System variables are settings that AutoCAD checks before it decides how to do something. For example, if you set the system variable SAVETIME to 10, AutoCAD automatically saves your drawing file every ten minutes; if you set SAVETIME to 60, the time between automatic saves is one hour. Hundreds of system variables control AutoCAD's operations.
To change the value of a system variable, just type its name at the AutoCAD command prompt and press Enter. AutoCAD displays the current value of the system variable setting and prompts for a new value. Press Enter to confirm the existing setting, or type a value and press Enter to change the setting.
Being able to change system variables by typing their names is a boon to power users and occasionally a necessity for everybody else. The only problem is finding or remembering what the names are. In most cases, you'll be told what system variable name you need to type — by me in this book or by the local AutoCAD guru in your office.
Note
Here are the three kinds of system variables:
Those saved in the Windows Registry: If you change this kind of system variable, it affects all drawings when you open them on your system, but not necessarily on other computers.
Those saved in the drawing: If you change this kind of system variable, the change affects only the current drawing, but on any computer.
Those that aren't saved anywhere: If you change this kind of system variable, the change lasts only for the current drawing session.
The System Variables section of the online help's Command Reference tells you which kind of system variable each one is. There are several hundred system variables, but understanding the ten listed in this chapter will go a long way to making you a happy CADster.
APERTURE controls how close the crosshairs must get to an object before an object snap marker displays (I discuss object snaps in Chapter 7). You can set APERTURE at the command line anywhere from 0 to 50 pixels, or you can set it in the Options dialog box by adjusting a slider to a maximum of 20 pixels. The initial value is 10, but I find 20 pixels is just about right for me. APERTURE is stored in the Windows registry.
DIMASSOC controls how AutoCAD's dimension objects are created (see Chapter 14 for dimensioning). When DIMASSOC is 0, AutoCAD creates exploded dimensions of separate lines, 2D solids, and text. You really don't want to work on DIMASSOC=0 drawings. When DIMASSOC=1, dimensions are referred to as non-associative — they're single objects, but they usually don't update if you change an object's size. When DIMASSOC=2 (the default), AutoCAD creates fully associative dimensions. You can set this value at the command line or select the Make New Dimensions Associative check box in Options
When MENUBAR is 1, AutoCAD's traditional classic menu bar displays below the program title bar. If MENUBAR is set to 0 (the default) the menu bar doesn't display. MENUBAR is set automatically when you switch between the AutoCAD Classic and 2D Drafting & Annotation workspaces.
Tip
If you want training wheels while you get used to the Ribbon, display the classic menu bar. (I describe the differences between the traditional AutoCAD Classic and the newer Ribbon-based workspaces in Chapter 2.)
You'll frequently want to build up a drawing by mirroring already-drawn components of it. That's what the MIRROR command does, but unfortunately, if text or dimensions are included in what you're mirroring, those end up being reversed as well. Using a mirror to read the text on your drawing is counterproductive, so you can invoke MIRRTEXT to fix things. When MIRRTEXT is 0 (the default), text is mirrored with everything else; setting it to 1 means everything gets mirrored but the text reads the right way around. MIRRTEXT is stored in the drawing.
The pickbox is the little square box that the crosshairs turn into when AutoCAD prompts you to select something. You control pickbox size by setting a value in pixels for the PICKBOX variable. The initial setting of PICKBOX is 3, which is fine for a lower screen resolution like 1024 × 768. But when you start running at higher resolutions — or when your hair is grayer, your face more wrinkled, and your vision not quite as acute as it once was, a higher setting might be more useful. My old laptop computer ran at 1600 × 1200 resolution, and I had to set PICKBOX to 5 to be able to use it effectively. My newer desktop runs at 1280 × 1024, and PICKBOX=4 is fine for that. But I've just acquired a new tablet PC that runs at 1400 × 1050, and I'm back to PICKBOX=5 for this one. As they say, your mileage may vary.
Sometimes you don't want AutoCAD to start in the same Documents folder where every other program stores its files. Maybe you work on a number of projects and want a separate desktop icon for each to start in a particular folder. REMEMBERFOLDERS is your tool. By default, it's set to 1, which means it defaults to storing files in My Documents (just plain Documents in Windows Vista). Set it to 0, and it will open from and save to the folder you specify in your desktop icon when you start, and the last folder you visit as you work. REMEMBERFOLDERS is a permanent setting that's stored in the Windows registry (permanent, until you change it to something else).
ROLLOVERTIPS controls the tooltip-like message boxes you see when you hover the mouse pointer over objects in the drawing editor. If ROLLOVERTIPS is 1, mousing over your drawing displays a temporary panel showing the object type with its layer, color, and linetype.
You can get double-duty from this command. Not only can you key it into AutoCAD, but if you have a dog named Tips, you can use it to teach him a neat pet trick! ROLLOVERTIPS is stored in the Windows registry. (And believe it or not — and I'll bet you won't — there's another system variable called NOMUTT! Could that be AutoCAD-ese for BAD DOG TIPS!?)
Tooltips themselves are the little text boxes that appear by default when you hover your mouse pointer over a toolbar or Ribbon button, or just about anywhere in a dialog box. Tooltips are useful when you're getting to know your way around, but when you have a sense of where things are, they can get in the way. By default, TOOLTIPS is set to 1, which means they display when you hover the pointer over something. Set TOOLTIPS to 0 if you don't want to see them again. The TOOLTIPS setting is stored in the system registry.
If you work with xrefs (refer to Chapter 18), you know the potential for competing layer properties. Sometimes you want your xref to look different from your current drawing so you can tell which is which. You can change layer colors of xrefs in the Layer Property Manager, but whether you have to do that every time you open a drawing with an attached xref depends on the setting of the VISRETAIN system variable. When VISRETAIN is set to 0, the layer properties in the xref take precedence, which means you would have to make those changes every time you opened the host drawing. When VISRETAIN is set to 1 (the default value), changes to layer properties you make in the host drawing remain intact every time you open that host drawing; however, those changes have no effect on the external file itself. The VISRETAIN value is stored in the current drawing.
Note
When working with layouts (see Chapter 5), sometimes it's handy to rotate a viewport so the model space objects inside the viewport fit on the sheet better. For example, maybe you draw elevator shafts, which (in my part of the world anyway) are vertical constructions. But your drawing sheet is probably oriented horizontally (that is, in landscape format). Before AutoCAD 2010, rotating a viewport left the model space geometry unrotated. You then had to switch to model space inside the viewport and change the user coordinate system or use the arcane DVIEW command's Twist option. The new VPROTATEASSOC variable lets you automatically rotate the view within the viewport as well as the viewport object itself.
This automatic rotation is the default behavior in AutoCAD 2010. If you want things to work the old way, set VPROTATEASSOC to 0.