500 Part V: On a 3D Spree

Select a shaded visual style such as Realistic, Conceptual, X-Ray, or Shades of Gray, from the Visual Styles drop-down on the Visual Styles panel of the View tab. (See Chapter 21 for the low-down on visual styles.)

Render the model, as described in Chapter 23.

AutoCAD LT doesn’t include visual styles because they’re not much use in 2D drafting. A drawing saved in a visual style in the full version of AutoCAD does display that style when it’s opened in AutoCAD LT. However, there’s no way of changing it to 2D wireframe (or anything else) so you can actually work on it.

From Drawing to Modeling in 3D

This section introduces three techniques for creating 3D objects: drawing 3D lines and polylines, creating 3D objects from 2D geometry, and creating solids. (In AutoCAD LT you can use the first two techniques only.) AutoCAD is also very capable at surface modeling, offering both freeform mesh and NURBS surfaces. If you’re interested in either of these, check out the online help system; navigate to User’s Guide; then choose Work with 3D Models, and then Create 3D models. Topics include creating surfaces and meshes from scratch, and creating solids and surfaces from 2D objects.

When you draw 3D objects, just like when you draw 2D objects, put them on appropriate layers and use precision techniques to specify each point and distance. (See Chapter 5 for more information.)

Drawing basic 3D objects

The most basic forms of 3D geometry are wireframe-like objects created by picking points or entering X, Y, Z coordinates. Such objects have no surfaces, so they look the same in 2D Wireframe mode or a photorealistic rendering. They are most useful as paths for sweeps and lofts, or as edges for surface creation. Such objects include

Lines: Lines are really 2D objects; although you can specify different Z coordinates for startand endpoints so they’re not coplanar with the world coordinate system, each individual segment is based on its own 2D plane. You can, however, use lines for constructing objects in 3D space.

3D Polylines: Created with the 3DPOLY command. Similar to the 2D polylines that we describe in Chapter 8, except the vertices of 3D polylines can have different Z coordinates: 2D polylines must be planar. 3D polylines are useful as paths for sweeps or for fly-throughs. (We don’t cover walk-throughs or fly-throughs in this book.)

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Chapter 22: From Drawings to Models 501

Splines: Splines are free-form curves, created with the SPLINE command and we describe them in a 2D context in Chapter 9. Splines are 3D objects, and vertices can have different Z values. Splines are a better option than 3D polylines for sweeps because they can have smoother curves.

Helices: Helices can be either 2D (think of a mosquito coil, or the element on an electric range) or 3D (think of Mr. Slinky). Helices are especially useful as paths for threaded objects.

You can find the LINE, 3DPOLY, and SPLINE commands on the Draw panel of the Ribbon’s Home tab; the HELIX command is on that panel’s slideout.

The 3DPOLY command is similar to the PLINE (plain old 2D polylines) command. Both commands draw a series of connected line segments, but they have different capabilities:

The 3DPOLY command accepts 3D points for the line segments’ vertices. The PLINE command requires that all vertices be on the same plane.

3DPOLY is limited to straight line segments. PLINE can draw arc segments and create segments with uniform or tapered width.

Segments created with 3DPOLY can’t display dash-dot linetypes; 3D polyline segments always display as continuous lines.

The command sequence for drawing 3D segments with the LINE or 3DPOLY command is the same as for drawing 2D segments with the LINE or PLINE command; see Chapter 8 if you need a refresher on drawing lines. The only difference is that you specify 3D coordinates instead of 2D ones. Figure 22-6 shows an example.

Creating 2D representations in this way is straightforward, although tedious, for all but the simplest objects. More important, a wireframe model becomes increasingly difficult to decipher as the complexity of the model increases.

You see a mass of lines representing the edges, and you have difficulty telling which parts of which edges are in front of others. To reduce this visual confusion, you need to graduate to surface or solid modeling commands. We introduce you to solid modeling in subsequent sections of this chapter.

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502 Part V: On a 3D Spree

Figure 22-6: Entering 3D coordinates to draw a 3D polyline.

Gaining a solid foundation

Solid modeling is in many ways the culmination of 3D CAD. Solids more accurately represent most real-world objects than do wireframes or surfaces. And even when representational accuracy isn’t the main issue, it’s easier to construct many kinds of models with solids.

Many special-purpose solid modeling programs use a combination of solid and surface modeling techniques for maximum flexibility in constructing and editing 3D models. These kinds of programs — and solid modeling in general — are especially popular in mechanical design.

Constructing the basic building blocks — or solid primitives — for a solid model in AutoCAD isn’t difficult. Just follow these steps:

1.Define a suitable UCS (user coordinate system).

The UCS controls the construction plane and basic 3D orientation of the solid. Read about changing planes in Chapter 21.

2.Click the leftmost button on the Modeling panel of the Home tab, and then choose a solid primitive from the lower half of the split button.

As shown in Figure 22-7, your choices are Box, Cylinder, Cone, Sphere, Pyramid, Wedge, and Torus.

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Chapter 22: From Drawings to Models 503

Figure 22-7: Everything you need for a solid foundation.

When you see a 3D object in a drawing, you can’t tell by looking whether it’s a 2D extruded object, surface mesh, or solid. If you want to find out, open the Properties palette and select the object. The drop-down list at the top of the palette shows the type of object that you selected.

Drawing solid primitives

Solids are the easiest kinds of object to work with if you’re new to 3D. The two types of 3D solid object are

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