A Primer on Screw Thread Systems by Peter Stuart

Meccano Modellers soon become familiar with the 5/32” Whitworth screws and nuts used to assemble models. In modern times these have been replaced by metric M4 (4mm) threads in boxed Meccano sets, but most of us continue to use the British Whitworth threads originally chosen by Frank Hornby, when we build traditional models.

This article seeks to explain some of the multitude of thread systems used in the world today and the problems they cause in engineering in this era of globalisation.

In the early days of the Industrial Revolution, manufacturers of stationary steam engines for factories each made their own screws, bolts and nuts to their own standards. There was little need for interchangeability when each engine was usually a one-off design. However, as steam engines became more widespread and designers settled on a range of standard models, and users started employing their own tradesmen to maintain them, there grew a need for thread standardisation. The growing development of steam locomotives and their builders and repair workshops around Britain hastened the need.

In 1841, James Whitworth, a clever engineer and successful industrialist, developed and lobbied for his own thread standard to be adopted. Eventually the British Government decided on Whitworth’s thread system and gave his name to it. This evolved into ”British Standard Whitworth” thread, or BSW for short.

British Standard Whitworth (BSW)

This thread became standard throughout all Commonwealth countries, including Australia. For every thread diameter there is an accompanying thread pitch, expressed in threads per inch (TPI) counted lengthwise along the screw or bolt. The thread angle is a constant 55 degrees. (This is the inside angle of the apex of the triangle of the thread shape when viewed from the side, regardless of whether it is measured at the crest or valley of the thread.) The 55 degree angle and the rounded crest and valley of the Whitworth thread give it a characteristic appearance when seen against other non-British threads.

The diameter range goes from 1/16” to 2”, although you are unlikely to come across anything smaller than 1/8”. Meccano threads are 5/32” diameter with 32 threads per inch.

Incidentally, Meccano worm gears are a short length of 9/16” BSW thread (12 TPI). Racks also have 12 teeth per inch because worms and racks both mesh with standard Meccano gears. In fact worms and racks can mesh with each other.

British Standard Fine (BSF)

Over time it became obvious that in the larger sizes, Whitworth thread pitches were a bit too coarse. The amount that the deep threads cut into the metal of bolts reduced the cross-sectional area of the bolt and hence its strength. So a fine series of threads was introduced, called British Standard Fine (BSF). In most diameters, the Whitworth pitches were retained but moved up one or two sizes to create a finer pitch thread on a given diameter bolt. The size range starts at 3/16”.

BSF threads were used extensively by British car manufacturers until the early 1960s.

British Association (BA)

Around 1900, the burgeoning radio and scientific instrument industries in Britain decided that the fractional inch range of threads available in small sizes of BSW and BSF threads (below ¼”) did not suit their needs. The diameter jumps were too great. This brought about the BA series threads which curiously, have a metric dimensional basis.

BA threads start at 6mm diameter and 1mm pitch. Note that the pitch is not specified as threads per unit of length (as in Whitworth’s threads per inch) but is instead described as the thread pitch in millimetres per thread (the inverse). The size range starts at 0BA (6mm diameter with 1.0 mm pitch) and decreases to 12BA (1.30 mm diameter with 0.28mm pitch), although smaller sizes have been encountered.

The BA size range formula is that each pitch dimension is 0.9 times less than the immediate bigger size. The thread diameter is then calculated to be 0.6 times the pitch. To cap it off, the thread angle is an odd 47.5 degrees. It is worth noting that the BA thread system was devised by scientists, not by engineers.

Despite its oddities, BA threads are still in use, being popular with builders of scale model locomotives (especially live steam), who desire to keep their fasteners true to scale. The close geometric progression of BA sizes greatly assists them.

American Threads: Unified National Coarse (UNC)

While Britain was working out its standards for screw threads, America was making similar progress but introducing its own standards, because there was little cross-Atlantic trade in engineering products in the 19th century. In 1864 William Sellers proposed a screw thread with a 60 degree thread angle and flattened crests on the threads.

This Sellers thread later became the Unified National Coarse (UNC) thread. The series specifies the thread pitch by the number of threads per inch (TPI).

Whether by design or by accident, the thread pitches for given diameters of UNC threads mostly correspond to the British Standard Whitworth series down to ¼”. However, there is one inexplicable exception. In the ½” size, the BSW thread has 12 TPI and the UNC thread has 13 TPI. Strange indeed.

Above ¼”, UNC and BSW bolts and nuts can be screwed together because of the correspondence between diameters and thread pitches, except for the ½” size as noted above. The difference in thread angle (55 degrees vs 60 degrees) does not matter for most practical purposes.

Below ¼” there is another anomaly with UNC threads. Whereas Whitworth threads continue with fractional sizes down to 1/16” in 1/32” steps, UNC threads are given a “Machine Screw Gauge” number from 12 down to 2 (for descending diameters) which are not fractional. They appear to be based on wood screw diameter gauges.
However, there is one useful aspect; the smaller UNC machine screws are nominated by gauge number and thread pitch; e.g., No.4-40 or No.10-24.

American Threads: Unified National Fine (UNF)

Similar to Whitworth threads acquiring a corresponding fine series (BSF) which was adopted by the British motor industry, the American UNC thread has a fine series (UNF) which was used by US car makers. Once again, increased bolt core strength was gained by using shallower, finer threads. However, there is no correspondence between diameters and pitches between BSF and UNF, so bolts and nuts from the two standards cannot be screwed together.

As with the UNC series, UNF threads below ¼” also revert to Machine Screw Gauges. In the example given above, typical UNF machine screws would be described as No.4-48 or No.10-32.

Metric Threads (ISO)

In 19th century Europe, different countries adopted their own metric thread standards. This caused problems during WW1 because it made interchangeability of military equipment difficult. In 1919, Germany developed the ISO Metric thread and most European and Asian countries adopted it. In 1948 this was later refined to the ISO metric thread standard which even Western countries eventually adopted. Most motor vehicles are now assembled with ISO metric fasteners. (ISO stands for International Standards Organisation.)

ISO metric threads have a 60 degree thread angle, use the prefix ‘M’ to denote the diameter from M2 to M40, and measure the thread pitch from crest to crest in millimetres. From M8 and above, there can be two or even three thread pitches for a given diameter, thus enabling a fine thread series to co-exist. For example, M8 threads can be specified as M8x1.25, M8x1.0 or M8x0.75 from coarsest to finest pitch.

ISO metric is actually the most organised thread series ever developed.