608 Part IV Drawing in Three Dimensions

Working with the User Coordinate System

Except for certain true solids, much of 3D work starts with a 2D shape. However, the 2D drawing commands can be drawn only on, or parallel to, the XY plane. The spool in Figure 21-15, for example, consists of three circles, all parallel to the XY plane. How do you draw a circle, or any other 2D object for that matter, that is not parallel to the XY plane?

The answer is to change the User Coordinate System (UCS), thereby changing the definition of the XY plane. You can move the UCS to any location to define the XY plane any way you want. After you do so, you can draw a 2D object at any angle. This section offers a brief review of UCS features that are particularly useful for 3D drawing. (See Chapter 8 for complete coverage of the UCS feature.)

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Using the UCS icon

Although you may have found the UCS icon an annoyance in 2D work, you should display it when working in 3D. Otherwise, it’s easy to lose track of which direction is which. Remember that you can also choose the Origin option, which displays the UCS icon, if possible, at the origin. Choose View Display UCS Icon Origin.

UCSs and viewpoints

A plan view of the World UCS (the default) is different from the plan view of a UCS you’ve created by rotating the UCS around the X axis, for example. The UCS defines the orientation and origin of the X, Y, and Z axes. On the other hand, the viewpoint shows your drawing from different angles without changing the orientation or origin of the axes. Understanding the difference between the UCS and viewpoints is important.

The broken pencil

When you choose a viewpoint that shows your drawing from the front, back, or one side instead of the top or an angle, you’re looking straight across the XY plane. This is like looking at a table (which represents the XY plane) by kneeling down so your eyes are exactly at the same height as the surface of the table. From this vantage point, you cannot easily pick objects on the XY plane. A circle looks like a line. In such a situation, if you’ve created a new UCS (not the World UCS) and if you’re using the 2D display of the UCS icon, the UCS icon becomes a brokenpencil icon to warn you that you’re looking at the XY plane of the current UCS edge on.

Figure 21-16 shows the same drawing from three viewpoints: plan view, the SE Isometric view (one of the preset viewpoints), and the front view. From the front view, you see the brokenpencil icon. Notice that the plan (top) view displays even fewer details of the drawing and would also make it difficult to select objects. The middle version uses the SE Isometric preset view, which displays the drawing from above and one corner. This view displays the drawing most clearly.

Express Tools has a command, RTUCS, that enables you to rotate the UCS icon by dragging with the mouse. You can set an incremental angle for rotation and specify an active axis around which you rotate the UCS. Choose Express Tools Real-Time UCS. For information on installing the Express Tools, see Appendix A.

UCS options

To change the UCS, choose one of the UCS menu items on the Tools menu. The following UCS options on the Tools New UCS submenu are useful for 3D drawing.

Object

You can align the UCS with any existing object. Because this option orients the UCS differently for different objects, this option can sometimes be confusing. However, to modify some objects, you must be on its XY plane — a good time to use the Object option. The overall principle is that the UCS’s XY plane stays parallel to the previous UCS, except for 3Dface objects. Table 21-1 explains how the Object option aligns the UCS for various kinds of objects.

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Figure 21-16: Looking at a drawing from different viewpoints in a custom UCS. When you’re looking at the XY plane edge on and are using the 2D UCS icon display, the broken-pencil icon appears.

Chapter 21 Specifying 3D Coordinates 611

Face

The Face option aligns the UCS on a face of a solid object. At the Select face of solid object: prompt, click within the boundary of a face or on its edge to highlight the face and align the X axis of the UCS with the closest edge of the first face it finds.

At the Enter an option [Next/Xflip/Yflip] <accept>: prompt, you can now refine the UCS. Right-click and choose Next to move the UCS to the adjacent face or the back face of the selected edge. Right-click and choose Xflip to rotate the UCS 180 degrees around the X axis.

Right-click and choose Yflip to rotate the UCS 180 degrees around the Y axis. When you have the UCS you want, press Enter to accept the current location of the UCS.

View

The View option aligns the X and Y axes with the current view. The current origin remains the same. The View option is most often used for creating text that you want to appear flat from your viewpoint of a 3D view of the drawing.

Origin

The Origin option creates a UCS parallel to the current UCS but with a new origin that you specify. You can use the Origin option for working at a new elevation, instead of changing the current elevation.

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Z Axis Vector

The Z Axis Vector option enables you to define an origin and then a point on the positive side of the Z axis. You can keep the previous origin to twist the UCS around its origin.

3Point

The first point you specify is the origin, the second point indicates the positive direction of the X axis, and the third point indicates the positive direction of the Y axis.

X

The X option maintains the current origin and rotates the Y and Z axes around the current

X axis at the rotation angle you specify. The most common rotation is 90 degrees or a multiple of 90 degrees, but you can specify any angle.

Y

The Y option keeps the current origin and rotates the X and Z axes around the current Y axis. You specify the angle.

Z

The Z option keeps the current origin and rotates the X and Y axes around the current Z axis. You specify the angle.

Apply

This option applies the current UCS to a viewport that you specify or to all viewports. This option is not available in AutoCAD LT because you can have only one UCS for all your viewports in AutoCAD LT.

STEP-BY-STEP: Creating UCSs

1.Open ab21-c.dwg from the CD-ROM.

2.Save it as ab21-04.dwg in your AutoCAD Bible folder. This drawing contains some centerlines based on measurements of a chair. OSNAP should be on. Set running object snaps for endpoint, midpoint, center, and quadrant. The UCS icon is set at the origin.

3.Choose Circle from the Draw toolbar. Use the From object snap to specify the center at @2,0 from 1 in Figure 21-17. Set the radius to 0.5.

4.Choose Properties on the Standard toolbar and select the circle you just drew. Change the thickness to 16. Press Enter. Close the Properties palette (or right-click its title bar and choose Auto-Hide).

5.Click ORTHO on the status bar to turn it on. Choose Mirror from the Modify toolbar. The circle should be selected. (If it isn’t, select it.) Specify the endpoint at 2 for the first point of the mirror line. Specify any point vertical to 2 for the second point of the mirror line. Choose not to delete the source object (the circle).

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0

Figure 21-17: These centerlines are the basis for drawing a chair.

6.Repeat the MIRROR command. Select the two circles. Pick the midpoint of the line at 3 for the first point of the mirror line. Pick any point horizontal to the first point for the second point of the mirror line. Press Enter. There are now four legs. Turn ORTHO off.

7.Type elev . Change the elevation to 16 and the thickness to 1.

8.Choose View 3D Views SE Isometric. You can now see the four chair legs.

9.Choose Polyline from the Draw toolbar. At the Specify start point: prompt, choose the From object snap. Use the Center object snap of the top of the circle near 1 in Figure 21-17 for the base point and an offset of @–2,0. (You may have to tab to get the Center object snap.) Continue to pick points 4 through 0. Notice that the Endpoint object snap symbol appears at the height of the first point. Then right-click and choose Close to close the polyline.

10.Choose Modify Object Polyline (or type pedit ). Select the polyline. At the prompt, right-click and choose Fit to fit the polyline. Press Enter to end the command.

11.To see the result, type hide . Remember that the polyline has no top or bottom surface. Imagine your model as a glass-bottomed chair. It should look like Figure 21-18.

12.Choose Tools Inquiry List. Select the front-left leg at 1 in Figure 21-18. Press Enter. The center (of the circle at the bottom of the leg) is X = 3.5000; Y = –7.0000; Z = 0.0000.

13.To see the effect of a different UCS option, choose Tools New UCS X. To rotate the UCS around the X axis, type 90 . Again, choose Tools Inquiry List and select the same leg of the chair. Now the center is X = 3.5000, Y = 0.0000, Z = 7.0000. Look at the UCS icon (which is at 0,0,0) and try to visualize why the coordinates are the way they are listed.

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Figure 21-18: Part of a 3D chair.

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14.Because you know the center of the circle of the front leg, you can move the UCS there.

Choose Tools New UCS Origin. At the Origin point <0,0,0>: prompt, type 3.5,0,7 . This places the X axis through the center of the circle.

15.Choose Tools New UCS View. The UCS is now parallel to your current view. Choose Draw Text Single-Line Text. Start the text at 2 in Figure 21-18. Set the height to

1.0 and the rotation to 0. Type A glass-bottomed chair. Press Enter twice to end the command.

16.Choose Tools New UCS Object. Pick the line at 3. Choose Tools New UCS Origin. Type 0,0,17 as the origin to place the UCS at the top of the seat.

17.Choose Tools New UCS X. Type –10 to rotate the UCS around the X axis by –10 degrees. This enables you to create the back of the chair at a 10-degree angle. Type plan and press Enter again to accept the default. (This command is covered in the next chapter.) It enables you to view your drawing from plan view (in this case, relative to the current UCS).

Tip Although it’s not necessary for this exercise, this would be a good UCS to save (choose Tools Named UCS and rename the Unnamed current UCS to Chair Back). Having gone through all these steps, it would be a shame to have to re-create the UCS again.

18.With no objects selected, choose Properties on the Standard toolbar. Set the thickness to 16. (The elevation is irrelevant because you’ll use OSNAPS in the next step.) Close the Properties palette.

19.Start the CIRCLE command. Choose the center of the lower circle of the top-left leg (it’s below the chair’s seat) for the center. Accept the default radius of 0.5. Use the same technique to draw a circle at the corresponding right circle. Choose View 3D Views SE Isometric to see the results. Then choose Zoom Previous on the Standard toolbar to return to your previous display.

20.Change the thickness to –5. Type hide so that you can see the circles you just created more clearly.