As I harp on a bunch of times elsewhere in this book, CAD programs are for precision drawing, so you'll spend a lot of your AutoCAD time drawing objects composed of straight-line segments. This rest of the chapter covers these commands:
The following additional straight-line drawing commands are also available in AutoCAD:
The RAY and XLINE commands are used to draw construction lines that guide the construction of additional geometry. Drawing construction lines is less common in AutoCAD than in some other CAD programs. AutoCAD's many precision techniques often provide more efficient methods for creating new geometry than adding construction lines to your drawing does.
The LINE command in AutoCAD draws a series of one or more visually connected line segments. Although the lines appear to be physically connected, in fact, each segment, or piece of a line with endpoints, is a separate object. This construction doesn't seem like a big deal until you try to move (or otherwise edit) a series of segments that you drew with the LINE command; you must select every piece separately. To avoid such a hassle, use polylines (described in the following section), not lines (or lines and arcs), when you want the connected segments to act as a single object.
If you're used to drawing lines in other programs, you may find it confusing at first that AutoCAD's LINE command doesn't stop after you draw a single segment. AutoCAD keeps prompting you to specify additional points so that you can draw a series of (apparently) connected segments. When you're finished drawing segments, just press Enter to finish the LINE command.
Unlike a lot of the AutoCAD drawing commands, LINE doesn't offer a bunch of potentially confusing options. It has a Close option to create a closed polygonal shape and an Undo option to remove the most recent segment that you drew.
Like all drawing commands, LINE puts the line segments that it draws on the current layer and uses the current color, linetype, lineweight, transparency, and plot-style properties. When you're doing real drafting, as opposed to just experimenting, make sure of the following:
Follow these steps to draw a series of line segments by using the LINE command:
The panels on the Home tab contain the most commonly used commands in AutoCAD.
AutoCAD starts the LINE command and prompts you to select the first point.
Remember to use one of the precision techniques I describe in Chapter 7 if you're doing real drafting. For the first point, object snap, snap, and typing coordinates all work well.
AutoCAD prompts you to specify the other endpoint of the first line segment. The command window prompt shows
Specify next point or [Undo]:
You can also see command prompts at the Dynamic Input tooltip beside the crosshairs by pressing the down-arrow key (press F12 or click the Dynamic Input status bar button to enable Dynamic Input mode). The arrow icon on the dynamic-input tooltip is your indicator that there are options available.
Again, use one of the AutoCAD precision techniques if you're doing real drafting. For the second and subsequent points, all the techniques mentioned in Step 4 work well, as do ortho, polar, object snap tracking, and direct distance entry.
After you specify the third point, AutoCAD adds the Close option. The command prompt shows
Specify next point or [Close/Undo]:
In either case, the blank command prompt indicates that the LINE command is finished:
Command:
The LINE command is fine for some drawing tasks, but the PLINE command is a better, more flexible choice in many situations. The PLINE command draws a special kind of object called a polyline. You may hear CAD drafters refer to a polyline as a pline (rhymes with beeline) because of the command name.
The most important differences between the LINE and PLINE commands are these:
Use PLINE instead of LINE in most cases where you need to draw a series of connected line segments. If you're drawing a series of end-to-end segments, there's a good chance that those segments are logically connected — for example, they might represent the outline of a single object or a continuous pathway. If the segments are connected logically, it makes sense to keep them connected in AutoCAD. The most obvious practical benefit of grouping segments together into a polyline is that many editing operations are more efficient when you use polylines. When you select any segment in a polyline for editing, the entire polyline is affected.
After you create a polyline, you can adjust its segments by grip-editing any of the vertex points. (The little squares on the vertices in Figure 8-3 are called grips; see Chapter 10 for details on grip editing.) For more complicated polyline-editing tasks, you can use the PEDIT command to edit the polyline, or you can convert the polyline to a collection of line and arc segments by using the EXPLODE command — although you lose any width defined for each segment when you explode a polyline. For more information on these two commands, look them up in the index of the online help.
Drawing polylines composed of straight segments is pretty much like drawing with the LINE command, as demonstrated in the following procedure. The PLINE command has lots of options, so watch the prompts! If Dynamic Input is on, use the down-arrow key to see the options listed at the crosshairs, or right-click to display the PLINE right-click menu, or simply read the command line.
To draw a polyline composed of straight segments, follow these steps:
AutoCAD starts the PLINE command and prompts you to specify a start point.
AutoCAD displays the current polyline segment line width at the command line and prompts you to specify the other endpoint of the first polyline segment:
Current line-width is 0.0000 Specify next point or [Arc/Halfwidth/Length/Undo/Width]:
Specify next point or [Arc/Halfwidth/Length/Undo/Width]: W Specify starting width <0.0000>: 0 Specify ending width <0.0000>: 0 Specify next point or [Arc/Halfwidth/Length/Undo/Width]:
Despite what you may think, a zero-width polyline segment isn't the AutoCAD equivalent of drawing with invisible ink. Zero width means “display this segment using the normal, single-pixel width on the screen.”
After you specify the second point, AutoCAD adds the Close option to the prompt. The command line shows
Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]:
In addition, you can view and choose options from the Dynamic Input options list, as shown in Figure 8-4, by pressing the down arrow on your keyboard.
AutoCAD draws the final segment. The blank command line indicates that the PLINE command is finished.
In the following procedure, I spice things up a bit and give you a preview of coming (curvy) attractions (in Chapter 9) by adding an arc segment to a polyline.
Just so you know, curved segments in polylines are circular arcs — pieces of circles with center points that you can draw with AutoCAD's ARC command. AutoCAD can draw other kinds of curves, including ellipses and splines, but not within the PLINE command.
To draw a polyline that includes curved segments, follow these steps:
The prompt changes to show arc segment options. Most of these options correspond to the many ways of drawing circular arcs in AutoCAD; see the section on arcs in Chapter 9. The command line shows
Specify endpoint of arc or [Angle/CEnter/CLose/ Direction/Halfwidth/Line/Radius/Second pt/Undo/Width]:
AutoCAD draws the curved segment of the polyline. The prompts continue to show arc segment options.
Your options at this point include
In this example, you return to drawing straight-line segments.
Perhaps the most useful of the alternative arc-drawing methods is Second pt. That gives you more control over the direction of the arc, but at the cost of losing tangency of adjacent segments. Sometimes it's best not to go off on a tangent, anyway.
Specify endpoint of arc or [Angle/CEnter/CLose/Direction/Halfwidth/Line/ Radius/Second pt/Undo/Width]: L
The prompt changes back to showing straight-line segment options.
Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]:
Figure 8-5 shows some of the things that you can draw with the PLINE command by using straight segments, arc segments, or a combination of both.
The LINE and PLINE commands work well for drawing a series of end-to-end single lines, but what if you want to draw a series of double lines to represent, for example, the edges of a wall or roadway? Here are some options:
You can use the PLINE or LINE command to draw a rectangle segment by segment. In most cases, though, you'll find it easier to use the special-purpose RECTANG command. The following procedure demonstrates how:
AutoCAD starts the RECTANG command and prompts you to specify a point for one corner of the rectangle. The command line shows
Specify first corner point or [Chamfer/Elevation/Fillet/Thickness/Width]:
AutoCAD prompts you to specify the other corner of the rectangle — the one that's diagonally opposite from the first corner.
Specify other corner point or [Area/Dimensions/Rotation]:
If you know the size of the rectangle that you want to draw (for example, 100 units long by 75 units high), type relative coordinates to specify the dimensions — for example, @100,75, or just 100,75 if Dynamic Input is enabled. (Chapter 7 describes how to type coordinates.)
AutoCAD draws the rectangle.
Unlike the neglected MLINE command, the RECTANG command has improved considerably since its debut. You can specify a rotation angle and — very handy for space planners — you can provide one dimension and an area. RECTANG will calculate the length of the other side and draw the rectangle.
Rectangles and other closed polylines are types of polygons, or closed figures with three or more sides. The AutoCAD POLYGON command provides a quick way of drawing regular polygons — polygons in which all sides and angles are equal.
The following procedure demonstrates the POLYGON command:
AutoCAD starts the POLYGON command and prompts you to enter the number of sides for the polygon.
Enter number of sides <4>:
Your polygon can have between 3 and 1,024 sides.
AutoCAD prompts you to specify the center point of the polygon.
Specify center of polygon or [Edge]:
AutoCAD prompts you to specify whether the polygon will be inscribed in (that is, the corners touch the circumference of the circle) or circumscribed about (that is, the sides are tangent to the circle) an imaginary circle whose radius you will specify in Step 6:
Enter an option [Inscribed in circle/Circumscribed about circle] <I>:
The command line prompts you to specify the radius of an imaginary circle.
Specify radius of circle:
AutoCAD draws the polygon.
If you type a distance or you click a point with ortho turned on, the polygon will be aligned orthogonally, as shown in Figure 8-6.
Figure 8-6 shows the results of drawing plenty of polygons — a practice known as polygony, which, as far as I know, is legal nearly everywhere.