What a Screw Thread Is: A Ramp Wrapped Round a Cylinder

Cut a long right-angled triangle from paper — say 200 mm along the base and 20 mm high — and wind it round a pencil starting from the tall end, keeping the base edge square to the pencil. The sloping edge traces a helix: exactly the shape of a screw thread. Wind a steeper triangle and the helix climbs faster, which is a coarse thread; a shallower one gives a fine thread. There is nothing else to it. Hold the paper model next to a real bolt and the two match. This is worth doing rather than reading, because the helix is genuinely hard to picture and instantly obvious once it is in your hand.

PITCH is the distance from one thread crest to the next. LEAD is how far the screw advances in one full turn. On an ordinary single-start thread they are the same number, which is why the words get used interchangeably. They part company on a multi-start thread, where two or more separate helices run side by side. A two-start thread has the same pitch but twice the lead, so it advances twice as fast per turn — which is how a bottle cap or a camera tripod screw comes off in half a turn. MAJOR DIAMETER is measured across the crests, MINOR across the roots. An M8 bolt is 8 mm across the crests, and the tapping drill for an M8 hole is much smaller because it must leave metal for the thread to be cut into.

A COARSE thread assembles fast, tolerates damage and dirt, cuts cleanly into soft material, and is far harder to cross-thread. It is the default, and it is right for most work. A FINE thread has a shallower helix, so it holds a set position better under vibration, allows finer adjustment, and gives slightly more strength in thin material because more threads are engaged in the same depth. It is also fragile: one dinged crest and it will not start. The practical rule is coarse unless something specifically demands fine — an adjustment screw, a thin-walled part, or a fastener that must resist vibration. Mixing them is the commonest workshop error: an M8 coarse bolt will start into an M8 fine nut for one turn and then jam solid.