

2. At the Specify start point: prompt, select a point.

3. At the Specify next point or [Arc/Halfwidth/Length/Undo/Width]: prompt, continue to select points. A line appears between each point you select.

4. Press when you finish selecting points, or right-click and choose Enter.

Your polyline doesn't look like much of a curve, but you can alter the way it's drawn using the Pedit command:

1. Choose Modify Object Polyline, or enter pe .

2. At the PEDIT Select polyline or [Multiple]: prompt, select the polyline you just drew.

3. At the Enter an option [Close/Join/Width/Edit vertex/Fit/Spline/Decurve/Ltype gen/Undo]: prompt, enter S to select the Spline option. The straight lines of the polyline change to form a curve, as shown in Figure 3.10.

Notice that the spline version of the polyline does not pass through the points selected to generate the polyline. Instead, the curve is "pulled" toward the points without passing through them. This is similar to the behavior of a spline drawn with the Fit Tolerance option set to 1.

Another way to turn a straight-line polyline into a curve is to use the Fit option in the Pedit command. Instead of using the Spline option in step 3 of the previous exercise, enter F to use the Fit option, which changes the straight-line polyline into a set of arcs, as shown in Figure 3.11.

Since arcs are used, the curve is not as smooth, but the polyline passes through each point that defined the corners of the original straight-line polyline.

If you've created a spline curve using the Polyline command but you decide you'd rather have a true spline, you can convert it using the Spline command. This lets you apply some of the spline-editing options to a curve generated using polylines.

To convert a polyline spline curve into a true spline curve, use the Object option in the Spline command:

1. Click the Spline tool in the 2D Draw control panel, or choose Draw Spline.

2. At the Specify first point or [Object]: prompt, enter O , or right-click and choose Object.

3. At the Select objects: prompt, select the spline-fitted polyline curves you want to convert.

4. Press to finish your selection. You can also right-click and choose Enter. The polylines are converted to splines.

These steps will work only on spline-fitted polylines. You will get an error message if you try to do this on other types of polylines.

WHAT ARE MULTILINES ?

Multilines are double lines that you can use to represent anything that needs parallel lines, such as walls or borders. You can also customize multilines to display solid fills, center lines, and additional linetypes . You can save your custom multilines as multiline styles, which are in turn saved in special files for easy access from other drawings.

Multilines are not flexible and are difficult to work with, so you might not encounter them often. If you do encounter multilines and find you need to edit them, you can explode and edit them using the standard AutoCAD editing tools. When a multiline is exploded using the Explode command, it is reduced to its component lines. Linetype assignments and layers are maintained for each component. If you are working as part of a team, you will want to consult with your team members before you explode multilines in an AutoCAD drawing. (See Chapter 4 for more about the Explode command.)