for screw threads, both external and internal. Two external helical gears operating on parallel axes must be of opposite hand. An internal helical gear and its pinion must be of the same hand. A left-hand helical gear or left-hand worm is one in which the teeth twist anticlockwise as they recede from an observer looking along the axis. Manufacture Worm wheels are first gashed to rough out the teeth, and then hobbed to the final dimensions. See also List of gear nomenclature Gear Linear actuator, some forms occasionally also referred to as worm gear or worm drive Rack and pinion Slewing drive References Bibliography External links Kinematic Models for Design Digital Library (KMODDL) Movies and photos of hundreds of working mechanical-systems models at Cornell University. Also includes an e-book library of classic texts on mechanical design and engineering. Formulae & Calculations for Worm Drive Various Metric Gears downloadable design specifications, 2D-3D models and catalogues Various Worm Gearboxes, 3D models Machining of Worm Shaft and Worm Gears Gears Greek inventions