1.First determine the positive direction of the axis. If you specify start and endpoints, the positive axis direction goes from the start point to the endpoint. If you pick an object, the positive axis direction goes from the pick point to the other endpoint. If you choose the X or Y axis, the positive direction is obvious.

2.Point your right thumb in the positive direction of the axis.

3.Look at the curl of your fingers on that hand. That’s the positive direction of rotation.

Creating Complex Solids

To create realistic objects, you usually need to edit the simple shapes that I have discussed in this chapter. You can create complex solids by adding them, subtracting them, or intersecting them. These processes are called Boolean operations, which in this context means using logical functions, such as addition or subtraction, on objects.

Figure 24-13: The results of UNION on two solids — shown after using the HIDE command.

762 Part IV Drawing in Three Dimensions

Subtracting solids

You use the SUBTRACT command to subtract one solid from another. This command is most commonly used to create holes. Figure 24-14 shows the result of subtracting a small cylinder from a larger one and then using the HIDE command.

Figure 24-14: You can create holes using the SUBTRACT command.

To subtract solids, follow these steps:

1.Choose Modify Solids Editing Subtract or choose Subtract from the Solids Editing toolbar.

2.At the following prompt, choose the solid (or region) that you want to subtract from (the one you want to keep):

Select solids and regions to subtract from...

Select objects:

3.At the following prompt, choose the solid (or region) that you want to subtract (the one you want to get rid of):

Select solids and regions to subtract...

Select objects:

Creating a solid from the intersection of two solids

You can also create a solid from the volume that two solids have in common. This volume is called their intersection. Figure 24-15 shows two solids before and after using the INTERSECT command. The solid on the right is shown after using the HIDE command.

Figure 24-15: A box and a sphere before and after using the INTERSECT command.

As you can see, you can create some very unusual models this way.

To use the INTERSECT command, choose Modify Solids Editing Intersect or choose Intersect from the Solids Editing toolbar. Just select the objects in any order. AutoCAD

creates the new solid.

Creating a new solid using INTERFERE

INTERFERE is similar to INTERSECT except that the original solids remain. AutoCAD creates a third solid from the volume that the two solids have in common. The INTERFERE command can also be used to highlight the common volume of several pairs of solids. Figure 24-16 shows a solid that was created using INTERFERE, after it was moved away from its original location. As you can see, the original solids remain intact.

Figure 24-16: When you use INTERFERE, the original solids remain intact.

INTERFERE is useful when you have a number of interfering solids. This command enables you to divide the selection set of solids into two sets so that you can compare one against the other. For example, you can compare a box with three other solids by putting the box in one set and the other three solids in the other set. INTERFERE highlights each pair of interfering solids in turn so that you can easily visualize your drawing.

764 Part IV Drawing in Three Dimensions

To use INTERFERE, follow these steps:

1. Choose Interference from the Solids toolbar.

2.At the Select first set of solids: prompt, select objects. If you want to compare only two objects, you can put them both in the first set. Otherwise, select solids for the first set and press Enter to end object selection.

3.At the Select second set of solids: prompt, select the second set of objects and press Enter to end object selection. If you don’t want a second set, press Enter. AutoCAD displays the number of solids and sets how many interfering pairs it finds.

4.At the Create interference solids? [Yes/No] <N>: prompt, press Enter if you don’t want to create a new solid. Right-click and choose Yes if you do.

5.If you’re testing more than two solids, you see the Highlight pairs of interfering solids? [Yes/No] <N>: prompt. Press Enter if you don’t need to see each pair of interfering solids highlighted. Right-click and choose Yes if you want AutoCAD to highlight the pairs. If you choose Yes, AutoCAD highlights the first pair of solids that interfere.

6.At the Enter an option [Next pair/eXit] <Next>: prompt — which appears only if there is more than one pair — press Enter to see the next pair highlighted. Continue to press Enter to cycle through the interfering pairs. Right-click and choose Exit to exit the command.

STEPS: Creating Complex Solids

1.Open ab-24-b.dwg from the CD-ROM.

2.Save the file as ab24-03.dwg in your AutoCAD Bible folder. Make sure that OSNAP is on. Set running object snaps for endpoint, midpoint, and center. This drawing is measured in millimeters. The solids have been created by drawing circles, using EXTRUDE, and moving the solids to the proper Z coordinate. The result is shown in Figure 24-17.

3.Display the Solids Editing toolbar by right-clicking any toolbar and choosing Solids Editing.

4.To create the six holes arrayed around the center plate, choose Subtract from the Solids Editing toolbar. Follow the prompts:

Select solids and regions to subtract from...

Select objects: Select the central plate at 1 in Figure 24-17.

Select objects: Right-click.

Select solids and regions to subtract...

Select objects: Select the six circles arrayed around the plate. Right-click to end selection.

5

4

1

32

Figure 24-17: These solids are the basis for the model.

5.To create the central tube, choose Subtract from the Solids Editing toolbar. Follow the prompts:

Select solids and regions to subtract from...

Select objects: Select the outer tube at 2.

Select objects: Right-click.

Select solids and regions to subtract...

Select objects: Select the inner tube at 3. Right-click to end selection.

6.To “carve out” the central disk, choose Subtract from the Solids Editing toolbar. Follow the prompts:

Select solids and regions to subtract from...

Select objects: Select the outer circle at 4.

Select objects: Right-click.

Select solids and regions to subtract...

Select objects: Select the inner circle at 5. Right-click to end selection.

7.Use the HIDE command to see the result. This enables you to check the effects of the subtraction operations. Your drawing should look like Figure 24-18.

8.Choose Union from the Solids Editing toolbar. Select the three solids at 1, 2, and 3 in Figure 24-18 and right-click to end selection.

9.Use the HIDE command again to see the result.

10.Save your drawing. It should look like Figure 24-19. If you’re continuing on to the next exercise, leave it open.

766 Part IV Drawing in Three Dimensions

1

3

2

Figure 24-18: The result of three subtraction operations.