Worm (module 2, single start)

A single-start worm in module 2 — 16mm pitch diameter, 20mm over the thread, four turns, 8mm bore, 6.28mm lead, 7.1° lead angle. Its axial section is the module-2 rack tooth, which is what makes it a gear rather than a bolt: the wheel it drives sees a rack sliding past it. Runs with the module-2 worm wheel below on 90° crossed axes at 38mm centres, for a 30:1 reduction in one stage — which is the reason to use a worm at all. At 7.1° the lead angle is well under the friction angle, so the drive is self-locking: the worm turns the wheel and the wheel cannot back-drive the worm. That is a genuinely useful property and it is also why a worm drive wastes a lot of its input as heat. Prints standing on its end. The thread's lower flank is a helicoid about 70° off vertical, which sounds unprintable and is not — it advances only about half a millimetre of radius per layer, so each layer lands almost entirely on the one below. The underside of the thread comes out rougher than the top; that is what a printed thread looks like and it does not stop it driving. Lying it on its side would put a real unsupported spiral under every turn, and that does fail. Nylon or PETG — a worm rubs rather than rolls, and PLA will not take the heat that generates.

from £2.18example, in PLA