What is CAD?

Monday, 9 April 2018

2:29 PM

Machine generated alternative text:
ENG1000 Civil and Environmental Engineering Strands 
What is CAD? 
CAD is an acronym for 
Computer Aided Drafting
. It has become the standard means by 
which engineering drawings are created, traditionally created by specialised drafts-
people. Engineering drawing is standard method by which an object, component or 
structure is specified. The rules and conventions that control engineering drawing are 
specified in the Australian Standard AS1100. Students are directed to the Australian 
Engineering Drawing Handbook published by the Institution of Engineers, Australia for 
more details. 
Some example CAD packages are: 
AutoCAD
, 
IntelliCAD, MicroStation
, 
TurboCAD
, 
RealCAD, Solidthinking, Struc-Plus 
 and a host of others, often with the an acronym CAD 
in the title. They almost exclusively use the vector based system of computer graphics, 
though many are capable of bitmap graphics as well. 
CAD software
 allows you to create a collection of drawing objects in vector format. 
Consequently there are many similarities between 
CAD software
 and typical drawing 
programs. These include such abilities as: 
 
Creation of many types of elements such as circles, rectangles, double lines and arcs 
 
Resize, relocate and reshape individual elements 
 
Group and ungroup 
 
Arrange items in sequences normal to the viewing plane 
 
Align objects to a grid or to each other 
What Makes a Drawing Program a CAD Program? 
A variety of software products create visual images, and they can all be lumped into the 
category of “graphics software.” CAD software belongs in this group, as well as a variety 
of other programs. Several characteristics distinguish CAD software from other types of 
graphics software: 
There are, however, significant additional features in 
CAD software
 that are not found in 
typical drawing programs. These include:
Machine generated alternative text:
 
Ability to perform sections, cross hatching, etc. 
 
Ability to specify a drawing scale (eg 1 in 1000, 10 in 1, 1 cm to 1 m etc) 
 
Ability to specify all graphics aspects in mm rather than points (pen thickness, cross 
hatching, text height etc) 
 
Ability to specify an objects dimensions using numerical input rather as well as the 
mouse 
 
Ability to specify many different size of drawing sheet (A0, A1, A2, A3, A4 etc) 
 
Assignment of objects to specific layers – these layers can be edited, hidden and 
deleted in isolation to the rest of the drawing 
 
Creation of objects in 3D 
model space
  
 
Dimensioning to ISO standards 
 
Locate grid points, vertices, midpoints, tangents and normals using 
snaps
 
 
Specification of scale so that a unit of measurement on paper is greater than or less 
than the real measurement 
Some graphics software products have one or two of these elements, but it takes all five 
to be a true CAD program. Many CAD packages also include 3D capabilities as well as 
rendering, animation and other solid surface modelling features. Other specific aspects of 
CAD packages are described below. 
Precision and Accuracy 
CAD is meant to replace manual drafting, so it must meet the standards of those who do 
drafting work. Part of the standard is mathematical precision. When you draw a line with 
CAD software
, the line has an exact length, which the program can calculate to a 
precision of ten decimal places. Every line, circle, etc., is on a coordinate grid, the exact 
location of which 
CAD software
 stores in memory. 
When drawing by hand, it’s easy to be sloppy and let a line go an extra millimetre or so, 
or to jiggle the compass when drawing a circle. But such imprecise actions are impossible 
in 
CAD software
. A 1 m line will measure exactly 1 m. Circles are true, angles are exact – 
even though a computer screen cannot display them accurately. Commands known as 
Snaps
 allow you to attach one object to another with extreme precision at exact locations.
Machine generated alternative text:
Automation 
CAD programs are designed to automate the manual drafting process which has been a 
demanding and labour-intensive. Flip through a textbook on manual drafting and you will 
see all the details that must be attended to before one can draw a single straight line. The 
right diameter pencil has to be sharpened in a certain way, the paper must be carefully 
attached to the drafting table, the T-Square and the triangle placed correctly, a 
measurement must be taken, construction points placed, and then finally a line can be 
drawn to connect the two points. Now consider drawing the same line using 
CAD 
software
. You install the program only once (the equivalent of setting up the drafting 
table and placing the paper). Line colour, line type, and line width are often already set 
(or you can change them easily). Select the Line tool, move the cursor to the point where 
the line is to begin (perhaps using a Snap command for exact placement). Click the left 
mouse button once, move the cursor to the end of the line and click again. The line is 
drawn, at the exact length and width, to the exact colour and style. 
Other graphics software products automate creating graphic images, but only a CAD 
program automates drafting. 
Real Scale 
Real scale means that as you draw, you can use the actual dimensions of the object you 
are representing on the screen, without regard to fitting it on the screen. When drawing a 
house plan, for example, you draw a 4 m wall as a 4 m wall, not as a 20 mm line that 
represents a 4 m wall. If you have done manual drafting, you can immediately appreciate 
the advantages of working with real scale. It is no longer necessary to figure out whether 
to draw your plans at 10 mm = 1 m or 1 mm = 10 mm, or any other scale. Using real 
scale is an easier, less intimidating way of drafting, because you focus on your ideas, not 
on how to make the drawing fit the paper. 
CAD
 is flexible, however, and offers the option 
of drawing at scale, if you prefer to work that way. 
Coordinate Geometry 
Everything you draw in 
CAD
 will be placed on a simple, flat, two-dimensional plane. 
Every possible location on the plane has a unique address, which can be written in X, Y
Machine generated alternative text:
coordinates. There is only one place (at any one time) in all of 
CAD
’s world that can have 
the address X = 1mm Y = 1 mm. To record both the length of a line and its position on 
the plane, the only thing 
CAD
 needs to know is the coordinates of each end of the line. A 
line with one end exactly at X = 5 mm Y = 5 mm and the other end at X = 11 mm Y = 5 
mm must be exactly 6 mm long, and must exist at one and only one place in the 
coordinate plane. When you use 
CAD
 for the first time (and select 
Metric
), the coordinate 
display is set to show distances in mm. As you move the pointer, the coordinate display at 
the bottom of the screen shows the X, Y coordinate position of the cursor as it moves 
across the drawing area. If the cursor is at X = 10 mm Y = 6 mm, then the cursor is 10 
mm to the right and 6 mm above of a location known as the origin point. This location, 
near the lower left corner of the drawing area, has the coordinate address of X = 0 Y = 0. 
All other coordinates are determined by how far they are from the origin point. Using the 
origin point as a consistent point of reference for coordinate display is known as 
Absolute 
Coordinates
. 
Vector Graphics Display 
A computer displays information in pixels, the little dots that make up an image on the 
screen. The more pixels, the higher the resolution of the image and the smoother the 
image. Paint programs and other types of graphics software are limited by the number of 
pixels available. Objects in these programs are defined by dots, so detail work can be no 
finer than the number of pixels available. 
Paint
 programs store images as a map of dots, 
and can’t define an image with detail that exceeds the number of available pixels. This is 
known as 
raster graphics
, raster referring to the technology that places the pixels on the 
screen. Paint programs are sometimes called device-dependent software, because the 
capability to create and reproduce a screen image is dependent on the display resolution 
in effect on the device in use when the image was created. If you create a drawing on a 
low-resolution device, then move it to a computer with higher resolution, the image will 
look grainy or lumpy. The file storing the image lacks the information necessary to 
display the image at the new, higher resolution. The limited information about resolution 
also affects printing the image. The higher the resolution offered by the printer, the lower 
the quality of the illustration if it was created at low resolution.
Machine generated alternative text:
CAD programs couldn’t care less about pixels. Objects in 
CAD
 are defined by their 
geometric characteristics, not by the points of light that display the objects. For example 
CAD
 calculates where the endpoints of lines are to go and then connects them with pixels 
to form an image on screen. The program makes this calculation each time the screen is 
redrawn. This form of display is called 
vector graphics
, since lines and other objects are 
defined by their geometric properties, which include vectors (headings). It is also known 
as 
device-independent
, since display resolution is not limited by the capabilities of the 
device on which the image was created. This feature is handy when sharing drawing files 
with other users, because it doesn’t matter what screen resolution is in use by either user. 
You might use 640 x 480 resolution on your 14” monitor, and your associate may use 
1200 x 1000 resolution on a 20” monitor. 
CAD
 will display your drawing correctly on 
both monitors, using every available pixel to generate the image. 
 
Acknowledgement:  These notes are prepared from lecture notes originally prepared by 
Dr Gareth Swarbrick, School of Civil and Environmental Engineering.

 

Created with Microsoft OneNote 2016.