MAYNARD, MASS. — Engineers used to spend hours with a calculator in one hand and a pencil in the other designing and revising parts for new products.Now, computer-aided design (CAD) software can automatically calculate the proportions, volume, center of gravity, and other features of the part - making new-product development much faster. Al Cassista of Digital Equipment Corporation's new products operation demonstrates how parts can be created in moments on a computer screen - complete with three-dimensional appearance. "If I try and do something stupid, the system scratches its head and says, 'hey dummy, you can't do that, he says. Companies based in the United States account for over 80 percent of worldwide sales of computer-aided design, manufacturing, and engineering systems, says Bruce Jenkins, who researches this market for Daratech Inc. in Cambridge, Mass. Mr. Jenkins forecasts industry revenue to grow 8.7 percent this year - down from 14 percent last year due to the recession and falling hardware prices. Top players in the market include IBM, Intergraph, Computervision, Mentor Graphics, and Hewlett-Packard. The CAD systems that Digital uses are linked directly to prototyping machines. Mr. Cassista can make a plastic prototype of a part at the push of a button. He sends the design to a $250,000 machine that uses laser light to solidify layers of liquid plastic. Meanwhile, at Carnegie-Mellon University in Pittsburgh, researchers have developed another machine that can make parts out of "any material which you can spray," be it metal, plastic, or ceramic, says Fritz Prinz, the director of the university's engineering design research center.