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.

 

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