Ww-thread
What is a WW thread?
WW thread refers to the Whitworth thread, a British inch-based thread system standardized in 1841. It was developed by engineer Joseph Whitworth and is considered the first widely accepted industrial thread standard. The defining feature of the WW thread is its 55-degree flank angle and the rounded crests and roots of the thread, which distinguishes it from, for example, the ISO metric coarse thread and the ISO metric fine thread, both of which have a 60-degree flank angle.
In machining, the WW thread belongs to the inch-based thread systems, and its sizing is based on the number of threads per inch, i.e. the TPI value (Threads Per Inch).
Structure and geometry of the WW thread
The WW thread profile is symmetrical, and its 55° flank angle together with rounded crests and roots reduces stress concentrations at the thread root. This structure improves the strength of the joint, especially under tensile load.
The thread pitch is specified as a TPI value, which differs from the metric system, where the pitch is given in millimetres. The most common nominal sizes are roughly between 1/16” and 2 ½”. The diameter and pitch together form the basic dimensions of the thread, on the basis of which the machinist selects the correct tap, single-point threading tool, or thread gauge.
The WW thread appears in several Whitworth derivatives, such as the BSW thread (British Standard Whitworth) and the BSF thread (British Standard Fine), which use different pitch densities.
WW thread in the machine shop industry
The WW thread was for a long time the dominant standard, especially in the United Kingdom but also more broadly in the machine shop industry, before the metric system became widespread. Today it is mainly encountered in older machine tools, steam engines, historical equipment, and spare parts where the original thread standard must be preserved.
In the machine shop, identifying a WW thread is particularly important in maintenance and refurbishment work. Using the wrong thread standard can lead to poor contact surfaces, play, or thread damage. In machine shops, WW threads are typically produced by single-point threading on a CNC lathe, by thread milling, or with taps, depending on the workpiece material and accuracy requirements.
Measurement uses thread gauges and thread micrometers to verify the pitch, pitch diameter, and the correct profile shape.
WW thread compared to other threads
The WW thread differs significantly from ISO threads and from UNC and UNF threads, particularly in its flank angle and profile. UNC and UNF threads are also inch-based, but they have a 60-degree flank angle and a profile that differs from the Whitworth form.
In practice, WW threads and 60-degree inch threads are not compatible with each other, even if the nominal diameter is the same. For this reason, the machinist must always confirm the standard used before threading or manufacturing a spare part.
Summary
The WW or Whitworth thread is a historically significant British inch-based thread standard whose 55-degree profile and rounded shape distinguish it from other thread systems. Although its use has declined with the spread of the metric ISO thread, the WW thread remains important in the maintenance of older machines and equipment in the machine shop industry. Knowing it is part of a machinist’s basic expertise, especially in refurbishment and spare part manufacturing.
