Sometimes a rough paper design already exists for a product. If so, the CAD operator can use the system's digitizing capabilities-the ability to "read" or trace the geometric shape from the paper drawing, and to use the computer to transform it into a series of geometric coordinates, or points-to put that design into the computer's memory. Then the CAD operator can manipulate the drawing on the screen. Or, perhaps the shape the operator wants is similar to something that's already been CAD-designed. If so, the CAD operator can call it up from computer memory, much as a secretary using word processing can call up a computer file containing a previously-typed letter. The old design is then placed into the computer, and the CAD operator can edit it, just as the secretary can make changes in the old letter. When the desired changes in the design are completed, the edited design can be stored again for later recall.
Typically, a CAD system has a library of designs and commands that have been stored. Much as word processing software has commands already stored that let the secretary delete, re-form paragraphs, or move blocks of type from one location to another in a manuscript, a CAD system contains commands that let the operator erase, redraw, or move portions of drawings around on the screen, as well as commands that let the operator perform many more functions.
As the Office of Technology Assessment (Congressional Board of the 98th Congress) reports, CAD enhances productivity of designers and drafters because of four basic functions: replication, translation, scaling, and rotation.
When a product or design has features that are repeated, CAD's ability to replicate means the operator can take part of the image and use it in several other areas of the design without having to redraw it each time.
CAD's ability to translate means the CAD operator can move images around from one location on the screen to another.
CAD can scale, by changing the proportions or size of one part of the image in relation to the others, and can zoom in on a desired portion of the drawing, much as a camera lens zooms in for portraits and close ups.
Finally, CAD's ability to rotate lets the CAD operator move the design around to see it from different angles or perspectives.
Because CAD systems allow operators to manipulate the images so quickly and effectively, they can accomplish in moments tasks that used to take hours and days with paper and pencil. That's why CAD has proved so effective in engineering and design environments, including architecture. CAD technology, however, is beginning to spread to other fields: mapping and cartography at the University of Illinois/Urbana; creative arts and set design at Purdue University; computer-generated medical diagnostics at Johns Hopkins University; and landscape architecture at Iowa State University.
2-D and 3-D CAD
Two-dimensional (2-D) CAD drawings are rather like an artist's sketch or a drawing made with paper and pencil. For certain applications, such as the design of electronic circuits, 2-D CAD is sufficient. When the CAD operator combines several 2-D CAD drawings from various perspectives, they can be used to model three-dimensional (3-D) objects.
Unlike the real world, the CAD world has what is called 2/2-D. "With 2l/2-D CAD, you can extrude your drawing into the 3rd dimension," explains Marmon Pine, president of CAD Design Systems Inc., "but you cannot alter its shape.
"I can draw a box. I can project the box as a square into the 3rd axis, but I cannot turn it into a pyramid, since that would alter its original shape. I can make a desk or a book, or can run a cylinder, be-cause the computer moves the drawing down on the Z axis without al-tering its shape. But it's not true 3-D CAD."
Although the image the operator draws on the computer screen is, of course, two-dimensional (having height and width, but not depth), there is-in the CAD world-a difference between 2-D and 3-D. The Office of Technology Assessment report* explains it this way: "The difference between a 3-D image as discussed here and any other 2-D drawing such as a painting, a photograph, or any drawing with perspective, is that this computer-screen image, unlike a paper drawing, can be manipulated as if it were a real 3-D object. For example, the operator can instruct the CAD system to rotate the object, so he or she then sees another face of the object."
There are many advantages to using true 3-D CAD in engineering and manufacturing, when it's appropriate.