UNM thread (USA)
The UNM thread is an American miniature thread. The standard dates back to 1955 when representatives from Great Britain, Canada, and the USA agreed on a urgently needed standard for very small miniature threads, such as those needed in the watch industry. Previously, many manufacturers had used company-specific standards, which was simply no longer contemporary in terms of interchangeability and the availability of spare parts. The abbreviation UNM stands for United National Miniature. The UNM thread is controlled by the American National Standards Institute (ANSI) and is standardized according to ANSI/ASME B1.10.
When designing the UNM thread, it was strongly based on the corresponding ISO miniature thread. Accordingly, the UNM thread is not an inch-based thread, as one would expect from a Unified National thread, but a metric one. However, compared to the ISO thread, it covers an extended size range up to an external diameter of 1.40 mm. From 1.60 mm, the UNF thread "takes over."
The UNM thread has a flank angle of 60°. Its pitch is given in mm. A conversion to the American thread specification of threads per inch is only approximately possible due to the odd values. Due to the small thread size in relation to the inch, this also results in impractically high values of sometimes several hundred threads per inch. Therefore, the much simpler metric values are usually used.
The designation for the UNM thread consists of the external diameter in mm and the acronym UNM. The only thing to note is that in the American system, decimal numbers are separated by a point instead of a comma. Thus, the designation for a UNM thread is, for example, 0.70 UNM.
The thread table for the UNM thread comprises 14 entries, significantly more than the corresponding table for the ISO miniature thread. This is due to the extended scope and some additional intermediate sizes. However, the correlating sizes are identical in their dimensions to the ISO template. The table lists external diameter and pitch in mm. For completeness, you will also find a conversion of the pitch to threads per inch in the table below. If you want to cut UNM threads yourself, you can reach the suitable UNM taps and dies by clicking on the links in our table.