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132 Drawing Parallel Lines

places the outermost element on the pick points. Logically, choosing Bottom justification does the reverse.

If you find the MLINE command’s justification labeling as confusing as we do, you’ll probably have better luck if you choose one justification (for example, Top) and one direction (for example, clockwise), and stick to it.

3.To change Scale from the current setting, type S.

The default value of 1.0 draws multilines exactly as specified. So if your multiline style has two lines with a 1-inch separation, that’s how far apart they’ll be in your drawing. If you wanted to use this multiline style to represent a 6-inch wide wall, you’d change the scale factor to 6.

Rather than try to do everything with a single multiline style, consider creating styles for real objects using real dimensions. For example, if you want a 6-inch wide wall, define the wall style so that the lines are 6 inches apart. That way you don’t have to even think about the Style option.

4.Type ST to switch the current multiline style to another style.

The new style must already exist in the drawing; you can’t create a new multiline style from the MLINE command prompt.

5.Pick a start point or enter coordinates.

The remaining prompts are identical to those for the LINE command. AutoCAD continues drawing multiline segments until you press Enter or Esc to terminate the command. After the second pick, there’s an Undo option, and after the third pick, a Close option.

Because multilines are single objects, they must be created according to a defined multiline style. By default, every AutoCAD drawing contains a multiline style definition called Standard. The default Standard style is used to create pairs of continuous parallel lines.

That’s how you create multilines and multilines. As for why you may want to use multilines, we leave that up to you but offer the following list of multiline shortcomings:

A sequence of multilines, such as the foundation drawing example, is a single AutoCAD object, and therefore difficult to edit; frequently users create geometry with multilines and then explode them to individual lines (we discuss the EXPLODE command in the next chapter).

Multilines can’t have curved segments; if you’re laying out a floor plan and need curved walls, you have to exit the command and draw the curved segments with regular lines and arcs.

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Multiline styles can’t be modified if there are any multiline objects drawn with that style in the drawing. If you want a slightly different style, you have to create a similar-looking multiline style and then re-create the lines.

You can’t change the style of an existing multiline object in the Properties palette as you can with other object types defined by styles.

Editing multilines requires a specialized set of tools and its own dialog box (although in AutoCAD 2005 and later you can use some regular editing commands on multilines).

It can be non-intuitive — not to say, downright tricky — to define multiline styles.

Complex Curves

Splines are smooth curves that pass near or through a series of control points. A spline object in AutoCAD can form either a closed or open shape. An ellipse is a symmetrical, oval shape that can be either closed or open (if it’s open, technically it’s an elliptical arc, but AutoCAD still classifies it as an ellipse). Ellipses are defined by a center point, a major axis, and a minor axis. In its early days, AutoCAD could only approximate spline curves and ellipses by creating polyline objects (we say more about polylines a little later). More recent versions have acquired the math skills to create what are known as Non-Uniform Rational B-Splines — or NURBS to their friends.

Lucy, you got some splining to do!

AutoCAD can create two different types of splines; one type is the spline-fit polyline, which we discuss in the next section. Industrial designers use splines to give form to organic shapes. Splines are created with the SPLINE command and are defined by an arbitrary number of control points. Depending on your needs, you can either eyeball your splines, or calculate the coordinates of their control points for absolute precision.

The following steps show you how to create splines and describe the options of the SPLINE command.

1.Click the Spline tool on the Draw toolbar, or choose Draw Spline, or type SPLine to start the SPLINE command.

AutoCAD prompts as follows:

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Specify first point or [Object]:

134 Complex Curves

2.Pick a point on-screen, or enter coordinates to specify a precise start point for the spline.

If you type O at this prompt, you can select a spline-fit polyline to convert to a NURBS spline.

3.Pick another point on-screen, or enter coordinates for the second point.

After you pick the second point, AutoCAD prompts:

Specify next point or [Close/Fit tolerance] <start tangent>:

4.Type F to specify a Fit tolerance.

Fit tolerance determines how closely the spline object comes to the points used to define it (see Figure 1-9). The default value of 0 forces the spline to pass precisely through the control points. Values greater than 1 allow the curve to pass that point within that tolerance value.

Figure 1-9:

A fit tolerance for your splines.

5. To create a closed spline, type C and press Enter.

AutoCAD makes its best guess at closing the spline shape, but needs one more piece of information from you. The command prompt reads:

Specify next point or [Close/Fit tolerance] <start tangent>: C

Specify tangent:

You can either pick a point on screen to set the tangency direction, or simply press Enter to accept the default tangent direction.

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6.To create an open spline, press Enter to accept the default choice and specify a start tangent direction.

AutoCAD prompts:

Specify start tangent:

Specify end tangent:

For each prompt, pick points on-screen, enter coordinates, or simply press Enter to accept the default tangency directions.

Solar Ellipses

Ellipses, like splines, are created as NURBS curves. Unlike splines, ellipses are closed, symmetrical shapes. Elliptical arcs are similar to ellipses, except that they’re open.

Drawing ellipses

AutoCAD provides two methods for drawing ellipses. You can define the ellipse by locating the two endpoints of its first axis and one endpoint of its second. Or, you can locate the center of the ellipse, and then one endpoint on each of its axes. The following steps show you how to create an ellipse:

1.Choose Ellipse from the Draw toolbar or Draw menu, or type EL to start the ELLIPSE command.

AutoCAD prompts:

Specify axis endpoint of ellipse or [Arc/Center]:

2.To draw an ellipse based on its center, pick a point or enter coordinates to specify the first endpoint of the first axis.

AutoCAD prompts:

Specify other endpoint of axis:

3.Pick a second point or enter coordinates for the second endpoint of the first axis.

AutoCAD prompts:

Specify distance to other axis or [Rotation]:

You have three choices here: You can specify a point by picking or typing a coordinate pair (the default method); you can specify a distance (that is, the length of the second axis); or you can use the Rotation option to specify an angle between the two axes.

Acceptable angle values range between 0 and 89.4°. The higher the rotation angle, the flatter the ellipse, and a 0° rotation will generate a circular ellipse.

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4.To draw an ellipse using the default method, pick a third point above or below the first axis, or enter a coordinate pair.

AutoCAD draws the ellipse.

Elliptical arcs

Drawing elliptical arcs is almost the same as drawing ellipses, except there are two additional inputs needed at the end of the process. The following steps explain how to draw an elliptical arc using the default construction method.

1.On the Draw toolbar, click the Elliptical Arc tool.

Use the toolbar button for the most direct route to this function, which is actually an option of the regular ELLIPSE command, as the command prompt shows:

Specify axis endpoint of ellipse or [Arc/Center]: _a Specify axis endpoint of elliptical arc or [Center]:

The Elliptical Arc tool is actually a command macro that’s programmed into the tool button to automatically choose the Arc option of the ELLIPSE command. The second line takes you to the first step of drawing a regular ellipse.

If you’re really attached to the menu system, you can choose Draw Ellipse Arc to run the same command macro (see Figure 1-10). But if you’re a dedicated keyboarder, you must type the A (for Arc option) yourself. Sometimes AutoCAD makes you work so hard!

2.Pick points on-screen to specify the first and second endpoints of the first axis, and the endpoint of the second axis.

AutoCAD prompts as follows:

Specify other endpoint of axis:

Specify distance to other axis or [Rotation]:

The final prompts allow you to set the internal angle of the elliptical arc.

3.Pick points on-screen to specify the start and end points of the elliptical arc.

AutoCAD prompts:

Specify start angle or [Parameter]:

Specify end angle or [Parameter/Included angle]:

For information on the Parameter option (which doesn’t do much that simply picking endpoints can’t handle), refer to the ELLIPSE command on the Index tab of the online Help.

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Figure 1-10:

Elliptical arcs are on today’s menu.

Complex Objects and Shapes

The last selection of commands and objects covered in this chapter are somewhat more complex than most of those covered previously. Some, like polylines, are extremely useful. Others, like 2D solids, are a lot less so.

2D Solids

The first thing to be said about 2D solids is that they’re not three-dimensional solid objects. We talk about working in 3D in Book V. Here we deal briefly with another ancient AutoCAD entity type, the 2D solid. Solid here refers to a solid-filled 2D outline.

A long, long time ago, AutoCAD was not able to solid-fill a closed area. If you wanted the effect, you had to use the SOLID command. A big drawback is that 2D solids can have only three or four sides — no more. So you might have had to create dozens of 2D solids to fill a randomly shaped area. The other problem with 2D solids is that their three or four sides are always straight.

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Ninety-nine times out of 100, you can get the effect you need with the HATCH command or a wide polyline. (We discuss hatching in Book III and polylines in the next sections.) If you still think you need 2D solids, check out the SOLID Command in the online Help.

Rectang, Polygon, Donut

No prizes for guessing what kinds of things that commands named RECTANG, POLYGON, and DONUT make. (Actually you may have to guess, but we don’t have any prizes for you!)

All three commands make polyline objects. A polyline is a very useful type of object, which can take almost any shape, as long as each part is connected to the other.

Rectangles

The RECTANG command creates four-sided closed polylines with horizontal and vertical sides. Command options allow you to draw rectangles in different ways. You can:

Locate two diagonal corners

Locate one corner, and then enter values for area and the length of one side

Locate one corner, enter values for length and width, and then specify direction

Locate one corner, specify a rotation angle, and then locate the other corner

In addition, before you pick the first point, you can

Set chamfer or fillet values so that all four corners are automatically chamfered or filleted

In drafting, a chamfer is a straight line that forms the transition between two 2D objects that meet at an angle; one example is a beveled edge at a corner. A fillet is a curved transition between two 2D objects; an example is rounded edge at a corner.

Set a width value so the sides of the rectangle have a 2D width

Set elevation and thickness values for the rectangle if you’re working in 3D.

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Polygons

Polygons, like rectangles, are closed polyline objects. Unlike rectangles, by default polygons are regular: They have equal length sides and equal internal angles.

If you want to draw irregular polygons, use the PLINE command.

AutoCAD’s POLYGON command draws regular polygons of a minimum of three and a maximum of 1,024 sides. After specifying the number of sides for your polygon, there are three options:

Edge: Locate an endpoint of the first edge, and then pick or enter a value to locate the second endpoint.

Inscribed: Locate the center of an imaginary circle, and then specify its radius. AutoCAD draws the polygon so that the corners touch the circumference of the circle (the circle is not actually drawn).

Circumscribed: Locate the center of an imaginary circle, and then specify its radius. AutoCAD draws the polygon so that the sides are tangent to the imaginary circle.

Donuts

Now for the biggest disappointment of the whole chapter. No, AutoCAD does not generate snack food when you run the DONUT command. Trust us — it’s better that way, anyway.

AutoCAD donuts are solid-filled polylines (as opposed to jelly-filled pastries). When you create donuts in AutoCAD, you specify an inside and an outside diameter, and then locate them by picking a center point.

A common use for donuts (the AutoCAD flavor, anyway) is in structural engineering drafting where you need to draw round reinforcing steel in a crosssection. You can specify an inside diameter of 0 to create a solid-filled donut. Just like asking for “Plain” at the coffee shop.

Polylines

A polyline is a single object that consists of one or (more usefully) multiple linear or curved segments. You create open or closed regular or irregular polylines with the PLINE command. Just like the LINE command, PLINE will continue adding line (or arc) segments until you terminate the command. Unlike the case with LINE, the objects you create with the PLINE command are single entities. Figure 1-11 shows geometry created with each command. The LINE command (on the left) created eight connected but separate line objects. The PLINE command (on the right) created one single object.

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Figure 1-11:

Drawing polylines (and lines).

The following sequence from the command prompt shows how the polyline in Figure 1-12 was drawn:

Command: _pline

Specify start point: 40,20

Current line-width is 0.0000

Specify next point or [Arc/Halfwidth/Length/Undo/Width]: W

Specify starting width <0.0000>: 0

Specify ending width <0.0000>: 2

Specify next point or [Arc/Halfwidth/Length/Undo/Width]:

@4<180

Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]: @4<180

Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]: W

Specify starting width <2.0000>: 2

Specify ending width <2.0000>: 4

Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]: @4<180

Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]: W

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Specify starting width <4.0000>: 4

Specify ending width <4.0000>: 0.5

Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]: @8<180

Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]: W

Specify starting width <0.5000>: 0.5

Specify ending width <0.5000>: 0

Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]: A

Specify endpoint of arc or [Angle/CEnter/CLose/Direction/Halfwidth/Line/Radius/Second

pt/Undo/Width]: @12<270

Specify endpoint of arc or [Angle/CEnter/CLose/Direction/Halfwidth/Line/Radius/Second

pt/Undo/Width]: <Press Enter to end the command>

Figure 1-12:

Constructing a polyline.

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