COMPUTER AIDED DESIGN

What is CAD?
This is an abbreviation for Computer Aided Design. It is computer-based system for creating
and manipulating drawings in the same manner an engineer, architect or designer might create
using traditional means.
It is used in many disciplines, including architecture, interior design, civil engineering,
mechanical engineering, electrical engineering and so on.

CAD involves the use of specialized software that incorporates state-of-the-art graphics
capabilities. The draughtsman can create, edit, save and print drawings through the use of ‘tools’
provided by the software.

APPLICATION CAD IN SOCIETY

CAD systems are used widely in:
1. Engineering: -Designing mechanical components
-Designing electrical components
-Designing civil engineering works
2. Architectural work: -Designing buildings

BENEFITS (ADVANTAGES OF USING CAD SYSTEMS)

(i) Production of high quality drawings: -The quality of lines, dimensions, symbols, etc are
independent of the individual skill of the draughtsman.
(ii) Creation of a library of commonly used symbols: -Once these symbols are created they can
be saved and recalled when needed. This saves time and ensures standards are adhered to at
all times
(iii) Greater accuracy: -Due to computer mathematical accuracy, a high degree of dimensional
control is obtained with reduction in the number of mistakes resulting in accurate material
and cost estimates.
(iv) Increased productivity: -Repetitive work on similar features or drawings is replaced by
automatic redrawing with consequent marked increase in speed and productivity.
(v) Use of layers: -The drawing may be drawn on any one of the available layers. Layers may
be considered as a stack of transparent sheets. They allow placement of different
items on separate ‘sheets’ e.g. drawing construction lines, dimensioning, hatching etc
(vi) Use of multicolored drawings: -This enhances the understanding of different parts of a
drawing.
(vii) Editing functions: -Powerful editing functions for correcting mistakes, deleting, inserting
new features, copying, moving rotating etc make object manipulation easier.

LIMITATIONS
(i) High cost during initial setup: -must acquire a computer and the necessary printing
hardware and software.
(ii) Continuous use may result in deskilling of technical staff. The manual draughting
skills acquired through training may fade with time.

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The Drawing Process
In many ways, computer-aided drafting (CAD) is analogous to traditional, or manual, drafting.
In manual drafting, a draftsman generates graphic objects using tools such as a ruler for straight
lines. In CAD systems, the draftsman, or the "user", uses various tools to draw. These tools are
usually represented in CAD programs as icons that are grouped together in toolbars that float
above the drawing window on the computer display. And, as in manual drafting, these tools
indicate what can be drawn—straight lines are drawn with a "line" tool, for example. 

A Comparison between Traditional Draughting Techniques and CAD Draughting

Techniques

Traditional Draughting Techniques CAD Draughting Techniques

1. Scale: 1. Scale:
A drawing of an object must be scaled up or There is no scaling while producing drawing in
down depending of the size of the object AutoCAD. All dimensions are entered in real
compared to the sheet of the paper. size (i.e. 1=1)

2. Paper size 2. Paper size

A sheet of paper for the drawing must be A paper size is selected during printing
selected e.g. A4, A3, A2 etc

3. Units 3. Units
Units depend on the convention expected by A command is available to set the drawing
engineers, designers or builders. The Units units.
may be metric or imperial. Or they may be
so small that one has to use ‘scientific’ or
‘exponential’ notation

4. Drawing Instruments 4. Drawing Instruments

T-square, pens, erasers, rulers, sets squares No instruments required. AutoCAD provides
etc must be available. Also precision- ‘tools’ for the entire drawing process.
drawing tools must be purchased.

5. Drawing board 5. Drawing board

A good drawing board is essential to aid in No drawing board required
accurate drawing of objects.

6. Drawing process 6. Drawing process

Takes a lot of time to draw accurately with a Takes less time. There is pre-defined objects
pen. Construction lines must be drawn and such as lines, arcs, circles etc
later erased. This adds further time wastage
7. Electronic paper size
While drawing in AutoCAD, an electronic

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 sheet of paper must be set up on the computer
big enough to hold the drawing at a scale of
1:1.

2D Co-ordinate Systems
In CAD systems, the user draws on a two-dimensional surface of infinite size, which has its
origin and two axes (x and y) perpendicular to each other, which are used to determine the
location of points relative to the origin. This is called a 2D Cartesian co-ordinate system, and is
fundamental to all 2D CAD systems. Under a Cartesian system, points are positioned in
Cartesian space by their x and y co-ordinates. All graphic objects are drawn by specifying point
locations within this rectangular co-ordinate system. 

Examine the figure below. The co-ordinates of the point shown is (10, 10). 

Many CAD systems also provide point specification using polar co-ordinates—i.e., point
location is described by its distance from the origin and an angle that the connecting line forms
with the x-axis.

CAD Drawing Aids

(i) Grids
CAD systems also feature grids, regular patterns of dots displayed across the drawing surface.
Grids are also used in manual drafting but there they are fixed—they cannot be changed as
needed during drawing.
(iii) Snaps
Drawing with the aid of grids, however, is imprecise—choosing grid points precisely is not easy.
To remedy this, CAD systems provide the snap feature. Snap forces the cursor to jump in exact
intervals on top of an invisible grid of points. A user may specify different intervals for grid and
snap—e.g., a grid interval may be set to 1 metre and the snap to 1 centimetre.

(iii) Object snapping

New objects can be drawn by object snapping, that is, by precisely selecting significant points
on existing objects: endpoints or midpoints of lines, etc.

(v) Construction lines 

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Construction lines—temporary lines that the user can snap to in drawing the model—also offer
time-saving benefits. When the construction lines are no longer needed they are usually deleted
or their layer made invisible ("switched off").

Executing Commands
Everything in AutoCAD is dependent on a command being issued first. To execute AutoCAD
commands three methods may be used: 

(i) Use of toolbars: -involves clicking an appropriate icon from the toolbars.
(ii) Use of pull-down menus: - an item is chosen from one of the pull-down menus
(iii) Typing the command at the command prompt in the command window—some
commands may be typed with an abbreviated alias—e.g., z for zoom

DRAWING BASIC OBJECTS

1. THE DRAWING UNITS
Before starting a drawing, the units of measurement must be specified and the limits of the
drawing area. The drawing units can be:
(i) Decimal: Millimetres
(ii) Engineering: Feet and inches (with decimals)
(iii) Architectural: Feet and inches (with fractions)
(iv) Fractional: Standard units and fractions
(v) Scientific: Units, in exponential form
The default drawing limits are 420 by 297. These are suitable for use with A3 paper (420 x
297mm), giving a simple translation of one drawing unit to one millimeter on paper.
For Imperial measurements, the default limits are 12 by 9.
Drawings can be started from templates or wizards, or from scratch.

To set the drawing units:

1. From the Format menu, select Units.
2. In the Drawing Units dialog box, set the unit values for your drawing. The Length and Angle
units are normally Decimal. As you change unit settings, AutoCAD shows examples under
Sample Output. Precision determines how accurate values are displayed.
3. Under Length, select a unit type and level of precision.
4. Under Angle, select an angle type and precision.
5. To specify an angle direction, choose Direction, and then select the base angle in the Direction
Control dialog box. The angle direction controls the point from which AutoCAD measures
angles and the direction in which they are measured. The default is 0 degrees on the right side of
the figure, measured counterclockwise. If you select Other, you can enter an angle, or choose
Angle to specify an angle using your pointing device.
6. Click OK to exit each dialog box.

To activate the drawing units through the Command line type UNITS

Drawing limits: -This is the user-defined rectangular boundary of the drawing area covered by
dots when the grid is turned on. Also called grid limits.
To set the drawing Limits:

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 1. On the Format menu, select Drawing Limits.
2. In the Command Line window, you will be prompted for the coordinates of the lower left
corner of the drawing area. The current values will be shown in angle brackets, e.g.
<0.00,0.00> Press [Enter] to accept these.
3. Enter the coordinates of the upper right corner. Give these in the form width, length.

The Zoom tool

The Zoom tool allows one to change the magnification of the drawing view. Increases or
decreases the apparent size of objects in the current viewport. The Zoom toolbar may be
accessed from the standard toolbar at the top of the screen or from the drop-down menu
View>Toolbars..>Zoom.
With some drawing limits, the grid is so small that you need to zoom in before you start work.
The Zoom options are:
(i) Zoom All: Zooms to display the entire drawing in the current viewport. In a plan
view, AutoCAD® zooms to the drawing limits or current extents, whichever is
greater. In a 3D view, ZOOM All is equivalent to ZOOM Extents. The display shows
all objects even if the drawing extends outside the drawing limits.
(ii) Zoom Center: Zooms to display a window defined by a center point and a
magnification value or height. A smaller value for the height increases the
magnification. A larger value decreases the magnification.
(iii) Zoom Dynamic: Zooms to display the generated portion of the drawing with a view
box. The view box represents your viewport, which you can shrink or enlarge and
move around the drawing. Positioning and sizing the view box pans or zooms to fill
the viewport with the image inside the view box.
(iv) Zoom Extents: Zooms to display the drawing extents. You can use ZOOM Extents
transparently, but it always regenerates the drawing.
(v) Zoom Previous: Zooms to display the previous view. You can restore up to 10
previous views.
(vi) Zoom Scale: Zooms the display at a specified scale factor. The value you enter is
relative to the limits of the drawing. For example, entering 2 doubles the apparent
display size of any objects from what it would be if you were zoomed to the limits of
the drawing.

If you enter a value followed by x, AutoCAD specifies the scale relative to the current
view. For example, entering .5x causes each object to be displayed at half its current
size on the screen.

If you enter a value followed by xp, AutoCAD specifies the scale relative to paper
space units. For example, entering .5xp displays model space at half the scale of
paper space units. The following illustration shows a number of viewports arranged in
paper space. The view in each viewport is scaled relative to paper space. The first
view is scaled 1=1 relative to paper space (1xp), the second is scaled .5=1 relative to
paper space (.5xp), and so on.
(vi) Zoom Window: Zooms to display an area specified by two opposite corners of a
rectangular window.
(vii) Zoom Real Time: Using the pointing device, zooms interactively to a logical extent.

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 The cursor changes to a magnifying glass with plus (+) and minus (–) signs.
The current drawing area is used to determine the zooming factor. ZOOM uses half of
the window height to move to a zoom factor of 100%. Holding down the pick button
at the midpoint of the window and moving vertically to the top of the window zooms
in to 100%. Conversely, holding the pick button down at the midpoint of the window
and moving vertically to the bottom of the window zooms out by 100%.

NOTE: If you place the cursor at the bottom of the window, hold down the pick
button, and move vertically to the top of the window, the zoom-in factor is 200%.
When you have reached the zoom-in limit, the plus sign in the cursor disappears,
indicating that you can no longer zoom in. When you have reached the zoom-out
limit, the minus sign in the cursor disappears, indicating that you can no longer zoom
out.
When you release the pick button, zooming stops. You can release the pick button,
move the cursor to another location in the drawing, and then press the pick button
again and continue to zoom the display from that location.

To exit zooming at the new position, press ENTER or ESC.

The Pan tool

Moves the drawing display in the current viewport.
AutoCAD displays the following prompt; you can pan the drawing display in real time.
Press ESC or ENTER to exit, or right-click to display shortcut menu.
If you enter -pan at the Command prompt, PAN displays alternative prompts on the command
line, and you can specify a displacement to pan the drawing display.

2. THE DRAWING TOOLS

AutoCAD provides the following drawing tools:
(i) Line
(ii) Circle
(iii) Rectangle
(iv) Arc
(v) Polyline
(vi) Polygon
(vii) Spline
(viii) Construction line
(ix) Ellipse
(x) Revision cloud

NB: -The toolbar holds only a few of the drawing tools. For full set of the tools, the Draw menu
or Command window should be used.
-Several tools have options which give extra level of control over the shape or appearance of
the object.

1. Line tool (Drawing a line)

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 The LINE (or simply L typed at the command line) will draw line entities in the drawing
window.
How the command works:
When the command is executed, AutoCAD will ask you to specify the starting and end
points of the line. You can use the Command line, the drop-down menu or the toolbar to
activate the Line tool.
Regardless of the method used to activate the command, you must look down at the
command line to see how AutoCAD is responding.

WHAT TO DO WHEN YOU MAKE A MISTAKE IN AutoCAD

(i) Pressing the ESC key
The Esc key at the top left of the keyboard will get you out of most problems you
encounter when using AutoCAD. Examples of scenarios when you can press the
Esc key include:
-When a command is not responding the way you expect.
-When you want to cancel a command you have started.
-When you have clicked a point on the screen unintentionally.
-When a dialogue box appears on the screen accidentally.
In all the cases above, pressing Esc once will free up the command line. When the
command line is blank, you can proceed to issue a new command. In some cases
you may need to press the Esc key two or three times. For example, if you are
dimensioning an object and you decide you would like to return to a blank
command line or if little blue boxes called grips appear on objects.

(ii) Using Undo

You can undo the last command by typing U at the command line and pressing
the Enter key, or by clicking on the Undo icon on the toolbar.
(iii) Using Redo
The Redo command will reinstate the last command you applied undo to. You
may undo as many commands as you like, but you may only redo once. Redo
must follow the Undo command

2. Circle tool (Drawing a circle)

You can use the command line, Menu bar or toolbar to draw a circle.
Command Line: Type the word circle or type simply c.
HOW THE COMMANDS WORKS
When the Circle command is issued, AutoCAD will need to know where the centre of the
circle is to be and its radius or diameter. The command line will prompt ‘Circle Specify
center point for circle or [3P/2P/Ttr (tan tan radius)]:’. Pick a point on the screen as the
centre of the circle. If you want to use a different option, you must type the capitalized
letter (s) of the option required. To draw a circle through three specified points you enter

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 3P at the command line and press the enter key. T must be typed to draw a circle with a
radius that is a tangent to two objects.
DEFAULT OPTIONS
The <angled brackets> shows the option that AutoCAD is offering you. To select an
option other than the default, you must type in the capitalized letter (s) offered at the
command point.

3. Rectangle tool (Drawing rectangle)

a) Activating the command:
(i) At the Command line, type rectang or rectangle
(ii) Fro the menu bar, click Draw, the select Rectangle or
(iii) Click the Rectangle icon on the toolbar.
b) Pick or type the coordinates of a corner
c) Drag and pick at the opposite corner or type its coordinates.
Rectangles are always drawn parallel to the axes. If you want one at an angle, you can rotate
it afterwards.
A rectangle can be drawn square, but it may be easier to create squares using the Polygon
tool.
RECTANGLE OPTIONS
When the command first starts-before the first point has been defined-you are offered the
following options:
Chamfer: The chamfer cuts off the corner at an angle. It takes two chamfer distances. If
these are the same, the angle is 45.
Elevation: This is the distance above the drawing surface. Most tools draw flat on the
surface unless you type in the z-coordinates. The effect is only visible in 3D views.
Fillet: Gives rounded corners. The radius of the arc must be set.
Thickness: Adds a third dimension to the rectangle, so that the lines become blocks. This is
only visible in 3D views.
Width: Sets the width of the line> The default is 0, which shows a hairline on the drawing.

After specifying the first point, you can use the Dimension option to define the rectangle by
its length and width, rather than giving the coordinates of the opposite corner.
The options are all set in much the same way: -Type the first letter of the option name.

4. Construction lines
A construction or xline is a line that runs to infinity in two directions. As in the
construction of any type of line, you need two points to define it. The first point you pick
becomes the ‘root’ of the construction line. There are many ways to pick the points for
constructing the xline, including the Object Snap modes.
The construction line is activated in three ways:
(i) By typing xline or xl at the command line
(ii) Through menu bar: Draw the select Construction line
(iii) Through toolbar by clicking its icon.
When the command is issued you have to select the first point or root. The default is to
just pick a point on the screen or use Object Snap. Five other options are offered. To
select one of these options type the capitalized letter from the list. The options are:

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 [Hor/Ver/Ang/Bisect/Offset]
Specific point: -Specifies the location of the infinite line using two points through which
it passes.
Hor: -Creates a horizontal xline passing through a specified point.
Ver: -Constructs a horizontal or vertical xline. Use Object Snap to draw it through a
Ang: -Creates an xline at a specified angle.
Bisect: -Creates an xline that passes through the selected angle vertex and bisects the
angle between the first and second line.
Offset: Creates an xline parallel to another object.

5. Ray
A ray is a line which has a starting point and extends off to infinity in a single direction. They

are used to help construct a drawing rather than be objects as part of the drawing. When the

command is issued, AutoCAD will ask for the ray’s starting point and then for another point

through which the ray will run.

The Ray command is activated by typing ray at the command line, through the menu bar
or through the toolbar (the icon does not appear by default but can be customized).

6. Polygons
A polygon is an enclosed shape with any number of equal sides. It can have from 3 to 1024

sides. You can use the polygon tool to create squares, triangles, pentagons, hexagons or

whatever any regular shape.

Polygons can be defined in three ways:

 By the position and size of one edge -as all the other edges will be the same
length, this is enough to define the polygon.
 By the centre and the radius of a circle that it could be circumscribed around –i.e.
the distance to the midpoint of an edge.
 By the centre and the radius of the circle that it could be inscribed in –i.e. the
distance to a vertex.
To define a polygon by an edge:
(i) Click the Polygon tool.
(ii) Type the number of sides required.
(iii) Type E to select Edge mode.
(iv) Specify the point at one end of an edge.
(v) Drag to draw the line –the polygon, based on this, will take shape. Pick or type
the coordinates to specify the endpoint.
To define a polygon by its centre and radius:
(i) Click the Polygon tool and type the number of sides.
(ii) Specify the centre point.

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 (iii) Type I to select Inscribed, or C for circumscribed mode.
(iv) Type the radius or
(v) Use the mouse to drag the polygon to size and rotate it as required, clicking when
it is in the right position.

7. Arcs
There are many ways to draw an arc. AutoCAD has 10 ways listed on the drop-down menu
for drawing an arc. Arcs are parts of circles. All arcs have a beginning and an end point.
AutoCAD uses these points in three techniques. All arcs have a centre; this is used in six
techniques. Three techniques use the idea of an angle to specify the distance the arc spans.
To draw an arc from three points:
(i) Click the Arc tool.
(ii) Specify the start point.
(iii) Specify another point anywhere on the arc –a straight line will (temporarily)
link it to the start point.
(iv) Specify the end point.

8. Ellipses
An ellipse has two axes that cross at right angles. The shape is defined by the length of those

axes and their angle to the grid.

To define an ellipse:
(i) Click the Ellipse tool.
(ii) Specify the position of the start point of an axis.
(iii) Drag and pick or type the coordinates to set the length and angle of the axis.
(iv) Specify the length of the other axis, from the centre to the edge –its angle will
be 90 degrees to the first.
9. Splines
This draws a continuous line with multiple curves. The line is defined largely by the points
on the cusps of the curves. The curves at the start and end are defined separately afterwards.
To draw a spline:
(i) Click on the Spline tool.
(ii) Click or type the coordinates to specify the start point.
(iii) Specify the points to locate the curves along the line.
(iv) Press [Enter] at the end.
(v) A straight line will appear, connecting the start point. Move this to form the
first curve. Repeat for the end curve.
10. Points
Points are mainly used to act as snap points to help in the construction of other objects.
Their screen appearance depends upon the point style, and can be highly visible or totally
invisible. The default style is a single pixel.

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 To place points:
(i) Click the Point tool.
(ii) Type or pick its coordinates.
(iii) Press Esc to end.
To set the point style:
(iv) Open the Format menu and select Point Style…
(v) Select a style
(vi) If required, set the size, either as relative to the screen or in absolute units.
(vii) Click OK.
The style will be applied to any existing points and to all that you draw from
that time.
11. Polylines
The Polyline tool is used in the same way as the Line tool, but there are several differences in
the result.
-Polyline-drawn lines form a single object while line-drawn lines are separate objects.
-The other difference arises from the greater flexibility of polylines. A polyline can be a
straight line or an arc, and it can be of any thickness- and the thickness can taper from
one end to the other.
Polyline options:
Polyline has a number of options which can be selected.

(i) Arc: -Draws an arc, instead of a straight line. Unless you specify otherwise, the
arc will be such that the previous line would meet it at a tangent. In practice, this
means that you can use it to draw rounded corners and ends without further
specification.
(ii) Line: Reverts to line-drawing mode; the rest are used for specifying the position
and angle of the arc. The simplest are Direction, which sets the tangent direction
and Second point and End point, which are used as they are with the Arc tool.
(iii) Close: Draws a line from the last endpoint to the start of the polyline, creating a
closed shape. This can be filled.
(iv) Halfwidth: Is –literally –half the width of the line. Use it in exactly the same way
as Width (but at half the value).
(v) Length: Draws a straight line of the given length, continuing at the same angle as
the previous line or as a tangent from an arc.
(vi) Undo: Undoes the last command within the polyline. This is very useful, as
things can go wrong with this complex feature, and it would be unfair to have to
start again from scratch because of one error.
(vii) Width: The start and end widths are set separately, and if they are different, the
lines thickness will smoothly taper from one end to the other over its length.

THE MODIFYING TOOLS

The Modify Toolbar
The modify toolbar contains all key tools for manipulating objects. Many of them work in
similar ways, so that once you have mastered one tool, the next is easier to get to grips with.
Notice in particular how objects are selected, and the use of a base point.

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The tools provided for modifying objects include:
(i) Erase
(ii) Copy
(iii) Mirror
(iv) Move
(v) Offset
(vi) Rotate
(vii) Trim
(viii) Stretch
(ix) Scale
(x) Array
(xi) Extend
(xii) Break
(xiii) Explode
(xiv) Chamfer
(xv) Fillet

1. The Erase tool

This is the simplest of the modify tools.
(i) Select the objects you want to remove.
(ii) Click the erase tool.
Or
(i) Click the erase tool.
(ii) Select the objects you want to remove.
(iii) Press [enter] or right-click to tell AutoCAD that you have finished selecting objects.
2. Copy
The standard Windows Copy And Paste technique works perfectly well, but it is worth
knowing about the copy command as it can give you better control when you place copies.
After selecting the objects, you must specify a base point. The copy is then located by giving the
position of its base point. This allows very accurate positioning.
By default, only one copy is made at a time. If several are wanted, switch on the multiple option
at the start of the command.

To copy an object:
(i) Select the object(s).
(ii) Click the Copy tool
Or
-Click the copy tool, select the objects and press [enter].
(iii) If multiple copies are needed, type M in the command line.
(iv) Pick a base point in or near the objects.
(v) Pick where you want to place the base point of the copy.
(vi) In multiple mode, place the other copies then press [enter].

3. Mirror
The mirror tool produces a mirror image, allowing one to create a symmetrical object by
drawing only half of it- or even a smaller fraction. You place the mirror line by picking two

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 points on it- the line will swivel after the first point has been set, and you will be able to see
how the reflection will look and where it will appear. Picking the second point fixes the line
and creates the new drawing- the original can be kept or removed as desired.
To create a mirror image:
(i) Select the objects(s)
(ii) Click the mirror tool
Or
 Click the mirror tool, select the objects and press [enter]
(iii) Pick the coordinates of one point the mirror line.
(iv) Pick the second point to fix the line
(v) The command line will prompt:
Delete source objects? [Yes/No] <N>:
Type ‘Y’ to delete the object, or press [Enter] to accept the default (no).
3. Offset
Offset creates copies of an object within or outside the source object. It is unusual in that you
cannot select an object first-you must start with the tool (or command) and specify the offset
distance before selecting the object.
(i) Select the Offset tool.
(ii) Type the offset distance into the Command Line.
(iii) Click on the source object.
(iv) Click inside or outside of the object to tell AutoCAD whether to make an inner or
outer offset.
(v) Repeat step (iv) to create more offsets of the same object, or go back to step (iii) to
select a new object.
(vi) Press [Enter] or right-click to end.

4. Array
This creates a regularly spaced array of copies of an object. The array can be a rectangular grid
or a polar pattern, radiating around a central point.
To create a rectangular array:
(i) Select the object (s)
(ii) Click the Array tool.
Or
 Click the Array tool, select the objects and press [Enter].
(iii) Select the Rectangular Array option.
(iv) Set the number of Rows and Columns.
(v) Enter the Row and Column offsets- the distance between the same points on
two copies.
(vi) The Angle of array this array from the horizontal. Set a value if required.
(vii) Click OK.

To create a polar array:

(i) Start as for a rectangular array, but selecting the Polar Array option in the Array
dialog box.
(ii) Type the Center point coordinates, or click and pick the point.
(iii) Set the Total number of items and the Angle to fill.

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 (iv) Tick or clear the Rotate items as copied checkbox on the bottom left, as required.
(v) Click OK.

5. Move
You can move an object by dragging. The main advantage of the Move tool is its greater
accuracy. It also avoids the possibility of distorting an object by dragging on a handle, instead of
a line.
(i) Select the objects(s).
(ii) Click the Move tool
Or
 Click the Move tool, select the objects and press [enter].
(iii) Pick a base point in or near the objects.
(iv) Type the coordinates or pick where you want to move the base point to.

6. Rotate
An object can be rotated about any point, either within its shape or elsewhere on screen.
(i) Select the object(s).
(ii) Click the Rotate tool.
Or
 Click the Rotate tool, select the objects and press [enter].

(iii) Type the coordinates or pick the point to rotate around.

(iv) Type the angle, or rotate the object with the mouse.

7. Scale
Scaling makes an object uniformly larger or smaller. It works by changing all the coordinates by
the same factor and in relation to given point. You can visualize it as projecting a shadow, and
the size of a shadow depends upon the relative positions of the object and the light source.

(i) Select the object(s)

(ii) Click the Scale tool.
Or
 Click the scale tool, select the objects and press [enter].
(iii) Pick a base point within the object to change its size without moving it, or outside to
change the size and position.
(iv) Type the scale factor in at the command line- the values are usually too small to set
accurately with a mouse.

8. Stretch
The stretch command lengthens or shortens objects. This will take part of an object and move it
in relation to the rest to stretch- or shrink- the overall shape. You could do the same job by
selecting the handles at one end dragging them, but this is a little simpler.

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As with the Offset command, you must start Stretch before you select the objects. Selection is
done using a ‘crossing window’- a dotted rectangle that appears when you drag across the screen.
Any object that is inside or is touched by this window is selected.

(i) Select the stretch tool.

(ii) Click to fix one corner of the crossing window.
(iii) Drag the window over the objects you want to select, and click to fix its other corner.
(iv) Press [enter] to end the select object routine.
(v) Type the coordinates or pick a point in the selected area.
(vi) Type the new coordinates or drag the point to where you want to stretch it.

9. Trim and Extend

The Trim tool allows one to clip off pieces of objects that intersect with other objects.
You can trim to where a line meets an existing line, or add new lines to act as the ‘cutting edge’.
Similarly, when extending lines, an existing or a temporary line can act as the boundary edge.
To trim a line or shape:
(i) If necessary, draw the cutting edge lines.
(ii) Click the Trim tool.
(iii) Select the lines to act as the cutting edge, then, press [Enter].
(iv) Click on the lines or shapes where they project beyond the cutting edge.
(v) Repeat to trim other lines or shapes and press [Enter] to end.
(vi) If the cutting edge line has on other purpose, delete it.

To extend lines:
(i) If necessary, draw the boundary edge lines.
(ii) Click the Extend tool
(iii) Select the lines to act as the boundary edge and press [Enter]
(iv) Click on the lines to extend to meet the boundary edge.
(v) Repeat to trim other lines or shapes. Press [Enter] to end.
(vi) If the boundary edge line has no other purpose, delete it.

10. Break
There are two versions of this tool. Break cuts a section out of a line or shape- use it for creating
openings in solid outlines or as an alternative to Trim. Break at point cuts through a line or a
shape, creating two lines through the ends still meet- use it to subdivide lines so that the sections
can be manipulated separately or as an alternative to Explode to cut one line out of a polygon or
polyline.
You must select the tool first in both versions.
To break a line:
(i) Select the break tool.
(ii) Click on the line at the point at the point where you want to break it this selects the
line and the break point.
(iii) Click where you want the second break point.
To break at a point:
(i) Select the break at point tool.
(ii) Click on the line or shape to select it.

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(iii) Click where you want the break point.

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11. Chamfer
The chamfer tool cuts off a corner at an angle. It works on a vertex of any shape or on any pair of
lines that can interest-though they do not have to actually meet or cross.
The chamfer angle is determined by the distances along the first and second depends on the order
in which you select them.

For example, to chamfer the corner below, set up the first chamfer distance as 5 and second
chamfer distance as 10, then select A as the first line and B as the second line.

5 A

10

 The chamfer option on a rectangle applies to all four corners. The chamfer tool only cuts
one corner at a time. The Multiple option will let you apply the same cut successively to
any number of corners. The Polyline option will apply the current chamfer settings to
every angle in a polyline object.
(i) Click the Chamfer tool.
(ii) The current chamfer distances will be shown in the command line. If these are
OK, go to step (v).
(iii) If you want to chamfer several corners, type U to turn on the multiple option.
(iv) Type D to select the Distance option.
(v) Enter the first then the second chamfer distances.
(vi) Select the line to apply the fist chamfer distance to.
(vii) Select the second line.
(viii) Repeat steps (vi) and (vii) for other corners if in Multiple mode.

12. Fillet
This tool produces a rounded corner, or draws an arc to join a pair of lines that interest or could
interest if extended.
 If the lines do not meet, and the radius of the fillet will not produce an arc big enough
to join them, the lines will be extended to connect to the arc.
 If the lines intersect, the parts of the lines beyond the fillet arc will normally be
trimmed. If you want to retain them, turn the trim option off.
 The Multiple and Polyline options are the same as in chamfer.
(i) Click the Fillet tool.
(ii) Check the current settings.
(iii) Type R then the value to change the Radius
(iv) Type T, then the setting if you want to change the trim mode.

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 (v) Type U if you want to turn on the multiple option.
(vi) Select the two lines to fillet- the order is irrelevant.

13. Explode
This will break up a polyline, polygon or rectangle into its component lines. Use it if you need to
delete or change the properties of one line in a compound object without affecting the rest.
It can also be used to break up blocks into their components.
(i) Select the objects(s)
(ii) Click the explode tool.
Or
 Click the Explode tool, select the objects and press [Enter].
An explosion is irreversible. If you need to reassemble the lines into a unit so that it can be
handled more conveniently, you can make them into a block.

LAYERS
WHAT ARE LAYERS?
Layers are the AutoCAD equivalent of overlay sheets in papers-rooted drawing offices. In a
building drawing, for example, you might have one layer for the brickwork and woodwork, one
for the wiring, one for the plumbing, and so on. As layers can be hidden, you can display or print
any single layer or combination of layers, to give an uncluttered view of selected features. You
can assign colour, linetype and lineweight to layer, which helps to identify sets of features and
gives consistency to their appearance.

The layers toolbar

At the center of the layers toolbar is the drop-down list where you can see and control the key
properties of layers.
Freezing makes layers invisible, but also have deeper impact. When a layer is frozen, AutoCAD
completely ignores it when regenerating and redrawing a model. Thaw the layer to bring it back
into active use. A layer can be frozen in the current view ports only, or in all viewports.

On/off simply toggles the visibility of layers. When turned off, a layer is not displayed or
plotted. Freeze layers that are not likely to be needed for a while; use on/off to hide layers on a
temporary basis.

Lock prevents objects on a layer from being edited or deleted

Colour shows the default colour for lines and hatches on the layer. You cannot change the
colour from here.

To change properties using the toolbar:

1. Click on any of the property icons, the layer name or the drop-down arrow on the right
to open the list.
2. Click on the layer name to switch to a different layer.
3. Click on the icon to toggle between on/off, freeze/thaw or lock/unlock. You cannot
freeze a layer while it is current-if necessary switch to another layer first.

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Creating Layers
When you start a new drawing, it has only one layer, ‘Layer o’. New layers can be created at any
stage. As objects can be moved from one layer to another, you can create a layer before drawing
the objects on it, or afterwards.

Layer 0 has the default settings for colour (white –or black if the background is white), linetype
and lineweight. When a layer is created, it will have the same settings as whichever existing
layer is selected at the time or the default settings if no layer is selected.

Layers are created in the Layer Properties Manager dialogue box. This has a small set of tools
for managing layers, and displays the full range of layer properties. These include Linetype,
Lineweight, Plot style and Plot (on/off) which are not shown on the Layers toolbar.
To create a new Layer:
(i) Click on the Format menu and select Layer…
Or
Click the Layer Properties Manager tool on the layers toolbar.
(ii) Click New
(iii) A new layer will be listed. Replace the default name ‘Layer1’ with a more meaningful
name
(iv) Set the layers properties.

To set layer properties:

(i) To set the Colour, Linetype or Lineweight, click on the icon to open the related
dialogue box, or open the dropdown lists in the Details area to select an option there.
(ii) To change the name, double-click on the name in the main list, or click into the Name
field in the Details area. Edit as with any other text.
(iii) If you want the full Details display, click the Show details button-and click Hide details
when they are no longer wanted.
(iv) With the toggles (on/off) –On, Freeze, Lock and Plot- click on the icon or the checkbox,
or open the drop-down lists in the Details area to select an option there.
(v) Click OK to close the Layers Properties Manager dialogue box when you have
finished.

To delete a layer:
(i) If there are any objects on the layer, delete them, or move them to other layers.
(ii) Open the Layer Properties Manager
(iii) Select the layer
(iv) Click Delete

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