AutoCAD 2005 for Dummies

the relationship between objects more clearly on the plotted drawing. Even where you must sacrifice accuracy, aim for precision.

In CAD, lack of precision makes later editing, hatching, and dimensioning tasks much more difficult and time consuming:

? Small errors in precision in the early stages of creating or editing a drawing often have a big effect on productivity and precision later.

? Drawings may guide manufacturing and construction projects; drawing data may drive automatic manufacturing machinery. Huge amounts of money, even lives, can ride on a drawing’s precision.

In recognition of this, a passion for precision permeates the profession. Permanently. Precision is one of the characteristics that separates CAD from ordinary illustration-type drawing work. The sooner you get fussy about precision in AutoCAD, the happier everyone is.

In the context of drawing objects, precision means specifying points and distances precisely, and AutoCAD provides a range of tools for doing so. Table 4-2 lists the more important AutoCAD precision techniques, plus the status bar buttons that you click to toggle some of the features off and on.

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 direction 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.

  Precision is especially important when you’re drawing or editing geometry — the lines, arcs, and so on that make up whatever you’re representing in the CAD drawing. Precision placement usually is less important with notes, leaders, and other annotations that describe, not show.

  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:

? 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.

Figure 4-10: Coordinating from the keyboard.