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Figure 23-26: The revolved surface.

Drawing an Extruded Surface

A simple way to create a 3D object is to start with a 2D object and extrude it (or thrust it out). In AutoCAD, extruding refers to creating a 3D object from a 2D object. The TABSURF command takes an outline, or profile, which AutoCAD calls a path curve, and extrudes it along a vector that defines the direction and distance of the extrusion. TABSURF creates a 3D polyline mesh. The type of surface created is called a tabulated surface. Figure 23-27 shows two examples of extruded surfaces.

Figure 23-27: Two extruded surfaces created using

TABSURF.

For the I-beam, you could have simply given the 2D polyline profile a thickness and achieved a similar result. However, the extruded surface on the left could only have been created with TABSURF because the extrusion is not perpendicular to the XY plane that contains the 2D polyline profile. TABSURF can extrude a shape in any direction.

When you select the vector object, your pick point determines the direction of the extrusion. AutoCAD starts the extrusion from the end of the vector closest to the pick point.

You use SURFTAB1 to control the number of lines AutoCAD uses to display the curve. If the curve is made up of polyline segments, AutoCAD displays one line at each segment vertex.

686 Part IV Drawing in Three Dimensions

Caution	Note the I-beam in Figure 23-27. If you create an object by mirroring, stretching, and so on,
	you’ll see extra tabulation lines at the separate segments in the polyline definition. If you
	want a clean look, you need to draw clean. You might need to use the original shape as a
	guide to draw a new polyline on top of the old one, and then erase the original.

You should use a nonplanar view when using TABSURF to check that you’ve accurately defined the extrusion vector into the third dimension. Any of the preset isometric views will be helpful.

To draw an extruded surface, follow these steps:

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1.Draw the object to extrude — a line, arc, circle, polyline, ellipse, or elliptical arc. This is the path curve.

2.Draw the vector, usually a line. If you use a 2D or 3D polyline, AutoCAD uses an imaginary line from the start point to the endpoint to determine the vector.

3. Choose Tabulated Surface from the Surfaces toolbar.

4.	At the Select	object	for	path curve: prompt, select the path curve object.
5.	At the Select	object	for	direction vector: prompt, select the line you’re using

for the vector.

The drawing used in the following Step-by-Step exercise on drawing tabulated surfaces, ab23-e.dwg, is in the Drawings folder on the CD-ROM.

STEP-BY-STEP: Drawing Tabulated Surfaces

1.Open ab23-e.dwg from the CD-ROM.

2.Save it as ab23-05.dwg in your AutoCAD Bible folder. You see a tabletop drawn at a Z height of 30. The current elevation is 30. You’re looking at the table from the SE isometric view. OSNAP should be on. Set running object snaps for endpoint, midpoint, and center. The current layer is Const. The drawing is shown in Figure 23-28.

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Figure 23-28: The tabletop.

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3.Start the CIRCLE command. Follow the prompts:

Specify center point for circle or [3P/2P/Ttr (tan tan radius)]:

Choose the From object snap.

_from Base point: Pick the endpoint at 1 in Figure 23-28.

<Offset>: @-1,3

Specify radius of circle or [Diameter]: .75

4.Start the LINE command. At the Specify first point: prompt, choose the Center

object snap of the circle you just drew. At the Specify next point or [Undo]: prompt, type @3,–3,–30 to draw a line flaring out from the circle and going down to the floor. End the LINE command.

5.Choose Tabulated Surface from the Surfaces toolbar. At the Select object for path curve: prompt, select the circle. At the Select object for direction

vector: prompt, select the line. (You can see only the top part of the line, but that’s the part you need to pick.) AutoCAD creates the tabulated surface.

6.Start the MIRROR command. Select the entire leg. Choose the midpoint of the bottom edge of both long sides of the table for the two points of the mirror line.

7.Repeat the MIRROR line and select both legs. Mirror them using the midpoints of the bottom edge of the short sides of the table for the two points of the mirror line.

8.Do a ZOOM Extents to see the entire table.

9.Save your drawing. It should look like Figure 23-29.

Figure 23-29: The completed table.

Drawing Ruled Surfaces

If you have two existing objects, you may want to define the surface that would extend between these objects. Use the RULESURF command to create a surface that extends between two objects. The objects can be lines, polylines (2D or 3D), circles, ellipses, elliptical arcs, splines, or points. The two objects must either be both open or both closed. Only one of the two can be a point.

688 Part IV Drawing in Three Dimensions

Use the SURFTAB1 system variable to control the number of lines AutoCAD uses to display the surface. Figure 23-30 shows some ruled surfaces.

The pick points of the two objects affect the resulting curve. If you pick them both on the same side, you get the type of curves shown in Figure 23-30. If you pick them on opposite sides, the curve intersects itself, as shown in Figure 23-31.

Figure 23-30: Ruled surfaces.

Figure 23-31: A self-intersecting ruled surface.

Follow these steps to draw a ruled surface:

1.Draw the two objects for the ruled surface.

2. Choose Ruled Surface from the Surfaces toolbar.

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3.At the Select first defining curve: prompt, choose the first object.

4.At the Select second defining curve: prompt, choose the second object.

The drawing used in the following Step-by-Step exercise on drawing ruled surfaces, ab23-f. dwg, is in the Drawings folder on the CD-ROM.

STEP-BY-STEP: Drawing Ruled Surfaces

1.Open ab23-f.dwg from the CD-ROM.

2.Save it as ab23-06.dwg in your AutoCAD Bible folder. You see a spline, as shown in Figure 23-32. In this exercise, you use the spline to draw some drapes.

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Figure 23-32: A spline.

3.Mirror the spline. For the mirror line, turn on ORTHO and use 1 and 2 as shown in Figure 23-32. Don’t delete the original spline.

4.Start the COPY command and select both splines. At the Specify base point or displacement, or [Multiple]: prompt, type 0,0,73 to copy the splines 73

units in the positive Z direction. Press Enter at the Specify second point of displacement or <use first point as displacement>: prompt.

5.Choose View 3D Views SE Isometric.

6.Choose Ruled Surface from the Surfaces toolbar. At the Select first defining curve: prompt, choose the top-right spline near its right endpoint. At the Select

second defining curve: prompt, choose the bottom-right spline near its right endpoint.

7.Repeat the RULESURF command. At the Select first defining curve: prompt, choose the top-left spline near its left endpoint. At the Select second defining curve: prompt, choose the bottom-left spline near its left endpoint.

8.Save your drawing. It should look like Figure 23-33.