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In this section, you will begin to draw the object shown in Figure 18.4.In the process, you will explore the creation of solid models by creating primitives and then setting up special relationships between them. ..



Displaying the Solids Toolbar

All of the commands you will use to create the solids primitives, and many of the commands you can use for editing solids, are accessible on the Solids ,

Right-click any toolbar and select Solids from the popup menu. The Solids toolbar appears.

Open the Solids toolbar as well. Now you’re ready to begin creating basic solids.

Creating Primitives

Primitives are the basic building blocks of solid modeling. At first, it may seem limiting to have only six primitives to work with, but consider the varied forms you can create with just a few two-dimensional objects. Let’s begin by creating the basic mass of our steel bracket. First, prepare your for the exercise.

  1. Create a new file called Bracket.
  2. Right-click the SNAP button on the status bar, then in the Drafting Settings dialog box, set the Snap spacing to 0.5 and turn on the Grid and Snap modes.
  3. Turn on the dynamic coordinate readout by pressing F6. You’ll use the read- out to help guide you in selecting points in the exercises that follow.

Now start building the solid model.

  1. Click the Box tool on the Solids toolbar, or enter BOX-J.You may also choose Draw > Solids > Box.
  2. At the Specify corner of box or [Center] <0,0,0>: prompt, pick a point at coordinate 3,2.5.
  3. At the Specify corner or [Cube/Length]: prompt, enter @7, create a box with a length of 7 and a width of 4.
  4. The Specify height: prompt that appears next is asking for the height of the box in the z-axis, Enter 1.
    You’ve now drawn your first primitive, a box that is 7 units long by 4 units wide by 1 unit deep. Now, let’s change the view so you can see the box more clearly. Use the Vpoint command to shift your view so you are looking at the WCS from the lower left.
  5. Open the Viewpoint Presets dialog box (choose View>3D Views Select), and then enter 225 in the From X Axis input box and 19.5 in the XY Plane input box.
  6. Click OK, and then adjust your view so it looks similar to Figure 18.5.
    FIGURE 18.5

    FIGURE 18.5


Turning a 2D Polyline into a 3D Solid

Now let’s add another box to form the lower lip of the bracket. ‘This time, you’ll create a box primitive from a polyline.

  1. Click the Polyline tool on the Draw toolbar.
  2. At the From point: prompt, start the polyline from the coordinate 5.2.5.
  3. Continue the polyline around to create a rectangle that is 1unit in the x-axis and 3 units in the y-axis, Your drawing will look like Figure 18.6.
  4. Click the Extrude tool on the Solids toolbar, or type EXT.
  5. At the Select objects: prompt, pick the polyline and press button.
  6. At the Specify height of extrusion or [Path]: prompt, type 1.
  7. At the Specify angle of taper for extrusion <0>: a to accept the default taper of 0°. (You’ll see what the Taper option does in a later exercise.) The polyline now extrudes in the z-axis to form a bar, as shown in Figure 18.7.
  8. Type R to redraw the screen.




.You’ve now drawn two box primitives using the Box and the Extrude options on the Solids toolbar. Just for variety’s sake, this exercise had you create the smaller box by converting a polyline into a solid, but you could just as easily have used the Box option for that as well. The Extrude option converts polylines, circles, and traces into solids. (Regular lines, 3D lines, 3D Faces, and 3D polylines cannot be extruded.)

Other Solids Options

Before you continue, let’s examine the commands for primitives that you haven’t had a chance to use yet. Refer to Figures 18.8 through 18.11 to understand the terms used with these other primitives.

Cone (Cone icon on the Solids toolbar) Draws a circular cone or a cone with an elliptical base. Drawing a circular cone is much like drawing a circle, with an added prompt asking for a height. The Ellipse option acts like the Ellipse command (on the Draw toolbar), with an additional prompt for height.



Sphere (Sphere icon on the Solids toolbar) Acts like the Circle command, but instead of drawing a circle, it draws a sphere.



Torus (Torus icon on the Solids toolbar) Creates a torus (a donut-shaped solid). You are prompted for two diameters or radii, one for the diameter or radius of the torus and another for the diameter or radius of the tube portion of the torus

FIGURE 18.10

FIGURE 18.10

Wedge (Wedge icon on the Solids toolbar) Creates a wedge-shaped solid. This Command acts much like the Box command you used to draw the bracket. You have the choice of defining the wedge by two comers or by its center and a corner.

FIGURE 18.11

FIGURE 18.11

In the following exercises you will be creating and combining solid primitives. The commands required to create complex solids are available on the Solids Editing toolbar.

Joining Primitives

Now let’s see how the two ,box objects you created are joined. First, you’ll move the new box into place, and then join the two boxes to form a single solid.

  1. Start the Move command, pick the smaller of the two boxes, and then press button.
  2. At the Base point: prompt, use the Midpoint Osnap override and pick the middle of the back edge of the smaller box, as shown in the top image of Figure 18.12.
  3. At the Second point: prompt, pick the middle of the bottom edge of the larger box, as shown in the bottom image of Figure 18.12.
    FIGURE 18.12

    FIGURE 18.12


  4. Choose Modify > Solids Editing >  Union, or type Uni. You may also click Union on the Solids Editing toolbar.
  5. At the Select objects: prompt, pick both boxes and press button Your drawing now looks like Figure 18.13.
    FIGURE 18.13

    FIGURE 18.13


As you can see in Figure 18.13, the form has joined to appear as one object. It also acts like one object when you select it. You now have a composite solid made up of two box primitives.

Now let’s place some holes in the bracket. In this next exercise, you will discover how to create negative forms to cut portions out of a solid.

  1. Click the Cylinder tool on the Solids toolbar, or type Cylinder . You may also choose Draw > Solids > Cylinder.
  2. At the Specify center point for base of cylinder or [Elliptical] <0.0.0>: prompt, pick a point at the coordinate 9,5.5.
  3. At the Specify radius for base of cylinder or [Diameter]: prompt, enter 25.
  4. At the Specify height of cylinder or [Center of other end]: prompt, enter 1.5 The cylinder is drawn.
  5. Copy the cylinder two inches in the negative direction of the y-axis, so your drawing looks like Figure 18.14.
    FIGURE 18.14

    FIGURE 18.14


You now have the cylinder primitive, but you still need to define its relationship to the composite solid you created from the two boxes.

  1. Choose Modify > Solids Editing > Subtract, or type Su. You may also click the Subtract tool on the Solids Editing toolbar.
  2. At the Select solids and regions to subtract from Select objects: prompt, pick the composite solid of the two boxes and press button.
  3. At the Solids and regions to subtract; Select objects: prompt, pick two of the cylinders and press button. The cylinder has now been subtracted from the bracket.
  4. To view the solid, choose View > Hide. You’ll see a Hidden Line view of the solid, as shown in Figure 18.15.
    FIGURE 18.15

    FIGURE 18.15


As you’ve learned in the earlier chapters in Part Iv, Wrreframe views, such as the one in step 3, are somewhat difficult to decipher. Until you use the Hide command (step 4), you cannot tell for sure that the subtracted cylinder is in fact a hole. Using the Hide command frequently will help you keep track of what’s going on with your solid model.

In step 3 of the previous exercise, you may have noticed that the cylinders changed shape to conform to the depth of the bracket. You’ll also recall that you drew the cylinder at a height of 1.5 units; not 1 unit, which is the thickness of the bracket. Having drawn the cylinder taller than needed, you can see that when AutoCAD performed the subtraction, it ignored the portion of the cylinder that doesn’t affect the bracket. AutoCAD always discards the portion of a-primitive that isn’t used in a Subtract operation.

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