AutoCAD For Dummies. Ralph Grabowski
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a number like 147.5, AutoCAD will understand it to be 147 1/2″ or 12′3 1/2″.
Decimal units are unitless — that is, they’re not based on any particular real-world unit, although this is the type used for metric drawings. With decimal units, each unit in the drawing could represent an inch, a millimeter, a parsec, a furlong, a fathom, a cubit (should you be into building arks in case that super rainy day comes), or any other unit of measure you deem suitable, from Danish alens to the Prussian zoll. An example would be 15.5.
Engineering units are in feet and inches and use decimals to represent partial inches: for example, 12′3.5″.
Fractional units, like decimal units, are unitless and show values as fractions rather than decimal numbers: for example, 15 1/2.
Scientific units, which are unitless and display values as exponents, are used for drawing really tiny or really large things. If you design molecules or galaxies, this is the unit type for you. Examples are 15.5E+06 (which is 15,500,000) and 15.5E–06 (which is 0.0000155).
FIGURE 4-1: The Drawing Units dialog box.
AutoCAD’s angle unit types are as follows:
Decimal Degrees show angles as decimal numbers and are by far the easiest to work with, if your type of work allows it.
Deg/Min/Sec is based on the old style of dividing a degree into 60 minutes and minutes into 60 seconds. Seconds aren’t fine enough to display AutoCAD’s precision capabilities, though, so seconds can be further divided into decimals. There is no degree symbol on a standard keyboard, so AutoCAD uses the lowercase letter d. An example would be 45d30′10.7249″. One nautical mile (6,076 feet) is approximately 1 minute of arc of longitude on the equator. David Letterman once said that the equator is so long that it would reach once around the world.
Grads and Radians are mathematically beautiful (so we’re told) but are not widely used in drafting. Apparently, the French artillery uses grads but as long as we’re friends with them we shouldn’t have to worry. Some of AutoCAD’s built-in programming language uses radians for angles. There are 400 grads, and 2*pi (6.2831…) radians, in a circle.
Surveyor’s Units type is similar to Deg/Min/Sec but uses north, east, west, and south quadrants (quarter circles) rather than an entire circle. An angle in Deg/Min/Sec might measure 300d0′.00″, while the same angle in Surveyor’s Units would be represented as S 30d0′0.00″ E.
When you use dash-dot linetypes (Chapter 11) and hatching (Chapter 15) in a drawing, it matters to AutoCAD whether the drawing uses an imperial (inches, feet, miles, and so on) or metric (millimeters, meters, kilometers, and so on) system of measure. The MEASUREINIT and MEASUREMENT system variables control whether the linetype and hatch patterns that AutoCAD lists for you to choose from are scaled with inches or millimeters in mind as the plotting units. For both variables, a value of 0 (zero) means inches (that is, an imperial-units drawing), whereas a value of 1 means millimeters (that is, a metric-units drawing). When you start from an appropriate template drawing (as described in the section “A Template for Success,” later in this chapter), the system variable values will be set correctly, and you won’t ever have to think about it. For an explanation of system variables and how to set them, see Chapter 26.
So why are there two variables? Simple. MEASUREINIT (short for measurement initial) sets the default value for new drawings, while MEASUREMENT sets the value for the current drawing.
Weighing up your scales
Somewhat surprisingly, you don’t need to consider scale when setting up a new drawing. In fact, rule number 1 in AutoCAD is to always draw everything full size. Hmmm, that makes quite a few rule number 1s, doesn’t it?
“Wait a minute! I want to draw a map of the known universe! If I draw full size, where am I going to find a sheet of paper big enough to print it — and who will help me fold it?”
Trust me. All will become clear shortly.
DRAFTING ON PAPER VERSUS ELECTRONICALLY
If you’ve ever done paper-and-pencil drafting (and there are fewer and fewer of us left who have), you’ll find that AutoCAD’s electronic paper works backward from dead-tree paper.
Dead-tree paper: In the (distant) past, we considered the approximate size of the object that we were drawing by hand and the views we wanted, selected a suitable sheet of paper from a set of standard sizes, and then scaled the drawing of the object to suit the sheet of paper. We were constantly translating sizes between the real-world object and our drawing of it. The height of text and the size of the components in dimensions were fixed. Again, scales were selected from a list of preferred values, such as 1:2. You rarely saw a paper drawing at a scale of, say, 1:2.732486921.
Electronic paper: Now we draw everything full size and then tell the PLOT command to grow or shrink things accordingly. This approach is much easier because we never have to translate sizes. Chapters 13–16 show you how AutoCAD now makes text and dimension sizing extremely simple.
“Okay,” you’re saying, “I understand that I need to print my drawings at a scale acceptable to the printer and discipline I work in. But when I’m drawing stuff full size, when do I need to worry about the scale factor?” Grab yourself a nice mug of cocoa and settle down ’round the fire because I’m going to tell you. By now you know (because I’ve told you so) that you draw real things full size, but drawings contain other things that are not real, such as text, dimensions, hatch patterns, title blocks, dash-dot linetypes, and so forth. And those nonreal things need to be legible on your plotted drawing.
Say, for example, you draw a plan of your big garage, and now you want to plot it on an 11-x-17-inch sheet of paper. No problem; just tell the PLOT command to scale everything down by a scale factor of 1:24, which architects would commonly represent as 1/2″=1′0″.
Oops, problem. Text annotations are typically about 3/32″ or 1/8″ high. Now, if you draw your 6-inch–wide wall full size, put a 1/8″-high title beside it, and then print the drawing at a scale of 1:24, the wall itself will measure 1/4″ on the sheet, and the note will be an illegible little speck beside it. You fix it by making the text 24 times larger, or 3 inches tall, so that it