A helical projection of uniform section on the internal or external surface of cylinder or cone. Also, the operation of cutting a screw thread.

BA Threads

A British system of threads with 47.5° included angle with rounded roots and crest.


A specialist thread form based upon the Whitworth thread and consisting of 26 threads per inch whatever the thread diameter.


A family of standard screw thread types that has been adopted internationally for interconnecting and sealing pipe ends.


Developed by Sir Joseph Whitworth in 1841.

Buttressed Thread

A screw thread with one vertical and one inclined flank.

Left Hand Thread

A screw thread that is screwed in by rotating counterclockwise.

Metric Threads

The most commonly used type of general-purpose screw thread.

Right Hand Thread

A screw thread that is screwed in by rotating clockwise. Most threads are right handed.

Siemens & Halske Thread

Used in telephones around 1900 AD.

Symmetrical Thread

Both flanks of the thread profile are inclined at the same angle.

Thread Angle

The angle formed by the two sides of the thread (or their projections) with each other.

Thread Axis

A line running lengthwise through the centre of the screw.

Thread Crest

The top surface joining the two sides of a thread.

Thread Depth

The distance between the crest and the root of a thread.

Thread Flank

The thread flanks join the thread roots to the crest.

Thread Height

This is the radial distance between the minor and major diameters of the thread.

Thread Length

The length of the portion of the fastener with threads.

Thread Micrometer

A micrometer in which the spindle is ground to a point having a conical angle of 60 degrees. The anvil, instead of being flat. has a 60 degree V-Shaped groove which fits the thread.

Thread Pitch

The distance from a point on one screw thread to a corresponding point on the next thread.

Thread Pitch Diameter

The diameter of a screw thread measured from the thread pitch line on one side to the thread pitch line on the opposite side.

Thread Root

The bottom surface joining the sides of two adjacent threads. On external threads the roots are usually rounded so that fatigue performance is improved.

Thread Runout

The portion at the end of a threaded shank which is not cut or rolled to full depth, but which provides a transition between full depth threads and the fastener shank or head.

Threads per Inch (tpi)

The number of threads in a linear inch along the length of a threaded component.


Standardised in 1918. Derived from British Standard Whitworth.


Unified Fine Thread, Standardised in 1918, Angle of Thread 60°.

Historical Notes

  1. 500 BC The philosopher Archytas first invented the screw. They were made out of wood and used for wine and olive oil presses.
  2. 1400′s Metal screws first appeared. Since they were handmade, they were very rare and considered a specialty tool.
  3. 1744 The screwdriver was developed.
  4. 1739 Henry Hindley designed and constructed a screw-cutting lathe featuring a plate guided tool and power supplied by a hand-cranked series of gears.
  5. 1789 Patent application for a machine to mass-produce screws.

See also: Acme Thread, American Standard Pipe Thread, Bolt, British Association Threads, British Standard Brass, British Standard Pipe, British Standard Whitworth, Buttressed Thread, Chasing Threads, C-Mount, CS-Mount, External Thread, Female Thread, Left Hand Thread, Lens Thread M39, Lens Thread M42, Lens Thread T2, Metric Threads, Multiple Thread Screw, Nut, Right Hand Thread, Rolled Thread, Screw, Screw Thread, Siemens and Halske Thread, Thread Rolling, Threads per Inch, Tripod Mounts, UNC, UNF.

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Subjects: Fixings and Fasteners