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Inside AutoCAD 14

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Creating Drawings with R14

by David M. Pitzer

To create accurate drawings with AutoCAD, you must understand how to specify and enter coordinates, and understand the points that they are composed of. This, in turn, requires a knowledge of AutoCAD's basic coordinate display systems, the World Coordinate Systems (WCS) and User Coordinate Systems (UCS). AutoCAD is an extremely accurate design and drafting package with the capability of 16 decimal places of precision stored in its database. To actualize this amount of accuracy, AutoCAD supports several drawing aids that enable you to draw, place, and edit objects in your drawings. This chapter will discuss AutoCAD's coordinate systems and the methods you can use to make drawing with accuracy and precision easier.

This chapter covers the following topics:

Coordinate Systems

No matter what kind of drawing you do in AutoCAD, you need a systematic method of specifying points. Points define the beginnings, midpoints, and endpoints of lines, the center of circles and arcs, the axis points of an ellipse, and so on. The capability to place points accurately is important. When an AutoCAD command prompts you for a point, you can either specify a point on the screen with the mouse or pointing device or you can enter coordinates at the Command: line. When entering points, AutoCAD uses a three-dimensional Cartesian, or rectangular, coordinate system. Using this standard system, you locate a point in 3D space by specifying its distance and direction from an established origin measured along three mutually perpendicular axes: the X, Y, and Z axes. The origin is considered to be at 0,0,0. Figure 6.1 illustrates such a coordinate system. Only two dimensions are depicted with the Z axis projecting up, perpendicular to the page. If you are concerned only with two-dimensional drawings, this is the presentation of AutoCAD's coordinate system that will be seen.

In figure 6.1, the 4,6 coordinate indicates a point 4 units in the positive X direction and 6 units in the positive Y direction. Points to the left or below the origin have negative X and Y coordinate components, respectively. Figure 6.2 illustrates the same coordinate system, only now the third dimension and the Z axis are shown. To specify 3D points, you add a third element to the coordinate designation. The point 4,6,6 in figure 6.2 is located 4 units in the positive X direction, 6 units in the positive Y direction, and 6 units in the positive Z direction. The system of reckoning coordinates is independent of the units used so that distances can be in any measurement; for example, the X direction could be English feet or inches, or metric centimeters or kilometers.

Later in this chapter, you will learn about the various ways that you can change the origin as well as the orientation of the three axes of AutoCAD's rectangular coordinate system. No matter how the coordinate system is oriented, you must know how to enter points.

Figure 6.1 The X and Y axes in a 2D coordinate system.

Figure 6.2 The 3D rectangular coordinate system.

Coordinate Point Entry Methods

Many of the drawings you make in AutoCAD--regardless of their eventual complexity--consist of a few relatively basic AutoCAD objects such as lines, circles, or text elements. These objects require that you enter points that specify their location, size, and direction. Many editing operations also require that you specify points. There are four ways to enter points or coordinates in AutoCAD:

Using Absolute Coordinates

Absolute rectangular coordinates are always measured from the origin point, 0,0,0. In AutoCAD, you specify an absolute coordinate from the keyboard by typing in the X,Y,and Z axes values separated by a comma--X,Y for 2D points or X,Y,Z for 3D points.

You don't need to use a plus sign (+) if the displacement from the origin is positive. You do, however, need to place a minus sign (-) in front of displacements in the negative direction: -2,3 or 4,-6,3.

Absolute polar coordinates also treat 2D coordinate entry as a displacement from the origin or 0,0, but you specify the displacement as a distance and an angle. The distance and angle are separated by a left-angle bracket (<) with no spaces:

distance<angle; for example, 25<135

Positive angles are measured counterclockwise from an assumed 0 degree that lies, by default, along the positive X axis as shown in figure 6.3.

NOTE: When entering the angle portion of polar coordinates, you can specify the angle as either positive (counterclockwise) or negative (clockwise). Thus:

37<90 is equivalent to 37<-270
This applies to both absolute and relative coordinate entry.

In the following exercise, you use both absolute rectangular and absolute polar coordinates to draw the outline of a fastener.

Figure 6.3 Default angle directions.

USING ABSOLUTE RECTANGULAR AND ABSOLUTE POLAR COORDINATE ENTRY

1. If necessary, start AutoCAD and begin a drawing using Wizard, Start from Scratch option.

2. Start the LINE command by either typing L or clicking on the Line tool on the Draw toolbar. Enter the following at the prompts:

From point: 4.5,4¯ To point: 6.5,5 ¯ To point: 9<38.34 ¯ To point: 7.8<25.36 ¯ To point: 6.5,4 ¯ To point: C ¯

3. Your drawing should look like figure 6.4.

Figure 6.4 Outline drawn using absolute coordinates.

This exercise demonstrates the limitations of absolute coordinates. Although absolute coordinates are adequate for designating the beginning of a line, measuring subsequent points in relation to the drawing's origin is cumbersome and often inaccurate. When the lines outlining an object (such as the fastener in the preceding exercise) are not orthogonal, absolute coordinates are inadequate if any degree of accuracy and efficiency is desired. The use of relative coordinates solves this problem.

Using Relative Coordinates

In almost any kind of drawing, once you have established the beginning of a line, you usually know the X and Y displacement or the distance and angle of the next point. Relative coordinates do not reference the origin point but are reckoned relative to the last point. You can use this more straightforward method with either relative rectangular or relative polar coordinates. To distinguish relative coordinate entry from absolute entry, you precede relative coordinates with the "at" symbol (@); for example, @1.5,3 for relative rectangular entry or @2.6<45 for relative polar entry.

In the preceding relative rectangular entry, the point specified lies at a displacement of 1.5 units in the X axis direction and 3 units in the Y axis direction from the previous point; in the relative polar entry, the point lies 2.6 units at an angle of 45 degrees from the previous point. In the following exercise, the utility of relative coordinates is shown in drawing a fastener similar to the one in the previous exercise.

USING RELATIVE COORDINATES

1. Continue from the previous exercise.

2. Start the LINE command by typing L or clicking on the Line tool on the Draw toolbar. Enter the following at the prompts:

From point: 9,5 ¯ (Note this is an absolute coordinate.) To point: @2<0 ¯ (Note this is a relative polar coordinate.) To point: @.8<46 ¯ To point: @0,-2.2 ¯ (Note this is a relative rectangular coordinate.) To point: @.8<134 ¯ To point: @2<180 ¯ To point: C

3. Your drawing should now resemble figure 6.5.

Figure 6.5 Outline drawn using relative coordinates.

NOTE: The angles involved or the availability of distance information will usually determine whether it is easier to enter the next point using relative polar or relative rectangular coordinates.

As you can see, relative coordinate point entry is much easier to use and permits more accuracy. Even when your drawing involves purely orthogonal displacements, relative coordinate entry is the superior and usually the only accurate method.

Direct Distance Entry

A variation of relative coordinate entry, called direct distance entry, is supported in Release 14. In direct distance entry, instead of entering coordinate values, you can specify a point by moving the cursor to indicate a direction and then entering the distance from the first point. This is a good way to quickly specify a line length. This method is used primarily when the displacements involved are orthogonal and you can therefore have the ORTHO drawing aid turned on. The following exercise demonstrates the use of direct distance entry.

USING DIRECT DISTANCE COORDINATE ENTRY

1. Continue from the previous exercise by starting the LINE command. Respond to the first prompt as follows:

From point: 7.5,7.5 ¯

2. Ensure that ORTHO mode is active by observing the ORTHO tile in the mode status bar at the bottom of the AutoCAD drawing window. If it is grayed-out (off), double-click it to turn it on.

3. With ORTHO mode on, move the crosshairs cursor any distance to the right of the point entered in step 1. Answer the current prompt as follows:

To point: 2.3 ¯

4. Note that the first polyline segment is drawn 2 units orthogonally to the right of point 7.5,7.5. Also note that your coordinate entry, although "relative" to the preceding point, did not require the "@" prefix.

5. Now respond to the current prompt with a standard relative polar coordinate as follows:

To point: @.8<46 ¯

6. Enter the next polyline segment using direct distance entry by moving the crosshairs cursor any distance below the last point and entering the following at the current prompt:

To point: 2.2 ¯

7. Note again that the preceding entry did not require the leading "@" symbol and that the line segment was drawn 2.2 units in the direction of the crosshairs cursor.

8. Again use standard relative polar coordinate entry for the next polyline segment:

To point: @.8<134 ¯

9. Respond to the current prompt by placing the crosshairs cursor to the left of the last point and entering:

To point: 2.3 ¯

10. Complete the outline by typing C and pressing Enter. The outline should be similar to that completed in the previous exercise.

Direct distance entry provides a more direct and easier method of entering relative coordinates when the point lies in an orthogonal relationship to the previous point--a common situation in most drawings. Of course, if the point you want to designate lies on a snap point, whether orthogonal to the previous point or not, you can bypass keyboard entry by simply snapping the cursor to and clicking on the point. (The concept of "snapping" is covered later in this chapter.)

NOTE: Direct distance entry was actually introduced in AutoCAD midway through the Release 13 cycle in Release 13c4. It was not, however, well documented at that time and will be a "new" feature to many Release 14 users.

Coordinate Display

The coordinate display window located at the bottom-left end of the mode status bar is useful when entering coordinates, whether you type them at the Command: prompt or pick points on the screen with the screen cursor. Figure 6.6 shows this display with two concurrent sessions of AutoCAD active.

Figure 6.6 The coordinate display window shows coordinates in the current drawing units.

The upper display shows decimal units while the lower display is in architectural units. The coordinate display has three modes of operation and, depending on the mode selected and the command in progress, can display either absolute or relative coordinates. You can cycle through the various modes in three ways: by pressing either F6 or Ctrl+D or by double-clicking in the display area itself. The three modes of coordinate display are as follows:

When Mode 0 is selected, the coordinate display appears grayed-out, although the coordinates of the last selected point are still visible. At an "empty" command prompt (no command in progress) or at an active prompt that does not accept either a distance or angle as input, you can only toggle between Mode 0 and Mode 1. At a prompt that does accept or require either a distance or angle as input, you can toggle among all three modes. Pay particular attention to the coordinate display window during the following exercise.

SWITCHING AMONG COORDINATE DISPLAY MODES

1. Continue from the previous exercise or open a new drawing. (Use the Wizard option, Start from Scratch if you are starting AutoCAD. You will not need to save this drawing.)

2. Ensure that SNAP mode is off by observing the SNAP tile in the mode status bar at the bottom of AutoCAD's application. If it is off, the window will be grayed-out. If necessary, double-click on the tile to make it appear grayed-out.

3. Ensure that the current mode setting of the coordinate display is "off" by pressing F6 until the display appears grayed-out.

4. Ensure that ORTHO mode is grayed-out (off) by pressing F8.

5. Now move the screen cursor within the drawing area. Note that the coordinate display remains static.

6. Double-click in the coordinate display area. Note that the display is no longer grayed-out.

7. Now move the screen cursor and notice that the coordinate display is continuously updated. The coordinate display is now in Mode 2.

8. Start the LINE command by typing L and pressing Enter. Because the From point: prompt does not accept distance or angle input, the display continues to update as you move the cursor.

9. Respond to the From point: prompt by typing in 4,7 and pressing Enter.

10. Notice that once a point has been specified, the coordinate display changes to show relative polar coordinates as you move the cursor. Type in the following absolute rectangular point:

To point: 6,7 ¯

11. The line is drawn between the two points. Now press Ctrl+D. The display changes to Mode 1 (on) and the display is again in absolute rectangular format as you move the cursor.

12. Press F6. The display is now in Mode 0 (off). The display is grayed-out and static as you move the cursor. Enter the following at the prompt:

To point: 2,3

13. The next line is drawn. Now double-click in the coordinate display window. The display is now back in Mode 2 with the read-out once again in relative polar format.

14. Press the Esc key to end the LINE command. Then press the U key and press Enter to undo the line. Note that the display reverts to absolute rectangular because no command accepting distance or angle input is active.

In this exercise, you saw the three modes of coordinate display and how they can aid you in selecting points. You also manually used the three methods available to you for toggling among the three modes. Although the absolute rectangular read-out mode is of limited usefulness, it can, for example, be used to specify the starting point of an object such as a wall or to specify a set of known points in a surveying data context.

Changing Coordinate Systems

The beginning of this chapter looked at AutoCAD's rectangular or Cartesian coordinates system from the standpoint of entering coordinates representing points in your drawing (refer to fig. 6.1). You learned about absolute and relative coordinate entry in both rectangular and polar formats. When you begin a new AutoCAD drawing you are, by default, using a rectangular coordinate system that is called the World Coordinate System or WCS. This is, indeed, the familiar coordinate system with the origin at or near the bottom-left corner of the "page," or AutoCAD's drawing window. This presentation of a standard rectangular coordinate layout is given a name because other coordinate systems and layouts are possible in AutoCAD. These coordinate systems are called Users Coordinate Systems, or UCS, because they may be defined in a number of ways by the user. UCSs make working in 3D space easier, although UCSs are frequently useful in 2D work as well.

World Coordinate System

The World Coordinate System is nothing more than a standard rectangular coordinate system with the origin in the lower-left corner of the screen, a horizontal X axis running left to right and a Y axis extending vertically from the top to the bottom of the screen. The Z axis is perpendicular to both the X and Y axes and is considered to extend in a direction perpendicular to the screen. To identify the WCS and establish its orientation, AutoCAD, by default, places the WCS icon at or near the origin. The WCS icon is shown in figure 6.7. Its defining characteristic is the "W" appearing on the icon; this tells you that you are in the World Coordinate System.

Figure 6.7 The World Coordinate System icon.

User Coordinate Systems

You can create your own coordinate systems called User Coordinate Systems, or UCSs. As the name suggests, a UCS is defined and controlled by you, the user. In a UCS, the origin as well as the direction of the X, Y, and Z axes can be made to move, rotate, and even align with drawing objects. Even though the three axes in a UCS remain mutually perpendicular, as they are in the WCS, a great deal of flexibility can be achieved in placing and orientating your UCS. The UCS command enables you to place a UCS origin anywhere in 3D space so that you can work relative to any point you want. You can also rotate the X,Y, and Z axes in 2D or 3D space. Figure 6.8 shows two UCS icons representing two different coordinate systems; one is the WCS, indicated by the "W" on the UCS icon; the other is a UCS defining a User Coordinate System. User Coordinate Systems are indispensable for working in 3D space.

The following exercise demonstrates how to create a User Coordinate System by aligning the UCS with two 2D points.

Figure 6.8 The World Coordinate System and a User Coordinate System.

ALIGNING A UCS WITH A 2D OBJECT

1. Start a new drawing and name it CHAPTER6.DWG. Use the UCS.DWG from the accompanying CD as a prototype or template. (See Chapter 3, "Setting Up the AutoCAD 14 Drawing Environment," for information on using prototype drawings.)

2. Your drawing should resemble figure 6.9. Note that the W in the UCS icon indicates the World Coordinate System is current.

3. Choose Tools, UCS, 3point. The following prompt appears:

Origin point <0,0,0>:

4. Shift+right-click to display the cursor menu and choose Endpoint. Then pick 1 in figure 6.9. The following prompt appears (the points given as a default may differ in your drawing):

Point on positive portion of the X-axis <6.58,2.04,0.00>:

5. Shift+right-click to display the cursor menu, choose Endpoint, and pick 2. The following prompt appears:

Point on positive-Y portion of the UCS XY plane <5.19,2.96,0.00>:

6. Pick anywhere near 3. Note that the UCS icon changes orientation to align with the new UCS and that the W disappears, indicating you are no longer in the WCS (see fig. 6.10).

Figure 6.9 Pick a point to change the UCS.

Figure 6.10 Arraying an object in the new UCS.

7. Choose Modify, Array. The following prompt appears:

Select objects:

8. Pick anywhere on the object at 1 and end the selection process by pressing Enter.

9. Answer the following prompts as shown:

Rectangular or Polar array (<R>/P): R ¯ Number of rows (---) <1>: 6 ¯ Number of columns (|||) <1>: ¯

10. The following prompt appears:

Unit cell or distance between rows (---):

11. Activate the cursor menu (Shift+right-click) and choose Intersection. Click at 2. Again activate the cursor menu, choose Intersection, and click at 3.

12. The array is carried out in a direction perpendicular to the X axis of the new UCS.

13. Return the UCS to the WCS with the USC command. Enter UCS and press Enter at the Command: prompt. When the following prompt appears, accept the default <World>:

Origin/ZAxis/3point/OBject/View/X/Y/Z/Prev/ Restore/Save/Del/?/<World>: ¯

Your drawing should resemble figure 6.11.

14. Save this drawing by pressing Ctrl+S.

Figure 6.11 The completed array in the UCS.

TIP: It is possible to change the UCS to the position and orientation you want in two or more ways. In the preceding exercise, for example, you could have rotated the UCS about its Z axis instead of using the 3point option. I generally prefer to use the 3point option because it seems more positive and easier to use.

Although defining new UCSs is most frequently used in 3D drafting, the preceding exercise demonstrated that the capability to change the UCS is helpful in 2D work as well. By aligning the UCS with the horizontal axis of the thread object in the drawing, a simple 6-row array could be quickly carried out with the "axis" of the array perpendicular to the horizontal axis of the object. The following exercise demonstrates two more options of the UCS command, re-establishing the most previous UCS and controlling the display of the UCS icon.

DISPLAYING THE PREVIOUS UCS AND CONTROLLING THE POSITION OF THE UCS ICON

1. Continue in the drawing from the previous exercise or open CHAPTER6.DWG. The drawing will resemble figure 6.11.

2. Choose Tools, USC, Previous. Note that the UCS reverts back to the UCS defined in the previous exercise.

3. Now, choose View, Display, UCS Icon, Origin. Note that the UCS icon moves to the origin point of the currently defined UCS as shown in figure 6.12. This is the origin point you defined in step 4 of the previous exercise.

4. Now again, choose View, Display, UCS Icon, and notice that a check mark appears beside the Origin selection. Click on Origin to toggle the icon back to its former position at the lower-left corner of the screen.

5. Repeat the preceding step except choose ON to remove the check mark. Note that the UCS icon is no longer visible.

6. Save this drawing by pressing Ctrl+S.

Figure 6.12 Placing the UCS icon at the current origin.

TIP: Although the preceding exercise uses shortcuts from the main menu bar, I prefer to turn the UCS icon on and off by typing UCSICON at the Command: prompt, and then typing in either On or Off. This method seems faster. You can also move the UCS icon to and from the current origin with the UCSICON command and typing either OR for origin, or N for no origin. This seems faster than traversing across three levels of cascading pull-down menus.

The UCS Command

The UCS command is the key to placing, moving, rotating, and displaying User Coordinate Systems. When called from the Command: line, the UCS command presents the following options:

Origin/ZAxis/3point/OBject/View/X/Y/Z/Prev/Restore/Save/Del/?/<World>:

You can use the following subset of these options for most 2D work:

In addition to being able to recall the most immediate UCS using the Previous option, as you did in the preceding exercise, using the Save option of the UCS command enables you to name and save various UCSs as you define them. Then if you want to re-establish a UCS, choose Tools, USC, Named UCS to display the UCS Control dialog box. From there you can name, re-name, delete, and restore previously named UCSs.

The commands involved with controlling UCSs are also available as tools on the Standard toolbar as shown in figure 6.13. By resting the screen pointer on each tool momentarily, the name of the tool is displayed. In most cases, the displayed tooltips closely follow the names of the UCS command's options.

Figure 6.13 The UCS tools on the Standard toolbar.

TIP: The tooltips displayed when you rest the screen pointer on a tool icon are, of course, rather short. As you work with toolbars, you will rely less and less on these tips. Keep in mind that for tools that you use less frequently, tooltips are also accompanied by a more extended "Help string" that is displayed at the lower-left portion of the screen. These Help strings are often more informative than the shorter tooltip.

The UCSICON Command

In the preceding exercise, you saw how the UCSICON command can be used to control the placement and visibility of the UCS icon. To round-out the discussion of User Coordinate Systems, here are the options for the UCSICON command. The UCSICON command displays the following prompt:

ON/OFF/All/Noorigin/ORigin <ON>:

TIP: Another capability of the UCS icon that you should know about is the system variable UCSFOLLOW. This variable controls whether or not a plan view will be automatically generated whenever you change the UCS. Setting this variable to 0 will not affect the view; setting it to 1 will cause the plan view to be generated. For 2D drafting, I find the automatic plan view setting to be helpful.

Setting Up Drawing Aids

AutoCAD has a number of Drawing Aids to help make your drawing more accurate and your work more efficient. These aids consist of a series of commands and system variables that you can use or set at the Command: prompt. You can also access them through the Drawing Aids dialog box. The Drawing Aids dialog box is shown in figure 6.14. It is accessed through Tools, Drawing Aids. This dialog box is also available from the Command: prompt by typing DDRMODES.

Figure 6.14 The Drawing Aids dialog box.

The Drawing Aids dialog box is divided into four sections: Modes, Snap, Grid, and Isometric Snap/Grid.

Modes

The Modes section contains the Ortho, Solid Fill, Quick Text, Blips, Highlight, Groups, and Hatch options. Each of these options is explained here:

Snap

Options in this section of the Drawing Aids dialog box control the Snap grid (refer to fig. 6.14). When the Snap grid is enabled (a check mark, or "X," appears in the On check box), the movement of the crosshairs cursor is restricted to incremental displacements across a grid of invisible "snap" points. This enables you to snap to and select points on this grid with a high degree of precision. You can enter both X and Y spacing for your snap grid by typing in the input boxes in this section. The other options--Snap Angle, X Base, and Y Base--control the angle at which the grid is oriented with respect to the current UCS and the origin's coordinates of the snap's grid.

The Snap grid settings can also be controlled at the Command: prompt with the SNAP command. You can toggle the Snap feature on and off with the F9 key, by pressing Ctrl+B, or by double-clicking on the SNAP tile on the mode status bar.

By carefully selecting the spacing of the Snap grid, you can usually make the picking of points much easier because you bypass the need to enter points at the keyboard.

Grid

In addition to a grid of invisible snap points, you can apply a grid of visible points to the drawing area. This visible drawing aid is simply called Grid. The controls found in the Grid section of the Drawing Aids dialog box control the appearance of this grid of visible points.

When the On option is checked, grid points are made visible with the spacing specified by the values (in drawing units) that you type into the X Spacing and Y Spacing edit boxes (refer to fig. 6.14).

It is common to link the spacing of the grid of visible points (the Grid) to the grid of invisible snap points. To establish this link, the X and Y spacing of the Grid points is set to 0. AutoCAD will then use the X and Y spacing of the Snap points and automatically apply these to the visible Grid. You can, of course, override this 1:1 relationship by explicitly entering values other than 0 for the Grid spacing. Keep in mind that regardless of the setting(s) of the Grid points, the origin and angle of the Grid is always kept the same as the origin and angle of the Snap points.

You can also control the visible Grid with the GRID command and toggle the Grid on and off with the F7 key, by pressing Ctrl+G, or by double-clicking the GRID tile on the mode status bar.

TIP: The Grid control section of the Drawing Aids dialog box largely duplicates functions available through the basic GRID command. Using the GRID command at the Command: prompt, however, offers an option, Grid spacing(X). Specifying a value followed by an "X" sets the Grid spacing to the specified value times the Snap interval. I often like to have my Snap spacing a fraction--say 1/4--of my Grid value. By using the "X" feature of the spacing setting available with the GRID command, this relationship between Grid and Snap remains in effect no matter how often I change the Grid setting.

Isometric Snap/Grid

You use the options in the Isometric Snap/Grid section of the Drawing Aids dialog box to draw a 2D isometric drawing. This section permits you to enable the isometric grid and set it to the standard Left, Top, or Right orientations.

TIP: When entering values in X and Y spacing text edit boxes in both the Grid and Snap sections of the Drawing Aids dialog box, entering a value in the X Spacing box followed by the Enter key will automatically transfer the value to the Y Spacing box. Because I usually want both the X and Y values of both of these settings equal, this shortcut eliminates the need to type anything in the Y boxes.

Object Snapping

No matter how carefully you set your Snap interval or how often you change that interval, it is highly unlikely that all the points in your drawings will conveniently fall on these snap points. This becomes increasingly true as your drawing becomes populated with various objects that, themselves, have important geometric features, such as endpoints, centers, and tangent points, to which you will want to relate other drawing objects. Most modern CAD applications, including AutoCAD, therefore, provide some means of identifying these geometric points. These tools make the construction of new geometry easier, the objects created are drawn more accurately, and the results can be consistently maintained with far more precision than is possible in traditional manual drafting. In AutoCAD, this capability is called Object Snapping and the Object Snap or Osnap. In AutoCAD, these modes consist of a set of tools that permit this accurate geometric construction.

Osnaps are used to directly and easily identify key points either on or in relation to your drawing objects. Figure 6.15 shows the Object Snap toolbar and the pop-up cursor menu.

Figure 6.15b The Object Snap toolbar and the pop-up cursor menu.
Figure 6.15b

The Osnap toolbar and the Cursor menu contain the same Osnap modes presented in essentially the same order. You can display or activate the pop-up cursor menu by simultaneously holding down the Shift button and pressing the right mouse button, commonly called Shift+right-click, or simply Shift+Enter because the right mouse button serves as an Enter button. If you use a three-button mouse, the middle button can be configured to "pop up" the cursor menu.

The Osnap modes are also represented on the Standard toolbar by a "fly-out" toolbar. Figure 6.16 displays both the Osnap toolbar and the Osnap fly-out. As shown, the Osnap toolbar can be displayed in a vertical format and placed or moved to a convenient position in your drawing area. Figure 6.15 showed the same toolbar displayed horizontally.

Figure 6.16 The Object Snap toolbar and fly-out.


TIP: Depending on personal preference and the type of drafting you are involved with, you can use the Osnap toolbar, in either a horizontal or vertical format (or a more compact rectangular arrangement), the Osnap fly-out from the Standard toolbar, or the pop-up cursor menu. The fly-out has the advantage of being present only during Osnap selections, the Osnap toolbar can be moved around and resized, and the cursor pop-up menu requires very little cursor movement. I almost always use the cursor menu because it seems to be the fastest means.

Osnap Modes

AutoCAD Release 14 has 13 Object Snap modes, including the new Tracking mode. Table 6.1 gives a description of each mode.

Table 6.1 AutoCAD Release 14 Object Snap Modes

Mode Description
Center Finds the center of a circle or an arc
Endpoint Finds the endpoint of a line or an arc
From Establishes a temporary reference point as a basis for specifying subsequent points
Insert Finds the insertion point of text objects and block references
Intersection Locates the intersection of two lines, arcs, or circles or the intersection of any combination of these
Midpoint Finds the midpoint of a line or an arc
Nearest Finds a point on an object that is nearest to the point you pick
Node Locates the location of a Point object
None Instructs AutoCAD not to use any Osnap modes
Perpendicular Returns a point at the intersection of the object selected and a line perpendicular to that object from either the last or the next point picked
Quadrant Finds the closest 0-, 90-, 180-, or 270-degree point relative to the current UCS on a circle or an arc
Tangent Locates a point that is tangent to the selected circle or arc from either the last or the next point picked
Tracking Specifies a point that is relative to other points, using orthogonal displacements

Running Osnap Toggle and Osnap Override

Running Osnap Toggle and Osnap Override are two new AutoCAD features. Although they were discussed in Chapter 1, "Exploring the New R14 Interface," they are important adjuncts to the overall operation of object snaps and are therefore briefly recapped here.

Running Object Snap Toggle

Running object toggle is an Osnap enhancement that enables you to toggle any running (continuing) Osnap off prior to selecting a point without losing the running Osnap settings. This feature is accessed by double-clicking the OSNAP tile on the mode status bar at the bottom of AutoCAD's screen. If this is done while no running Osnaps are in effect, the Osnap Settings dialog box is displayed, giving you the opportunity to set a running Osnap.

Object Snap Override

AutoCAD Release 14 provides an option that enables you to explicitly enter coordinate data that has priority over any running Osnaps that may be in effect. This enhances direct coordinate entry and you can be certain that such entries have precedence over any other settings.

AutoSnap

You saw AutoCAD Release 14's new AutoSnap feature in Chapter 1, "Exploring the New R14 Interface," and learned about controlling some of its features. This is an important new feature in AutoCAD and a closer look is taken in this chapter.

With AutoSnap, you can visually preview snap point candidates before picking a point. Depending on how you have AutoSnap's features set, AutoSnap will display a Snap Tip placard similar to the toolbar's Tool Tip feature. A marker distinctive to each Osnap mode can also be displayed in the color of your choice. You can also enable a "magnet" feature that snaps the marker into place much like the action of AutoCAD's Grips feature.

In the following exercise, you use some of AutoCAD's Osnap modes and the three methods of invoking them. In addition, you will see how the AutoSnap feature makes looking for and confirming Osnap points an unambiguous, efficient means of picking Osnap points.

BISECTING AN ANGLE USING OBJECT SNAPS WITH AUTOSNAP

1. Start a New drawing using the Wizard, Start from Scratch option. You will not need to name or save this drawing.

2. Choose View, Toolbars. The Toolbars dialog box appears.

3. In the Toolbars dialog box, scroll to and choose Object Snap. Note that the Object Snap toolbar appears (see fig. 6.17). Click on Close to dismiss the dialog box.

Figure 6.17 The Toolbars dialog box.

4. Use the DDOSNAP command to display the Osnap Settings dialog box and then choose the AutoSnap tab to display the AutoSnap page (see fig. 6.18). Ensure that the Marker, Magnet, and Snap Tip features are all enabled and that the Display aperture box is disabled as shown in figure 6.18. Click on OK to close the dialog box.

Figure 6.18 The Osnap Settings dialog box's AutoSnap tab.

5. Use the LINE command to draw a line from 1 to 2 to 3 as shown in figure 6.19.

Figure 6.19 Creating an arc using AutoSnap modes.

6. Choose Draw, Arc, the Center, Start, End. You see the following prompt:

arc Center/<Start point>: _c Center:

7. Click on the Snap to Endpoint tool from the displayed Osnap toolbar. Then move and rest the screen cursor to a point near 4 in figure 6.19. Notice the AutoSnap marker displayed at 2 and the Snap Tip identifying the lines endpoint.

8. Pick a point near 4. Then at the Start point: prompt, click and hold the Osnap fly-out on the Standard toolbar (see fig. 6.19) and then move to the Snap to Nearest tool and release the pick button. Move the cursor near to the lower line at 5. Note the appearance of the Nearest AutoSnap Marker as you approach 5.

9. Pick at 5. At the Angle/Length of chord/<End point>: prompt, Shift-Enter to display the screen pop-up menu. Then choose Endpoint and move the cursor toward 6. Note the appearance of the Endpoint Marker.

10. Pick the upper line at 6. AutoCAD draws the arc as shown in figure 6.20.

Figure 6.20 Bisecting an angle using Osnaps.

11. Type L and press Enter at the Command: prompt. At the From point: prompt, type endp and press Enter. Note the appearance of the Endpoint AutoSnap marker as you approach 7 in figure 6.20.

12. Click at 7. Then at the To point: prompt, click on the Midpoint tool on the Osnap toolbar and move the cursor to any point on the arc. Note the appearance of the Midpoint marker on the arc. With the marker showing, pick any point on the screen.

13. At the To point: prompt, press Enter to end the LINE command. The bisector line is drawn.

You do not need to save this drawing.

As seen in the preceding exercise, the use of Osnaps and the AutoSnap feature gives a definite, unambiguous indication of the geometry to which you are snapping. Even in crowded areas of a drawing, such as shown in figure 6.21, positive identification of which point is the current snap target is possible--out of several near the cursor.

Figure 6.21 AutoSnap provides positive identification of target geometry.

AutoSnap also supports a feature that enables stepping through the object snap points of objects lying within the target aperture when it is enabled. The Tab key is used to cycle from the closest to the furthest Osnap point from the center of the aperture box. The target geometry is highlighted to further aid in identification. This highlighting feature is shown in figure 6.22 where the midpoint of two objects lie very close together and both fall within the Osnap aperture box. Repeated pressing of the Tab key cycles among the objects, highlighting the target geometry so that you can snap to the correct object's midpoint.

Tracking

Figure 6.22 In crowded areas, AutoSnap highlights target geometry.

A new Osnap feature called Tracking is introduced in AutoCAD Release 14. As with many new AutoCAD features, this feature was first introduced in AutoCAD LT where it has been very popular. Although not an object snap in the strict sense, Tracking is used with standard Osnaps to enhance your ability to find points relative to another object's geometry.

You can use Tracking whenever AutoCAD prompts for a point. If you try to use Tracking at the Command: prompt, AutoCAD displays an error message. When you start Tracking and specify a point, AutoCAD constrains the next point selection to an orthogonal path that extends vertically or horizontally from the first point. The orthogonal direction determines which of the old point's values, X or Y, is replaced with the new point's X or Y value. If the rubber band is constrained to horizontal, then the X value is replaced. If the rubber band is constrained to the vertical, then the Y value is replaced. If you select a second point and press Enter to end Tracking, AutoCAD locates the new point at the intersection of an imaginary orthogonal path extending from the first two points.

In the following exercise, you use Tracking and Point Filters to find the center point of a rectangle.

FINDING THE CENTER OF A RECTANGLE USING TRACKING AND POINT FILTERS

1. Open TRACKING.DWG from the accompanying CD-ROM. You will use this drawing to practice the Tracking feature, but will not need to save your work. Your screen will resemble figure 6.23. Ensure that ORTHO mode is on.

Figure 6.23 Two rectangles.

2. You will draw a line from the center of one rectangle to the center of the other rectangle. First, start the LINE command. Respond to the prompt by entering a point filter as follows:

From point: .y ¯

The following prompt appears:

Of:

3. Invoke the Midpoint Osnap mode by typing mid and pressing Enter. Then move the cursor close to 1 in figure 6.23. Note that the midpoint AutoSnap marker appears. Click at 1. Respond to the following prompt as shown:

mid of (need XZ): mid ¯

4. At the mid of: prompt move the cursor near 2. Note the midpoint AutoSnap marker appears. Pick at 2. The from point of the lines is established at the intersection of the midpoints of the adjacent sides of the rectangle--the center on the rectangle. The following prompt appears:

To point:

5. Respond to this prompt by invoking Tracking; type tk and press Enter. The following prompt appears:

First tracking point:

6. Invoke the midpoint snap by using Shift+right-click to display the cursor menu, and then choose Midpoint. At the mid of: prompt, move the cursor near 3 and notice the AutoSnap midpoint marker. Pick at 3.

7. Move the cursor to the right to establish horizontal tracking. The following prompt appears:

Next point (Press ENTER to end tracking):

8. Invoke the midpoint Osnap again and move the cursor toward 4 until the midpoint AutoSnap marker appears as shown in figure 6.24. Pick at 4 and respond to the following prompt as shown:

Next point (Press ENTER to end tracking): ¯

9. AutoCAD draws the line between the two center points. End the LINE command by pressing Enter.

NOTE: You will continue in this drawing in the next exercise.

Figure 6.24 A line between the center points of two rectangles.


As you saw in the preceding exercise, object snaps are useful in identifying points on a drawing's object. You used the .Y Point Filter to temporarily store the Y coordinate value of the side of the first rectangle's midpoint, then you supplied the X coordinate value by snapping to the midpoint of the adjacent side. This established the intersection of the midpoints of the two sides, or the center of the rectangle. This point was then supplied to the LINE command as the initial From point. In this respect, Point Filters enable you to identify points that bear some relationship to other points, such as a common Y coordinate.

Next Tracking was turned on and the midpoint of one side (it would have made no difference which) was established as the first tracking point. With possible point selection now restrained to points orthogonal to this point, you established horizontal tracking by moving the cursor to the right and supplied the X point by choosing the midpoint of the adjacent side. When tracking was then turned off, the end of the first line segment was completed at the intersection of the midpoints of the two adjacent sides of the second rectangle or its center point.

Both Tracking and Point Filters must be invoked while a command requesting point entry is in progress. They each gather and store coordinate data, which is then fed to the suspended command.

From and Apparent Intersection Osnaps

Much like Point Filters and the Tracking feature, the "auxiliary" Osnaps From and Apparent Intersection supply data points that stand in some relationship to points on drawing objects. The From object snap establishes a temporary reference point as a basis for specifying subsequent points. From object snap is normally used in combination with other object snaps and relative coordinates. For example, at a prompt for the center point of an arc, you could enter from endp, select a line, and then enter @4,5 to locate a point four units to the right and five units up from the endpoint of the line; the center of the circle would then be located at this point.

Apparent Intersection snaps to the apparent intersection of two objects that might or might not actually intersect in 3D space. In 2D drafting, Apparent Intersection is usually involved with the projected intersection of two line elements.

The following exercise demonstrates both the From and Apparent Intersection Osnaps as you center a circle at a distance from the apparent intersection of two lines.

BEGINNING A LINE AT A DISTANCE FROM AN APPARENT INTERSECTION

1. Continue from the previous exercise. Start the LINE command and enter the following at the prompt:

From point: from ¯

2. At the Base point: prompt, invoke the mid Osnap:

Base point: mid ¯

3. At the Of: prompt, move the cursor near to 1 in figure 6.25 and click.

4. At the Offset: prompt, type the relative coordinate @1.5,1 and press Enter. This establishes the starting point of the line.

5. At the To point: prompt, activate the Apparent Intersection Osnap as follows:

To point: appint ¯

Figure 6.25 Using From to snap to relative geometry.

6. At the Of: prompt, move the cursor near to 2, note the appearance of the apparent intersection AutoSnap marker, and pick.

7. At the And: prompt, move the cursor near to 3, note the position of the intersection AutoSnap marker, and pick.

8. The end of the line segment is placed at the apparent intersection. Press Enter to end the LINE command. Your drawing should resemble figure 6.26.

Figure 6.26 A line drawn with the From and Apparent Intersection Osnaps.

NOTE: Although you typed in the Osnap modes in the preceding exercise, you could, of course, use any of the three other methods of setting Osnaps--namely, the cursor pop-up menu, the Osnap toolbar, or the Osnap fly-out on the Standard toolbar.

When used in conjunction with the other Osnaps, the From and Apparent Intersection--as well as the Tracking and Point Filter features--provide a powerful set of tools that help you establish points and accurately place geometry in your drawing.

Construction Lines and Rays

With the existence of Point Filters, Apparent Intersection, From Osnaps, and the Tracking feature, little need exists for the "construction lines" used in traditional "pencil" drafting. Once you become competent with these drawing aids, the time and effort required to draw and subsequently erase traditional construction lines will seem inefficient.

There may be occasions, however, when the inclusion of construction lines may be indicated to assist in visually presenting the relationship among elements of a drawing. AutoCAD has two special line objects, xlines and rays, that function as traditional construction lines.

The XLINE command creates infinite lines, which are commonly used as construction lines. Xlines can be placed vertically, horizontally, at a specified angle, offset a specified distance, or as an angle bisector. Although xlines extend infinitely in both directions, they are ignored for the purpose of calculating the drawing's extents.

The RAY command creates "semi-infinite" lines commonly used as construction lines. A ray has a finite starting point and extends to infinity. As with the xline, the infinite length of a ray is ignored for the purpose of determining a drawing's extents.

In line with their use as a largely visual element, both xlines and rays are often placed on separate layers with a distinctive linetype and color assigned. Figure 6.27 shows a typical application of xlines.

Figure 6.27 Xlines and Rays serve as construction lines.

Summary

In this chapter, you learned about AutoCAD's coordinate system and the methods you can use to enter coordinate points in your drawings. Absolute coordinate entry enables you to specify points relative to the drawing's fixed 0,0 point, or origin. Relative coordinate entry, on the other hand, enables you to specify points relative to the previous point you entered. Relative coordinates are expressed either as an x and y distance or a distance and angle from the last point. You also learned how to change the orientation of AutoCAD's coordinate system and how to configure drawing aids such as Snap and Grid.

This chapter also covered the important concept of snapping to specific geometry in your drawings, such as a circle's center or the intersection of two lines. You learned that AutoCAD Release 14's new AutoSnap feature makes snapping to such points both easier and less ambiguous than in previous releases.

In the next chapter, you will learn how to draw the elementary objects that, taken together, comprise almost all of the drawings you will make in AutoCAD.

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