Righty-tighty is a convention, and left-handed threads exist for clever reasons
Almost every screw tightens clockwise, possibly because right-handers turn a screwdriver that way using the arm's stronger muscle. But some threads deliberately run backward: on welding gas lines, circular saw blades, and light bulbs in certain railway stations, where a reversed thread makes them useless to anyone tempted to steal one.
At heart, a screw is a narrow ramp wrapped around a cylinder. Turn it, and rotation becomes straight-line motion, with a gentle twist producing a powerful push along the shaft. The closer together the threads, the greater that multiplication. It was among the last simple machines to appear, surfacing in Mesopotamia during the Neo-Assyrian period, between 911 and 609 BC. Egyptians used screw pumps before Archimedes analysed the device around 234 BC, and Heron of Alexandria later described a tap for cutting internal threads.
Handedness describes which way the helix winds. A right-handed thread follows the right-hand grip rule: curl your fingers in the direction of turning and your thumb points where the shaft travels. Left-handed threads earn their place where spinning would loosen a normal nut, as on grinder wheels. Turnbuckles carry one thread of each kind so a single twist tightens both ends. Fuel gas hoses for welding use left-handed fittings so nobody connects them to the oxygen supply by mistake. Tradition even holds that coffin lids were fastened with left-handed screws.
Thread shape is a trade-off between grip and glide. Fasteners want friction so they stay put, and most use a steep 60 degree profile. Machine screws that must move loads easily, such as jacks and lathe lead screws, use flatter profiles: the square thread wastes least effort but is awkward to cut, so the 28 degree Acme thread usually replaces it. Light bulbs use rounded knuckle threads, cheaply rolled from sheet metal.
Because intricate helices had to be cut by hand, screw fasteners stayed rare until screw-cutting lathes arrived; they first showed up in 15th-century clocks. Galileo worked out the full theory of simple machines, screw included, in 1600.
Source: Screw mechanism
