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CAD drawing of a NASA hyper-X plane

Computer-aided design (CAD)

by Chris Woodford. Last updated: September 30, 2013.

People have worried about machines taking over their lives since the beginning of the Industrial Revolution—and with good reason: lots of jobs that used to be done by humans are now automated. In farming, for example, machines such as tractors and combine harvesters have slashed the fraction of the US population working the fields from about 90 percent in 1800 to just 2 percent today.

The same fears were often expressed about computers during the early part of the 20th century. While it's true that these amazing electronic machines have brought major changes to the way we work, attitudes have changed too. Now we're more likely to see computers as partners—tools that can help us work quicker and smarter, cutting out drudgery and leaving us time for more interesting things instead. That's particularly true of fields like architecture and the development of new products, where computer-aided design (CAD) helps to maximize creativity. CAD sounds hi-tech and modern but, amazingly, it's been in use now for over a half century. It first appeared back in 1959, when IBM and General Motors developed Design Augmented by Computers-1 (DAC-1), the first ever CAD system, for creating automobiles on a computer screen. So what is CAD exactly and how does it work? Let's take a closer look!

Photo: Designing a plane? CAD makes it quicker and easier to transfer what's in your mind's eye into reality. Photo by courtesy of NASA Langley Research Center (NASA-LaRC).

What is computer-aided design?

We know from cave paintings that the urge to draw things goes right back to prehistoric times. But scribbling on stone is no way to work in the modern world. Even paper makes life harder than it needs to be. Suppose you're designing a new car and you scribble some ideas on a giant pad with your markers (felt-tips). That's great, but what will it look like in green, as a coupé, or with four doors instead of two? Making changes on paper usually means drawing your design all over again. Playful creativity can suddenly start to feel like a chore.

A Wax model of a plane made from CAD drawing.

Drawing on a computer screen with a graphics package is a whole lot easier, because you can modify your sketch really easily. But that's not all there is to CAD. Instead of producing a static, two-dimensional (2D) picture, usually what you create on the screen is a three-dimensional (3D) computer model. You do this either by drawing the surface freehand, by building it up from pre-drawn basic elements (such as circles, cylinders, and polygons), or by "importing" it from scanned photographs or artworks. Unlike a basic bit of 2D computer art, a 3D model is not made from separate points (pixels) or colored-in areas but from lines called vectors that link all the key points (edges or corners) together. At this stage, the model is a kind of line-drawn skeleton called a wireframe, which looks a bit like an object wrapped in graph paper.

Photo: CAD is not just a matter of drawing lines on a computer screen—it involves making a detailed 3D model with the help of a powerful workstation, like this classic Silicon Graphics computer from the 1990s. Photo by courtesy of NASA Langley Research Center (NASA-LaRC).

Once the outside of the model's done, you turn your attention to its inner structure. This bit is called rigging your model (also known as skeletal animation). What parts does the object contain and how do they all connect together? When you've specified both the inside and outside details, your model is pretty much complete. The final stage is called texturing, and involves figuring out what colors, surface patterns, finishes, and other details you want your object to have: think of it as a kind of elaborate, three-dimensional coloring-in. When your model is complete, you can render it: turn it into a final image. Ironically, the picture you create at this stage may look like it's simply been drawn right there on the paper: it looks exactly like any other 3D drawing. But, unlike with an ordinary drawing, it's super-easy to change things: you can modify your model in any number of different ways. The computer can rotate it through any angle, zoom in on different bits, or even help you "cutaway" certain parts (maybe to reveal the engine inside a plane) or "explode" them (show how they break into their component pieces).

A Wax model of a plane made from CAD drawing.

CAD/CAM: Making an object with CAM from a CAD drawing.

What is CAD used for?

From false teeth to supercars and designer dresses to drink cartons, virtually every product we buy today is put together with the help of computer-aided design. Architects, advertising and marketing people, draftsmen, car designers, shipbuilders, and aerospace engineers—these are just some of the people who rely on CAD. Apart from being cheaper and easier than using paper, CAD designs are easy to send round the world by email (from designers in Paris to manufacturers in Singapore, perhaps). Another big advantage is that CAD drawings can be converted automatically into production instructions for industrial robots and other factory machines, which greatly reduces the overall time needed to turn new designs into finished products. Next time you buy something from a store, trace it back in your mind's eye: how did it find its way into your hand, from the head-scratching designer sitting at a computer in Manhattan to the robot-packed factory in Shanghai where it rolled off the production line? Chances are it was all done with CAD!

Photo: Turning CAD into reality Left: 3D drawing to 3D model: Once you've completed your CAD drawing, you can use it to make a physical model. This wax model was automatically machined using the data from a CAD file. Photo by courtesy of NASA Langley Research Center (NASA-LaRC). Right: Computer-aided manufacturing (CAM) involves using CAD drawings (the blue wireframe model shown on the screen, upper left) to drive computerized machines that faithfully reproduce the objects in 3D (bottom). Photo by courtesy of NASA Glenn Research Center (NASA-GRC).

CAD architecture: from paper to mind's eye, from computer screen to computer model

Architectural model of a building made from white cardboard.

If you were like me and you went to a practical, technology-oriented kind of school in the 1970s or 1980s, you might have learned a subject called technical drawing, which involved sitting in front of 3D objects and learning how to turn them into precise 2D drawings on paper—invaluable if you planned to be a draftsman or an architect when you grew up, but a complete waste of time for every aspiring doctor, teacher, scientist, or road-sweeper.

In those days, technical drawing was the best solution to a maddening problem architects and engineers knew only too well: how to communicate the amazing three-dimensional constructions they could visualize in their mind's eye with clarity and precision. Even with three-dimensional drawings (such as orthographic projections), it can still be hard to get across exactly what you have in mind. What if you spent hours drawing your proposed building, airplane, or family car... only for someone to say infuriating things like: "And what does it look like from behind? How would it look from over there? What if we made that wall twice the size?" Having drawn their projections, architects would typically build little models out of paper and board, while engineers would whittle model cars and planes out of balsa wood. But even the best models can't answer "What if...?" questions.

Computer-aided design (CAD) solves these problems in a particularly subtle way. It doesn't simply involve drawing 2D pictures of buildings on the screen: what you produce with CAD is effectively a computer model of your design. Once that's done, it's easy to rotate your design on-screen or change any aspect of it in a matter of moments. If you want to make a wall twice the size, click a button, drag your mouse here and there, and the computer automatically recalculates how the rest of your model needs to change to fit in. You can print out three dimensional projections of your model from any angle or you can demonstrate the 3D form to your clients on-screen, allowing them to rotate or play with the model for themselves. Some models even let you walk through them in virtual reality. CAD has revolutionized architecture not simply by removing the drudge of repetitive plan drawing and intricate model making, but by providing a tangible, digital representation of the mind's eye: what you see is—finally—what you get.

Photo: Architectural models are traditionally made from paper or cardboard, but they're laborious and expensive to make, fragile and difficult to transport, and virtually impossible to modify. Computer models don't suffer from any of these drawbacks. Photo by Warren Gretz courtesy of US DOE/NREL.

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Woodford, Chris. (2010) Computer-aided design (CAD). Retrieved from http://www.explainthatstuff.com/computer-aided-design.html. [Accessed (Insert date here)]

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