Miniature Screw Threads When I was delivering training courses on bolting, I was often surprised what each company regarded as a small fastener. In some industries M16 was regarded as being small whereas in others, an M6 screw is regarded as large. What this article looks at is very much at the small end of the size spectrum. The smallest screw size that we have done joint calculations and tests upon is a joint with M2 screws. Even an M2 screw, fully tightened can deliver a meaningful load, of the order 1.5 kN (larger than most men's weight) with a tightening torque of 0.5 Nm. As the thread diameter reduces, the relaxation loss increases as the percentage of preload. This often necessitates some mitigating measure to be put in place. Standard metric threads go down to M1. There are requirements, such as in watch making, instrumentation, optical equipment etc., to use smaller diameter screws than M1. The Thury thread was the first standardised miniature thread whose thread dimensions was first published in 1878 in Switzerland. This thread was based upon measurements made of screws from Swiss factories making miniature screws. The starting point was a thread of pitch 1 mm whose most appropriate diameter was decided to be 6 mm, the thread angle being 47.5 degrees. Based upon mathematical ratios the Swiss committee derived 26 thread sizes. There is presently no authorised standard for this thread, and it is now only of historical interest. In 1884, the British Association for the Advancement of Science largely adopted the Thury thread form with some small deviations. Such threads are referred to as BA threads and start with a 0 BA thread (whose diameter is 6 mm) down to 25 BA whose diameter is 0.25 mm. The current standard (British Standard BS 93) now limits the sizes to 0 BA to 16 BA (0.79 mm diameter). The BA thread has been used in instrumentation, electrical connections and aerospace applications. Even through a 0 BA thread has the same diameter and pitch as that of a metric M6 thread, the thread angle differences make them incompatible. They may mate together but severe thread bending will occur under loading. The BA thread is now superseded by the metric thread and for small screw diameters by the metric miniature thread form. Miniature threads are typically 1 mm and below. The relevant ISO standard is ISO 1501 (ISO miniature screw threads), this standard covers threads from 1.4 mm diameter down to 0.3 mm. Unlike ISO general purpose metric threads, there is only one pitch size associated with each diameter. The thread basic and design profiles differ from that of an ISO metric thread (which is based upon ISO 68-1 that defines the basic and design profiles of metric threads). The minor diameter and root radii are larger. Between nominal diameters of 1 mm and 1.4 mm the user has the choice of using a standard metric profile or using the miniature screw thread profile. With the ISO miniature thread since the minor diameter is increased, thread stripping can more easily occur when compared with the ISO general purpose thread. Since there is a larger core diameter in the screw, and not as much material is being removed in the tapped hole, the chances of a torsional overload failure of the tap is reduced relative to the standard metric profile. For very small diameter threads, tap breakage can easily occur. Accordingly, miniature metric threads are given a different designation than ISO general purpose metric threads. The letter S rather than M is used, so S 0.8 would designate an ISO miniature thread of nominal diameter 0.8 mm. Not all countries follow this designation; Germany designates a miniature thread as other metric threads with the letter M and the US (ASME) with UNM (Unified National Miniature). Even in countries that have fully adopted the ISO miniature thread standard, the letter M instead of S is frequently used for miniature screws. Besides differences in the design profile of miniature threads compared to general purpose ISO threads, the tolerancing system also differs. For internal threads, there are four tolerance classes, 3G5, 3G6, 4H5 and 4H6, for external threads, there is only one tolerance class, 5h3. So, for example, the full designation for an external thread of nominal 0.8 mm diameter would be S0.9 - 5h3. The 5h is the tolerance class for the pitch diameter and the 3 is the tolerance grade for the major diameter. (With the 5h, the 5 denotes the tolerance grade which is the size of the tolerance band, and h indicates the thread position relative to the design profile; h indicates that the thread is on the design profile.) The US standard ASME B1.10M Unified Miniature Screw Threads is similar to but not identical to the ISO 1501 standard. The nominal thread sizes and pitches are as per the ISO standard but the tolerances of the ASME standard are different from the tolerances in the ISO standard. The ASME standard does not offer any choice of the tolerancing. So, for a ASME miniature screw designated as 0.8 UNM fully defines the screw. That is, diameter, the pitch (like the ISO standard, there is no choice) and the tolerance class (for which there is no choice). Because of the lack of choice on the internal thread tolerance classes, even in the US, the ISO rather than the ASME standard is often used for miniature screws. New designs should use miniature screw threads from the first choice columns of ISO 1501, that is a nominal diameter of 0.3, 0.4, 0.5, 0.6 or 0.8 mm diameter. The ISO Technical Committee responsible for screw thread standards (TC 1) has no current plans (July 2025) to change/amend the ISO 1501 standard. Social Media Links Linkedin