Metrology & Threads
The most important factors when threaded parts must meet engineering specifications, are accuracy and consistency. I realize that many of you are already familiar with threads and thread measurement but thought it might be helpful to get some quick reference diagrams and a few definitions here on our blog. It's a quick cheat sheet.
The fineness or coarseness of a screw's threads are defined by two closely related things:
- The lead is defined as the linear distance a screw travels in one complete revolution (360°) of the shaft. The lead determines the mechanical advantage of the screw; the smaller the lead, the higher the mechanical advantage.
- The pitch is defined as the axial distance between the crests of adjacent threads.
Most threads are "single start", which have a single helical thread wrapped around them, the lead and pitch are equal. "Multiple start" threads have several intertwined threads. In these screws the lead is equal to the pitch multiplied by the number of starts.
There are quite of number of thread forms in use today and lots of times what you think you recognize may not be what is. The Acme form, which is 29 degrees included, is only 1 degree different from the ISO Metric Trapezoidal form which is has a 30 degree included angle.
Most thread forms are designated by published standards standards. I've put the entire list of here so you can see just how many spec's there are out there
- Unified Thread Standard (UTS), which is still the dominant thread type in the United States and Canada. This standard includes:
- Unified Coarse (UNC), commonly referred to as "National Coarse" or "NC" in retailing.
- Unified Fine (UNF), commonly referred to as "National Fine" or "NF" in retailing.
- Unified Extra Fine (UNEF)
- Unified Special (UNS)
- National pipe thread (NPT), used for plumbing of water and gas pipes, and threaded electrical conduit.
- NPTF (National Pipe Thread Fuel)
- British Standard Whitworth (BSW), and for other Whitworth threads including:
- British Standard Fine (BSF)
- Cycle Engineers' Institute (CEI) or British Standard Cycle (BSC)
- British standard pipe thread (BSP) which exists in a taper and non taper variant; used for other purposes as well
- British Standard Pipe Taper (BSPT)
- British Association screw threads (BA), primarily electronic/electrical, moving coil meters and to mount optical lenses
- British Standard Buttress Threads (BS 1657:1950)
- British Standard for Spark Plugs BS 45:1972
- British Standard Brass a fixed pitch 26tpi thread
- Glass Packaging Institute threads (GPI), primarily for glass bottles and vials
- Power screw threads
- Acme thread form
- Square thread form
- Buttress thread
- Camera case screws, used to mount a camera on a photographic tripod:
- ¼″ British Standard Whitworth (BSW) used on almost all small cameras
- ⅜″ BSW for larger (and some older small) cameras
- Royal Microscopical Society (RMS) thread, also known as society thread, is a special 0.8"-36 Whitworth thread form used for microscope objective lenses.
- Microphone stands:
- ⅝″ 27 threads per inch (tpi) Unified Special thread (UNS, USA and the rest of the world)
- ¼″ BSW (not common in the USA, used in the rest of the world)
- ⅜″ BSW (not common in the USA, used in the rest of the world)
- Stage lighting suspension bolts (in some countries only; some have gone entirely metric, others such as Australia have reverted to the BSW threads, or have never fully converted):
- ⅜″ BSW for lighter luminaires
- ½″ BSW for heavier luminaires
- Tapping screw threads (ST) – ISO 1478
- Aerospace inch threads (UNJ) – ISO 3161
- Aerospace metric threads (MJ) – ISO 5855
- Tyre valve threads (V) – ISO 4570
- Metal bone screws (HA, HB) – ISO 5835
- Panzergewinde (Pg) (German) is an old German 80° thread (DIN 40430) that remained in use until 2000 in some electrical installation accessories in Germany.
- Fahrradgewinde (Fg) (English: bicycle thread) is a German bicycle thread standard (per DIN 79012 and DIN 13.1), which encompasses a lot of CEI and BSC threads as used on cycles and mopeds everywhere (http://www.fahrradmonteur.de/fahrradgewinde.php)
- CEI (Cycle Engineers Institute, used on bicycles in Britain and possibly elsewhere)
- Edison base Incandescent light bulb holder screw thread
- Fire hose connection (NFPA standard 194)
- Hose Coupling Screw Threads (ANSI/ASME B1.20.7-1991 [R2003]) for garden hoses and accessories
- Löwenherz thread, a German metric thread used for measuring instruments
- Sewing machine thread
Although threads have been around for thousands of years and we've the standards let us have interchangeable parts, there still some threads, like Ballscrew and Worm threads, aren't defined in detail by any standards organizations.
I'm working on another blog post that talks about mechanical advantage of threads and how much load is exerted on each thread. I think you'll find it interesting and helpful.