Screw threads

The essentials

A screw thread is a helical ridge formed on the outside of a cylinder (external thread, as on a bolt) or on the inside of a hole (internal thread, as in a nut). Threads are used to fasten parts together and to transmit motion or power, as in a screw jack or the lead screw of a lathe. The shape of the thread seen in an axial section is its profile, and each profile suits a particular use.

Key points

• Major diameter: the largest diameter of the thread; it is the nominal size (for example M20 means 20 mm major diameter).
• Minor diameter: the smallest diameter, at the roots of an external thread.
• Pitch (P): distance between corresponding points on adjacent threads, measured parallel to the axis.
• Lead: distance the screw advances along its axis in one complete turn. Lead = number of starts × pitch.
• Square thread: flanks perpendicular to the axis; depth and thread width about 0.5P. Used for power transmission (screw jacks, vices).
• Knuckle thread: a square-type thread with well-rounded crests and roots; suits rough use and easy cleaning, e.g. railway carriage couplings and bulb holders.
• BSW (British Standard Whitworth): V thread with 55° thread angle, rounded crests and roots, depth about 0.64P.
• ISO metric thread: V thread with 60° thread angle; the external thread depth is about 0.61P. Written as M20 or M20 × 2.5 (pitch 2.5 mm).
• Right-hand threads advance when turned clockwise; left-hand threads advance when turned anticlockwise.

Exam tips

• When drawing profiles, mark the pitch first along the axis and then build the thread shape.
• For a multi-start thread, always multiply the pitch by the number of starts to get the lead.

Common mistakes

• Mixing up the 55° (BSW) and 60° (metric) angles.
• Treating pitch and lead as the same for a double-start thread.