736 Part IV Drawing in Three Dimensions

14.Choose Pyramid from the Surfaces toolbar. Follow the prompts to draw a pyramidal pepper shaker:

Specify first corner point for base of pyramid: 2'6,2'6 Specify second corner point for base of pyramid: @1,0 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 slightly through the plate.

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.

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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.

Axis of rotation

Path curve

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.

738 Part IV Drawing in Three Dimensions

Pick point

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.

Setting the number of segments

You use the SURFTAB1 and SURFTAB2 system variables to determine how AutoCAD creates 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 that you want to see.

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

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

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Using the REVSURF command

To create a revolved surface, follow these steps:

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

2.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.

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

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

STEPS: 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 that is 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 View 3D Views SE Isometric.