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BSF vs BSB threads

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Matt Stevens 122/01/2021 01:12:46
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Is there a difference between British Standard Fine (BSF) and British Standard Brass (BSB)? They are both 55 degree threads.

More specifically i am wondering if a 1/4" x26 BSF is the same as a 1/4"x26 BSB.

Not sure if there is any difference in the thread form...

Paul Kemp22/01/2021 01:31:01
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I would say they are the same without getting the books out but I am sure someone will be along to disagree lol.

Paul.

Edited By Paul Kemp on 22/01/2021 01:31:52

Hopper22/01/2021 02:14:33
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It;s the same. Same thread profile angle and same TPI and same diameter. Same same.

The difference comes in across the range. BSB uses 26TPI across all diameters. You have lucked out and picked the one BSF diameter that uses 26tpi also.

To add to the fun, British Cycle Thread aka Cycle Engineers Institute thread also uses 26tpi across many diameters, but uses a 55 degree thread.

Edited By Hopper on 22/01/2021 02:15:30

Michael Gilligan22/01/2021 04:48:26
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Posted by Hopper on 22/01/2021 02:14:33:

It;s the same. Same thread profile angle and same TPI and same diameter. Same same.

The difference comes in across the range. BSB uses 26TPI across all diameters. You have lucked out and picked the one BSF diameter that uses 26tpi also.

To add to the fun, British Cycle Thread aka Cycle Engineers Institute thread also uses 26tpi across many diameters, but uses a 55 degree thread.

Edited By Hopper on 22/01/2021 02:15:30

.

Oops !

You were explaining that nicely, Hopper ... until you wrote : “but uses a 55 degree thread”

MichaelG.

.

https://britishfasteners.com/threads-bsc

https://www.nwmes.org.uk/wp-content/uploads/2020/02/British-Standard-Cycle-Thread-Chart.pdf

Edited By Michael Gilligan on 22/01/2021 04:54:47

Hopper22/01/2021 04:54:20
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Yes 60deg for cycle thread. Obvioiusly a typo on my part.

Michael Gilligan22/01/2021 04:55:18
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yes

Clive Brown 122/01/2021 09:40:11
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An explanation for Brass Thread that I've heard, but never really seen confirmed, is that the 26 tpi came about from hand-turning of small brass fittings. The turner only had to repeat the same tool movement for any thread diameter.

Possibly a better explanation is that lathes needed only one change-gear set-up.

SillyOldDuffer22/01/2021 10:01:06
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As an amusing aside, there isn't a British Standard covering 'British Standard Brass'. Technically there is no such thing as BSB, though everyone calls it that, ho ho.

26 tpi Whitworth was widely used in the UK on brass tube because 26tpi BSW is well suited to thin walled pipe. It's a trade application of an existing thread, and was originally called 'British Brass Thread'.

I guess chaps in the past assumed brass thread was as formally defined as BSW & BSF, and promoted it to 'British Standard Brass' by common usage. And we still refer to BA despite this genuinely being a British Standard thread.

Historic engineering terminology is a bit of a muddle!

Dave

Hopper22/01/2021 10:05:40
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And I think they probably used a set TPI for the same reason as bicycle manufacturers: you only needed one set of thread rollers to make fittings in all diameters. Cost saving.

Neil Wyatt22/01/2021 10:14:20
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Some of the thread series use the same forms but different sequences of pitch and diameter, which can create overlaps at some sizes but not others.

The result of retrofitting standards to past practice rather than starting from scratch.

Neil

Tim Stevens22/01/2021 14:25:43
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Machinery's Screw Thread Book of 1972 states:

Whatever the diameter, it consists of 26 TPI of Whitworth Form.

The dimension tables they offer show the Minor Diameter of the BSF version as 0.2008", and for Brass it is 0.2007". I wouldn't worry about the odd tenth of a thou, even if I could measure it.

Cheers, Tim

Georgineer22/01/2021 16:43:18
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Posted by Clive Brown 1 on 22/01/2021 09:40:11:

An explanation for Brass Thread that I've heard, but never really seen confirmed, is that the 26 tpi came about from hand-turning of small brass fittings. The turner only had to repeat the same tool movement for any thread diameter...

This is what my father told me. He started his turning and fitting apprenticeship in Portsmouth Dockyard in 1926, in the days when hand turning was a lot more common than it is now, and brass was often turned freehand like wood. Filing in the lathe was also both common and accepted, unlike now. The male and female threads were fitted to each other, so parts weren't interchangeable.

My guess is that the brass turners' lathes didn't have leadscrews . Does anybody know? Another advantage is that they would only need one pair of chasers so no effort was needed to decide which was the right thread for the job. I've never tried doing it, but I believe it's quite easy to start and pick up a thread using only a hand chaser.

I can't give the reference off the top of my head, but one of my early 20th century metalworking books gives details of turning steel by hand, with very long handled tools. I'll see if I can find it.

George B.

Martin Kyte22/01/2021 18:00:44
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Posted by Georgineer on 22/01/2021 16:43:18:

I can't give the reference off the top of my head, but one of my early 20th century metalworking books gives details of turning steel by hand, with very long handled tools. I'll see if I can find it.

George B.

I often turn both Brass and Steel by hand using short handled turning tools with no top rake. My hand turning rest allows for the tool to be supported very close to the work and sets the tool at centre height. It's surprisingly easy to do although it's more like a scaping action than what you would think of as turning.

regards Martin

Matt Stevens 122/01/2021 18:26:18
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Slightly different topic....but i have always wondered why standard dies have lead in on both sides. It would be much more ideal to have lead in on one side only then you can flip the die around to tap right up to shoulders rather than create a relief area..... particularly relevant on small parts/threads

Mike Poole22/01/2021 18:55:11
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Posted by Matt Stevens 1 on 22/01/2021 18:26:18:

Slightly different topic....but i have always wondered why standard dies have lead in on both sides. It would be much more ideal to have lead in on one side only then you can flip the die around to tap right up to shoulders rather than create a relief area..... particularly relevant on small parts/threads

Most dies I have encountered have the lead on one side and traditionally the lettered side has the lead, if threading to a shoulder I turn the die round as suggested.

Mike

duncan webster22/01/2021 21:29:20
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Even tho' the pitch is constant the helix angle will change with diameter, so do you really get away with just one set of thread rollers? Not that I'm any expert of thread rolling

DC31k22/01/2021 22:00:01
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Posted by duncan webster on 22/01/2021 21:29:20:

Even tho' the pitch is constant the helix angle will change with diameter, so do you really get away with just one set of thread rollers?

That is a very good question.

I went looking here:

https://www.threadtools.com/technical-information

and found

https://www.threadtools.com/uploads/pdf/technical-specs/alco-r-type-dimensions-capacities.pdf

If you look at the second page of the document, for BSP threads, the closest to a constant pitch easily to hand, the rolls only cover a limited range, so I think your observation on helix angle is well made.

Special mention of the technical info. page above as there is a lot of stuff there on Coventry die heads, particularly the sharpening thereof.

Hopper23/01/2021 01:47:19
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Posted by duncan webster on 22/01/2021 21:29:20:

Even tho' the pitch is constant the helix angle will change with diameter, so do you really get away with just one set of thread rollers? Not that I'm any expert of thread rolling

Me neither. It's an interesting thought. Most thread rollers though are much larger diameter than the thread they are rolling, but have the same TPI, so must have a helix angle much less than the finished job. I suppose they could roll up to the same diameter as the roller and then after that the helix angle would become a problem and bind on the flanks?

 

 

Edited By Hopper on 23/01/2021 01:54:06

Hopper23/01/2021 08:48:22
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PS Looking at the Landis catalog for capstan lathe (remember them?) roller heads, it looks like one set of thread rollers can be used on a range of diameters, but it involves a changeable axle plate arrangement that looks like it changes the skew of the roller axles to change resulting helix angle as the diameter setting is changed.

So in ancient Cycle Engineers Institute times, it is possible they used one set of 26tpi rollers for all diameters but held them in holders that varied the axle alignment to achieve the right helix angle. A holder for rolling 1/4" thread would have the roller axles at a different angle from a holder for a 3/8" thread for instance.

Rollers were a high wear item in production conditions so this standardisation would have made it simpler for semi-skilled toolsetters or capstan operators to simply dig into a bin of standard rollers and replace the worn ones with no need to look up complex codes to get the right roller for the right tpi and diameter.

CEI did use 20tpi as well for larger and coarser threads, which rollers could be distinguished at a glance as much coarser. I suppose they could have made the axle hole larger or the rollers longer on these so they were not readily interchangeable with 26tpi rollers too.

Pure conjecture of course, but its one way using standard 26tpi rollers across multiple diameters could have been done.

Tim Stevens23/01/2021 09:56:55
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In my experience of using roller dies to put a thread on spokes, the rollers are fairly loose on their spindles, and so can swivel a bit to accommodate the minor differences in helix angle for different diameters.

The thread-rolling machines used for bolts, though, do not rely on circular rollers, but flat plates between which the steel rod is rolled. Changing the angle of a plate is easier than moving an axle - especially when it comes to measuring the exact angle.

Cheers, Tim

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