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INTRODUCTION TO AUTOCAD

    AutoCAD is a general purpose computer aided drafting application program designed for use on single-user, desktop personal computers and graphic workstations. It was initially developed in the early 1980's by Autodesk Inc., Sausalito, California. It is currently distributed in Australia by Autodesk Australia, now located in Sydney. These notes have been updated to describe Release 12 of that package.

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CONCEPTS AND DEFINITIONS

    AutoCAD is an interactive drawing system designed to permit a user to construct or edit a drawing on a graphics display screen. To this extent, it is analogous to a wordprocessing program, except that in this case the thing being processed is a drawing. Each drawing is stored on a disk file, and AutoCAD is only able to edit one drawing (or file) at a time. This similarity to word processors is reflected in the fact that the principal functional component of AutoCAD is known as the drawing editor.
    Up until Release 10, AutoCAD was essentially a two-dimensional drawing system. Following that release, it now supports a full three-dimensional database. This has had the effect that the features of AutoCAD that support two-dimensional drawing are fully self-contained and can still be used as a 2D system without being concerned with its 3D features. In this course we will focus on the 2D drafting aspects of AutoCAD since that better reflects the way in which this system is generally used in practice at the present time. Several sections in these notes explain how the 3D features have been incorporated into AutoCAD, but provide little detail of the 3D operations.

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WHAT IS IN AN AUTOCAD DRAWING

    An AutoCAD drawing is made up of entities. These can be either simple graphic primitives (such as lines, arcs, circles, text, and so on) or blocks (which are groups of entities).
    The graphic primitives are defined geometrically in terms of the normal cartesian coordinate system (right-handed system with positive X-axis to the right, positive Y-axis up the screen and positive Z-axis coming out of the screen towards the user). Hence, for example, lines are defined by their end- point coordinates (x, y and z), while circles are defined by their centre coordinates and radius. Each entity also has certain attributes associated with it, such as line style, text font or colour.
    A block is a group of entities that can be manipulated as a single unit. Once created, a block may be moved, scaled, rotated, copied or deleted. A block can be created by collecting together a group of entities from the current drawing (the drawing being currently edited) and assigning a single name to that group. Alternatively, an existing drawing (from disk) can be inserted into the current drawing as a block. Equally, a block from the current drawing can be written out to a file as a new drawing.
    It is important to understand that a block, in AutoCAD, is unique to a specific drawing. That is, when an existing drawing is inserted into the current drawing as a block, AutoCAD simply copies the graphics from that drawing and includes it as a single object. The existing drawing is unaffected by the action. Indeed, if that existing drawing (from which the block was created) were subsequently edited then those changes would not affect the inserted block. It is possible, however, to update a block definition in a drawing by having AutoCAD redefine the block using the current version of the original drawing.
    As an alternative to inserting a drawing into another drawing as a block, AutoCAD now provides the facility to attach a drawing to the current drawing as an external reference. The external drawing is then treated like a block except that it is automatically updated each time the drawing to which it is attached is loaded. Naturally, if the external drawing is altered in the meantime, then the external reference will be updated to reflect those changes.
    Another important feature of a block (including external reference blocks) is that it can be duplicated many times within a drawing, with each copy pointing to the one graphic description, but having its own unique position, scale and rotation factor. This is known as instancing. There are two advantages to this approach:
    space saving - the graphics description is only stored once; and ease of re-definition - the graphics description of a block can be altered and then re-defined in order to update each instance in the drawing.
    It is also possible to associate named attributes with a block. For example, a block representing a door might have attributes such as type, manufacturer, finish, lock-type, etc. Each instance of the block would have a value associated with it for each defined attribute. AutoCAD provides facilities for scheduling the attribute data associated with inserted blocks in a drawing.
    The final concept to be explained about blocks is the notion of exploding. Any instance of a block may be exploded in order to reduce it back to its separate graphic entities. Similarly, an external reference block can be unbound so that it becomes a local block and then, in turn, can be exploded. Units, scales and paper sizes.
    A clear understanding of the way that AutoCAD handles units of measurement, scale and paper size is necessary before a drawing can be created or edited. AutoCAD provides complete control over these things by distinguishing between model space and paper space. All drawings, whether two-dimensional or three-dimensional should be thought of as models of a real-world entity (either a drawing or a 3D description of a real-world object). In either case, the description is maintained in model space. Paper space can be thought of as a model of a standard-size piece of paper on which you can establish viewports (rectangular regions) in which AutoCAD will display scaled views of the drawing you have created in model space. You can, of course, add additional linework in paper space to form borders and annotation.
    In order to understand this, it is best to picture model space as a very large piece of paper located somewhere in two-dimensional space. (AutoCAD can actually draw anywhere in 3D space, but let's ignore that for the moment!) Since, the size and location of that piece of paper is entirely up to you, it would make sense to choose something that makes drawing as simple as possible. Now, Australian architects typically use millimetres as their unit of measurement and would draw objects in their true size. This means that when drawing a plan of a building that is, say, 15 metres square, the "paper" size (in model space) would be set at something greater than 15,000 x 15,000 units (where "units" equals millimetres). Strictly speaking, it is not necessary to set the model space drawing size, but it is helpful to do so in order to establish a context for the drawing.
    When working on the drawing in model space, you should think of the graphics screen as a window through which you can view all or part of that drawing sheet. Thus, when viewing a drawing, a larger display scale will allow you to view only a part of the drawing on the screen. This is referred to as zooming in on the drawing: as you increase the scale, you can see less of your drawing, but what you can see will be in greater detail. If you zoom out, then you will be able to see more of the drawing, but in lesser detail. AutoCAD provides some fairly sophisticated tools for zooming in and out of your drawing and for panning back and forth across it. These are described later.
    Although it is possible to plot from model space, it is normal to set up a sheet in paper space, and create one or more viewports showing different views of the drawing in model space. Each view is displayed at a fixed scale, and the paper space drawing is then plotted at a scale of 1:1. In this case, text notes and other annotation on the drawing is best added in paper space, and plotted at its actual size. Any text that is entered in model space has to be drawn at an artificially large size so that when it is scaled down for plotting, it ends up at the size required. For example, text on a 1:100 drawing might be set at 250 millimetres high in model space to create plotted text that is 2.5mm high on the plot.
    Drawing Layers
    AutoCAD also supports the standard notion of layering (refer to the general notes on CAD). This means that any drawing entity can be assigned to any layer of the drawing. This allows separation of portions of the drawing. For example, all brickwork could be assigned to one layer, all electrical work to another, and so on. In that way, when plotting, only selected layers need be included in the physical drawing that is produced from the one AutoCAD drawing.
    Layering can also be used to advantage while editing a drawing. For example, suppose a plan drawing is substantially complete with only the brickwork cross-hatching to go. If all the brickwork is on one layer, each other layer could be turned off so that only the brickwork is visible and selectable. That makes it possible to cross-hatch the brickwork without inadvertently affecting something else.
    Layering in AutoCAD is manipulated through the properties that can be assigned to each layer. Once the interaction between layer properties is understood, then these can be manipulated to great advantage. The following notes summarise the properties of layers.
    Layer name.
    An unlimited number of layers may be created. Each is identified by a unique name assigned by the user. The name may be up to 31 characters long, made up from uppercase letters, digits and three special characters ($ - _).
    Current layer.
    AutoCAD maintains the notion of a current layer, being that layer upon which all newly created entities are placed. Any of the existing layers may be selected as current at any time.
    Visibility.
    Each layer can be either visible (ON) or invisible (OFF). Only visible layers are displayed or plotted. Layers can be turned on or off at will, in any combination. Entities can still be added to an invisible layer (indeed it is possible to have a current layer that is invisible), but entities that are not visible cannot be selected for editing.
    Colour and linestyle.
    Each layer has associated with it both a colour and a linestyle. As entities are drawn, they are assigned, by default, the colour and linestyle of the current layer. (This can be overridden by assigning each entity its own colour and linestyle.) By default, new layers are assigned colour 7 (white) and continuous linestyle.
    Freeze/Thaw state.
    At any time the user may freeze one or more layers of a drawing. A frozen layer is made invisible. Any entities on a frozen layer are completely ignored or forgotten by AutoCAD during the editing process. This has the effect of speeding up many editing operations. Naturally, a frozen layer can be thawed again at any time.
    The above discussion refers to the manipulation of layers generally, whether in model space or paper space. AutoCAD also permits layers to be manipulated (turned on / off or frozen) independently in each viewport in paper space. This means that a layer may be visible in one viewport, but not another. AutoCAD also allows you to manipulate layers that are contained within external reference drawings.
    Integrity of the displayed drawing
    Clearly, with any drafting system, it is important that the image displayed on the screen for the user is an accurate representation of the drawing that is stored by the computer. At the same time, it is equally important that the system respond quickly to the user. In order to achieve a fine balance between operating speed and maintaining the integrity of the drawing, AutoCAD sets up a display database separate from its complete drawing database. The display database allows very fast screen graphics interaction, but needs to be "regenerated" from time to time in order to maintain that integrity. Hence, in AutoCAD we distinguish between screen redraws and drawing regens.
    Screen redraws are faster (but less thorough) than regens and AutoCAD will automatically offer to do the latter only when it is actually necessary. You will find that you will want to redraw the screen fairly regularly while you are using AutoCAD, while, on the other hand, you will very rarely need to force AutoCAD to do a regen. On those occasions when it does offer, you should generally allow the required regen to be carried out without interference. It is actually possible to disable regens entirely, but that should be done with caution, as it severely restricts the integrity of the displayed image.
    In the pursuit of speed versus integrity, there are three other facilities in AutoCAD that can be used to achieve a balance between interactive speed and the quality of the displayed image. These are things that you will not normally need to worry about unless your drawing is particularly complex. Text can be represented on the screen merely as rectangular boxes rather than the fully formed letters (this is referred to as quick text). Circles and arcs can be displayed as a series of straight line segments rather than smooth curves. Solid filled areas and thick lines can be shown hollow (drawn in outline only).
    Prototype drawings
    As already discussed, there are many settings in AutoCAD which form an integral part of any drawing to which they apply. Things such as layer names and associated attributes, scale, drawing size and display parameters can be saved (to file) as a drawing, even if there is no graphics in that drawing.
    This facility allows prototype drawings to be set up which can be used as "templates" in the production of other drawings. This is a powerful concept as it permits firms to set up office standards which are accessible to all users of the system.
    3D graphics concepts
    All graphic entities in AutoCAD are defined fully within 3D space. However, in the normal case, where the user makes no attempt to define them as 3D objects, they are assumed to lie on the base (Z=0) plane of the world coordinate system (WCS) defined by AutoCAD. In that case, the z- value of every point is assumed to be zero and the user has no need to be concerned about it.
    In order to support the 3D user, AutoCAD provides five basic ways to enter 3D information:
    firstly, most of the basic graphic entities (lines, arcs and curves) can have a thickness associated with them which is effectively a height in the z-direction (this has always been a feature of AutoCAD and is sometimes referred to as 2.5D);
    secondly, when specifying the position of any point during the drawing process, all three coordinate values can be entered by the user (again, if only two coordinates are specified, the point is assumed to lie in the base plane - note also that some entities must be perfectly planar, so if you attempt to enter a 3D point that would make such an object non-planar, then AutoCAD will display an error message);
    thirdly, AutoCAD supports a number of specifically 3D graphic entities, including 3D polylines, faces and meshes, along with a set of commands that can be used to create 3D objects with them;
    fourthly, and most significantly, AutoCAD allows the draftsman to set up a temporary user coordinate system (UCS) which can be positioned and orientated anywhere in space (relative to the WCS) - thus, you can picture the standard X, Y and Z axes of the WCS and then imagine an equivalent set of axes (the UCS) being moved away from the origin and then tilted and rotated to any orientation that you wish - once a UCS is established, all 2D drawing is done relative to those coordinates until a new UCS is defined or the user switches back to the WCS;
    lastly, from release 11 onwards, AutoCAD has provided an extension package called AME (Advanced Modelling Extension) which provides 3D solid modelling capabilities - these allow you to form complex solid objects by adding, subtracting and intersecting a set of simple solid primitives (such as, boxes, cone, cylinders, spheres, etc) - for example, you can form a box with a circular hole by subtracting a cylinder from a box, and so on - we will not be concerned with these techniques in these notes.
    Notice that the 3D characteristics of AutoCAD merge fairly neatly with the traditional 2D facilities such that it is possible (and not uncommon) to use AutoCAD as if it had no 3D facilities at all.
    That now concludes the discussion of general concepts in AutoCAD. The remainder of this chapter is concerned with how to interact with AutoCAD in order to produce drawings.
    Interacting with AutoCAD
    When you run AutoCAD, you are immediately switched to what has become known as the Drawing Editor. In this mode, the display screen is set up as shown diagrammatically in Figure 5.1 over the page.
    If AutoCAD is running inside a windowing system (such as OpenLook on the SUN graphics workstation), then it may not occupy the whole screen and will include a window bar across the top of the window (as illustrated). On a DOS-based PC, AutoCAD occupies the whole screen. The major portion of the AutoCAD window is assigned to the drawing area. It will display the same portion of the drawing as was visible the last time the present drawing was edited. (If you are starting a new drawing then, naturally, the drawing area will be clear.) Above the drawing area is the status area where various items of status information are displayed (such as the current layer, current colour, coordinates of the last point entered, etc.). On a DOS-based PC, whenever the graphics cursor is moved into the status area, the status information is temporarily replaced by a row of pull-down menu names: the use of these will be explained later in the notes. On some platforms, such as the SUN graphics workstation running the Openlook graphics user interface (GUI), the pull-down menus are displayed permanently above the status line (not shown in Figure 5.1). To the right of the screen is displayed the permanent screen menu. Along the bottom of the screen is the command echo region, signified by the "Command:" prompt on the left side.
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