Drawing in 3D Using Solids and Surfaces


You can work with two types of 3D objects in AutoCAD: solids and surfaces. You can treat both solid and surface objects as if they were solid materials. For example, you can create a box and then remove shapes from the box as if you were carving it, as shown in Figure 6.6.

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Figure 6.6: Solid modeling lets you remove or add shapes.

With surfaces, you create complex surfaces shapes by building upon lines, arcs, or polylines. For example, you can quickly turn a series of curved polylines, arcs, and lines into a warped surface, as shown in Figure 6.7.

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Figure 6.7: Using the Loft tool, you can use 2D objects to define a complex surface.

First, you'll learn how to create a solid box, and at the same time, I'll introduce you to some common modeling features. Next, you'll learn how to create a surface model. You'll get a chance to see how surface modeling differs from solid modeling, and you'll learn how to best use each modeling method.

If you have some experience with AutoCAD 3D (in versions earlier than AutoCAD 2007), you'll notice that the new surface-modeling features are not the same as the surface objects you are used to using, which were created in the older version of AutoCAD. You can edit the new surface objects using Boolean functions, which you'll learn about later in this chapter. They can also interact with solid primitive objects. You can still create the old-style 3D surface objects, but they are not the same types of objects as those presented in this chapter. You can convert them to the new style surfaces using the Convtosurface command.

Then, the rest of the chapter provides reference material on the individual tools for 3D modeling and editing that you were introduced to earlier. You won't find a comprehensive explanation of all the 3D modeling tools, but I'll provide enough information to get you started. If you're doing a simple study model, you might find all you need in this chapter.

Creating a 3D Solid

In this section, you'll create a simple box using a 3D solid primitive. You'll then learn how to manipulate the box and how to get around in the 3D workspace.

Start by creating a box using the Box tool in the 3D Make control panel:

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

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  2. Click a point near the location shown in Figure 6.8 near coordinate 0,15. You can use the coordinate readout to select a point near 0,15. Once you've clicked, you'll see a rectangle follow the cursor.

  3. Click another point near coordinate 20,0, as shown in the left image of Figure 6.8. Now as you move the cursor, the rectangle is fixed and the height of the 3D box appears.

  4. Enter 4 for a height of 4 units for the box. You can also click to fix the height of the box.

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Figure 6.8: Drawing a 3D solid box

You've just created a 3D solid box. By default, the box takes on the current layer color. If you use the acad3D.dwt or acadiso3D.dwt template file to create a new drawing, the default color is a light gray. If you use the acad.dwt or acadiso.dwt template file, the default color is black. If your sample box appears as a black box, do the following:

  1. Choose Format è Layer, or click the Layer Properties Manager tool in the Layers control panel.

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  2. In the Layer Properties Manager dialog box, click the color swatch in the Layer 0 listing, as shown at the top of Figure 6.9.

  3. In the Select Color dialog box, select cyan, as shown in the bottom of Figure 6.9.

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Figure 6.9: Changing the color of the current layer

If you don't want to change the color of your solid, you can choose Conceptual from the Visual Styles drop-down list or choose View è Visual Styles è Conceptual. This will help you see black 3D objects more easily.

You used three basic steps to create the box. You first clicked one corner to establish a location for the box, and then you clicked another corner to establish the base size. Finally, you selected a height. You can use a similar set of steps to create any of the other 3D solid primitives found in the 3D Make control panel. For example, for a cylinder, you select the center, then the radius, and then a height. For a wedge, you select two corners, as you did with the box, and then select the height of the wedge, as shown in Figure 6.10.

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Figure 6.10: Examples of how to create 3D objects using the same basic three steps

Editing 3D Solids with Grips

Once you've created a solid, you can fine-tune its shape by using grips. If you click the solid to select it, you might see a message alerting you to the new editing options. Dismiss the message, and you will see that grips appear on the 3D solid, as shown in the left image of Figure 6.11.

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Figure 6.11: Grips appear on a 3D solid

The square grips at the base of the solid allow you to adjust the location of those grips. The arrow grips allow you to adjust the length of the side to which the arrows are attached. If you click an arrow grip and you have Dynamic Input turned on, a dimension appears at the cursor. You can enter a new dimension for the length associated with the selected grip, or you can drag the arrow to adjust the length. Try the following to get a firsthand feel for how this works:

  1. Click the arrow grip toward the front of the box, as shown on the left in Figure 6.11. Now as you move the cursor, the box changes in length.

  2. Press the Esc key to clear the grip selection.

You can also move individual edges by using Ctrl+click:

  1. Ctrl+click the top-front edge, as shown on the right in Figure 6.11. You might see a Selecting Subobject dialog box; dismiss it. You'll see that the edge is highlighted, and a grip appears at its midpoint.

  2. Click the edge's grip, and move the cursor. The edge follows the grip.

  3. Hold down the Shift key, and "pull" the grip forward away from the box's center. The Shift key constrains the motion in the x-, y-, or z-axis.

  4. Click a point to fix the edge's new position.

  5. Click the Undo button to return the box to its original shape.

As you can see, you have a great deal of flexibility in controlling the shape of the box. The Shift key lets you constrain the motion of the grip.

Constraining Motion with the Grip Tool

Another handy tool for grip editing 3D objects is the Grip tool. Its icon looks like the UCS icon, and it appears whenever you hover over a grip. Try the next exercise to see how the Grip tool works:

  1. Ctrl+click the front-top edge of the box again to expose the edge's grip.

  2. Place the cursor on the grip, but don't click. The Grip tool appears (see Figure 6.12).

  3. Place the cursor on the z-axis of the Grip tool, but don't click. A blue line appears that extends across the drawing area, and the z-axis of the Grip tool changes color.

  4. Click the z-axis. Now as you move the cursor, the grip motion is constrained in the z-axis.

  5. Click again to fix the location of the grip.

  6. Press the Esc key to clear your grip selection.

  7. Click the Undo tool to undo the grip edit.

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Figure 6.12: Using the Grip tool to constrain motion

Here you'll see that you can use the Grip tool to change the Z location of a grip. You can use the Grip tool to modify the location of a single grip or the entire object.

Rotating Objects in 3D Using Dynamic UCS

Most of the discussion in this book centers on a 2D drawing area. Coordinates in this area reside in what is known as the world coordinate system (WCS). This is the default coordinate system that AutoCAD uses in new drawings, but you can also create your own coordinate systems that are subsets of the WCS. A coordinate system that you create is known as a user coordinate system (UCS).

UCSs are significant in 3D modeling because they can help you draw in 3D space. You can set up a UCS, for example, that is on a vertical face of the 3D box you created earlier. You could then draw on that vertical face just as you would on the WCS of the drawing. Figure 6.13 shows a cylinder drawn on the side of a box. The vertical grid shows the orientation of the UCS. Also notice that the UCS icon is in the corner of the box.

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Figure 6.13: Drawing on the side of a box

The UCS is a great tool for 3D modeling, but it can be cumbersome to use. AutoCAD 2007 introduced Dynamic UCS, which automatically changes the orientation of the x-, y-, and z-axes to conform to the flat surface of a 3D object.

If you created the new 3D file using the acad3D.dwt template, you might have noticed that the cursor looked different. Instead of the usual cross, it has three intersecting lines. If you look carefully, you'll see that each line of the cursor is a different color. In its default configuration, AutoCAD shows a red line for the x-axis, a green line for the y-axis, and a blue line for the z-axis. This color scheme mimics that of the UCS icon, as shown in Figure 6.14.

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Figure 6.14: The UCS icon at the left and the cursor in 3D to the right are color matched.

As you work with Dynamic UCS, you'll see that the orientation of these lines changes when you point at a surface on a 3D object. To see how this works firsthand, try the following exercise. Here you'll use Dynamic UCS to help you rotate the box around the x-axis.

  1. Make sure that the Dynamic UCS feature is turned on by checking the DUCS button at the bottom of the AutoCAD window. It should be in the on, or "down," position. If it isn't, click it to turn it on.

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  2. Choose Modify è Rotate.

  3. At the Select objects: prompt, click the box, and then press to finish your selection.

  4. At the Specify base point: prompt, don't click anything, but move the cursor from one surface of the box to a side of the box. As you do so, pay attention to the orientation of the cursor. It changes depending on the surface to which you are pointing.

  5. Place the cursor on the left side, as shown in Figure 6.15.

  6. With the cursor on the left side of the box, Shift+right-click, and select Endpoint from the shortcut menu.

  7. Place the osnap cursor on the lower-front corner of the box, as shown in the left image of Figure 6.15. Click this corner. As you move the cursor, the box rotates around the y-axis.

  8. Enter -30 for the rotation angle. Your box should look like the image in the right of Figure 6.15.

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Figure 6.15: Selecting a rotation point and the resulting box orientation

Here you'll see that you can hover over a surface to indicate the plane about which the rotation is to occur. You aren't limited to using the surfaces of the object you are rotating. If you have multiple objects in your model, you can use the surface of any object to indicate the rotational plane.

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USING OBJECT SNAPS IN 3D SPACE

If you need to place objects in precise locations in 3D, such as endpoints or midpoints of other objects, you can do so using object snaps, just as you would in 2D. But you must be careful when using object snaps as far as Dynamic UCS is concerned.

In the exercise in the "Rotating Objects in 3D Using Dynamic UCS" section, you were asked to make sure that you place the cursor on the side of the box that coincides with the rotational plane before you select the Endpoint osnap. This ensures that the Dynamic UCS feature has selected the proper rotational plane; otherwise, you might find that the box rotates in the wrong direction.

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Now, suppose you want to add an object to one of the sides of the rotated box. The next section will show you another essential tool you can use to do just that.

Drawing on a 3D Object's Surface

In the rotation exercise, you saw that you can hover over a surface to indicate the plane of rotation. You can use the same method to indicate the plane on which you want to place an object. Try the following exercise to see how it's done:

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

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  2. Place the cursor on the top surface of the rectangle, as indicated in Figure 6.16, and hold it there for a moment. The cursor will align with the angle of the top surface.

  3. With the cursor aligned with the top surface of the box, click a point roughly at the center of the box. The base of the cylinder appears on the surface.

  4. Adjust the circle so it is roughly the same 6-unit radius of the one shown in the right of Figure 6.16, and then click to set the radius. You can also enter 6.

  5. Enter 4 for the height of the cylinder.

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Figure 6.16: Drawing a cylinder on the surface of a 3D solid

This demonstrates that you can use Dynamic UCS to align objects to the surface of an object. Note that the Dynamic UCS works only on flat surfaces. You can't, for example, use Dynamic UCS to place an object on the curved side of the cylinder.

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WORKING WITH USER COORDINATE SYSTEMS

Dynamic UCS is a great addition to AutoCAD, and it might be all you need to create your 3D models. But at some point, you might find you want a bit more control over the UCS. That's where the standard UCS comes in handy.

A UCS is more permanent than Dynamic UCS, and you can use a UCS if you need to create an object that is aligned with another object's surface but is not on the object.

In the section "Rotating Objects in 3D Using Dynamic UCS," you learned how to use Dynamic UCS to align your working coordinate system to the side of a box. To create a UCS that remains in place, do the following:

  1. Choose Tools è New UCS è Face.

  2. Click the surface of an object that represents your working plane. You will see the UCS icon move to the surface you select.

  3. At the Enter an option [Next/Xflip/Yflip] <accept>: prompt, select Accept from the Dynamic Input display, or press to accept this location. If it is the wrong location, click Undo, and try again. You also have the option to try the Next face, flip the x-axis (Xflip), or flip the y-axis (Yflip).

To return to your previous UCS, choose Tools è New UCS è Previous. If you want to return to the WCS, choose Tools è New UCS è World.

You can also use three points to define a UCS. To do this, you'll need to use osnaps and a set of objects to which you can snap. You can then take the following steps:

  1. Choose Tools è New UCS è 3 Point.

  2. At the Specify new origin point <0,0,0>: prompt, use an osnap, such as the Endpoint osnap override, and then select an object, such as the corner of a box, as shown here in the left image:

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  3. At the Specify point on positive portion of the X-axis: prompt, select another corner along the x-axis, as shown earlier in the middle image. As you do so, notice the rubber-banding line. This visually shows you the x-axis of your newly defined UCS.

  4. At the Specify point on positive-Y portion of the UCS XY plane: prompt, select another corner along the y-axis from the origin point, as shown in the earlier middle image. Again, notice the rubber-banding line indicating the y-axis of your new UCS.

  5. At the Specify point on positive-Y portion of the UCS XY plane: prompt, you don't have to select a location exactly on the y-axis of your new UCS. The point you select to define the x-axis will define the UCS's x-axis orientation.

The point you select in step 5, the positive Y location, can be anywhere as long as it defines the general direction of the y-axis.

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Sculpting Objects with Union and Subtraction

Now suppose you want to create a cylindrical void at the center of the cylinder of the solid you just edited. To do so, you create a 3D solid in the shape of the void and then use the Subtract tool to remove the 3D solid.

Start by combining the cylinder and the box into a single solid object:

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

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  2. Select the cylinder and the box, and then press .

Although they haven't changed in appearance, both objects are now joined into a single 3D solid. Now create a cylinder that will become the hole in the center of the combined box and cylinder:

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

  2. Place the cursor on the top of the cylinder, and wait unit the cursor orients itself to the angle of the cylinder's top surface.

  3. Shift+right-click, and choose Center from the shortcut menu.

  4. Point to the edge of the cylinder, and when you see the osnap center mark at the center of the cylinder, click. (See the top-left image in Figure 6.17.)

  5. Enter 2 for the cylinder radius.

  6. Move the cursor downward until you see the cylinder appear below the box, as shown in the top-right image of Figure 6.17, and then click to finish the cylinder.

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Figure 6.17: Creating and subtracting a cylinder from the 3D solid

You have the void shape in place. To finish this solid, try using the Subtract tool to make a hole in the solid:

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

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  2. At the Select solids and regions to subtract from .. Select Objects: prompt, select the box, and then press .

  3. At the Select solids and regions to subtract .. Select Objects: prompt, select the newly created cylinder, and then press . A hole appears in the solid that is centered on the cylinder (see Figure 6.17).

In these exercises, you created a cylinder and subtracted it from the original 3D solid to form a void in the shape of a cylindrical hole. With the tools you've learned how to use so far, you can begin to build nearly any shape you might need.

Making Changes to Your Solid

When you're creating a 3D model, you will hardly ever get the shape right the first time. Suppose you decide that you need to modify the shape you've created so far by moving the hole from the center of the larger cylinder to its edge. This next exercise will show you how you can gain access to the individual components of a 3D solid to make changes.

The model you've been working with is composed of three objects: a box and two cylinders. These individual components of the solid are referred to as subobjects of the main solid object. When you use the Union, Subtract, or Intersect tool on a set of objects, the objects merge into a single solid, or at least that is how it seems at first. You can gain access and modify the shape of the subobjects from which the shape is constructed just by holding down the Ctrl key while clicking the solid. Try the following to see how this works:

  1. Place the cursor on the solid you've made so far. You'll see that the entire object is highlighted as if it were one object. If you were to click it (but don't do that yet), the entire object is selected.

  2. Hold down the Ctrl key, and move the cursor over the various parts of the solid. First, place the cursor on the hole, then on the cylinder, and finally on the box. As you do this, you will see that the individual parts of the solid are highlighted instead of the whole solid (see Figure 6.18).

  3. While still holding the Ctrl key down, click the center of the hole. The grips for the hole appear as shown on the left side of Figure 6.19. As you might guess, you can use these grips to change the shape and location of the selected solid.

  4. Click the center grip of the cylinder and move your cursor. If you find it a bit uncontrollable, turn off Polar mode. As you move the cursor, you'll see that the hole moves with the cursor.

  5. Place the hole in the location shown on the right side of Figure 6.19, and click. You've just moved the hole from the center to the edge of the cylinder.

  6. Press the Esc key to clear the selection.

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Figure 6.18: Individual objects are highlighted when you use the Ctrl key.

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Figure 6.19: You can move the hole-forming cylinder to a new location using its grip.

This example shows you that the Ctrl key can be an extremely useful tool when you have to edit a solid because it allows you to select the solids that form your model. Once the solid is selected, you can move it, or you can use the arrow grips to change its size.




Introducing AutoCAD 2008
Introducing AutoCAD 2008
ISBN: 0470121505
EAN: 2147483647
Year: 2007
Pages: 147
Authors: George Omura

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