Perciuliar American threads trivia question

Hi,
What do the numbers in those peculiar American threads like 2-56, 4-40 and 6-32 mean ?
Thanks,

--
Boo

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Boo wrote:

Diameter (wire gauge IIRC, same as the number drills) and threads per inch, so a 2-56 screw has a number two diameter(.060+.013+.013=.086) with 56 threads per inch. There are both coarse and fine series in the number series of threads
The sizes are based on a progression from 0 (zero) upwards in thirteen thou steps from a .060 base size. IIRC there are are sizes going smaller on the same progression, 00,000, 0000, etc.
Not commonly used in sizes beyond 1/4 inch diameter.
Sorta like BA screws, over here, the sizes are never going to go away, because of the amount of legacy equipment.
The number size screws also fill in very well at the small end of the sizes where you would start getting into awkward fractions in order to have a reasonable progression of sizes.
For what it's worth, the pronunciation of 6-32 is six thirty two, rather than six thirtyseconds , etc. for the rest of the series.
That's all from memory, so it may be off a tiny bit in the numbers, but the gist of it is as I know it.
Wanna explain BA in trade?
Cheers Trevor Jones
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You are pretty much spot on! They are known as "machine screws".
Steve R.
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To be pedantic that's not strictly correct, going by Machinery's Handbook, as socket cap screws with those threads are classified as being "Cap Screws" (which are a different category to "Machine screws"). To add to the confusion, "Cap screws" include some types of slotted countersunk and round head screws and grub screws are called set screws!
Bob
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Well, where I live cap screws are made from stronger stuff than ordinary screws. The type of head has no bearing on it whatever. As a general rule, machine screws have fine threads, rather than the NC series. Then, just to confuse the matter further, we have the NS (National Special) series. If you check in Machinery's, you will find that some are listed as National fine, and NS, for the same screw size! One interesting point is that between the number sizes, and NS almost all of the diameters and pitches used in the Model Engineer threads are covered. As an example, I have some 1/4-40 NS taps and dies, which are correct for model spark plugs. British 1/4-40 ME boiler fittings will often fit in bushings threaded with the NS taps! If not, a quick run over the ME threads with the NS die settles the matter. Yes, we can get them in 32, and 40 TPI. The one good thing that came out of the Unified system, is the slightly rounded root, and peak on the threads. This removed the stress risers of the older sharp V standard for American threads. When I was growing up in Victoria, B.C., in the 40s and 50s, British, and American threads were encountered in roughly equal quantities.
Steve R. (Doddering 66 year old)
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    --Lucid description; thanks from one who uses 'em all the time! :-)     An addendum: the 4-40 thread is considered by many as an abominable choice and should be avoided. There's something about the shank diameter/thread combination that makes it prone to failure. 5-40 is a much better choice but it's uncommon to find it in a hardware store; go figure..     Another addendum: the hole depth to recess a socket head cap screw is equivalent to the clearance drill diameter, so you can use one as a spacer to set depth.
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In the US, the 6-32 thread is also very common but is also pretty weak because the pitch is fairly coarse for the diameter. Lots of 6-32 taps get broken. The 5-40 thread can rarely be found in loose hardware but for many years has been standard for captive terminal screws on electrical outlets. The most common in the US are 6-32, 8-32, and 10-24 with quite a few 2-56, 4-40, and 10-32. Others are much rarer.
Don Young
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On or around Thu, 1 Mar 2007 22:05:34 -0600, "Don Young"

it's used on (some) alloy bows for fixing the bowsight. coarse thread is an advantage in cast alloy.
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Not the full story but for all BA threads the pitch is 0.9mm to the power of the BA number eg 2 BA pitch = .81mm. I think the rest of the dimensions are derived by a formula too. Machinery's screw thread handbook tells all but my copy is not to hand at the moment.
Bob The thread angle is 47.5 degrees
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Thanks for the comprehensive answer, I'd often wondered what they were about.

Someone's thankfully beaten me to it :-). The wikipedia article at <http://en.wikipedia.org/wiki/British_Association_screw_threads says they are "largely obsolete", again thankfully.
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Boo
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Thankfully? They are excellent for model making, far better than metric fasteners; long may they go on.
David
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>Thanks for the comprehensive answer, I'd often wondered what they wer about.

Thankfully? They are excellent for model making, far better tha metric fasteners; long may they go on.
Why does model making need BA threads? The Americans use there numbere threads and Europe uses metric how come they manage?
Steve Larne
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Because most of them have coarse threads and look quite inappropriate; also the various members of these series are too far apart. The 10% steps in the BA series gives a very fine degree of choice.
Have you actually *used* BA threads? I ask because most US model engineers have no idea what they are, or at least never get to see them. Their loss.
David
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When I first started aeromodelling in the 70s BA threads is what was stocked by model shops and I used to use them for mounting engines etc. Personally I find the proliferation of extremely similar threads to be a complete nonsense. It's also extremely aggravating when you manage to fit (say) a 2-56 nut onto a bit of M2 studding and then find you've chewed it up as a result.
There should be a law making all designers fill in a request to a central agency every time they use a non-ISO metric thread. I mean each nut and screw sold, not each design. That'd soon stop the rot.
--
Boo

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On Sat, 03 Mar 2007 18:58:40 +0000, Boo

Which one? there are four different ISO metric thread pitches for some diameters.
Mark Rand RTFM
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True, I meant ISO metric coarse. And before I'm jumped on, I was not serious :-)
--
oo

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Why does model making need BA threads? The Americans use there numbered

Because most of them have coarse threads and look quite inappropriate; also the various members of these series are too far apart. The 10% steps in the BA series gives a very fine degree of choice.
Have you actually *used* BA threads? I ask because most US model engineers have no idea what they are, or at least never get to se them. Their loss.
David -- David Littlewood
Yes I have used BA.
If you are making a scale model why change the tread angle so much 60Deg is close to 55 than 47.5?
Using metric coarse as a comparison
0 - 10BA is 11 thread sizes with, OBA at 6.0OD x 1mmP 10BA at 1.7OD x 0.34mmP Pitch reduces in .9 steps.
M6-M1.6 is 8 threads with, M6 at 6mmOD x 1mm pitch, M1.6 at 1.6mmOD x0.35mmP Average pitch reduction 0.86 steps
Doesnt look that big a change and thats without using fine series.
Stev
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[snip]
I know there are two options for fine threads in some diameters, plus coarse makes three. What's the fourth?
Tim
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wrote:

There are three separate ISO fine threads for, at least, 12mm:- 1mm, 1.25mm, 1.5mm Plus the coarse thread at 1.75mm
Mark Rand RTFM
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wrote:

Actually the ASME threads offer reasonable equivalents to BA down to 0-80 and 11BA. Of course, If you are doing small stuff then there is no American equivalent below that and down to 23BA. But then everything in the states is bigger than over here!
A more appropriate answer to Steve Larner's question might be "Why did the Americans and Europeans need ASME or SI threads in the first place?". BA predates SI by 14 years and ASME by 23 years and has a better thread form then either.
Mark Rand RTFM
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