Other Threads

§1The specialists’ drawer

Every form so far optimised for clamping, moving or sealing. The threads on this page optimise for stranger things — filth, mass production in sheet metal, gear ratios, one-handed opening, split-second locking — and their shapes follow.

The pattern to watch is the same dial the whole section has turned, pushed to its odd corners. Where the fastening vee maximised wedge and the power trapezoid minimised it, the knuckle thread (§2) abolishes corners altogether because its enemy is not friction but grit and abuse. Where the metric system tuned pitch for strength, the closure thread (§5) tunes lead for the human wrist, and the interrupted thread (§6) deletes most of itself to buy engagement speed. And one member of the drawer — the worm (§4) — stops being a fastener entirely and becomes a gear, carrying the systems page’s lead-angle arithmetic into transmission design. None of these will ever appear on a bolt; all of them appear daily in the built world, usually unrecognised, and each earns its place with one computable trick this page works through.

§2Round and knuckle threads

Build a thread entirely of arcs — deep round crests, deep round roots, no corner anywhere — and it becomes nearly impossible to clog, chip or cross-thread: the form for the dirtiest jobs in engineering.

The round (knuckle) thread, designated Rd in its standardised form, is the hero’s wave: generous radii where every other form has flats or points. The consequences are exactly what its homes demand. Dirt tolerance: a vee groove packs with grit and jams; an arc groove has no corner for grit to wedge into, and coupling squeezes contamination out along the smooth flanks — which is why railway wagon coupling screws, fire hydrant caps and hose couplings that live in mud all run knuckle forms. Damage tolerance: there is no fine crest to bruise; a dinged knuckle thread still assembles, where a dinged vee needs a file. Speed and feel: the deep, coarse, rounded engagement starts by feel in the dark with gloves on, the design requirement of every coupling made up daily in the field. The price is everything the vee was good at — the knuckle centres loosely, wedges little and holds no precision — and its strength economics favour few, deep, coarse threads over many fine ones. The table below places it among its fellow specialists; §3 meets its most-manufactured member.

§3The Edison thread

The most manufactured screw thread on earth is a knuckle form rolled into brass sheet a fraction of a millimetre thick — the lamp cap, unchanged in principle since Edison adopted it.

Look at a lamp base and the §2 form is all there: fully rounded crests and roots, coarse and deep, formed — not cut — by rolling the profile into thin sheet metal, so the thread appears as a corrugation with the female socket shell made the same way as its mirror. The designation is the diameter in millimetres with an E prefix — E27 and E14 for the common mains caps, E10 and below for instruments — the same name-is-the-major-diameter convention as the metric page, applied to a form the metric page would not recognise. Every property is a manufacturing property. Rolled sheet costs nothing per thousand and holds no precision, which the knuckle form forgives; the rounded engagement self-centres a cap inserted askew by feel; the springy sheet thread grips with elastic preload no rigid form could keep in such thin material; and the whole geometry survives the abuse of a century of hands changing bulbs in the dark. It is worth holding up as this page’s emblem: a thread with no strength figures worth computing, whose entire engineering is the match between a forgiving form and a near-free process — and the thread-rolling page just ahead is the story of that process grown up.

§4The worm — a thread that is a gear

Mesh a screw thread with the teeth of a wheel and the thread becomes a worm: one revolution advances the wheel one tooth per start, and the section’s lead-angle arithmetic becomes transmission design.

§5Closure threads

The thread on a jar or bottle is engineered around a wrist: coarse, rounded, multi-start, and dimensioned so the whole engagement clears in about one comfortable turn.

The requirement is stated in human units — open one-handed, in one motion, ten thousand times without thought — and the geometry follows from the multi-start arithmetic of the systems page. Suppose the closure needs 6 mm of axial thread engagement for a secure seal. Cut single-start at a 3 mm pitch and opening takes 2 turns of the wrist; cut the same profile two-start, doubling the lead to 6 mm, and the cap clears in 1 turn — same engagement, same seal, half the motion, which is why moulded closures routinely run two, three or four starts. Everything else about them is process and duty. The forms are rounded knuckle-family profiles because they are moulded — glass finishes and plastic caps must strip cleanly from their tooling, and arcs demould where corners lock — and because a cap cross-threaded by a hurried hand must forgive it. And where the cap also holds pressure, the buttress page’s bookend returns: the fizzy-drink closure carries a rounded buttress-style form, loaded one way, on a thin-walled plastic nut that cannot afford radial burst. A jar lid is a dense little seminar in this whole section; it simply never says so.

§6Interrupted threads

Cut away alternate sectors of a thread — on both members — and the two parts slide together axially through the gaps, then lock with a part-turn: the fastest full-strength engagement a thread can offer.

The hero’s end view shows the trick. With the thread divided into three threaded and three blank sectors, the male’s ridges pass axially through the female’s gaps, and a rotation of 360/(2×3) = just 60° brings every ridge under every land — fully engaged, along the entire length, in a sixth of a turn. Two sectors per side gives 90°; the sector count is a dial between locking speed and manufacturing fuss. The costs are exactly two and both are computable. Half the circumference is thread, so the engaged shear area is half a continuous thread’s — answered by doubling the engagement length, cheap on the long breech-style connections the form suits. And the sectors must be clocked: the part-turn only works from the one axial alignment, so interrupted joints carry stops and index marks as part of the design. The homes follow from the virtue: the artillery breech block (buttress-form sectors, the two specialist ideas of this section combined), quick-release couplings and clamps, autoclave and pressure-door closures — anywhere a full-strength thread must engage now. Its light-duty cousin, the bayonet fitting, drops the thread entirely and keeps only the part-turn; the interrupted thread is the version that still has to hold like a thread.

§7Quick reference

The working core of the page on one card rack.