Always confused over threads and tapping

I sympathise Hopper; needs must. It's also true that some suppliers do try to send UNC in place of BSW on the basis that close enough is good enough - and presumably of loose tolerance, or BSW heads on UNC threads (these probably of chinese manufacture)

I wish they would send us UNC threads with BS heads! At least you only need the one set of spanners per vehicle then. As it is now, all too often an old bike will have a mixture of BS, AF and metric hexes dotted about it.

But I am guilty of creating my three penn'eth of confusion, making tooling for my Myford I find myself making things like a rear toolpost mounting stud with a 7/16 UNF thread because that is what taps and dies I have, but I machine my own nut for it with BS sized hexagon so it matches the Myford standard spanner set on the wall behind it. Someone in the future will be scratching their head over that one.

From what Hopper shows in the pics it seems I have to dump my old 1/4" W taps as they must be worn beyond the limits, or buy some low cost UNC screws when needed.

Emgee

Also guilty of that Hopper. My lathe has metric threads, but my mods (dro , carriage stop etc) are attached with bs cap screws because I had them.

Much depends on what the nuts and bolts are for. The penalty of mixing 55° and 60° threads of the same TPI is a reduction in strength, coupled with an increased chance of jambing or coming loose. I couldn't find any comparative numbers, practical or theoretical, quantifying the difference.

Found a couple of convincing examples saying 55° BSW threads perform better in cast iron than 60° threads (UNC or Metric) but it's a special case. For most purposes 60° threads are better than 55°.

As ordinary nuts and bolts are plenty good enough for almost everything I do intermixing BSW and UNC is a useful trick. But I wouldn't do it if the fixing had to be strong or vibration proof. Very unwise to replace the Grade 8.8 high tensile bolts on my Engine Crane with ordinary DIY Store bolts because the latter are at least half as strong and could be much weaker. Strong nuts and bolts are made of higher specified steel with tighter fitting close tolerance threads: it makes a big difference.

Dave

I really have problem with this discussion as some of you are actually looking to mismatch fasteners, why do you think the designers of these fittings made both parts match..I could not in all honesty make stainless or any other material fasteners that did not match. Let’s be fair all the information required for thread sizes /rates /tapping sizes/ clearance/ and ods in the case of numbers is all out there pull it off print and read it.If a jobs worth doing.!!

Bernard, as you say you have the 'problem', in the real world BSW & UNC will work together with no problem apart from 1/2" diameter. If you are making both obviously match the angles etc etc, otherwise in reality it matters little, an engineer will put accuracy/precision where it's needed.

Tony

Not sure if it's been mentioned but for camera tripod 1/4" and 3/8" threads the past standard was BSW and the modern one is UNC, but usually they are made 'loose' enough that either is an easy fit.

Neil

.

Not previously mentioned on this thread, Neil … but it’s a good point, and a prime example of the expedient use of the appropriate UNC ‘Tolerance Class’

MichaelG.

The OP lives in Italy and has limited access to any fasteners or taps/dies other than metric. As he said, he already has a 1/4 BSW tap but is expecting a new part of some sort to arrive that needs to use a 1/4 UNC thread. Importing a UNC tap for him to do this one job will cost him a large bundle of beer tokens.

So for his purposes, it makes sense to use his existing BSW tap to make a thread to house the UNC thread when it arrives. I was merely pointing out that as I live under similar circumstances in Australia where BS fasteners and taps/dies are no longer commonly sold and cost a motza to order in specially, I routinely mix the two threads out of necessity and have never had a problem in many decades of doing so.

But once again, the thread seems to have been sidetracked on microscopic minutiae, standards drawings and hypothetical "you can't do that"s, So the practical solution to the OP's simple practical question has been lost in the fog of theory

Unless of course the OP's impending job is attaching the wings to the space shuttle. .

Edited By Hopper on 21/02/2022 22:50:47

Neil Wyatt points out (above) that camera tripod threads were BSW, but are now UNC. The user guide for my JVC camcorder specifies the tripod thread as 6.35mm x 1.25p, with no mention of the thread form or angle.

A rose by any other name?

Dick

If camera owners were to measure what they actually have, I suspect they'd find a mixture, including threads that aren't to any particular standard.

Cameras need a short quick action thread that doesn't have to be strong or vibration resistant. 4 or 5 turns of any coarse thread on a not too tiny diameter will do. In Victorian times the obvious standard thread for this purpose was ¼" Whitworth. (Or ⅜" BSW for giant cameras.) In practice there's no need for the thread to be accurate - all is well provided it grips.

BSW lasted nearly a century, but during WW2 considerable trouble was caused by incompatibilities between US and British nuts and bolts. Two almost identical systems that didn't reliably fit together so a broken down American truck couldn't be fixed with a British bolt or vice versa. This is serious when millions of vehicles and machines are deployed in a shooting war. To reduce this problem in the event of another major war the British and Americans adopted a common system - Unified Threads. In practice the Unified System is being superseded by Metric/

After unification BSW and BSF were, at least in theory, not to be used for new manufacture. In consequence the standard thread for camera mounts was changed from BSW to UNC. Thing is, I doubt camera threads were ever made to a tight specification: they approximate BSW / UNC rather than match it slavishly. Moderate errors in TPI, diameter, flank angle and thread form don't matter. A real BSW will fit a real UNC and both will fit a metric approximation like 6.35mm x 1.25p. And so would US Sellers ¼ - 20 of 1868, which later became NC ¼ - 20.

Be interesting to survey exactly what threads are out there! For example:

• Outer diameter should all be about 6.35mm
• Flank angle. 55°=Whitworth, 60°=Sellers, NC, UNC or Metric)
• Male thread shape - probably, but note some standards allow optional variations
  • rounded peaks and troughs=Whitworth
  • flat peaks and troughs = Sellers
  • flat peak, rounded troughs = NC, UNC or Metric
  • Sharp V peak or trough - non standard.

No wonder pgrbff is 'Always confused over threads and tapping'. He lives in Metric Italy, and is trying to fix a pre-war British saw on a small budget. Unlike camera threads, getting the nuts and bolt exactly right on a machine is more important, and extra hard to do where he lives.

Dave

Probably be close enough.

Same thing at my local steel dealer recently: "We don't have imperial sized stock anymore. Gone completely metric."

Yeah.

Not sure if there is an ISO standard for 9.53mm bar, or 7.94mm or 6.35mm?

I guess the OP would be even more confused if someone mentioned the class of fit for a prticular thread (oops too late).

Example in the link :-

**LINK**

regards Martin

Edited By Martin Kyte on 22/02/2022 14:57:00

I have read every reply. I'm not confused, just happy to have created some semi-heated debate on the subject.

I had to order one tap from the UK, so I ordered both, postage VAT and local charges will far outweigh the cost of the 2 taps.

And Microphone stands use 1/4", 3/8" and 1/2" BSW, obviously deliberately chosen to be at odds with camera tripods.

I have some internally and externally threaded adaptors - some of them are for cameras ,some for microphones... much potential for confusion!

Neil

Hopper,

You are not alone.

My Record vice is fixed to the bench with home made nuts, 17 mm A/F but UNF threads.

The hexagon allows the torque to be applied and the thread applies the tensile load to hold things together.

On the Bristol RE bus, with manual transmission, the clutch was held to the Gardner engine (Which used Whitworth form threads ) with 5/16 BSW bolts, with 5/8 A/F heads!

All the other fittings on the vehicle were A/F, (Unified ) so that was the logic.

Chiefie did not seem to be aware of this, thank goodness, see below..

We found that Whitworth form bolts would not screw into the Unified tappings in Clayton Dewandre air brake valves, so used to tap them out. Our Chief Engineer, at the time, would not countenance having Unified threads, anywhere!.

Howard