Inventing a Screw

Is harder than it seems, unless you're talking about the figurative kind of screws your average employer puts to its employees. Read how Kenneth LeVey invented new kinds of screws for plastic and concrete:

Screws are made, oddly enough, by squeezing metal rather than cutting it. A steel or alloy blank, a cylinder with no threads, is rolled between two heavy dies that are grooved with diagonal lines. The blank is put under so much pressure that metal is squished into the diagonal grooves, forming a threaded spiral. Manipulating the shape of the threads using this method, called thread-rolling, was thought to be impossible because it would be too hard to control the structure of the screw if metal oozed into odd shapes...

He was flabbergasted by how archaic screw design was. On rare occasions when a new screw length or width was needed, an engineer would consult a 300-page manual dating from 1936 that explains the relationships between certain heights and pitches of threads and the lengths and widths of the resulting screws. "They would go do math for a couple of days and come back with an answer," LeVey says--to how the grooved dies should look, how much pressure should be applied to the blank, and what the diameter of the blank should be.

LeVey had a handful of interns spend three months putting the mummified math of the old screw guide into software. Meanwhile, he grabbed an old thread-rolling machine out of a nearby factory and wired it to operate very slowly to let him observe exactly what was happening. Using three-dimensional solid-modeling software, LeVey gleaned a finer understanding of how the metal moved when it was squished. Possibilities opened up. LeVey could design intricate dies that, on a computer at least, could wrap screws with a helix of shaped threads.

To make dies capable of pressing tiny, intricate patterns onto the threads, LeVey had to borrow a technology often used to create injection molds for detailed plastic parts. The pattern of the die is milled into a soft, graphitelike carbon. The carbon is placed next to the steel die form, and very high voltage is sent between the carbon and the steel, creating a powerful arc of heat, which vaporizes the steel in the desired pattern. "No one had bothered to take advantage of all of this new technology available to us and apply it to this very old product," LeVey says.

Lead via Kottke.org.