Editing Solid Models in AutoCAD 2000

31 Jul, 1999 By: John E. Wilson

With the introduction of AutoCAD 2000, Autodesk finally provides some real editing tools for 3D solid models. Of course, you always could make changes to solid models by using Boolean operations and occasionally making a strategic slice through a model. In fact, with some ingenuity and skill, you could do a surprising amount of modification to your solid models. However, there were no specific editing tools that, for example, would move holes and other components of a model, lengthen or shorten components, change the radius of arc and cylindrical shaped components or even remove unwanted components. You had to improvise, and it was often tedious and difficult work.

To many people, the lack of editing tools signified that AutoCAD was not a serious contender in the field of 3D solid modeling, and so they ignored the solid modeling features of the program entirely. If you are in that group, you will want to take a look at the 3D solid editing capabilities of AutoCAD 2000 to see if you need to reconsider your position. And, if you've been using the solid modeling features of earlier versions of AutoCAD, you will be anxious to learn how the new editing tools can help you.

Let's examine each of the 3D solid model editing tools that are in AutoCAD 2000 to give you an idea of what each tool is, what it can do and how you can use it. All of the tools are within a single command-Solidedit. Primarily, the tools edit by modifying faces on a 3D solid. You cannot restore the primitives you used in Boolean operations to build a model. In some ways, you will find that this approach gives you more freedom and creativity in modifying your solids; in other ways, you will find it to be restrictive and awkward. For example, if you want to change the diameter of a cylinder, you must calculate the difference between the current and the desired radius of the cylinder and offset the face of the cylinder by that amount. You cannot simply specify a new diameter. Also, there is no satisfactory way to change the radius of fillets. You will most likely edit fillets by deleting and remaking them.

The Solidedit Command
Solidedit issues a command line prompt that divides its editing options into

Figure 1. Each face on this 3D solid has a color that is different from its neighboring faces. This was done by the Color option of AutoCAD's new Solidedit command. Notice that the slot has four separate faces-one of which is hidden in this figure. The red area on the front of the model is a single face, even though it is curved, because it is based on a spline.
three categories. One category of options modifies faces. A face is a surface area on a 3D solid that has either a sharp edge with adjacent faces or is tangent to adjacent faces. The rather garish 3D solid shown in Figure 1 has each of its faces in a color that is different from those of its neighboring faces. Notice that the slot in this solid has four faces-two for straight section (one of which is hidden in the figure) and two for the rounded ends of the slot-while the round hole just has a single face. The curved face on the front of the model is a single face despite its curves. This is because the curves continually blend together so that you can't determine precisely where one curve ends and another begins. Extruding a spline curve made this face.

A second category of Solidedit options modifies edges, which are defined simply as the boundary between two faces. The options in the remaining category modify bodies. A body, as far as this option is concerned, is the solid object itself-including the components that make up the solid. Autodesk made an odd choice by electing to use the word body for this category of solid editing options because a body is an AutoCAD object type, just as is a line, a circle and even a 3D solid. However, bodies are unusual objects because you cannot make them directly. Body objects are the curved faces of exploded 3D solids. The Body category of Solidedit does not modify AutoCAD body objects.

Face Editing Options
Most editing operations on 3D solids involve faces. When you select this category, AutoCAD displays the following options on the command line:

Extrude/Move/Rotate/Offset/Taper/ Delete/Copy/coLor/Undo/eXit

Undo reverses the previous action, while eXit returns to the main Solidedit prompt. The other options are described in the following paragraphs.

Extrude. This option uses the edges of a face as a profile that is extruded in the same way that AutoCAD's Extrude command extrudes a wireframe profile. The face that is to be extruded must be flat. Just as with the command, you can extrude the profile for a specified distance or extrude it along a path that is defined by a wireframe object. Positive extrusion distance values push the profile away from the existing solid, while negative values push the profile into

Figure 2. These are seven before-and-after examples of some of the face editing options of Solidedit. Most of your editing operations will be done with these options.
the existing solid. You can also have the sides of the extrusion taper inward or outward provided a path is not used to define the extrusion. Figure 2 shows a face that has been extruded along a path.

Move. Use this option to move one or more faces. Just as in AutoCAD's Move command, you will be prompted for a base point and a destination point. You cannot move the faces to a different plane.

In Figure 2, the Move option was used to make the slot shorter by moving one of the curved end faces on the slot closer to the other curved face. You could also move the entire slot by selecting all of its faces, and you could make the box-shaped solid longer, shorter or taller by moving an appropriate rectangular face.

Rotate. You can rotate faces with this option. In Figure 2, for example, the slot has been rotated 90 degrees. AutoCAD gives you numerous choices for defining the rotation axis. You can select two points, use an object, use the view line-of-sight or use an axis parallel to the x, y or z axis as the rotation axis. After selecting a rotation axis, specify the rotation angle by entering an absolute angle or a reference angle. The faces can be rotated only within their neighboring face. Thus, the slot shown in Figure 2 could not be rotated onto the vertical face of the solid.

Offset. This option moves every point on a face a specified distance in a direction that is perpendicular to the face. For flat faces, the Offset option is often similar to the Move option. The radius of curved faces, though, becomes larger or smaller as they are offset. After you select the faces to be offset, AutoCAD prompts for the offset distance. Positive distance values offset faces away from the solid, while negative distance values offset faces into the solid. In Figure 2, the slot has been reduced in size by offsetting its four faces away from the solid. Unfortunately this option does not work well in changing the radius of fillets and rounds. Trying to offset their faces to increase their radius results in an error, and offsetting their faces to decrease their radius removes the tangency between the arc shaped surface of the fillet and its adjacent faces, as shown in Figure 2.

Taper. You can add a taper angle, which is often also referred to as a draft angle, to individual faces with this option. After selecting the face (or faces) to be tapered, you will be asked to specify a base point for the taper and then to specify the direction of the taper. The face (or faces) will be rotated about an axis that is on the plane of the base point and is perpendicular to the direction from the base point to the second point. In the first example of Taper shown in Figure 2, the base point is in the middle of the selected face, which causes the face to swivel. The second example of Taper in Figure 2 shows how all four sides of the slot can be tapered.

Delete. Use this option to remove unwanted features from your 3D solid. In the images shown in Figure 2, you can delete the fillet and any of the slots. If necessary, AutoCAD will fill in missing volumes to match the adjacent faces.

Copy. This option makes copies of one or more faces. Similar to AutoCAD's Copy command, you can place the copy by using base and destination points or by specifying a displacement vector. The object type of copied faces will be a region when the 3D solid face is planar and a body when the face is non-planar. The example in Figure 2 shows a copy of the four faces of the slot. Unfortunately, you can only make copies of the slot's faces; you cannot copy its hole.

coLor. This option allows you to assign a specific color to a specific face. Figure 1 is an example of how the option can color individual faces.

After you have selected one of the face editing options, you are prompted to select one or more faces, which turns out to be the trickiest part of the editing operation. All of the face-editing options use the same procedure for selecting faces. It starts with the command line prompt:

Select faces or [Undo/Remove]:

Neither of the two options of this prompt have any real effect, so you will most likely ignore them and select a face. AutoCAD will then issue the prompt:

Select faces or [Undo/Remove/ALL]:

This prompt is repeated as each face is selected until your press the <Enter> key. You must individually select a face by picking a point on it. Window and crossing selections are not allowed, and object selection cycling (in which you hold down the <Ctrl> key as you pick points) does not work. The point you pick can be on the edge of a face or on an isoline. You can also select a face by picking a point on its surface, even when surfaces are transparent (as they are in wireframe viewing modes). When a face is selected, AutoCAD will highlight its edges.

Selecting faces that are behind other faces from your viewpoint, such as the upper rectangular face of the slot in Figure 1, can be a problem. Your first pick will probably select the face that is in the foreground, so you must make a second pick in the same point to select the face you really want. Then you will have to remove the face you do not want edited. You can also set up multiple viewports with each viewport showing your model from a different viewpoint to give you an unobstructed view of every face you intend to edit.

The ALL option of the prompt selects all of the faces on the 3D solid. This is convenient for geometries in which it is easier to specify the faces that you don't want edited than it is to specify those you do want edited. You can select all of the faces and then deselect the ones that you don't want edited. The Remove option of the prompt is for deselecting faces. It displays the prompt:

Remove faces or [Undo/Add/ALL]:

Selecting faces to be removed from the selection set works the same as adding them. The All option removes all faces from the selection set, while the Add option returns to the Select faces prompt. The Undo option in both prompts cancels the last selection.

Edge Editing Options
Your options for editing edges are limited to making copies of them and adding color to them. With either option you can select as many edges as you want. Similar to AutoCAD's COPY command, you can use base and destination points or relative displacements in positioning copied edges. The AutoCAD object type of copied edges will be lines, arcs, circles or splines (depending on the shape of the edge).

Body Editing Options
> When you choose this Solidedit category, AutoCAD issues a command line prompt that has the following options:

Imprint/seParate solids/Shell/ cLean/Check/Undo/eXit

Undo reverses the most recent editing operation, and eXit returns to the main Solidedit prompt. The other options are described in the following paragraphs.

Imprint. You will use this option to divide a face on a 3D solid into two faces. After you select a 3D solid, the option asks you to specify an imprint object and then whether or not you want to delete it. The imprint object can be a wireframe object, such as a line, arc, circle or spline; a region or a body; or another 3D

Figure 3. The Imprint option can divide a face into two faces. In the top half of this figure, a line is imprinted onto the curved face of the solid. In the lower half, one of the two resulting faces has been tapered to demonstrate that the curved area is truly two faces.
solid. You can only select one object as the imprint object. The boundary of the resulting face will be where the imprint object touches or intersects a face on the selected 3D solid. In the top half of Figure 3, a vertical line that lies on the curved face is imprinted on the face to divide it into two faces. In the bottom half of Figure 3, one of the resulting two faces has been tapered to demonstrate that it is truly separate from the other face.

seParate Solids. This option creates separate 3D solids from a 3D solid that has empty space between two or more individual components. As soon as you select a 3D solid, AutoCAD creates a separate 3D solid from each individual component without any additional user input. This option does not decompose a solid into its primitives as the Solsep command of Autodesk's defunct AME solid modeling program did.

Shell. You are going to like this option. It hollows out a 3D solid, leaving a relatively thin-walled shell. You can also specify that some faces are to be

Figure 4. Using the Shell option of Solidedit, you can easily create models having relatively thin walls. The model on the left was constructed by extruding a profile and filleting its lower edges. The Shell option was then used to transform the solid into the one shown on the right.
open. On the right in Figure 4 is the shelled version of the 3D solid shown on the left. You are prompted to select one 3D solid, then to specify the faces to open and, lastly, to provide the shell wall thickness. Unlike the Mechanical Desktop shell feature, you can't specify a different wall thickness for each different face.

cLean. This is a utility option that removes imprints, any redundant edges and geometry that a 3D solid might have.

Check. When the new system variable solidcheck is set to 1, AutoCAD automatically inspects a 3D solid for internal errors and defects after every 3D solid editing operation. Turn off automatic checking by setting solidcheck to 0. If automatic checking is not on or if you want to validate a solid, the Check option will instruct AutoCAD to specifically inspect a solid. The option asks you to select a solid to check and then report its findings on the command line.

That completes our review of the 3D solid editing tools of AutoCAD 2000. If you are experienced in creating AutoCAD 3D solid models, you will probably wish there were more of them. But at the same time, you will appreciate and make use of those that are available.

About the Author: John E. Wilson

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