MIT 4.202 Geometric Modeling Fall 2003
Exercise #2 Supplemental Handout
AutoCAD Basics

Units
Before you start drawing, make sure you set your units as desired [Format>Units...]. The distances in AutoCAD correspond to whatever unit you choose. A distance of 1.0 could be 1.0 inches, 1.0 feet, 1.0 meters, etc... Typically, people used decimal format for metric units and architectural format for feet and inches.

Layers
Each element in your drawing is associated with a layer. Each drawing, by default, has the layer 0 which you should never use (unless you know what you're doing). It is best to create a new layer before drawing new elements [Format>Layers...]. Layers are used to control the properties of all elements on that layer. To draw on a particular layer, you have to set the current layer to the desired layer [Format>Layers...>select a layer>Current>OK]. There are several properties of a layer: on, freeze, lock, color, linetype, lineweight, plot style, plot.

Coordinates
To specify coordinates when drawing, you can use 2 basic methods. The first method is absolute (x,y,z or distance<angle) which means the coordinates are referencing the grid system of the entire drawing. The second method is relative (@x,y,z or @distance<angle) which places the new point relative to the current point.

Object Snaps
Once you have the current layer set, you are ready to start drawing. AutoCAD is a precise drafting tool. 1.0 is not the same as 1.00001. To ensure that your lines are abutting against each other end to end, always use the object snaps [F3 or ctrl+right-click]. Never eye-ball the location of your lines. The object snap menu options appear if you right-click on OSNAP button just below the command line window. The most useful settings are endpoint, center, node, and intersection. Try not to have too many of the options set as you will most likely end up snapping to everything.

Selecting
The easiest way to select an element is to click on it. To pick more than one element, simply click on another item. AutoCAD has a unique method for selecting several items at once. There are two basic modes: window and crossing. In window mode, only elements that are completely inside the window are selected. To get into window mode, you can click and drag a window from left to right (you should see a solid rectangle) or by pressing "w" while in a command. In crossing mode, any element that intersects the window will be selected. To get into crossing mode, you can click and drag a window from right to left (you should see a dashed rectangle) or by pressing "c" while in a command. While in a command, you can also use "a" to add more items to the selection and "r" to remove items from the selection.

Undo
AutoCAD remembers every move you make in a drawing while the drawing is in use. You can undo [Edit>Undo] as many times as you like. But you can only redo [Edit>Redo] an undo once. Once you close a drawing, all the moves you made are lost.

Saving
AutoCAD saves your drawing as a *.dwg file. It also generates a *.bak file every time you save. The *.bak file is the original version of the file before you saved. As a general rule of thumb, you should save your drawing using incrementally different filenames. For example, the first drawing could be mybuilding01.dwg. After a major achievement in the drawing/model, save the drawing using the name mybuilding02.dwg [File>Save As...]. This way, if something goes wrong, you know you have some of your hard work saved.