Surfaces and Solids

31 Mar, 2001 By: John E. Wilson

Although most people concentrate on Mechanical Desktop’s parametric solid-modeling tools, the program also has a decent set of surface-modeling tools that can make smoothly curved shapes—such as those found on automobile body panels, telephones and other consumer products, plastic bottles and yacht hulls—that can’t be made with conventional solid-modeling tools. Furthermore, Mechanical Desktop surfaces and solids can interact to create solid geometry that is impossible to make directly. We will review Mechanical Desktop’s surface-modeling tools in this month’s column, and tell you how to use them to shape solids.

Surface Modeling
Typically the edges, surfaces and cross sections of surface models are in the shape of splines, which are natural curves similar to those formed by bending flexible thin strips (or small diameter rods) of spring steel or plastic. The characteristic property of splines is that their curves have continually changing radii along their length. The acronym NURBS, which stands for non-uniform rational b-spline, is commonly used in references to both wireframe curves and surfaces of Mechanical Desktop.

Virtually any shape can be reproduced as a surface model. The price for this power, though, is that surface models are often more difficult to construct than solid models. One reason for this disparity is that the spline-based geometry you are modeling is more subjective than is geometry based on just arcs, circles and lines. A second reason is that surfaces are usually built from wireframe boundary and edge objects. As a result, you must visualize the surface you intend to make and then draw curves that twist and turn through 3D space to create the surface you are visualizing.

Surface models are not parametric. Although surfaces can be edited and modified, these operations are not nearly as straightforward as those for parametric solid models. Also, while multiview 2D drawings can be made from surface models, they are not automatically dimensioned as are parametric solid models. In fact, since radius and diameter dimensions have no meaning in spline-based geometry, 2D drawings of surface models often can only be partially dimensioned.

When you are sizing up the design of an object, you will almost always first consider modeling it as a solid. But, if the geometry you expect the object to have can’t be made using solid-modeling operations, then you will build it as a surface model or else create surfaces you can use in shaping the solid model. Fortunately, the gap between the geometry that surface and solid-modeling techniques can make is decreasing. Mechanical Desktop is

Figure 1. This figure shows examples of the seven Mechanical Desktop commands for creating surfaces from wireframe curves. In each example, the wireframe objects are shown on the left and the resulting surface object is shown on the right.
now able to make some shapes as solid models that could only be made as surface models a couple years ago.

Table 1 gives a brief description of the Mechanical Desktop commands for creating surfaces, while Figure 1 shows examples of surfaces made from those commands. The three commands that transform the path of a wireframe profile moving through space into a surface are similar to the revolve, extrude and sweep operations for creating solid geometry. The profiles for creating solids, however, always represent an area, and must be planar and closed. The profiles for creating surfaces, on the other hand, represent wires and can have virtually any shape.

Table 1. Mechanical Desktop Commands for Creating Surfaces
Command Description
A flat surface is created by either drawing a rectangular boundary or by using a 2D closed wireframe boundary of any shape. The
surface can contain holes.
AMREVOLVE An open or closed curve is revolved about an axis to form a surface. The angle of revolution can be less than 360 degrees.
AMEXTRUDEA surface is created by pushing a wireframe profile linearly. A draft angle, or taper, is allowed.
AMSWEEPSFA surface is formed by moving an open or closed profile curve along a rail, which is a wireframe object that can be a 3D curve.
Additional profiles can be placed along the rail to further shape the surface. Also, two rails can be used to provide additional
sweep control.
AMRULEA surface is formed between two wireframe boundary objects. If the boundary objects have shapes that differ from each other,
the surface is blended to match them. The boundary objects can be either open or closed. If they are open, each side of the
surface is linear.


A surface is created from a set of open or closed wireframe cross-section objects that are roughly parallel to one another.

A surface is created from two sets of open or closed wireframe cross-section objects. In each set the wireframe objects are
roughly parallel to one another, and the two sets are approximately perpendicular to each other.

Because wireframe objects—lines, circles, arcs, splines and so forth—are so important in creating surfaces, Mechanical Desktop has some specialized commands for creating and editing them. These commands can create curves from sections and cross sections of surfaces (AMSECTION); change lines, polylines and arcs into splines (AMFITSPLINE); join wireframe objects, even if there are gaps between them (AMJOIN); and, create a set of curves (called flow lines) across a surface (AMFLOW). Enhanced commands for filleting (AMFILLET3D) and offsetting (AMOFFSET3D) wireframe objects are also available.

Mechanical Desktop also has a special wireframe object, called an augmented line, that has lateral direction in addition to lengthwise direction. Lateral direction is indicated by vector lines that are spaced along the length of an augmented line and are perpendicular to it. Despite their name, the shape of augmented lines can vary from a straight line to a 3D-spline curve. They are used for a variety of purposes, including serving as rails and as supplementary cross sections for swept surfaces. The AMAUGMENT command creates augmented lines from surfaces. And, the AMEDITAUG command transforms lines and polylines (but not splines) into augmented lines. The AMEDITAUG command also has options for editing the vectors, including rotating them, of an augmented line.

On your computer screen, surfaces are shown as a matrix of nodes connected by a mesh of lines and curves in wireframe views. Their appearance in hidden-line and shaded viewing modes is controlled by the same system variables and commands that control the display of solids. Surfaces, though, have a front and back side, which may cause some surfaces to disappear in hidden-line and shaded viewing modes. To ensure that both sides of surfaces are always visible, open AutoCAD’s Option dialog box, select the System tab, click the 3D Graphics Display Properties button and clear the Discard Back Faces checkbox.

Once you have created one or more surfaces, Mechanical Desktop has a variety of commands for editing them. The editing commands you are most likely to use are listed in Table 2.

Table 2. Mechanical Desktop Commands for Editing Surfaces
Command & Description

Trims two intersecting surfaces. As an option, you can create a 3D polyline on the intersection boundary

.Creates an arc-shaped surface (a fillet) between two adjacent surfaces. Variable radius fillets are allowed.
AMCORNERCreates a rounded corner fillet. Three edge fillets must exist and must intersect.

Smoothly fills a gap between two or more existing surfaces with a new surface that is a blend of the existing surfaces.
Splits a surface into two separate surfaces.
AMJOINSFJoins two or more adjacent surfaces. Small gaps between surfaces are allowed.
AMLENGTHENExtends or shortens a surface.
AMSCALEIncreases or decreases the size of a surface by a specified scale factor.
AMPROJECTProjects a wireframe profile onto a surface to make a hole.
AMOFFSETSF Offsets an existing surface to create a new one.
Edits surfaces on a node-by-node basis.

Release 5 Surfacing Enhancements
After having almost totally ignored surfaces for several revisions, Autodesk has included some significant surfacing enhancements to Release 5 of Mechanical Desktop. The command names for invoking these tools, along with a description of what the tool accomplishes, are listed in Table 3.

Table 3. Release 5 Surfacing Enhancements
Command & Description
Constructs a spline curve between and tangent to the edges of two existing surfaces. This command is helpful in creating opposite edges for a surface that is to connect two dissimilar surfaces.
Provides improved editing of splines through a dialog box.

Closes a gap between two surfaces by adjusting the edges of the surfaces.
Analyzes surface curvature, the effects of draft angle and mold pull directions, and the fit and finish of adjoining surfaces. This command asks you to specify the surfaces to be analyzed, and then displays them in a separate window, using color shading to depict the results. Figure 2 shows an example of a curvature analysis. With such an analysis, you can readily spot slight bumps and depressions in a surface that are virtually impossible to detect visually.

Figure 2. Release 5’s AMANALYZE command uses color shading to depict the relative curvature of surfaces. The command also uses shading to indicate the effects of mold pull direction and to study the fit and finish of adjoining surfaces.

Adds thickness to a surface and transforms it to a solid. This enables you to create solid geometry that can’t be made directly as a solid. The twisted portion of the steel strap solid model shown on the right in Figure 3, for example, can’t be made directly as a solid, but as shown on the left it can be easily made as a surface and then converted to the solid shown on the right. The resulting solid is a Mechanical Desktop base solid and, therefore, is not parametric. You can, though, add parametric features to a base solid.

Figure 3. Mechanical Desktop’s AMTHICKEN command, which is new to Release 5, adds thickness to a surface and transforms it into a solid. The twisted portion of the steel strap solid model, shown on the right, can’t be made directly as a solid, but it can be easily made as the surface shown on the left and then converted to a solid.

Shaping Solids with Surfaces
You can also use surfaces for shaping 3D parametric solids to create smoothly sculpted faces that can’t be made by conventional solid-modeling operations.

Figure 4. Surfaces can carve 3D parametric solids to create faces that can’t be made by conventional solid-modeling operations. The sculpted face shown in the left-hand panel of this figure could well be a surface model, but the back side of the model, as shown in the right panel, reveals that it is indeed a solid model.
An example is shown in Figure 4. The sculpted face shown in the left-hand panel of this figure could well be a surface model, but the back side of the model, as shown in the right panel, reveals that it is indeed a solid model.

To carve a 3D solid, you create a surface having the geometry you need, place it within the solid you intend to shape, invoke the AMSURFCUT command, select the surface and specify which side of the surface material is to be removed from the solid. The surface and a portion of the solid will disappear, leaving a sculpted surface on the remaining portion of the solid. The resulting 3D feature, which is listed in the Desktop Browser as a surfcut feature, can subsequently be used and modified (including additional surface cuts) like any other 3D solid

The rules for using a surface to cut a solid with AMSURFCUT are as

  • The surface must be a Mechanical Desktop surface. AutoCAD surfaces are not accepted.
  • The surface must be a single object. You can’t use two or more surfaces with a single call to AMSURFCUT.
  • All surface edges must

    Figure 5. As shown on the left in this figure, all edges of the surface used to make a surfcut feature must be outside the solid. The resulting surfcut feature is shown in a faceted hidden-line viewing mode on the right.
    be outside the solid, with the intersecting boundary between the solid and the surface forming a closed loop. See Figure 5 for an example of a valid surfcut surface.
  • Multiple intersections of the surface with the solid are allowed, provided a closed loop is formed with each intersection. Even though multiple faces are created on the solid from these intersection cuts, Mechanical Desktop treats them as a single feature.

    If you delete one face, the others are deleted as well. Figure 6 shows an example of a surface having multiple intersections with a solid.
  • The surface can’t contain a hole in the intersecting region of the solid.

Two examples of invalid surfaces for making a surfcut feature are shown in Figure 7. The surface on the left in this figure does not extend completely through the solid, and, therefore, the intersection of the surface and solid does not form a closed loop. The surface on the right in Figure 7 does extend completely through the solid, but it contains
a hole.

The AMSURFCUT command also has a mode, called protrusion, in which empty space between a solid and its surface is filled to become a 3D solid feature, while the rest of the solid disappears. The surface must completely enclose a space

outside the solid. Figure 8 shows an example of a feature made from the surfcut protrusion mode.

Even though surfcut features cannot be fully dimensioned, you can control their location by tying the surface to a work point that has been constrained to the solid by dimensions. Then, as the work point is moved relative to the solid, the surfcut feature will move with it. Furthermore, if the work point is on a work plane that is based on an angle or offset modifier, the surfcut feature will move as the work plane
is moved.

You can also make changes to a surfcut feature through the Surfcut option of the AMEDITFEAT command. When you select this option, Mechanical Desktop will restore the surface used for the surfcut and roll back the 3D solid to its original condition along with features created after the surfcut temporarily disappear. You can then change the shape of the surface to change the shape of the surfcut feature. Typically, you will use stretching operations, performed either by moving grips or by moving a crossing window, to change the shape of a surface.

About the Author: John E. Wilson