Working with Advanced Surfacing Commands

AutoCAD provides five true Surfacing commands. These commands make use of existing geometry—usually polylines—to create surfaces. These surfacing tools include the following commands:

• EDGESURF

• RULESURF

• TABSURF

• REVSURF

• 3D MESH

The EDGESURF Command

The EDGESURF command is used to create a 3D surface between four connected lines or polylines. Figure 29.6 shows you an example of the result of using EDGESURF.

The EDGESURF command can be accessed by choosing the Draw menu, Surfaces, Edge Surface or by typing EDGESURF at the Command: prompt.

The resolution of the 3D surface generated by EDGESURF, as well as by the other surfacing commands, is controlled by two system variables: SURFTAB1 and SURFTAB2. These two variables represent the number of mesh faces in the M and N directions respectively. The letters M and N are used to reduce the confusion with the standard X and Y axes. The M direction, however, is generally considered to coincide with the X-axis direction and the N direction with the Y-axis direction The default value of both SURFTAB1 and SURFTAB2 is 6. By increasing these values, you get more accurate surfaces because more individual faces are generated between the bounding edges. Figure 29.7 shows you the difference between a surface with both variables set to 6 and the same surface with both variables set to 24.

The following exercise shows you how to use EDGESURF to create the canopy of an airplane.

Exercise 29.3 Using EDGESURF to Create the Canopy

1.	Load the file 29Canopy.DWG from the accompanying CD-ROM. The file contains the outline of an airplane canopy, as shown in Figure 29.8. Figure 29.8. The outline drawing of an airplane canopy ready for the EDGESURF command.
2.	At the Command: prompt, type SURFTAB1 . Set the value to 24 .
3.	Repeat step 2, setting Surftab2 to a value to 24 .
4.	From the Draw menu, select Surfaces, Edge Surface.
5.	Select the four lines in the drawing. The surface is then created, as shown in Figure 29.9. Because the SURFTAB variables are set the same, the order of selection is not important. Figure 29.9. The canopy with surface applied.
6.	Close this drawing without saving changes.

In this exercise, you set Surftab1 and Surftab2 to relatively high values. This resulted in a smoother surface at the expense of longer regeneration times and a larger drawing. If you change Surftab1 and Surftab2 to either higher or lower values, you must erase or undo the previous mesh and perform the surfacing operation again. Also note that the surface is generated on the current layer. As you work with EDGESURF, remember that the command neither removes your edges from the drawing nor gets attached to the mesh.

The RULESURF Command

The RULESURF command creates a ruled surface. Unlike the EDGESURF command, RULESURF requires only two defining edges instead of four. Because RULESURF works only between two lines, it makes use of just the SURFTAB1 system variable to establish the surface mesh density. Figure 29.10 shows examples of typical surfaces created with the RULESURF command. The defining edges can be points, lines, splines, circles, arcs, or polylines. If one of the edge objects is closed—a circle, for example—then the other object must also be closed.

If you generate a surface between two open boundary edges, you must be careful about the points you use to select the boundaries. RULESURF starts generating the mesh at each boundary by dividing the boundary curve into a number of segments equal to the current setting of Surftab1, starting from the endpoint nearest the pick point. If you use pick points on opposite sides of the two boundary curves, the resulting curve will be self-intersecting as shown in Figure 29.11.

The following exercise shows you how to use RULESURF to surface the contours of a site.

Exercise 29.4 Using RULESURF in a Mapping Application

1.	Load the file 29Edge.DWG from the accompanying CD-ROM. The file contains four contours, as shown in Figure 29.12. Figure 29.12. Four contour lines that you surface with RULESURF.
2.	From the Draw menu, select Surfaces, Ruled Surface.
3.	Select the top two lines in order then of Figure 29.12. The ruled surface is generated, as shown in Figure 29.13. Figure 29.13. The first surface generated between the first two contour lines.
4.	The smoothness of this curve is inadequate. Type SURFTAB1 and set the value to 24 .
5.	Use the ERASE command to erase the surface. Repeat step 3 with the new Surftab1 setting. The surface should now resemble Figure 29.14. Figure 29.14. The first surface generated between the first two contour lines at higher density.
6.	Select the new surface and use the PROPERTIES command to change its layer from 0 to Surface. Note that the Surface layer is currently frozen. Click OK in the frozen layer warning dialog box. The surface disappears when transferred to the frozen layer.
7.	Repeat step 3 selecting at and of Figure 29.12. RULESURF generates a second surface.
8.	Use the same procedure as in step 6 to transfer the second surface to the Surface layer.
9.	Use RULESURF and the boundaries at and to generate the third surface.
10.	Transfer the third surface to Surface layer and thaw Surface layer. Your drawing should resemble Figure 29.15. Figure 29.15. The completed contour surfaces.
11.	Close this drawing without saving changes.

This exercise demonstrates that the amount of detail necessary in a RULESURF mesh often depends upon the boundary object(s). With curved boundaries, Surftab1 may need to be set to a higher value to cause the surface to follow the boundary edge more closely. With sequential boundaries such as in the preceding exercise, it is a good idea to create the surfaces on a separate layer and then transfer them to a frozen layer. This allows the boundary edges to be more available for selecting.

The TABSURF Command

The TABSURF command creates a tabulated surface, or a surface that is extruded along a linear path. TABSURF is accessed by choosing Draw, Surfaces, Tabulated Surface or by typing TABSURF at the Command: prompt.

To create a surface with TABSURF, you must have two elements: an outline, or curve, to be extruded and a direction vector indicating the direction and distance the curve is to be extended. Figure 29.16 shows several examples of tabulated surfaces.

The path curve can consist of a line, arc, circle, ellipse, or a 2D or 3D polyline. TABSURF draws the surface starting at the point on the path curve closest to the selection point.

If the direction vector is a polyline, TABSURF considers only the first and last vertices of the line in determining the length and direction of the vector. In other words, TABSURF will extrude only a straight line. The end of the vector line chosen determines the direction of the extrusion. As with RULESURF, only Surftab1 has meaning with TABSURF.

The following exercise shows you how to use the TABSURF command to create a stair railing.

Exercise 29.5 Using TABSURF to Create a Stair Railing

1.	Load the file 29Tab.DWG from the accompanying CD. This drawing is shown in Figure 29.17. Figure 29.17. The path curve and direction vector for use in constructing a stair railing.
2.	From the Draw menu, select Surfaces, Tabulated Surface.
3.	Select the outline of the handrail.
4.	Select the vertical line near in Figure 29.17. The curve is extruded, as shown in Figure 29.18. Figure 29.18. The completed railing.
5.	Close this drawing without saving changes.

The REVSURF Command

REVSURF is perhaps the most useful of the 3D surfacing commands. REVSURF generates a 3D mesh object in the form of a surface of revolution by taking an outline—the path curve—and revolving it about an axis of revolution. As shown in Figure 29.19, the path curve is revolved around the axis of revolution to create the surface. The REVSURF command is accessed by selecting the Draw menu, Surfaces, Revolved Surface, or by typing REVSURF at the Command: prompt.

The path curve is the outline that will be revolved. It must be a single object: a line, arc, circle, ellipse, elliptical arc, polyline, polygon, spline, or donut.

The axis of revolution is the axis about which the path curve is revolved. The axis can be a line or an open 2D or 3D polyline. If a polyline is selected, the axis is assumed to be a line running through the first and last vertices.

REVSURF is made even more powerful by the fact that it can revolve the path curve through an included angle that can range from 0 to a full 360 degrees. The default angle is a full circle that results in the generation of a closed surface of revolution such as that shown in Figure 29.19. If you specify an angle less than 360 degrees, then the surface is generated in a counterclockwise direction. If you specify a negative angle less than 360 degrees, then the surface is generated in a clockwise direction.

You can also specify the start angle, which is the angular offset from the path curve at which the surface of revolution begins. The default value, 0, indicates that the surface of revolution will begin at the location of the path curve.

Determining the Positive Direction of Rotation

To have a start angle other than 0 degrees or an included angle other than a full circle, you must be able to determine the positive direction of rotation. REVSURF follows these conventions: A negative value dictates an angular distance in the clockwise direction, and a positive value dictates an angular distance in a counterclockwise direction. You can determine the direction of rotation by applying the so-called “right-hand rule.”

According to the right-hand rule, if you point your right thumb in the positive direction of the axis about which you are rotating and wrap your fingers of your right hand around the axis, the curl of your fingers indicates the direction of positive rotation. But how do you determine the positive direction of the axis?

The positive direction along the axis of rotation runs from the endpoint of the object nearest the pickpoint used to select the object to the other endpoint. For example, in Figure 29.20, if you select the line at point , then the positive direction of the axis runs from to . If you select the line near , then the positive direction of the axis runs from to .

Unlike the surfaces generated by TABSURF and RULESURF, but similar to the surfaces generated by EDGESURF, REVSURF generates a two-dimensional mesh and, therefore, both Surftab1 and Surftab2 influence the density of the resulting mesh. Again, keep both Surftab values as low as is consistent with a mesh that meets your needs.

The following exercise shows you how to use REVSURF to create a rivet.

Exercise 29.6 Using REVSURF to Create a Rivet

1.	Load the file 29Rev.DWG from the accompanying CD-ROM. The file contains an outline of the piston and its axis of rotation as shown in Figure 29.21. Figure 29.21. The outline of a piston and the axis of rotation line.
2.	At the Command: prompt, type SURFTAB1 . Set the value to 24 . Type SURFTAB2 . Set its value to 24 .
3.	Change the current layer to Mesh.
4.	From the Draw menu, select Surfaces, Revolved Surface.
5.	When prompted for the object to revolve, pick the piston outline.
6.	When prompted to select the object that defines the axis of revolution, pick the vertical line next to the outline.
7.	Press Enter to accept the default start angle of 0 degrees.
8.	When prompted for the included angle, press Enter to accept the default of 360 degrees. REVSURF creates the revolved surface. Your drawing should now resemble Figure 29.22. Figure 29.22. The revolved piston surface.
9.	From the Modify menu, select 3D Operation, Rotate 3D.
10.	Type All and then press Enter to select all objects for rotation. Press Enter again.
11.	Type X and then press Enter to specify the X-axis as the axis of rotation.
12.	When prompted for a point on the X-axis, use a Midpoint osnap to pick the midpoint of the line representing the axis of rotation.
13.	When prompted for the rotation angle, type 90 .
14.	The piston, its outline, and the axis line are rotated 90 degrees around the X-axis.
15.	From the View menu, select 3D Views, SE Isometric. AutoCAD switches to an isometric view. Notice that the original piston outline and the axis line are visible.
16.	From the View menu, select Shade, Flat Shaded. Your model should now resemble Figure 29.23. Figure 29.23. The revolved piston surface in a shaded rendering.
17.	Close this drawing without saving changes.

REVSURF offers more options and parameters than the other commands presented in this chapter. Although it is a somewhat complicated surfacing command, it is one of the most flexible and useful 3D tools available.