BA threads. Why the tpi?

[MWParticipant @mw27036]

Posted by SillyOldDuffer on 22/10/2017 18:12:25:

I'm curious as to why any system of measurement should gather a fan base, become a national icon, or need defending. Engineers and scientists surely understand that measurement is utilitarian, not emotional. The best system is the one that's most useful and straightforward. When I point out that Imperial has more defects than the Metric system, I'm not attacking a religion, personal beliefs, or an individual, its just the truth. I hope we all welcome that!

Dave

I don't think reality has ever worked that simply!

Michael W

[EmgeeParticipant @emgee]

I was expecting to read some logic for the BA thread angle, guess it's designed to provide a given number of TPI, or is there another reason ?

Emgee

[MWParticipant @mw27036]

Posted by duncan webster on 22/10/2017 19:03:49:

. For most jobs whole numbers of mm are quite good enough, not messing about with 1/64 and 1/32.

Fractions aren't an exclusive feature of the imperial system. You'd still have to learn them even with a metric system.

Michael W

[MWParticipant @mw27036]

Posted by Mick B1 on 22/10/2017 19:16:36:

Posted by SillyOldDuffer on 22/10/2017 18:12:25:

I'm curious as to why any system of measurement should gather a fan base, become a national icon, or need defending.

Dave

The answer to that is that people who've made substantial intellectual investment learning idiosyncratic systems as schoolchildren are naturally protective of the effort it cost them.

The metric system is based on measurements of prime meridians on arbitrary points of a map. You could equally call that idiosyncratic, anything that's based on a perception of importance, even if it's as simple as "the rule of thumb", is still swayed to towards somebody's entirely human opinion.

For example, if I decided that my system of measurement was to be based on a given fraction of the average distance between the earth and the sun, It's still realisable just like the metric system, who's to say this is any less valid. 

Michael W

Edited By Michael-w on 22/10/2017 20:00:16

[MikelkieParticipant @mikelkie]

Automotive sparkplug holes M14 x 1.25 and waterways 1/4, 3/8 and 1/2" BSP ?

[Mick B1Participant @mickb1]

Posted by Michael-w on 22/10/2017 19:54:55:

Posted by Mick B1 on 22/10/2017 19:16:36:

Posted by SillyOldDuffer on 22/10/2017 18:12:25:

I'm curious as to why any system of measurement should gather a fan base, become a national icon, or need defending.

Dave

The answer to that is that people who've made substantial intellectual investment learning idiosyncratic systems as schoolchildren are naturally protective of the effort it cost them.

The metric system is based on measurements of prime meridians on arbitrary points of a map. You could equally call that idiosyncratic, anything that's based on a perception of importance, even if it's as simple as "the rule of thumb", is still swayed to towards somebody's entirely human opinion.

Michael W

Not disputing that.

[thomas oliver 2Participant @thomasoliver2]

If you want a complete record of all threads, go to the http://www.watchman.dsl.pipex.com/thread,html website and download the 8 pages. Measure diameter and tpi and go list to identify any thread. I have a small Dormer Tap and Die handbook. In the Metric Coarse list a 3mm thread is given as 0.5mm pitch. In the Metric Fine list 3mm has a pitch of 0.36mm but under this is given 3mmm with a 0.6mm pitch which is coarser than the coarse thread, Similarly 4mm is given with two pitches in the Fine list, the second again being coarser than the 4mm coarse pitch Can anyone explain this? The 3mm Coarse at 0.5mm pitch is decidedly a fine thread. I have never come across a 3mm Fine thread yet. The 6mm Coarse is decidely a coarse and hence a weak thread, but some Chinese engines have the 6mm fine thread used on the carb, intake.

[Max ToleranceParticipant @maxtolerance69251]

Before asking why the imperial system is so illogical it should be remembered where it came from. Originally before the nineteenth century standards as we know them did not exist. The English system of weights and measures were conceived to make trade etc. easy for a largely illiterate population. So each trade had its own system and each district or town had a local "standard" that could be used to check any suspicious weights or measures.

From the Roman mile (one thousand steps for soldiers on the march) to the sheep's fleece (7 pounds) the barrel, hogs head etc. all meant something to the people who used them. Some still remain with us even today Carat (used for gem stone measurement) Hand's used for horses height, Timber measures to show how much useful timber can be taken from a round log all these were devised to suit a specific purpose. The Acre used for land measure (the amount of land needed to produce a given quantity of crop) etc. etc.

Then engineers and philosophers started to look for a standard that could be used for their work and that is when things get interesting. In Britain Joseph Whitworth was responsible for the first truly scientific definition of the inch. He applied this to the threads that still bear his name. The thread angle and pitch was carefully chosen to suit the iron or steel materials available in his day. He did not have access to the high tensile steels we are used to.

The French were responsible for the metric system which was devised on a "scientific" principal involving one degree of arc of the earth at the latitude of Paris. This attempt which was somewhat better than previous systems but even this was not perfect. All that remains of this standard are the names. The current metre was defined by the International Standards Organisation and was applied through out the world, even the "metric" countries had to revise their systems to comply since they all had slight variations in their previous standards.

It may be interesting when talking of logic that the French also tried the decimal week consisting of ten days. It was not a success.

Curiously enough even the metric French still cling to their old measures for wine. Think Magnum, demi-john etc.

[vintagengineerParticipant @vintagengineer]

Try making the male and female tools for cutting a drill pipe knuckle thread!

Posted by jimmy b on 22/10/2017 16:44:02:

One of my late shift searches for a thread spec did lead me down a rabbit warren, it was whilst going down a blind alley that I found all about the "German bottle closure thread"…

jim

Edited By jimmy b on 22/10/2017 16:44:19

Edited By jimmy b on 22/10/2017 16:50:28

[Speedy Builder5Participant @speedybuilder5]

Machinery's SCREW THREAD BOOK 19th Edition ?

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

Posted by thomas oliver 2 on 22/10/2017 20:20:37:

http://www.watchman.dsl.pipex.com/thread,html

No access to website? Go to this here —> **LINK**

[jimmy bParticipant @jimmyb]

This sounds like just the book I need.
Machinery’s Screw Thread Book

Edited By jimmy b on 23/10/2017 04:45:03

[john carruthersParticipant @johncarruthers46255]

.>>.. small, well-intentioned threads.

What a delightful concept !! <<

and very polite.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 22/10/2017 19:14:28:

Posted by SillyOldDuffer on 22/10/2017 18:12:25:

… Remind me again why there are 14lbs in a Stone?

…

7pounds being the accepted weight of a typical sheep clip.

Shear genius

MichaelG.

Shear genius? Absolutely NOT!

Baaah humbug.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 23/10/2017 09:22:17:

Baaah humbug.

.

… Heston Blumenthal's version of Lamb with Mint Sauce

MichaelG.

[Peter LayfieldParticipant @peterlayfield]

I thought when we left the E.U. we were going to revert back to the imperial system of weights and measurement

that is what Boris said

[Martin KyteParticipant @martinkyte99762]

Every time this question raises it's head it splits into three.

Could I suggest that the headings are.

Imperial and Metric measurement systems.

Thread forms and series. (Shapes, sizes and pitch)

Decimal and fractional representations of numbers.

regards Martin

[LambtonParticipant @lambton]

May I draw attention to the article in MEW 256 pages 46 & 47 "Lehman Archer and BA Taps" which explains the origin of BA threads.

I have been a model ( and full size ) engineer for over 50 years and in the whole of that time I have screw cut of all types of threads except BA. I have never seen the need to even attempt to do so as I have always used a good quality tailstock die holder together with British made HSS dies with every satisfaction.

Eric

[not done it yetParticipant @notdoneityet]

I was wondering how many would lathe cut threads on anything less than about 5mm diameter? Is cutting with a die, securely held by the tailstock, any less accurate than likely needing a travelling steady? I've seen longer, small diameter threaded rod made using a travelling guide before but I've never even considered it.

[jimmy bParticipant @jimmyb]

A screwcut thread will be perfectly true to the other dias.

I’ve screwcut external threads down to M3 and internal down to M5, at work.

The actual parts I need to make will only have two threads, so as long as the shoulder is square, a die will be fine.

Like I said 3 pages ago, just curious about the BA thread in general.

Jim

[Andrew TinsleyParticipant @andrewtinsley63637]

Yes another thread sinking into the sands!

I really don't care what standards people choose to adopt, or not, as the case maybe. I do get a bit fed up with the chopping and changes that go on. I learnt Imperial measures. Then as a scientist I had to learn cgs (remember them?). Then it became MKS units and now it is SI Units. So that is 4 different systems in my lifetime.

I have tooling in both metric and Imperial so I really don't care. Each system has its good and bad points. I truly get fed up with one group knocking the others measurement units!

No one is asking anyone to use units that they don't like! I hope Jimmy B now understands the basis of the BA system . Can we just leave it at that and stop this pointless bickering?

Andrew.

[Tim StevensParticipant @timstevens64731]

Further to the comment about German Bottle-closure threads, you may be surprised to note that within this series are threads specifically for Honey Jars Only.

The reason that BA threads are metric but not as you know it, is that they were formalised before many other countries produced their own standards. The same reason answers the question 'why do BA threads not have 60 degree flanks?' The BA system was designed on rational principles, which were not followed by others.

Cheers, tim

[Anonymous]

What's the rationale for a 47½° flank angle?

Andrew

[Russell EberhardtParticipant @russelleberhardt48058]

The BA screw thread is not Imperial. It is based on a Swiss metric system. All is explained by these two papers published by the British Association for the Advancement of Sciencein the 19th century:

**LINK**

**LINK**

Russell

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 23/10/2017 10:43:56:

I was wondering how many would lathe cut threads on anything less than about 5mm diameter? Is cutting with a die, securely held by the tailstock, any less accurate than likely needing a travelling steady? I've seen longer, small diameter threaded rod made using a travelling guide before but I've never even considered it.

.

One aspect not [so far as I am aware] yet mentioned is the cost

It's rather like the pros & cons of shaper vs vertical mill for cutting dovetails.

Screwcutting uses a single-point tool, but decent quality small dies [say, less than 1.5mm] are 'not inexpensive'

MichaelG.

[Author]

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