Creating 3D Forms from 2D Shapes

The 3D solid primitives are great for creating some basic shapes, but in many situations, you will want to create a 3D form from a more complex shape. Fortunately, you can extrude 2D objects into a variety of shapes using additional tools found in the 3D Make control panel. For example, you can draw a shape such as a puzzle piece and then extrude it into a third dimension, as shown in the top of Figure 6.25. Or you can use several strategically placed 2D objects that can form a flowing surface such as the wing of an airplane, as shown in the bottom of Figure 6.25.

Figure 6.25: You can use the closed polylines on the left to construct the 3D shapes on the right.

Extruding a Polyline

You can insert a variety of solid shapes into your drawing. For example, you can create a sphere by choosing Sphere from the 3D Make control panel. You can then select a center point and a radius, just as you would for a circle, but you end up with a solid sphere.

Another way to create solids is to extrude them from closed polylines. This is a more flexible way to create shapes because you can create a polyline of any shape and extrude it to a fairly complex form.

In the following exercise, you'll start with a pentagon, which is actually a simple closed polyline. From that pentagon, you'll use a variety of tools to form a box.

1. Open a new file in AutoCAD using the acad3D.dwt template.

2. Choose Draw è Polygon.

3. At the Enter number of sides <4>: prompt, enter 5↲.

4. At the Specify center of polygon or [Edge]: prompt, enter 0,0↲.

5. At the Enter an option [Inscribed in circle/Circumscribed about circle] <I>: prompt, press ↲.

6. At the Specify radius of circle: prompt, enter 17↲. You now have a 2D polygon similar to the one shown on the left side of Figure 6.26.

Figure 6.26: Drawing a 3D polygon

If you extrude an object that is not closed, such as a line or an arc, the Extrude tool will create 3D surfaces.

Now you're ready to change this polygon into a 3D object:

1. Click the Extrude tool in the 3D Make control panel, or enter Extrude↲ at the command prompt.

   

2. At the Select objects: prompt, select the polygon, and then press ↲.

3. At the Specify height of extrusion or [Direction/Path/Taper angle]: prompt, you will see that the polygon is now a 3D solid whose height follows the cursor. Enter 3↲ to make the height of the extrusion 3 units. The polygon expands to become a 3D pentagon, as shown on the right side of Figure 6.26.

In this exercise, you extruded a simple polygon, but you can extrude any shape you can dream up using open or closed polylines. For example, you can extrude the complex curved shapes, as shown in Figure 6.27.

Figure 6.27: Examples of extruded closed polylines

Extruded shapes created from open polylines instead of closed polylines create 3D surfaces that have no thickness. You can give such an object thickness by using the Thicken Surface tool in the 3D Make control panel. You can find this tool in the expanded 3D Make control panel (see Figure 6.28).

Figure 6.28: The Thicken Surface tool in the expanded 3D Make control panel, which is shown at the right

Click the Thicken Surface tool, and then select the object or 3D surfaces you want to thicken. At the Specify thickness <0.0000>: prompt, enter the desired thickness. Figure 6.29 shows two examples of thickened surfaces.

Figure 6.29: Sample of surfaces before and after using the Thicken Surface tool

Checking Interference and Finding the Intersection of 3D Shapes

In an earlier exercise, you saw how you can combine solids in various ways to form more complex shapes. Another useful tool for editing solids is the Interfere tool. Interfere lets you find the intersection of two solids. This can be helpful if you need to reproduce a section of a solid or if you want to check to see whether two or more 3D objects come into contact.

Interfere works by creating a third 3D solid that is the shape of the intersection of two other solids. Figure 6.30 shows how you can use an extruded cornice and a simple box to make a copy of a small piece of the cornice.

Figure 6.30: Making a copy of a section of a 3D solid using Interfere

CONVERTING 3D OBJECTS FROM EARLIER VERSIONS TO 3D SURFACES

Thicken is not the same as the Thickness property of an object. If you have experience with AutoCAD 3D, you know you can change the Thickness property of an object to make it into a 3D object. But an object with a thickness cannot be edited using the tools described in this chapter. You can convert a simple 2D object with thickness to a full 3D surface object by using the Convert To Surface tool in the expanded 3D Make control panel.

Click the Convert To Surface tool, select the objects you want to convert, and then press ↲. The objects will become true 3D surfaces. Once you've done this, you can use the Thicken Surface tool.

To work with the Interfere command, follow these steps:

1. Click the Interference Checking tool in the expanded 3D Make control panel, or enter Interfere↲ at the command prompt.

   

2. At the Select first set of objects or [Nested selection/Settings]: prompt, select the cornice, and press ↲.

3. At the Select second set of objects or [Nested selection/checK first set] <checK>: prompt, select the box, and press ↲. The Interference Checking dialog box appears.

   

4. In addition, the selected objects will temporarily become transparent, and only the intersection of the selected objects will be visible.

5. Uncheck the Delete Interference Object Created On Close option, and then click Close.

6. Delete the box to reveal the interference object.

As you can tell from the name of the Interference tool, it is designed to check for interference between 3D solids and surfaces. In this example, you're using it to create a new 3D shape from two existing ones. If you want to check only for interference, you can visually inspect the intersection of the solids in step 3. The Interference Checking dialog box offers the Zoom, Pan, and 3D Orbit tools to let you inspect the interference. The Previous and Next buttons move to the next interference shape when multiple intersections exist. The Zoom To Pair option causes the view to automatically zoom in on the interference shape.

If you just want to create the intersection of two solids and discard the original solids, you can use the Intersection command. Click the Intersect tool in the 3D Make control panel or choose Modify è Solid Editing è Intersect, and then select the two objects whose intersection you want.

Filleting a Corner

One useful editing command for 3D solids is more commonly used in 2D drawings. If you want to round the corner of a 3D solid, you can do so using the Fillet command. But Fillet behaves in a slightly different way when applied to 3D solids.

Issue the Fillet command by selecting it from the Modify menu or the 2D Draw control panel, and then select the solid edge you want to fillet (see Figure 6.31). The selected edge is highlighted, and you are prompted for a fillet radius. Once you've entered a radius, you can select other contiguous edges to fillet. Press ↲, and the edges are filleted, as shown in Figure 6.31.

Figure 6.31: Selecting the edges to fillet and the result

You can also use the Chamfer command to chamfer the corners of 3D solids. Chamfer works like the Fillet command.

Extruding along a Path

You've seen how you can extrude a polyline into the third dimension. This method let you create some basic 3D shapes, but what if you want to create something a bit more complex?

A tool called Sweep allows you to extrude a 2D object along a complex path. You can define the path by using a polyline or a spline.

Figure 6.32 shows how to create the exterior wall of a building using a polyline outline of the wall's profile and a polyline path of the wall's footprint on the ground.

Figure 6.32: The 2D drawings of a building wall section and footprint

To see how the Path option works, draw the outline of the shape you want to extrude, and then draw the path you will use to extrude the outline. Figure 6.32 shows an example of an outline and a path in an isometric view.

The sample outline in Figure 6.32 is a profile or wall section of a building. The path is the building footprint. The footprint has a gap to provide a place to locate the outline, as you will see later.

Once you have both the outline and path drawn, do the following:

1. Click the Sweep tool in the 3D Make control panel, or enter Sweep↲ in the command line.

   

2. At the Select objects to sweep: prompt, select the outline.

3. At the Select sweep path or [Alignment/Base point/Scale/Twist]: prompt, enter b↲, and then select the exact location on the outline that is to follow the path. In the wall example, the outer-bottom corner will form the footprint of the building.

4. Select the path for the outline. The outline is extruded along the path. The right side of Figure 6.32 shows the outline and path after using the Sweep tool.

Although you used a building as an example in Figure 6.32, you can use this method to create any number of extruded shapes along a path. The shape can be as simple as a circle extruded along a curve to form a tube or a wall section along a footprint, as in this example. The path doesn't have to be restricted to a plane. You can create a path that goes in any direction in 3D space. AutoCAD also offers the Helix tool, which you can use to create a helix path to draw screw threads or rounded parking garage ramps.

Creating a Helix

Perhaps one of the most frequently asked for 3D tools is one that will allow you to draw a screw thread. AutoCAD 2007 introduced the Helix tool that you can use to create a spiral path. You can then create a cross section of a screw thread and use the spiral path to create a 3D screw thread. To create a spiral path, do the following:

1. Click the downward-pointing arrows in the 3D Make control panel. The 3D Make control panel expands to display some additional tools.

   

2. Click the Helix tool in the expanded area of the 3D Make control panel. You'll see the following prompt:

    Number of turns = 3.0000    Twist=CCW Specify center point of base: 

3. Point to a location for the center of the helix, and then click.

4. At the Specify base radius or [Diameter] <1.0000>: prompt, enter a radius for your helix.

5. At the Specify top radius or [Diameter] <1.0000 >: prompt, you can enter a different radius value if you want the helix to gradually change its radius from bottom to top.

6. At the Specify helix height or [Axis endpoint/Turns/turn Height/tWist] <1.0000>: prompt, you can select the height of the helix. Figure 6.33 shows a helix with the base and top radius set to 9 and the height set to 6.

Figure 6.33: A helix path and a circle applied to the helix using the Sweep tool

Once you've created the helix, you can use it with the Sweep tool to create your spiral form. If you decide that you want to alter the dimensions of an existing helix, you can do so by using the Properties palette. Select the helix, and then click the Properties tool in the Standard toolbar, or just double-click the helix. You can edit the radii, height, and number of turns in the helix. Remember that you can also specify the number of turns, the turn height, and the direction that the helix turns at the prompt in step 6. For example, if you want to set the direction of the turns, enter W for the Twist option at the prompt in step 6, and then enter CW for clockwise or CCW for counterclockwise.

You can find the total length of a helix or the turn slope by looking at the Properties palette for the helix.

Lofting through Different Shapes

In the earlier Figure 6.25, you can see a wing-shaped object that was constructed from a set of 2D cross sections. The tool used to actually create the curved surface is the Loft tool. Loft allows you to create sculpted, free-form shapes such as a wing, a faucet handle, or any number of other sculpted shapes. If you can identify and draw a set of 2D cross sections for a 3D object, Loft will create the 3D shape for you. The object doesn't have to be a solid either. You can loft open polylines, arcs, and lines to form surfaces such as a 3D terrain model from contour lines.

The following example uses a pair of arcs as an example of how the Loft tool works. It also shows you can employ a box to help you quickly lay out an object.

1. Use the Box tool from the 3D Make control panel to create a box similar to the one shown in Figure 6.34.

2. Choose Draw è Arc è 3 Points, and then using the Dynamic UCS feature described earlier in this chapter, draw an arc on the side of the box similar to the one in Figure 6.34. Remember that the cursor will change to indicate which surface you are aligned with, and a dashed line will also appear around the surface.

3. Draw another arc, as shown in Figure 6.34.

4. Choose Draw è Polyline, and then draw the three-segment polyline shown in Figure 6.34. Press ↲ when you have drawn the three line segments to exit the Polyline tool.

Figure 6.34: Drawing a box with arcs and a polyline on its side

Here you'll see how you can use a box to help you draw an item in 3D space. The box in conjunction with Dynamic UCS lets you orient the arcs in a vertical plane that is perpendicular to the regular WCS.

Now move the arcs apart so you can practice using the Loft tool:

1. Click the box, and press the Delete key to erase it. You used it as an aid only to draw the 2D objects on a plane perpendicular to the XY plane.

2. Click the top arc so that it is the only object selected, and then place the cursor on its center grip. The Grip tool appears on the center grip.

   

3. Place the cursor on the green y-axis of the Grip tool. You'll see a green line appear along the axis you are pointing to on the Grip tool.

4. With the green line showing, click the Grip tool. Now, as you move the cursor, the arc follows and is constrained on the green axis.

5. Place the cursor so that the arc is farther to the right, as shown in Figure 6.35, and then click. You don't have to be exact, since this is just to show generally how drawing in 3D works.

6. Press the Esc key to clear the selection of the arc.

7. Repeat steps 2 through 5, but this time, select the other arc, and move it to the location shown in Figure 6.35.

Figure 6.35: Moving the arcs

In this example, you'll see that you can constrain the movement of an object along an axis just by clicking the axis of the Grip tool.

With objects such as the polyline, which has no single grip that controls the location of the entire object, you must use the Move tool to move it. You cannot use the Grip tool to move it.

Now that you have the arcs and polyline in place, you can create a surface that flows through them:

1. Click the Loft tool in the 3D Make control panel.

   

2. At the Select cross-sections in lofting order: prompt, select the arcs and polyline in order from left to right. Press ↲ when you've selected all three objects.

3. At the Enter an option [Guides/Path/Cross-sections only] <Cross-sections only>: prompt, press ↲, or if you have Dynamic Input turned on, select Cross Section Only from the Dynamic Input display. The Loft Settings dialog box opens.

   

4. Make sure Smooth Fit is selected, and click OK. A surface appears that smoothly transitions through the three objects, as shown in Figure 6.36.

Figure 6.36: The surface created using the Loft tool

Once you've created a lofted 3D object, you can make changes to its shape by adjusting its grips:

1. Click the lofted shape to expose its grips. Although it might not be obvious at first, the grips you see are those of the three objects you used to create the loft.

2. Place the cursor on the top grip of the middle polyline, but don't click quite yet (see the left image in Figure 6.37). The Grip tool moves to the grip to which you point.

3. Point to the blue Z coordinate on the Grip tool. A blue line appears telling you that you can now click and your motion will be constrained to that axis.

4. Click the Z coordinate of the Grip tool, and then move the cursor upward. The loft shape is pulled upward. Click again to fix the grip in a new, higher location. (See the right image in Figure 6.37.)

Figure 6.37: Moving a grip with the aid of the Grip tool

In this example, you used open objects, but you can use closed polylines, circles, and closed splines as well.

Revolving a Polyline

You've seen how the Extrude and Sweep commands let you create solids in two ways: you can extrude in a straight line or along a path. The Revolve command lets you create a 3D solid by rotating a shape. It extrudes a circle, an ellipse, a closed polyline, or a region along a circular path up to 360°. You can also rotate lines, arcs, or any other open shape to get 3D surfaces. You draw the profile of the shape you want, define an axis of rotation around which the shape will be "rotated," and then use Revolve to turn the shape into a rounded 3D object.

Figure 6.38 shows the progression from 2D objects to revolved 3D solids.

Figure 6.38: An ellipse that has been revolved into a solid

First I drew an ellipse and a line. The ellipse is the shape that is to be revolved, and the line is the axis of revolution.

To revolve the ellipse, follow these steps:

1. Choose the Revolve tool in the 3D Make control panel.

   

2. At the Select objects to revolve: prompt, select the ellipse, and press ↲.

3. At the Specify axis start point or define axis by [Object/X/Y/Z] <Object>: prompt, indicate the axis of revolution by either entering X, Y, or Z or by selecting a line that represents the axis around which the ellipse is to be revolved.

4. At the Specify angle of revolution or [STart angle] <360>: prompt, you can press ↲ to revolve the ellipse a full 360°, or you can enter a specific value.

For the revolved shape in Figure 6.38, I used an angle of 180°. Figure 6.38 shows an ellipse as an example, but you can use any shape you want, such as the cross section of a car wheel or pulley. Just make sure that when you draw your shape, the outline does not cross over itself, like an 8. AutoCAD will return an error message if you attempt to revolve such a shape.