Chapter 20

Using Advanced 3D Features

AutoCAD for Mac’s extended set of tools, menu options, and commands for working with 3D drawings let you create 3D objects with few limitations on shape and orientation. This chapter focuses on the use of these features, which help you easily generate 3D forms and view them in both perspective and orthogonal modes.

In this chapter, you’ll learn to do the following:

• Master the User Coordinate System
• Understand the UCS options
• Use viewports to aid in 3D drawing
• Create complex 3D surfaces
• Create spiral forms
• Create surface models
• Move objects in 3D space

Setting Up AutoCAD for This Chapter

Before you start, I’d like you to set up AutoCAD in a way that will make your work a little easier. You’ll use the Modeling Tool Sets panel you were introduced to in Chapter 19, “Creating 3D Drawings.” To do so, follow these steps:

In Chapter 19, you started a new 3D model using a template set up for 3D modeling. Here you’ll start to work with the default 2D drawing. Now you’re ready to get to work.

Mastering the User Coordinate System

The User Coordinate System (UCS) enables you to define a custom coordinate system in 2D and 3D space. You’ve been using a default UCS, called the World Coordinate System (WCS), all along. By now, you’re familiar with the L-shaped icon in the lower-left corner of the AutoCAD screen containing a small square and the letters X and Y. The square indicates that you’re currently in the WCS; the X and Y indicate the positive directions of the X and Y axes. WCS is a global system of reference from which you can define other UCSs.

It may help to think of these AutoCAD UCSs as different drawing surfaces, or two-dimensional planes. You can have several UCSs at any given time. By setting up these different UCSs, you can draw as you would in the WCS in 2D yet draw a 3D image.

Suppose you want to draw a house in 3D with doors and windows on each of its sides. You can set up a UCS for each of the sides, and then you can move from UCS to UCS to add your doors and windows (Figure 20-1). In each UCS, you draw your doors and windows as you would in a typical 2D drawing. You can even insert elevation views of doors and windows that you created in other drawings.

In this chapter, you’ll experiment with several views and UCSs. All the commands you’ll use are available from the Tools menu on the menu bar and at the command line. Many are also available from the bottom of the Tool Sets palette in the Coordinates tool group (Figure 20-2).

Defining a UCS

In the first set of exercises, you’ll create a 3D model of the Barcelona chair, which is a fairly well-known chair design. In creating this chair, you’ll be exposed to the UCS as well as to some of the other 3D capabilities available in AutoCAD:

Notice that the UCS icon appears in the same plane as the current coordinate system. The icon will help you keep track of which coordinate system you’re in. Now you can see the chair components as 3D objects.

Next, you’ll define a UCS that is aligned with one side of the seat:

The Orthographic tab of the UCS dialog box offers a set of predefined UCSs for each of the six standard orthographic projection planes. Figure 20-5 shows these UCSs in relation to the WCS. You may notice that there is no top view. The World Coordinate System (WCS) can be used in place of a top view.

Because a good part of 3D work involves drawing in these orthographic planes, AutoCAD supplies the ready-made UCS orientations for quick access. But you aren’t limited to these six orientations. If you’re familiar with mechanical drafting, you’ll see that the orthographic UCSs correspond to the typical orthographic projections used in mechanical drafting. If you’re an architect, you’ll recognize that the Front, Left, Back, and Right UCSs correspond to the south, west, north, and east elevations of a building.

Before you continue building the chair model, you’ll move the UCS to the surface on which you’ll be working. Right now, the UCS has its origin located in the same place as the WCS origin.

You can move a UCS so that its origin is anywhere in the drawing where it’s needed:

You just created a new UCS based on the Front UCS you selected from the UCS dialog box. Now, as you move your cursor, the origin of the UCS icon corresponds to a 0,0 coordinate. Although you have a new UCS, the WCS still exists; you can always return to it when you need to.

Saving a UCS

After you’ve gone through the work of creating a UCS, you may want to save it, especially if you think you’ll come back to it later. Here’s how to save a UCS:

Your UCS is now saved with the name 3DSW. You can recall it from the UCS dialog box or by using other methods that you’ll learn about later in this chapter.

Working in a UCS

Next, you’ll arrange the seat and back and draw the legs of the chair. Your UCS is oriented so that you can easily adjust the orientation of the chair components. As you work through the next exercise, notice that although you’re manipulating 3D objects, you’re really using many of the same tools and commands you’ve used to edit 2D objects.

Follow these steps to adjust the seat and back and to draw legs:

The new UCS orientation enabled you to use the grips to adjust the chair seat and back. All the grip rotation in the previous exercise was confined to the plane of the new UCS. Mirroring and scaling will also occur in relation to the current UCS.

Building 3D Parts in Separate Files

As you work in 3D, your models will become fairly complex. When your model becomes too crowded for you to see things clearly, it helps to build parts of the model and then import them instead of building everything in one file. In the next set of exercises, you’ll draw the legs of the chair and then import them to the main chair file to give you some practice in the procedure.

I’ve prepared a drawing called legs.dwg, which consists of two polylines that describe the shape of the legs. I did this to save you some tedious work that isn’t related to 3D modeling. You’ll use this file as a starting point for the legs, and then you’ll import the legs into the barcelona1.dwg file:

Next, you need to change the Thickness property of the two polylines to make them 2˝ or 5 cm wide. You’ll use the Chprop command to do this:

As you’ve just seen, you can convert polylines that have both a width and a thickness into 3D solids. Thickness is another property both open and closed 2D objects have in common, and it is a property that can be edited through the Chprop command. Now you’re ready to add the legs to the rest of the chair:

In these last few exercises, you worked on the legs of the chair in a separate file and then imported them into the main chair file using the Clipboard. By working on parts in separate files, you can keep your model organized and more manageable. You may have also noticed that although the legs were drawn in the WCS, they were inserted in the 3DSW UCS that you created earlier. This shows you that imported objects are placed in the current UCS. The same would have happened if you inserted an Xref or another file.

Understanding the UCS Options

You’ve seen how to select a UCS from a set of predefined UCSs. You can frequently use these preset UCSs and make minor adjustments to them to get the exact UCS you want.

You can define a UCS in other ways. You can, for example, use the surface of your chair seat to define the orientation of a UCS. In the following sections, you’ll be shown the different ways you can set up a UCS. Learning how to move effortlessly between UCSs is crucial to mastering the creation of 3D models, so you’ll want to pay special attention to the command options shown in these examples.

Note that these examples are for your reference. You can try them out on your own model. Figure 20-2, shown earlier, will help you locate the tools mentioned in this section.

UCS Based on Object Orientation

You can define a UCS based on the orientation of an object. This is helpful when you want to work on a predefined object to fill in details on its surface plane. The following steps are for information only and aren’t part of the tutorial. You can try this at another time when you aren’t working through an exercise.

Follow these steps to define a UCS this way:

When you create a UCS using the Object option, the location of the UCS origin and its orientation depend on how the selected object was created. Table 20-1 describes how an object can determine the orientation of a UCS.

Table 20-1: Effects of objects on the orientation of a UCS

Object Type	UCS Orientation
Arc	The center of the arc establishes the UCS origin. The X axis of the UCS passes through the pick point on the arc.
Circle	The center of the circle establishes the UCS origin. The X axis of the UCS passes through the pick point on the circle.
Dimension	The midpoint of the dimension text establishes the UCS origin. The X axis of the UCS is parallel to the X axis that was active when the dimension was drawn.
Face (of a 3D solid)	The origin of the UCS is placed on a quadrant of a circular surface or on the corner of a polygonal surface.
Line	The endpoint nearest the pick point establishes the origin of the UCS, and the XZ plane of the UCS contains the line.
Point	The point location establishes the UCS origin. The UCS orientation is arbitrary.
2D polyline	The starting point of the polyline establishes the UCS origin. The X axis is determined by the direction from the first point to the next vertex.
3D polyline	Returns the message This object does not define a coordinate system.
Spline	The UCS is created with its XY plane parallel to the XY plane of the UCS that was current when the spline was created.
Solid	The first point of the solid establishes the origin of the UCS. The second point of the solid establishes the X axis.
Trace	The direction of the trace establishes the X axis of the UCS, and the beginning point sets the origin.
3D Face	The first point of the 3D Face establishes the origin. The first and second points establish the X axis. The plane defined by the 3D Face determines the orientation of the UCS.
Shapes, text, blocks, attributes, and attribute definitions	The insertion point establishes the origin of the UCS. The object’s rotation angle establishes the X axis.

UCS Based on Offset Orientation

At times, you may want to work in a UCS that has the same orientation as the current UCS but is offset. For example, you might be drawing a building that has several parallel walls offset with a sawtooth effect (Figure 20-12).

You can easily hop from one UCS to a new, parallel UCS by using the UCS Origin option. Click Define UCS Origin from the Coordinates tool group. You can also choose Tools ⇒ New UCS ⇒ Origin on the menu bar or type UCS↵ O↵. At the Specify new origin point <0,0,0>: prompt, pick the new origin for your UCS.

Another UCS option, called Move, will move an existing, named UCS to a new location and keep its original orientation. You won’t find the UCS Move option on any tool group or menu bar, but you can use it by entering UCS↵ M↵ at the Command prompt.

The steps in the following section are for information only and aren’t part of the tutorial. You can try this at another time when you aren’t working through an exercise.

UCS Rotated around an Axis

Suppose you want to change the orientation of the X, Y, or Z axis of a UCS. You can do so by using the X, Y, or Z flyout on the Tool Sets palette’s Coordinates tool group (Figure 20-13). These are perhaps among the most frequently used UCS options.

Similarly, the X and Y options enable you to rotate the UCS about the current X and Y axis, respectively, just as you did for the Z axis earlier. The X and Y tools are helpful in orienting a UCS to an inclined plane. For example, if you want to work on the plane of a sloped roof of a building, you can first use the Set UCS Origin tool to align the UCS to the edge of a roof, and then use the X icon to rotate the UCS to the angle of the roof slope, as shown in Figure 20-15. Note that the default is 90°, so you only have to press ↵ to rotate the UCS 90°, but you can also enter a rotation angle.

Finally, you align the Z axis between two points using the Z-Axis Vector option. This is useful when you have objects in the drawing that you can use to align the Z axis. Here are the steps:

Figure 20-16 shows the resulting UCS if you use the bottom of the barcelona1 chair leg to define the Z axis.

Because your cursor location is in the plane of the current UCS, it’s best to pick a point on an object by using either osnap overrides or coordinate filters.

Orienting a UCS in the View Plane

Finally, you can define a UCS in the current view plane. This points the Z axis toward the user. This is useful if you want to switch quickly to the current view plane for editing or for adding text to a 3D view.

Click the View tool from the Coordinates tool group of the Tool Sets palette. You can also choose Tools ⇒ New UCS ⇒ View on the menu bar or type UCS↵ V↵. The UCS icon changes to show that the UCS is aligned with the current view.

AutoCAD uses the current UCS origin point for the origin of the new UCS. By defining a view as a UCS, you can enter text to label your drawing, just as you would in a technical illustration. Text entered in a plane created this way appears normal.

You’ve finished your tour of the UCS command. Set the UCS back to the WCS by clicking the World UCS tool in the Tool Sets palette’s Coordinates tool group, and save the barcelona1.dwg file.

You’ve explored every option in creating a UCS, except one. In the section “Creating a Curved 3D Surface” later in this chapter, you’ll learn about the 3-Point option for creating a UCS. This is the most versatile method for creating a UCS, but it’s more involved than some of the other UCS options.

Saving a UCS with a View

AutoCAD has the ability to save a UCS with a view. Type V↵ E↵ U↵. The prompt Save current UCS with saved views? [Yes/No]: appears. The default setting is Yes, which saves the current UCS along with a saved view so that when the view is restored, the UCS is also restored with it.

Using Viewports to Aid in 3D Drawing

In Chapter 15, “Laying Out Your Printer Output,” you worked extensively with AutoCAD’s floating viewports in Paper Space. In this section, you’ll use tiled viewports to see your 3D model from several sides at the same time. This is helpful in both creating and editing 3D drawings because it enables you to refer to different portions of the drawing without having to change views.

Tiled viewports are created directly in Model Space, as you’ll see in the following exercise:

You’ve set up your viewports. Let’s check to see that your viewport arrangement was saved:

Now, take a close look at your viewport setup. The UCS icon in the orthogonal views in the two left viewports is oriented to the plane of the view. AutoCAD enables you to set up a different UCS for each viewport. The top view uses the WCS because it’s in the same plane as the WCS. The side view has its own UCS, which is parallel to its view. The isometric view to the right retains the current UCS.

Look at the 3D Views menu on the Viewport Controls for each viewport. Notice that they show the view name for each view: Top, Front, and SW Isometric. You can change these settings individually for each viewport.

Another Viewports dialog box option you haven’t tried yet is the Apply To pop-up menu in the New Viewports tab (Figure 20-19).

This pop-up menu shows two options: Display and Current Viewport. When Display is selected, the option you choose from the Standard Viewports pop-up menu applies to the overall display. When Current Viewport is selected, the option you select applies to the selected viewport in the sample view in the right side of the dialog box. You can use the Current Viewport option to build multiple viewports in custom arrangements.

You have the legs for one side of the chair. The next step is to mirror those legs for the other side:

Your chair is complete. Let’s finish by getting a better look at it:

Creating Complex 3D Surfaces

In the previous example, you drew a chair composed of objects that were mostly straight lines or curves with a thickness. All the forms in that chair were defined in planes perpendicular to one another. For a 3D model such as this, you can get by using the orthographic UCSs. At times, however, you’ll want to draw objects that don’t fit so easily into perpendicular or parallel planes. In the following sections, you’ll create more-complex forms by using some of AutoCAD’s other 3D commands.

Laying Out a 3D Form

In this next group of exercises, you’ll draw a butterfly chair. This chair has no perpendicular or parallel planes to work with, so you’ll start by setting up some points that you’ll use for reference only. This is similar in concept to laying out a 2D drawing. As you progress through the drawing construction, notice how the reference points are established to help create the chair. You’ll also construct some temporary 3D lines to use for reference. These temporary lines will be your layout. These points will define the major UCSs needed to construct the drawing. The main point is to show you some of the options for creating and saving UCSs.

To save time, I’ve created the 2D drawing that you’ll use to build your 3D layout. This drawing consists of two rectangles that are offset by 4˝ (10 mm for metric users). To make it more interesting, they’re also off center from each other (Figure 20-24).

The first thing you’ll need to do is set up the drawing for the layout:

Now you need to move the outer rectangle in the Z axis so its elevation is 30˝ (76 cm for metric users):

As an alternate method in step 3, after choosing Move from the Grip Edit shortcut menu, you can turn on Ortho mode and point the cursor vertically so it shows –Z in the coordinate readout. Then enter 30↵, or 76↵ for metric users.

Using a 3D Polyline

Now you’ll draw the legs for the butterfly chair by using a 3D polyline. This is a polyline that can be drawn in 3D space. Here are the steps:

All objects, with the exception of lines and 3D polylines, are restricted to the plane of your current UCS. Two other legacy 3D objects, 3D Faces and 3D meshes, are also restricted. You can use the Pline command to draw polylines in only one plane, but you can use the 3DPoly command to create a polyline in three dimensions. 3DPoly objects can’t, however, be given thickness or width.

Creating a Curved 3D Surface

Next, you’ll draw the seat of the chair. The seat of a butterfly chair is usually made of canvas, and it drapes from the four corners of the chair legs. You’ll first define the perimeter of the seat by using arcs, and then you’ll use the Loft tool to form the shape of the draped canvas. The Loft tool creates a surface based on four objects defining the edges of that surface. In this example, you’ll use arcs to define the edges of the seat.

To draw the arcs defining the seat edges, you must first establish the UCSs in the planes of those edges. In the previous example, you created a UCS for the side of the chair before you could draw the legs. In the same way, you must create a UCS defining the planes that contain the edges of the seat.

Because the UCS you want to define isn’t orthogonal, you’ll need to use the three-point method. This lets you define the plane of the UCS based on three points:

You’ve defined and saved a UCS for the front side of the chair. As you can see from the UCS icon, this UCS is at a non-orthogonal angle to the WCS. Continue by creating UCSs for two more sides of the butterfly chair using the 3-Point UCS option:

Next, you’ll mirror the side-edge spline to the opposite side. This will save you from having to define a UCS for that side:

Finally, let’s finish this chair by adding the mesh representing the chair seat:

You’ve got the beginnings of a butterfly chair with the legs drawn in schematically and the seat as a 3D surface. You can add some detail by using a few other tools and commands, as you’ll see in the next set of exercises.

Converting the Surface into a Solid

In the previous example, you used the Loft tool to create a 3D surface. Once you have a surface, you can convert it to a solid to perform other modifications.

You’ll want to round the corners of the seat surface to simulate the way a butterfly chair hangs off its frame. You’ll also round the corners of the frame and turn the frame into a tubular form. Start by rounding the seat surface. This will involve turning the surface into a solid so you can use solid editing tools to get the shape you want:

The seat surface appears to lose its webbing, but it has just been converted to a very thin 3D solid.

Shaping the Solid

The butterfly chair is in a fairly schematic state. The corners of the chair are sharply pointed, whereas a real butterfly chair would have rounded corners. In this section, you’ll round the corners of the seat with a little help from the original rectangles you used to form the layout frame.

First, you’ll use the Fillet command to round the corners of the rectangles. Then, you’ll use the rounded rectangles to create a solid from which you’ll form a new seat:

Next, create a 3D solid from the two rectangles using the Loft tool:

Finding the Interference between Two Solids

In the next exercise, you’ll use a tool that is intended to find the interference between two solids. This is useful if you’re working with crowded 3D models and you need to check whether objects may be interfering with each other. For example, a mechanical designer might want to check to make sure duct locations aren’t passing through a structural beam.

You’ll use Interference Checking as a modeling tool to obtain a shape that is a combination of two solids: the seat and the rectangular solid you just created. Here are the steps:

As mentioned earlier, the Interference Checking tool is intended to help you find out whether objects are colliding in a 3D model; as you’ve just seen, though, it can be an excellent modeling tool that can help you derive a form that you may not otherwise have the ability to create.

A number of other options are available when you’re using the Interference Checking tool. Table 20-2 lists the options in the Interference Checking dialog box.

Table 20-2: Interference Checking dialog box options

Option	What It Does
First Set	Specifies the number of objects in the first set of selected objects
Second Set	Specifies the number of objects in the second set of selected objects
Interfering Pairs Found	Indicates the number of interferences found
Delete Interference Objects Created On Close	Deletes the interference object after the dialog box is closed
Zoom To Pair	Zooms to the interference object while you’re using the Previous and Next options
Previous	Highlights the previous interference object
Next	Highlights the next interference object if multiple objects are present
Pan Realtime	Closes the dialog box to allow you to use Pan Realtime
Zoom Realtime	Closes the dialog box to allow you to use Zoom Realtime
3D Orbit	Closes the dialog box to allow you to use 3D Orbit

The prompt for the Interference command also showed some options. The prompt in step 2 shows Nested selection/Settings. The Nested selection option lets you select objects that are nested in a block or an Xref. The Settings option opens the Interference Settings dialog box (Figure 20-40).

This dialog box offers settings for the temporary display of interference objects while you’re using the Interference command. Table 20-3 lists the options for this dialog box.

Table 20-3: Interference Settings dialog box options

Option	What It Does
Visual Style	Controls the visual style for interference objects
Color	Controls the color for interference objects
Highlight Interfering Pair	Highlights the interfering objects
Highlight Interference	Highlights the resulting interference objects
Visual Style	Controls the visual style for the drawing while displaying the interference objects

Creating Tubes with the Sweep Tool

One more element needs to be taken care of before your chair is complete. The legs are currently simple lines with sharp corners. In this section, you’ll learn how you can convert lines into 3D tubes. To make it more interesting, you’ll add rounded corners to the legs.

Start by rounding the corners on the lines you’ve created for the legs:

The chair is almost complete, but the legs are just wireframes. Next you’ll give them some thickness by turning them into tubes. Start by creating a set of circles. You’ll use the circles to define the diameter of the tubes:

Now you’re ready to form the tubes:

Using Sweep to Create Complex Forms

Although you used circles with the Sweep command to create tubes, you can use almost any closed or open shape. Figure 20-44 shows some examples of closed shapes you can use with the Sweep command.

In step 3 of the previous exercise, you may have noticed some command-line options. These options offer additional control over the way Sweep works. Here is a rundown on how they work:

Alignment This option lets you determine whether the object to sweep is automatically set perpendicular to the sweep path. By default, this option is set to Yes, which means the object to sweep is set perpendicular to the path. If set to No, Sweep assumes the current angle of the object, as shown in Figure 20-45.

Base point By default, Sweep uses the center of the object to sweep as the location to align with the path, as shown in Figure 20-46. Base Point lets you set a specific location on the object.

Scale You can have Sweep scale the sweep object from one end of the path to the other to create a tapered shape, as shown in Figure 20-47. This option requires a numeric scale value.

Twist You can have the object to sweep twist along the path to form a spiral shape, as shown in Figure 20-48. This option requires a numeric value in the form of degrees of rotation.

These options are available as soon as you select the sweep object and before you select the path object. You can use any combination of options you need. For example, you can apply the Twist and Scale options together, as shown in Figure 20-49.

Creating Spiral Forms

You can use the Sweep tool in conjunction with the Helix tool to create a spiral form, such as a spring or the threads of a screw. You’ve already seen how the Sweep tool works. Try the following to learn how the Helix tool works firsthand.

In this exercise, you’ll draw a helicoil thread insert. This is a device used to repair stripped threads; it’s basically a coiled steel strip that forms internal and external threads. Here are the steps:

In step 6, you used the Turns option to specify the total number of turns in the helix. You also have other options that give you control over the shape of the helix. Figure 20-51 shows you the effects of the Helix command options. You may want to experiment with them on your own to get familiar with Helix.

Now, use the Sweep tool to complete the helicoil:

If the space between the coils is too small for the cross section, you may get an error message. If you get an error message at step 3, make sure you created the helix exactly as specified in the previous exercise. You may also try increasing the helix height.

In step 3, instead of selecting the sweep path, you can select an option to apply to the object to sweep. For example, by default, Sweep aligns the object to sweep at an angle that is perpendicular to the path and centers the object to sweep. See “Using Sweep to Create Complex Forms” earlier in this chapter.

Creating Surface Models

In an earlier exercise, you used the Loft command to create the seat of a butterfly chair. In this section, you’ll return to the Loft command to explore some of its other uses. This time, you’ll use it to create a 3D model of a hillside based on a set of site-contour lines. You’ll also see how you can use a surface created from the Loft command to slice a solid into two pieces, imprinting the solid with the surface shape.

Start by creating a 3D surface using the Loft command:

Once the loft surface has been placed, you can make adjustments to the way the loft is generated by using the arrow grip that appears when you select the surface:

In the butterfly chair exercise, you used the Guides option of the Loft command. This allowed you to use the polyline curves to guide the loft shape from the front arc to the back arc. In this exercise, you didn’t use the command options and went straight to the multifunction grip menu. The Ruled option that you used in step 3 generates a surface that connects the cross sections in a straight line.

Slicing a Solid with a Surface

In the barcelona1.dwg chair example, you converted a surface into a solid using the Thicken command. Next, you’ll use a surface to create a solid in a slightly different way. This time, you’ll use the surface to slice a solid into two pieces. This will give you a form that is more easily read and understood as a terrain model:

You may have noticed that as you raised the box height, you could see how it intersected the contour surface. Next you’ll slice the box into two pieces:

In step 3, you saw a prompt that offered a variety of methods for slicing the box. The Surface option allowed you to slice the box using an irregular shape, but most of the other options let you slice a solid by defining a plane or a series of planar objects.

Finding the Volume of a Cut

A question I hear frequently from civil engineers is, How can I find the volume of earth from an excavated area? This is often referred to as a cut from a cut and fill operation. To do this, you first have to create the cut shape. Next, you use the Interfere command to find the intersection between the cut shape and the contour surface. You can then find the volume of the cut shape using one of AutoCAD’s inquiry commands. The following exercise demonstrates how this is done.

Suppose that the contour model you’ve just created represents a site where you’ll excavate a rectangular area for a structure. You want to find the amount of earth involved in the excavation. A rectangle has been placed in the contour drawing representing such an area:

Here you used the Selection Cycling feature to help you select the rectangle, which is overlapped by the contour solid. Selection Cycling lets you determine the object you are about to select by highlighting an object that overlaps another object. If the highlighted object is not correct, you can press -spacebar and AutoCAD will highlight the next object it thinks you might want to select, thereby filtering out other objects nearby that might be selected accidentally. Once the object you want to select is highlighted, click in the drawing area.

With the excavation rectangle in place, you can use the Interfere command to find the volume of the excavation:

Understanding the Loft Command

As you’ve seen from the exercises in this chapter, the Loft command lets you create just about any shape you can imagine, from a simple sling to the complex curves of a contour map. If your loft cross sections are a set of closed objects like circles or closed polygons, the resulting object is a 3D solid instead of a surface when the Mode option is set to solid. Setting Mode to Surface causes the Loft command to create a surface instead of a 3D solid when lofting closed objects.

The order in which you select the cross sections is important because Loft will follow your selection order to create the surface or solid. For example, Figure 20-60 shows a series of circles used for a lofted solid. The circles are identical in size and placement, but the order of selection is different. The solid on the left was created by selecting the circles in consecutive order from bottom to top, creating an hourglass shape. The solid on the right was created by selecting the two larger circles first from bottom to top; the smaller, intermediate circle was selected last. This selection order created a hollowed-out shape with sides that are more vertical.

In addition to the selection order, several other settings affect the shape of a solid created by the Loft command. In the contour-map example, you selected the Ruled option from a multifunction grip after you had completed the Loft command. You can also set the Ruled option through the Loft Settings dialog box (Figure 20-61). This dialog box appears during the Loft command when you select the Settings option after you’ve selected a set of cross sections.

You can radically affect the way the Loft command forms a surface or a solid through the options in this dialog box, so it pays to understand what those settings do. Take a moment to study the following sections, which describe the Loft Settings dialog box options.

Ruled and Smooth Fit

The Ruled option connects the cross sections with straight surfaces, as shown in the sample on the left in Figure 20-62.

The Smooth Fit option connects the cross sections with a smooth surface. It attempts to make the best smooth transitions between the cross sections, as shown on the right in Figure 20-62.

Normal To

Normal To is a set of four options presented in a pop-up menu. To understand what this option does, you need to know that normal is a mathematical term referring to a direction that is perpendicular to a plane, as shown in Figure 20-63. In these options, Normal refers to the direction the surface takes as it emerges from a cross section.

If you use the All Cross Sections option, the surfaces emerge in a perpendicular direction from all the cross sections, as shown in the first image in Figure 20-64. If you use the End Cross Section option, the surface emerges in a direction that is perpendicular to just the end cross section, as shown in the second image in Figure 20-64. The Start Cross Section option causes the surface to emerge in a direction perpendicular to the start cross section. The Start And End Cross Sections option combines the effect of the Start Cross Section and End Cross Section options.

Draft Angles

The Draft Angles option affects only the first and last cross sections. This option generates a smooth surface with added control over the start and end angle. Unlike the Normal To option, which forces a perpendicular direction to the cross sections, Draft Angles allows you to set an angle for the surface direction. For example, if you set Start Angle to a value of 0, the surface will bulge outward from the start cross section, as shown in the first image of Figure 20-65.

Likewise, an End Angle setting of 0 will cause the surface to bulge at the end cross section (see the second image in Figure 20-65).

The Start and End Magnitude settings let you determine a relative strength of the bulge. The right image in Figure 20-65 shows a draft angle of 0 and magnitude of 50 for the last cross section.

Close Surface or Solid

The Close Surface Or Solid option is available only when the Smooth Fit option is selected. It causes the first and last cross section objects to be connected so the surface or solid loops back from the last to the first cross section. Figure 20-66 shows the cross sections at the left, a smooth version in the middle, and a smooth version with the Close Surface Or Solid option turned on on the right. The Close Surface Or Solid option causes the solid to become a tube.

Moving Objects in 3D Space

AutoCAD provides three menu bar options specifically designed for moving objects in 3D space: 3D Align, 3D Move, and 3D Rotate. You can find all three commands in the Modify ⇒ 3D Operations submenu on the menu bar. These options help you perform some of the most common moves associated with 3D editing.

Aligning Objects in 3D Space

In mechanical drawing, you often create the parts in 3D and then show an assembly of the parts. The 3D Align command can greatly simplify the assembly process. The following steps show how to use 3D Align to line up two objects at specific points:

Using the 3D Move command

In Chapter 19, you saw how you can use a gizmo to help restrain the motion of an object in the X, Y, or Z axis. AutoCAD offers a 3D Move command specifically designed for 3D editing that includes a gizmo to restrain motion.

You don’t need to perform these steps as an exercise. You can try the command on your own when you need to use it.

Here’s how it works:

Alternatively, in step 3, you can hover over and click a plane indicator on the gizmo to restrain the motion along one of the planes defined by two of the axes (Figure 20-70).

Rotating an Object in 3D

The 3D Rotate command is another command that is like an extension of its 2D counterpart. With 3D Rotate, a Rotate gizmo appears that restrains the rotation about the X, Y, or Z axis.

You don’t need to perform these steps as an exercise. You can try the command on your own when you need to use it.

Here’s how it works:

You can also just select one of the circles in step 3 instead of selecting a base point. If you do this, then the selected object begins to rotate. You don’t have to select a start and end angle.

The Bottom Line

Master the User Coordinate System. The User Coordinate System (UCS) is a vital key to editing in 3D space. If you want to master 3D modeling, you should become familiar with this feature.

Master It Name some of the predefined UCS planes.

Understand the UCS options. You can set up the UCS orientation for any situation. It isn’t limited to the predefined settings.

Master It Give a brief description of some of the ways you can set up a UCS.

Use viewports to aid in 3D drawing. In some 3D modeling operations, it helps to have several different views of the model through the Viewports feature.

Master It Name some of the predefined standard viewports offered in the Viewports dialog box.

Create complex 3D surfaces. You aren’t limited to straight, flat surfaces in AutoCAD. You can create just about any shape you want, including curved surfaces.

Master It What tool did you use in this chapter’s chair exercise to convert a surface into a solid?

Create spiral forms. Spiral forms frequently occur in nature, so it’s no wonder that we often use spirals in our own designs. Spirals are seen in screws, stairs, and ramps as well as in other man-made forms.

Master It Name the commands or tools used in the example in the section “Creating Spiral Forms,” and name two elements that are needed to create a spiral.

Create surface models. You can create a 3D surface by connecting a series of lines that define a surface contour. You can create anything from a 3D landscape to a car fender using this method.

Master It What is the tool or command used to convert a series of open objects into a 3D surface?

Move objects in 3D space. You can move objects in 3D space using tools that are similar to those for 2D drafting. But when it comes to editing objects, 3D modeling is much more complex than 2D drafting.

Master It What does the Rotate gizmo do?