As in mechanical threads, not the spider or web threads: a helical projection of uniform section on the internal or external surface of cylinder or cone. Also, the operation of cutting a screw thread.


  • ACME-C – Acme Thread-Centralizing
  • ACME-G – Acme Thread-General Purpose

Axial Load

On the axis of the bolt thread. A force extending or compressing the bolt.

BA nuts and bolts

BA Threads

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

A selection of BA nuts and bolts in sizes from 2BA to 12BA and in steel and brass.

Bolt Strength Grade

eg G8.8


A fixed pitch standard Whitworth form thread used for brass fittings. The thread pitch is 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.

  • BSPP – British Standard Pipe (Parallel) Thread
  • BSPT – British Standard Taper Pipe Thread – the taper is 1:16 and so for each 16 units of distance along the axis of the thread, the diameter increases by 1 unit of measurement.

The thread form is Whitworth.


Developed by Sir Joseph Whitworth in 1841. Series specified by BS 84:1956.

Buttress Thread

A screw thread with one vertical/undercut and one inclined flank.

polar duck mini tripod and camera

Camera Threads

There are many different threads in use from the trigger release to the 42mm lens thread in old SLR cameras.

The standard tripod thread is 1/4″ x 20 tpi UNC. The larger tripod thread is 3/8-16 UNC thread.

ISO 1222:2010 Photography — Tripod connections

Historically the tripod mounts were 1/4″ x 20 BSW and 3/8″ x 16 BSW.

Die Holder

The die holder can be a hand tool or a tailstock die-holder (eg Soba Tailstock Die-holder).

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, one of the first international standards agreed when the International Organization for Standardization was set up in 1947.

Nut Dilation

The wedge shape of the thread results in a radial force. The radial force on the nut results in it increasing in diameter and so the thread is not so well engaged.

Right Hand Thread

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

Screw-Cutting Lathe

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.

Siemens & Halske Thread

Used in telephones around 1900 AD. German electrical engineering company that later became part of Siemens AG.

Symmetrical Thread

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

19th century tapwrench

Tap Wrench

A hand tool that can be locked onto the square post on the tap and used to apply a torque to cut an internal thread.

When using a tap wrench it is usual to turn the tap 180° clockwise and then back off in the opposite direction before cutting the next 180°. This allows the tap to clear the swarf.

Thread Angle

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

The thread angle is quite an important factor that should be taken into account when looking at the application of the thread.

thread nomenclature

Thread Axis

A line running lengthwise through the centre of the screw.

The axis should run parallel to the crest and root. Sometimes it is difficult to machine accurately over a long length of thread.

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.

Important to look at the thread depth, pitch and angle when considering certain types of materials and applications.

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.

Tightening Technique

This can just be by hand applied torque, a measured torque, a measured torque and then an angular displacement or even a force measured in the bolt.

Transverse Load

Normal to the bolt thread, a force trying to shear the bolt.


Unified Coarse Thread Series. Evolved from the USS or Sellers series originally proposed in 1864.


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

  • UNEF – Unified Extra Fine Thread Series

Wood Screw

500 BC The philosopher Archytas first invented the screw. They were made out of wood and used for wine and olive oil presses.