Chapter 23 Creating 3D Surfaces 681

Specify third corner point for base of pyramid: @0,1

Specify fourth corner point for base of pyramid or [Tetrahedron]:

@-1,0

Specify apex point of pyramid or [Ridge/Top]: t

Specify first corner point for top of pyramid: @1/4,1/4,3 Specify second corner point for top of pyramid: @-1/4,1/4,3 Specify third corner point for top of pyramid: @-1/4,-1/4,3 Specify fourth corner point for top of pyramid: @1/4,-1/4,3

15.Choose View Hide. You can now visualize the drawing better.

16.Save your drawing. It should look like Figure 23-22. If you look carefully, you’ll see that the edge of the cheese wedge goes through the plate slightly.

Figure 23-22: The table with a plate, wedge of cheese, bowl, orange, and nonmatching salt and pepper shakers.

Drawing a Revolved Surface

A common way to define a surface is to revolve an outline around an axis. You can create some very complex surfaces in this way. The REVSURF command takes an object that defines an outline or profile — AutoCAD also calls it a path curve — and revolves it around an axis, creating a 3D polygon mesh. Figure 23-23 shows two examples of revolved surfaces.

The path curve must be one object — a line, arc, circle, polyline, ellipse, or elliptical arc. It can be open, like the path curves shown in Figure 23-23, or closed. A closed path curve creates a model that is closed in the N direction.

Tip	If you have several adjoining objects that you’d like to use as one path curve, remember that
	you can use PEDIT to change lines and arcs to polylines and join them together. For more
	information, see Chapter 16.

682 Part IV Drawing in Three Dimensions

Axis of rotation

Path curve

Revolved 90o

Revolved 360o

Figure 23-23: Two revolved surfaces.

Determining the angle of rotation

You can start the angle of rotation at any angle; it doesn’t have to start on the plane of the path curve. You can rotate the path curve to any angle. Of course, rotating the path curve 360 degrees closes the model (in the M direction of the mesh).

When you rotate the path curve less than 360 degrees, you need to know which way to rotate. You can specify a positive (counterclockwise) or negative (clockwise) angle.

The point at which you pick the axis of rotation object affects the positive direction of rotation. Then you use the right-hand rule to determine which way the path curve will rotate around the axis. To do this, point your right thumb along the axis in the opposite direction from the endpoint closest to where you pick the axis. The direction in which your other fingers curl is the positive direction of rotation. Figure 23-24 shows the same model revolved in different directions. In the left model, the line of the axis was picked near the bottom endpoint. In the right model, the line of the axis was picked near the top endpoint.

Pick point

Pick point

Path curve

Figure 23-24: From the viewer’s point of view, the left revolved surface was rotated back 125 degrees, and the right revolved surface was rotated forward 125 degrees.

Chapter 23 Creating 3D Surfaces 683

Setting the number of segments

You use the SURFTAB1 and SURFTAB2 system variables to determine how AutoCAD displays the mesh. AutoCAD calls this the wireframe density.

SURFTAB1 affects how the M direction — the direction of revolution — is displayed.

SURFTAB2 affects how the N direction — the path curve — is displayed.

The higher the setting, the more lines AutoCAD uses to display the model. However, if the path curve is a polyline with straight segments, AutoCAD just displays one line at each segment vertex.

In Figure 23-24, SURFTAB1 is 6 and SURFTAB2 is 12. Figure 23-25 shows the same model with SURFTAB1 at 20 and SURFTAB2 at 5 to show the contrast between the two.

Note Although you count M and N mesh sizes by vertices, you specify SURFTAB1 and SURFTAB2 by the number of surface areas you want to see.

To set these system variables, type them on the command line and specify the new value you want.

Figure 23-25: The same model re-created with SURFTAB1=20 and SURFTAB2=5.

Using the REVSURF command

To create a revolved surface, follow these steps:

1.First create the path curve, which must be one object.

2.Then draw the axis of revolution, usually a line.

3. Choose Revolved Surface from the Surfaces toolbar.

4.At the Select object to revolve: prompt, select the path curve object.

5.At the Select object that defines the axis of revolution: prompt, select the axis of revolution object.

684 Part IV Drawing in Three Dimensions

6.At the Specify start angle <0>: prompt, press Enter to accept the default of 0 (zero) or type in a start angle.

7.At the Specify included angle (+=ccw, -=cw) <360>: prompt, press Enter to revolve the surface 360 degrees or type in a positive or negative angle.

You may need to create the path curve and the axis in a different plane than the one you use when revolving them. You can draw the path curve and axis in one UCS and use REVSURF in another. If the object doesn’t come out in the right direction, you can rotate the entire object when completed. Rotating objects in 3D is covered in the next chapter.

REVSURF retains the original path curve and axis objects. It helps to draw them in a different layer and color so you can easily erase them afterward. Otherwise, they’re hard to distinguish from the revolved surface. Having them on a separate layer also helps if you need to redo the revolved surface — you can more easily avoid erasing them when you erase the revolved surface.

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

STEP-BY-STEP: Drawing Revolved Surfaces

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

2.Save it as ab23-04.dwg in your AutoCAD Bible folder. The path curve and axis are already drawn in a UCS revolved around the X axis of the World Coordinate System (WCS) by 90 degrees.

3. Choose Revolved Surface from the Surfaces toolbar.

4.At the Select object to revolve: prompt, select the polyline to the right.

5.At the Select object that defines the axis of revolution: prompt, select the line.

6.At the Specify start angle <0>: prompt, press Enter. At the Specify included angle (+=ccw, -=cw) <360>: prompt, press Enter to revolve the path curve in a full circle.

7.Choose Tools New UCS World to return to the WCS.

8.Choose Top View from the View flyout of the Standard toolbar to return to plan view of the WCS.

9.Choose View 3D Views SE Isometric.

10.Choose View Hide to see the shape more clearly.

11.Save your drawing. It should look like Figure 23-26.