Best "v" thread form

Yesterday I needed some 7.5 " coach bolts, but not having any I cut up some 8mm threaded rod and added washers and nuts rather than make a trip to get some. This reminded me of a question I have often thought of asking. I have a lifetimes collection of Whitworth, Unified, BA and Metric nuts and bolts. Assuming that they are made from the same quality steel which thread form is the strongest and least likely to fail? I understand that finer threads tend to be strongest, although this seems counter intuitive to me.
i am also curious why smaller threads were not small Whitworth rather than BA. Did BA have advantages.
I understand why we have standardised, but i guess I am asking if we have standardised on the best thread form or just the most universal.
I have tried to answer this question with a google search, but failed to find an answer. perhaps it is just too complex and I need a good book. I did see one recommended, but the cost of £50 -90 did seem a lot to satisfy my curiosity.
Thanks in advance to any that share their wisdom.

Edited By Keith Wyles on 14/11/2021 18:14:11

The best thread is the one that does the job with a safety factor as required and at a reasonable cost. With the vast range available it should be easy to make your choice dependant on the job in hand. How long is a piece of string ? Noel

When you look at the BA sizes, it appears they were designed by a mathematician and not by an engineer. I personally like the 'look' of the BA threads as opposed to the small (1/8 and 3/32" whitworth threads that are rarely found today.

Its a bit like the metric series - for some reason, its the metric coarse which you normally find in DIY stores, and not the fine series. Whilst on holiday in NSW Australia, I came across a seaside hardware shop that seemed to have nuts and bolts of every thread form. I asked why they had such a good range - answer, that they provided for the repair of old boat engines, newer British engines, American and Japanese.

Bob

The BA thread-form better suits use in thin plates ... unsurprising, as it derives from the Swiss Thury thread.

MichaelG.

A finer thread is stronger than coarse for the same bar diameter and material by having a larger root cross-sectional area.

The full BA series is in geometrical progression from 0 down to 24BA - broken by omitting the so-called "non-preferred" sizes. It is a metric thread but specified in British Standards and many text-books, in inch dimensions.

The Americans invented their own "Unified" system for no obvious reason other than NIH - just as they adopted other things invented in Britain but changed the names! Their fastenings though look neat, with fairly compact and often machined hexagons; unlike the rough old ISO-M Coarse stampings.

One strking difference between the BSW and BSF system and the ISO-metric and UN, is that its spanner sizes are always designated by the thread form and size; or by BA-number. The rest leave you to guess, look up or remember which inch or mm AF spanner fits what fastening.

And that falls down with flanged nuts and in some other cases when the thread is standard ISO-M, but the hexagon is not the appropriate standard mm-AF at all.

Briefly, in the beginning there were no standard threads. Joseph Whitworth was the first to recognise the advantages of having a standard set of threads that could be used on fasteners and so make the mass production of nuts and bolts possible and cheaper. Joseph based his thread to give the best results on typical steels and cast metals of his time. These were the days before high tensile and tool steels etc. were available.

These were a real improvement on previous threaded fasteners and since the British tool and manufacturing industries were the largest in the world . So these threads predominated through out the Empire and then the world.

It was recognised that there were problems when the bolt size got below about 1/8th of an inch . When the pitch became too course relative to the diameter. So, in the late nineteenth century a scientific team got together to design threads for small mechanisms, scientific instruments, time pieces etc. They came up with the BA series. which is actually a metric thread form and does follow a fixed ratio between thread diameter and pitch.

Around the same time separate series of threads where brought out for bicycles etc. Where large diameters but fine threads were needed Brass ware and pipe fitting threads where also needed. Auto manufacturers also needed special threads for various components and most developed their own standards.

During the second world war problems were found when US equipment (built to US imperial standards) needed to be repaired in Europe. Where a mixture of old metric and UK imperial was the rule. So was born the unified series meant to be an approximation between the old metric and UK / US imperial.

By the time the ISO standards came out it was decided to standardise on a small number of metric threads designed to offer the greatest strength in the more modern steels now available. There were proposals for around twenty different thread pitches to cover all sizes between 1 mm up to 100 mm. Of course they then needed the "specials" for cameras etc. and so we end up with the wonderful variety of 'Standard ' threads we see today.

Generally the ISO metric thread form offers the best choice for modern steels. Cast iron is probably better with Whitworth form though metric is good too. if you are repairing old cars them you are stuck with whatever was used during manufacture. And watches etc. BA if older and metric if newer. Though BA gives a larger range in the smaller sizes.

For many years BA threads were very often used in electrical applications, such as terminals.

It could be argued that BA offers a smaller increment between sizes than Metric, in the small sizes, but are now non preferred, if only because of the different thread form.

BSW and BSF have what appear to be strange A/F sizes, and the wish to reduce metal useage during WW2 led to the hexagon sizes changing to one size down can lead to confusion between "old" and "new"

Becoming of academic interiest in many respects as Metric Coarse becomes the probably, the most commonly available thread.

Whitworth form threads live on in BSP ("Gas" in Europe ) and M E, and in British Standard Brass , threads

Personally, I like the 40 tpi M E threads as a means of fine adjustment, although 1/2 UNF is very useful in larger sizes, as would be any 20 tpi thread for adjusters in Imperial sizes.

In a Metric environment a 0.5 or 1 mm pitch thread would serve the same purpose.

AND, in general, Metric provides a smaller selection of pitches that BSW, BSF or Unified..

With regard to pitches, the finer threads are stronger (Unless you try pulling a heavy Tailstock along the lathe bed with a ME 40 thread! ). Fine threads of any type can produce some great tensile loads, using the correct grade of material. A 1/2 UNF thread in W range steel does not yield until a load of 9 tons is applied, wheras, in my youth, I was very prone to wrecking 1/4 BSW bolts and studs!

If you think of a thread as a lever, wrapped around a bar, the finer threads provide a greater mechaniical advantage, (As a longer lever per unit of movement ) and so allow a greater tensile or compressive load to be applied.

Coarse threads tend to be used in softer materials because of the greater thread depth, and flank thickness.

Howard.

Sir Joseph Whitworth, Dr. C. W. Siemens, Sir Frederick Bramwell, Mr. A. Stroh, Mr. Beck, Mr. W. H. Preece, Mr. E. Crompton, Mr. E. Rigg, Mr. A. Le Neve Foster, Mr. Latimer Clark, Mr. Buckney, and Mr. H. Trueman Wood (Secretary)

Fair few engineers on the BA small screws commitee of 1882. including Joe Whitworth himself.

regards Martin

Strange that nobody has mention the thread angles. 60° for Metric, 55° for Whitworth, 47.5° for BA, and 29° for Acme. Maybe there are some other too.

Gary, it was the angle that I was thinking of in my OP. Somehow I omitted to mention it. Thanks for the comments so far. The reason i said that finer threads being stronger seemed counter intuitive was that if you take it to an extreeme with tpi approaching infinity you effectively get no thread depth. As well as being impossible to thread without cross threading. So there must be an optimum beyond which getting finer shows no benefit.

Gary

Just being pedantic, You missed the 45 degree Buttress thread for uni directional loads, or the square threads that are on so many of our machines.

And there are may more, some quite specific and non standard, to our minds

One or more instances of Horse for Courses.

(Who would want to use an Imperial Micrometer with a 19 tpi thread? )

Howard

The question asks about the 'best' and limits its inquiry to the strength of the thread form. But in the real world, the answer must depend on other factors too. The material involved, for one, and the proportion of the thread to the size of material, and whether the thread is on bar or tube. Not least of these factors, is the meaning of strength. In practice, resistance to failure is more important in many cases, and this means resistance to coming loose through vibration as well as the effect of sharp corners on fatigue failure of the bar.

Other factors not related to strength itself are the ease of production in the particular case itself, the relative importance of service needs and standardisation, and the relation between first cost, servicing costs, and the consequences of failure (which bear directly on safety factors).

In my case, working on vehicles made before WW2, in a market with strict rules about 'originality', but laws which require compliance at the same time with modern standards, it gets quite complicated. In some cases, then, I need to make parts with pre-war metric threads, but whitworth spanner sizes. My answer to your question, then? Well, it all depends. Best for what?

Keep at it. After 50 years it begins to make sense, but only until you look closely ...

Regards, Tim

PS: Don't forget the German Honey-pot thread, and the British Standard Rusty Gate-hinge Bolt thread.

Tim

Edited By Tim Stevens on 15/11/2021 16:49:25

I could never see the the stupidity of the BA thread,the peculiar choice of thread angle,the odd pitches starting from 0 BA at 6mm dia at 1mm pitch.and the smaller dias and pitches not based on logic or basic engineering , why not when at the time our empire used whit form stick to a small thread system ,based on whit and using round number imperial pitches,with imperial eqipment screwcutting and making taps and dies would have been easier,Then our electrical industry used odd number BA which was possibly chosen as odd numbered screws/nut were not readily available,then at one time Imperial Typewriters mainly used odd no BA with regular fractional hex sizes.I remember from my apprenticeship days being told to avoid the use of 1/4 whit as it was the weakest known common thread size. But I cannot really understand how the use of number and letter drills carried on so long where there is no logic to progression of sizes,where as the metric system has a regular progression, even way back in the good old days when imperial dims were used ,a set of metric drills would have been easier to use with the 0.1 steps ,roughly 4 thousandths of an inch,

I think you'll find that the electrical industry used even number BA threads. ie. 0,2,4,6,8, etc

Roy

The GPO used odd BA numbers as they were rarer and less likely to get nicked. For the logic of the system maybe you should get your answers from the horses mouth as it were. 5BA is conveniently die cut on the end of 1/8 rod.

Here are the links to the first and second reports from the BA Small Screws Committee from 1882 and 1884

**LINK**

**LINK**

regards Martin

.

Not sure why you think that ^^^ strange, Gary

I had presumed that Keith was aware of the angles, and was asking which we considered "best"

MichaelG.

I thought it strange that despite asking about the best V thread form, the most basic difference, ie. the V angle, was ignored.

Gary, you are correct i was mainly thinking about the role of the angle, and the actual profile as well.
But i guess that it is also a question of manufacturing costs as well. For the majority of application it probably doesn't matter. I was interested in why we went from a combination of 55 and 47.5 deg to 60deg. Is 60 better or cheaper to make. Or was it simply Unified and Metric happened to be 60 and became standard.
Martin, thanks for sharing the BA minutes, they do make interesting reading. I had forgotten that I had briefly read them before. It would be interesting to see a similar discussion of Whitworth and the 60 deg threads. Although I understand that Whitworth arrived at his standard by examining threads already in use.Perhaps he arrived at a course thread because that was the norm then, and thus the need for BSF. Perhaps Unified and metric became 60 deg, just to be different and that is were we have arrived, or is their an argument for 60 deg being better.
Like many things perhaps the original reasons have been lost to history. I was, am, just curious why we arrived were we are. Is it more to do with standardisation and ease of manufacture than sound engineering principles.
Equally Whitworth spanner size shrunk to match BSF or at least a new standard allowed that to save steel (money), was the initial size because of the quality of steel, or an extravagance or for added strength or to reduce rounding from badly fitting spanners.

I will continue to use whatever I happen to have to hand, usually over-engineered, Mainly because I get fed up of fixing commercial products that have been built to a cost rather than longevity.

Again thanks to everyone who has given up their time to add their contribution.

The V angle is important if the screw is meant to come out for maintenance etc.

Some of my tangential toolholder metric clamping screws had 90 degree V countersunk head and they clearly needed extra violence to give up grip and You can easily hear it.

The american countersunk are 80 to85 degree from memory and are a little more civilised.

But still unpleasant.

I then went for 60 degree and it is much nicer but to day I use the seat from a modified spiral drill 125 degree mating the 55 degree countersunk on some home modified Unbrako screws.

The wedge action between Screw head and seat is the same as between male and female thread profile and Withworth was best all the time.

For no locking tendency zero degre ACME is best