AutoCAD 2005 For Dummies (2004)

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Table 4-2	Precision Techniques
Technique		Status Bar Button	Description
Coordinate entry		—	Type exact X,Y coordinates.
Single-point object snaps		—	Pick points on existing
			objects (lasts for one point
			pick).
Running object snaps		OSNAP	Pick points on existing
			objects (lasts for multiple
			point picks).
Snap		SNAP	Pick points on an imaginary
			grid of equally spaced “hot
			spots.”
Ortho		ORTHO	Constrain the cursor to
			move at an angle of 0, 90,
			180, or 270 degrees from the
			previous point.
Direct distance entry		—	Point the cursor in a direc-
			tion and type a distance.
Object snap tracking		OTRACK	Helps the cursor locate
			points based on multiple
			object snap points.
Polar tracking		POLAR	Makes the cursor prefer
			certain angles.
Polar snap		—	Causes the cursor to prefer
			certain distances along
			polar tracking angles.

Before you draw objects, always check the status bar’s SNAP, ORTHO, POLAR, OSNAP, and OTRACK buttons and set the buttons according to your precision needs.

A button that looks pushed in indicates that the feature is on.

A button that looks popped up indicates that the feature is off.

Keyboard capers: Coordinate entry

The most direct way to enter points precisely is to type numbers at the command line. AutoCAD uses these keyboard coordinate entry formats:

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Absolute rectangular coordinates in the form X,Y (for example: 7,4)

Relative rectangular coordinates in the form @X,Y (for example: @3,2)

Relative polar coordinates in the form @distance<angle (for example: @6<45)

AutoCAD locates absolute rectangular coordinates with respect to the 0,0 point of the drawing — usually its lower-left corner. AutoCAD locates relative

rectangular coordinates and relative polar coordinates with respect to the previous point that you picked or typed. Figure 4-10 demonstrates how to use all three coordinate formats to draw a pair of line segments that start at absolute coordinates 2,1; go 3 units to the right and 2 units up; then go 4 units at an angle of 60 degrees.

AutoCAD also understands absolute polar coordinates in the form distance<angle, but this format is almost never useful.

Relative rectangular coordinates

Absolute rectangular coordinates

Relative polar coordinates

Figure 4-10:

Coordinating

from the

keyboard.

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You can view coordinate locations by moving the cursor around in the drawing area and reading the Coordinates area at the left of the status bar. The X,Y coordinates should change as you move the cursor. If the coordinates don’t change, click the Coordinates area until the command line says <Coords on>. Although it’s not apparent at first, there are in fact two <Coords on> display modes: absolute coordinates and polar coordinates. If you start a command such as LINE, pick a point, and then click the Coordinates area a few times, the display changes from coordinates off to live absolute coordinates (X,Y position) to live polar coordinates (distance and angle from the previous point). The live polar coordinates display mode is the most informative most of the time.

When you type coordinates at the command line, do not add any spaces, because AutoCAD interprets them as though you’ve pressed Enter. This “Spacebar = Enter” weirdness is a productivity feature that’s been in AutoCAD forever. It’s easier to find the spacebar than the Enter key when you’re entering lots of commands and coordinates in a hurry.

If you’re working in architectural or engineering units, the default unit of entry is inches, not feet.

To specify feet, you must enter the symbol for feet after the number, for example:

6’ for 6 feet

You can enter a dash to separate feet from inches, as architects often do:

6’–6” is 6 feet, 6 inches.

Both the dash and the inch mark are optional when you’re entering coordinates and distances:

AutoCAD understands 6’6” and 6’6 as the same as 6’–6”.

If you’re typing a coordinate or distance that contains fractional inches, you must enter a dash — not a space — between the whole number of inches and the fraction:

6’6–1/2 (or 6’–6–1/2) represents 6 feet, 61⁄2 inches.

If all this dashing about confuses you, enter partial inches by using decimals instead:

6’6.5 is the same as 6’6–1/2 to AutoCAD, whether you’re working in architectural or engineering units.

Grab an object and make it snappy

After you’ve drawn a few objects precisely in a new drawing, the most efficient way to draw more objects with equal precision is to grab points, such as

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endpoints, midpoints, or quadrants, on the existing objects. AutoCAD calls this object snapping, because the program pulls, or snaps, the cursor to a point on an existing object. The object snapping feature in general and object snap points in particular often are called osnaps.

I’d like to make just one point

AutoCAD provides two kinds of object snapping modes:

Single point (or override) object snaps

A single point object snap lasts just while you pick one point.

Running object snaps

A running object snap stays in effect until you turn it off.

Here’s how you draw precise lines by using single point object snaps:

1.Open a drawing containing some geometry.

2.Turn off running osnap mode by clicking the OSNAP button on the status bar until the button appears to be pushed out and the words <Osnap off> appear on the command line.

Although you can use single point object snaps while running object snap mode is turned on, you should turn off running osnap mode while you’re getting familiar with single point object snaps. After you’ve gotten the hang of each feature separately, you can use them together.

3.Start the LINE command by clicking the Line button on the Draw toolbar.

The command line prompts you to select the first endpoint of the line:

Specify first point:

4.Hold down the Shift key, right-click anywhere in the drawing area, and release the Shift key.

If you find the Shift+right-click sequence awkward, you can avoid it by using the Object Snap toolbar instead. To turn the toolbar on, point to any toolbar button, right-click, and choose Object Snap. Now you can activate a single point object snap by clicking its toolbar button.

The object snap cursor menu appears, as shown in Figure 4-11.

5.Choose an object snap mode, such as Endpoint, from the object snap cursor menu.

The object snap cursor menu disappears, and the command line displays an additional prompt indicating that you’ve directed AutoCAD to seek out, for example, endpoints of existing objects:

_endp of:

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Figure 4-11:

The object snap cursor menu.

6.Move the cursor slowly around the drawing, pausing over various lines and other objects without clicking yet.

When you move the cursor near an object with an endpoint, a colored square icon appears at the endpoint, indicating that AutoCAD can snap to that point. If you stop moving the cursor for a moment, a yellow ToolTip displaying the object snap mode (for example, Endpoint) appears to reinforce the idea.

7.When the endpoint object snap square appears on the point you want to snap to, click.

AutoCAD snaps to the endpoint, which becomes the first point of the new line segment that you’re about to draw. The command line prompts you to select the other endpoint of the new line segment:

Specify next point or [Undo]:

When you move the cursor around the drawing, AutoCAD no longer seeks out endpoints because single point object snaps last only for a single pick. Use the object snap cursor menu again to snap the other end of your new line segment to another point on an existing object.

8.Use the “press Shift, right-click, release Shift” sequence described in Step 4 to display the object snap cursor menu, and then choose another object snap mode, such as Midpoint, from the object snap cursor menu.

The command line displays an additional prompt indicating that you’ve directed AutoCAD to seek, for example, midpoints of existing objects:

_mid of:

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When you move the cursor near the midpoint of an object, a colored triangle appears at the snap point. Each object snap type (endpoint, midpoint, intersection, and so on) displays a different symbol. If you stop moving the cursor, the ToolTip text reminds you what the symbol means. Figure 4-12 shows what the screen looks like during this step.

9.Draw additional line segments by picking additional points. Use the object snap cursor menu to specify a single object snap type before you pick each point.

Try the Intersection, Perpendicular, and Nearest object snaps. If your drawing contains arcs or circles, try Center and Quadrant.

10.When you’re finished experimenting with single point object snaps, right-click anywhere in the drawing area and choose Enter from the cursor menu.

New line segment

Already object-snapped to endpoint

About to object-snap to midpoint

Figure 4-12:

A snappy line.

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There’s a difference between right-clicking and Shift+right-clicking in the drawing area:

•Right-clicking displays menu options for the current command (or common commands and settings when no command is active).

•Shift+right-clicking always displays the same object snap cursor menu.

AutoCAD 2005 includes a new Mid Between 2 Points (M2P) object snap mode, which finds the point lying exactly halfway between two other points. You should use precision techniques, such as other object snaps, to select the two points.

Run with object snaps

Often, you use an object snap mode (such as endpoint) repeatedly. Running object snaps address this need. These steps set a running object snap:

1.Right-click the OSNAP button on the status bar.

2.Choose the Settings option.

The Object Snap tab on the Drafting Settings dialog box appears, as shown in Figure 4-13.

3.Select one or more object snap modes by checking the appropriate boxes.

4.Click OK to close the dialog box.

Figure 4-13:

Grabbing multiple object features is an osnap.

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You click the OSNAP button on the status bar to toggle running object snap mode off and on. After you turn on running object snap, AutoCAD hunts for points that correspond to the object snap modes you checked in the Drafting Settings dialog box. As with single-point object snaps, AutoCAD displays a special symbol — such as a square for an endpoint object snap — to indicate that it has found an object snap point. If you keep the cursor still, AutoCAD also displays a ToolTip that lists the kind of object snap point.

Use single point or running object snaps to enforce precision by making sure that new points you pick coincide exactly with points on existing objects. In CAD, it’s not good enough for points to almost coincide or to look like they coincide. AutoCAD knows the difference between “looks the same” and “is the same,” and will cause you untold amounts of grief if you try to make do with “looks the same.” You lose points, both figuratively and literally, if you don’t use object snaps or one of the other precision techniques covered in this chapter to enforce precision.

Other precision practices

The following are some other AutoCAD precision techniques (refer to Table 4-2, earlier in this chapter):

Snap: If you turn on snap mode, AutoCAD constrains the cursor to an imaginary rectangular grid of points at the spacing that you’ve specified. Follow these steps to turn on snap mode:

1.Right-click the SNAP button on the status bar.

2.Choose the Settings option.

The Snap and Grid tab on the Drafting Settings dialog box appears.

3.Enter a snap spacing in the Snap X Spacing field and then click OK.

Click the SNAP button on the status bar or Press F9 to toggle snap mode off and on. To use snap effectively, change the snap spacing frequently — changing to a smaller spacing as you zoom in and work on smaller areas. You often need to toggle snap off and on, because selecting objects and some editing tasks are easier with snap off.

Ortho: Ortho mode constrains the cursor to move at right angles (orthogonally) to the previous point. Click the ORTHO button on the status bar or Press F8 to toggle ortho mode off and on. Because technical drawings often include lots of orthogonal lines, you’ll use ortho mode a lot.

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Direct distance entry: This “point and type” technique is an easy and efficient way to draw with precision. You simply point the cursor in a particular direction, type a distance at the command line, and press Enter. AutoCAD calls it “direct distance entry” because it avoids the indirect command line method of specifying a distance by typing relative or polar coordinates. (I describe this older method earlier in this chapter.) You can use direct distance entry any time the crosshair cursor is anchored to a point and the command line prompts you for another point or a distance. You’ll usually use direct distance entry with ortho mode turned on, to specify a distance in an orthogonal direction (0, 90, 180, or 270 degrees). You also can combine direct distance entry with polar tracking to specify distances in non-orthogonal directions (for example, in angle increments of 45 degrees).

Object snap tracking: This feature extends running object snaps so that you can locate points based on more than one object snap point. For example, you can pick a point at the center of a square by tracking to the midpoints of two perpendicular sides. (AutoCAD LT lacks the object snap tracking feature.)

Polar tracking: When you turn on polar tracking, the cursor jumps to increments of the angle you selected. When the cursor jumps, a ToolTip label starting with Polar: appears. Right-click the POLAR button on the status bar and choose the Settings option to display the Polar Tracking tab on the Drafting Settings dialog box. Select an angle from the Increment Angle drop-down list and then click OK. Click the POLAR button on the status bar or press F10 to toggle polar tracking mode off and on.

Polar snap: You can force polar tracking to jump to specific incremental distances along the tracking angles by changing the snap type from Grid snap to Polar snap. For example, if you turn on polar tracking and set it to 45 degrees and turn on polar snap and set it to 2 units, polar tracking jumps to points that are at angle increments of 45 degrees and distance increments of 2 units from the previous point. Polar snap has a similar effect on object snap tracking.

To activate polar snap, follow these steps:

1.Right-click the SNAP button on the status bar.

2.Choose the Settings option.

The Snap and Grid tab on the Drafting Settings dialog box appears.

3.Click the Polar Snap radio button, type a distance in the Polar Distance edit box, and then click OK.

When you want to return to ordinary rectangular snap, as described at the beginning of this list, select the Grid Snap radio button in the Drafting Settings dialog box.

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If you’re new to AutoCAD, its wide range of precision tools probably seems overwhelming at this point. Rest assured that there’s more than one way to skin a cat precisely, and not everyone needs to understand all the ways. You can make perfectly precise drawings with a subset of AutoCAD’s precision tools. I recommend these steps:

1.Get comfortable with typing coordinates, ortho mode, direct distance entry, and single point object snaps.

2.Become familiar with running object snaps and try snap.

3.After you have all these precision features under your belt, feel free to experiment with polar tracking, polar snap, and object snap tracking.

It’s easy to confuse the names of the snap and object snap (osnap) features. Remember that snap limits the cursor to locations whose coordinates are multiples of the current snap spacing. Object snap (osnap) enables you to grab points on existing objects, whether those points happen to correspond with the snap spacing or not.