Screw specification - Page 1
Forum section: electronics / beginners

author: tkamiya
author_rank: Super Contributor
timestamp: December 14, 2021, 01:24:54 am
content: I have a dumb question. Although I've been in US for almost 40 years, I never got familiar with imperial system. Having said that, I have in my hand a post which screws for 9 pin serial connector housing screws into. According to my Mitutoyo pitch guage, it's "40." The diameter is 2.75mm or .01080 inches.I have to buy nuts for these. Question is, what is the normalized and standard expression for nuts?Thank you.

author: ve7xen
author_rank: Super Contributor
timestamp: December 14, 2021, 01:35:08 am
content: D-sub connectors are threaded Imperial "#4-40". Nominal diameter #4 - 0.112", 40 TPI. e.g.https://www.mcmaster.com/nuts/thread-size~4-40/18-8-stainless-steel-hex-nuts-10/

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 14, 2021, 02:12:59 am
content: Everything is specified by diameter followed by pitch in threads per inch. Diameters under 1/4" or so are numbered, smaller number=smaller. In the event there are two common pitches you might also see "fine" or "coarse" in place of pitch. It's fairly straightforward.In your case you need "#4-40 nuts", this would normally be called out as "four forty" if you're asking in a proper hardware store.

author: tkamiya
author_rank: Super Contributor
timestamp: December 14, 2021, 02:36:41 am
content: Thank you! Order has been placed. Appreciate both of your help!

author: TimFox
author_rank: Super Contributor
timestamp: December 14, 2021, 04:48:50 am
content: In "conventional" US units for screw fasteners, there are two "normal" series: "UNC" = unified national coarse and "UNF" = unified national fine. These determine the thread pitch (threads per inch) for a given diameter. Therefore, 4-40 is a #4 diameter with 40 tpi, and is UNC. 4-48 is the UNF with a finer thread. #4 denotes a diameter of 0.112 inch. These quantities are best looked up in a suitable table. One convenient pitch is 32 tpi, as in 6-32 UNC, 8-32 UNC, and 10-32 UNF, since if you should strip the threads in a 6-32 tapped hole, you can re-tap it to 8-32 and from there to 10-32.There is another series, commonly used for larger screws with very fine thread in electronics applications. For example, a BNC connector body thread is 3/8-32 UNEF, where the diameter is 3/8 inch and the "extra fine" thread pitch is 32 tpi. Seehttps://www.gewinde-normen.de/en/unified-extra-fine-thread.htmlfor larger UNEF diameters, with < 32 tpi.

author: Renate
author_rank: Super Contributor
timestamp: December 16, 2021, 10:32:31 am
content: Let's not forget 5/8-27 microphone stand thread!

author: TimFox
author_rank: Super Contributor
timestamp: December 16, 2021, 02:44:34 pm
content: Is that 5/8-27 or 5/8-24 UNEF?Ed.: I looked it up, and it is 5/8-27.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 16, 2021, 03:44:37 pm
content: The vintage 1963 Lafayette here was made in Japan.Most of the internal screws seem to be metric (M3)But the steel cabinet screws are not metric.Maybe the cases were made in USA?The bottom case screws diameter measures between 4-40 and 5-40 according to the charts.abouit 2.9 mmI am about to buy a packet of both from RTL to try.

author: tkamiya
author_rank: Super Contributor
timestamp: December 16, 2021, 06:43:17 pm
content: Nuts from McMaster just arrived. It fits perfectly. Thanks for the help! I bought one indicated above and one with built in star lock washer. Great stuff.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 16, 2021, 07:43:38 pm
content: Quote from: Renate on December 16, 2021, 10:32:31 amIf you want goofy threads I think the Delco (and perhaps other US designed automotive alternators) alternator/generator pulley thread wins. The shaft is finished just under 17mm for a slip fit in the typical 6203-2RS bearing used on the pulley end. To save the machining step of stepping it down to 5/8 for the pulley and nut thread they just ran a 20tpi die head over it, making the unholy .668(17mm)-20 thread.

author: Teledog
author_rank: Regular Contributor
timestamp: December 17, 2021, 04:26:03 am
content: At 2.75mm, it seems like a 4-40 (common American standoff/screw)There is UNF, NPT, BPT , etc., etc..threads and more..sometimes takes a while to find it ..thread gauge helps ( both Imperial & metric..sometimes..)Look up the bewildering array of thread charts.In my old age, I've collected boxes of both Imperial 2-56, 4-40, 1/8", #6 , #8, etc. , and Metric M1.5 thru M5 nut/bolts, in both plated metal and nylon.And Metric & Imperial tap & die sets. (+ thread gauges)If one asks for Christmas presents...More sh*t to put in the head...

author: TheWildMan
author_rank: Contributor
timestamp: December 17, 2021, 04:41:12 am
content: We have so many different bolts that we need to match or get nuts for around our property that I bought a screw gauge.https://www.amazon.com/gp/product/B07K7W3NQQ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1They are only like $9USD, but that thing has saved me a number of times from buying the wrong bolt or nut. One note: it is made of plastic, but has held up well because I am really careful not to cross thread or force a bolt into a hole (no matter how much I want it to fit). I have some old thread gauges, but this thing is just really handy because it has the size and thread pitch marked.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 17, 2021, 05:10:46 am
content: Quote from: mag_therm on December 16, 2021, 03:44:37 pmIn repairing OLD guns I found that in some cases none of the screws were any kind of standard, either in pitch or diameter. Many of the early manufacturers simply made their own screws for their own use so they didn't bother with any "standard". I first ran into that on the take down screw on a Stevens "English Frame" 44 1/2 model. I doubt that gun manufactures were the only ones that did that. Note that I'm talking about production guns not custom made one offs.Something no one has mentioned yet is the "class" of threads. Basically the tolerance of the dimensions of the threads for any given screw size.The old Machinery's Handbooks have great detail about screw threads, forms, diameters, pitches, strengths of, friction of and everything else that you could wish to know on the subject. I highly recommend that anyone that involved with anything mechanical have a copy and read through it. They print a new, updated edition every year so used copies are readily available and I think that in many cases the old books give better descriptions than the newer ones do.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 17, 2021, 04:52:04 pm
content: I've no experience on gunsmithing but have a lathe here and have made adapters for vintage camera lenses and bicycle parts.The British were first and exported their Whitworth lathes everywhere.So some camera threads originating in EU are metric diametrically with helix in turns per inch !Bikes are metric now but some of the threads are still turns per inch eg pedals and steerer.My 1939 South Bend 9A has a full gearbox of TPI and I have a compound 63:64 tooth gear for metric too.And double ended threading tools from UK, one end is 60 degree for USA and Metric, the other end is 55 degree Whitworth for some camera threads.

author: TimFox
author_rank: Super Contributor
timestamp: December 17, 2021, 05:08:43 pm
content: Your mention of camera threads (e.g. the classic M42 lens mount with 1 mm pitch) reminds me that the standard tripod mount thread for small cameras is 1/4-20 UNC. Some of my camera manuals refer to it as M6.35, but it is a coarser thread than most metric screws. I have also seen the larger tripod screw (for larger cameras or to mount the head on the tripod), 3/8-16 UNC, referred to as the “metric size”.

author: richard.cs
author_rank: Super Contributor
timestamp: December 17, 2021, 05:41:40 pm
content: Quote from: TimFox on December 17, 2021, 05:08:43 pm1/4 and 3/8 UNC are sizes that happen to interchange acceptably with Whitworth. Certainly some older UK tripods, etc. had Whitworth, I am not really sure which came first.

author: tszaboo
author_rank: Super Contributor
timestamp: December 17, 2021, 05:48:16 pm
content: Yes, this is a lot more easier, than having M2, M3, M5, M6, and M8 screws, that a everyone who ever held a wrench can visually distinguish from each other in the blink of an eye. This is the freedom version of screws!

author: jpanhalt
author_rank: Super Contributor
timestamp: December 17, 2021, 07:13:36 pm
content: @ richard.csGlad you mentioned Whitworth threads. Well into the 60's they were common in Britain. My toolroom lathe (Smart & Brown, 1966) has them. Conventional American threads may be threaded into Whitworth holes, but the fit is loose. On the bright side, Whitworth screws are still available in the US. I have a small supply for my lathe.Now for a bit of trivia, the Royal Microscopical Society specification for microscope objectives is a Whitworth thread (https://www.sizes.com/library/technology/RMS_thread.htm). I have only run into them one other place. Two of the screws in my Harbor Freight 4" horizontal bandsaw (bought circa 2003) were Whitworth. I dropped one, and not suspecting it was Whitworth, I tried UNC and UNF of the correct diameter and neither fit. My experience with the lathe caused me to suspect Whitworth. Now, why would some Chinese engineer choose that? Because of the deeper cut, thay have some advantages. Fortunately, I found the missing screw. Except for ancient stuff, if you are certain of the pitch, and it doesn't match, consider Whitworth. I checked the manual for the current HF model, and it does not mention them.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 17, 2021, 10:23:58 pm
content: Can't talk about Whitworth without mentioning the 47.5° BA thread, which was used for small screws similar to the US numbered screws, just to keep it weird it's technically metric!https://en.wikipedia.org/wiki/British_Association_screw_threads