Miscellaneous notes on screws
Terms for metric and English screw size standards
These terms are all equivalent for most purposes:
metric = ISO metric = ANSI mm
English (US) = imperial (UK) = UTS = ANSI inch
Major diameter
Metric screw sizes are easy to figure out.
M3: The “M” is for metric, and the “3” is the major (nominal) screw diameter in mm.
English screw sizes are easy for large screw sizes.
1/4-20: 1/4 inch major diameter
For smaller screw sizes, read the next section.
The numbers for English screws
Two common sizes of small machine screws are #4-40 and #8-32. What do these numbers mean? As it says above, the second number refers to the thread pitch, and is reported as the threads per inch. However, the first numbers, the major (nominal) diameters, are a bit more complicated. Here’s how to convert them into inches:
major diameter in inches = 0.060 + (0.013 * N)
Where N is the first number in the name of the screw.
Thread pitch
To denote the thread pitch, the two main systems use different measures which are actually the inverse of each other. For metric screws, the pitch is measured in mm per thread. For example, M3 screws are typically 0.5 mm per thread. For English/imperial screws, the thread pitch is measured in threads per inch. For example, 1/4″-20 screws are 20 threads per inch.
TLAs (Three Letter Abbreviations)
UNC, UNF, UNEF
These are from the UTS (Unified Thread Standard). It’s for English/Imperial screw sizes. Each screw size (nominal diameter/major diameter) is available in either a coarse (UNC), fine (UNF), or extra fine (UNEF) thread pitch; e.g., UNC = UNified thread standard Coarse.
BSW, BSF, BSC
BSW stands for British Standard Whitworth. Joseph Whitworth authored the first national screw standard in 1841, and it still carries his namesake. BSF is the variant with fine thread pitch. BSC is a variant used for Cycles (motorcycles and bicycles). UNC is based off of the BSW standard, but with a different shape to the threads. In some applications, UNC and BSW are interoperable. For example, the screw mount at the bottom of SLR cameras for tripod attachement is a BSW 1/4-20 standard, but UNC 1/4-20 screws can often be used in them.
Fun fact for the day
One time in London, I needed a fine pitch tap and die set and the cheapest I could find was a BA set, so I bought that. The BA screw standard isn’t used much any more, but it’s an interesting standard.