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How do I read this gauge

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Wolfie23/11/2011 19:06:31
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502 forum posts
I was looking for a thread gauge so I could determine BA threads etc and I picked one up.
 
However I don't know what the sizes mean.
 
Each separate thread gauge has on it something like this
 
18 G 1/2 5/8"
 
I assume the 5/8" means inches but what about the rest. Some of them don't have the middle fraction at all and some of them go into whole numbers eg 2 1/2. One of them doesn't have the inches either, simply 13G.
 
What does it all mean?

Edited By Wolfie on 23/11/2011 19:08:05

Peter G. Shaw23/11/2011 19:55:00
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Off top of my head, I suspect that you have got one for the BSW/Whitworth range of threads.
 
Like you, I wanted to be able to determine what particular threads were so I obtained three sets of thread gauges, one each for Metric, BA and BSW/Whitworth and mine do have lots of strange numbers & letters on the BSW/Whitworth ones.
 
As far as I can tell, the first number is the TPI, eg 18 tpi. The "G" I don't know even though mine has it. However, my 18G gauge is stamped 5/16 which is correct for BSW.
 
And yes, I also have one labelled simply 13 G.
 
Be interesting to find out!
 
Regards,
 
Peter G. Shaw
 

Clive Farrar23/11/2011 19:55:36
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With out seeing an image of the gauge it is difficult to be sure.
 
But if it is circular , like a die, then I would suspect it relates to the class of fit or accuracy of the thread. if it is that type it is way more accurate than you are likely to need in our field of work.
 
Regards Clive
Clive Farrar23/11/2011 19:57:04
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If it is just a saw tooth pitch gauge then I don't know either.
 
Regards Clive
Wolfie23/11/2011 20:23:09
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502 forum posts
OK I think I have deciphered the second fraction.
 
Looking at the table here http://en.wikipedia.org/wiki/BSW
 
Some of the TPI figures have more than one core measurement eg 11G is 5/8 or 11/16
 
The gauge for 11 reads
 
11G 5/8 11/16"
 
So presumably it simply gives you the tpi count and notes that the core diameter has two possibilities.
 
Interestingly there is no 13 in that table

Edited By Wolfie on 23/11/2011 20:24:04

John Coates23/11/2011 20:29:22
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20G 1/4
 
means 20 tpi for a standard Whitworth 55 degree 1/4" thread. Never worked out what the G was for though
chris stephens23/11/2011 20:31:58
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Hi Guys,
I suspect that if you have 13 tpi on your gauge you are looking at a UNC one, not a BSW one.
chriStephens
Wolfie24/11/2011 00:11:23
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It has Whitworth 55* stamped on it
Anthony Knights24/11/2011 00:42:07
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260 photos
Looking at my Whiworth thread table, I see that both 5/8" and 11/16" have 11 TPI.
Anthony Knights24/11/2011 00:43:58
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260 photos
Please excuse dyslexic keyboard. Should have typed WHITWORTH
chris stephens24/11/2011 01:07:33
1049 forum posts
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Hi Wolfie,
In that case they are hedging their bets. Over a certain range UNC and BSW are the same pitch/diameter, although the angle is different. I suspect that they gave you a 13 to cover both.
chriStephens
Anthony Knights24/11/2011 01:10:57
681 forum posts
260 photos

This is what I know as a thread gauge. Pehaps its really a TPI finder outerer.
Terryd24/11/2011 03:31:12
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1946 forum posts
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Hi Wolfie,
 
If it is stamped 'Whitworth' it is an imperial gauge with a Whitworth thread form of 55 degree thread angle and it is measured in 'teeth' per inch (tpi). Each thread form (Whit, BA, UN, ISO metric etc) is different and really needs a different gauge. That sort of problem is one of the reasons I have plumped for metric when I had to re-equip my workshop.
 
No matter what the figures mean you cannot use that gauge to read BA threads you need a BA gauge. The BA series is a metric base thread with a 47.5 degree thread angle and the largest is 0BA at 6mm diameter with a 1mm pitch (note - not tpi). to find the pitch of the next size down multiply the pitch by 1.11r. E.g the pitch of 0BA is 1mm and the pitch of 1BA is 1.11mm, 2BA pitch = 1.111x1.11 = 1.234, etc down to 22BA (the smallest size defined - but rarely , if ever used). Modellers like it because of the head size but metric bolts are available with reduced head size from standard which don't look too bad.
 
Regards
 
Terry
 
Versaboss24/11/2011 12:02:10
512 forum posts
77 photos

Ok, to the rescue!

I think it should be clear now that the gauge(s) are for imperial threads.

Now to that mystic 'G'. G means Gang (or Gänge in plural), not in English but in German! Gang is the same as your TPI. So 13G means 13 turns (Gänge), same as 13 TPI.

Maybe the gauge was made in Germany.

And as far as I know the Imperial threads, it is not usual that the same TPI is used for different diameters (or fine/coarse). So that's where sometimes you see 2 diameters. 13 TPI is not used in the Whit. system then it seems? Too lazy to go to the workshop and check my gauge...

Greetings, Hansrudolf

chris stephens24/11/2011 12:50:20
1049 forum posts
1 photos
Hi Terry,
"to find the pitch of the next size down multiply the pitch by 1.11r. E.g the pitch of 0BA is 1mm and the pitch of 1BA is 1.11mm, 2BA pitch = 1.111x1.11 = 1.234, etc down to 22BA"
Foolish me had always thought that the pitch of BA threads gets smaller as the diameter reduces and the number increases. Are you sure you are correct by saying that it increases? No wait, I was not wrong, each subsequent thread is 90% of the next larger. 1mm(0BA), 0.90mm(1BA), 0.81mm(2BA) etc.
 
I guess it goes to show that you have to be senior, to have a senior moment.
chriStephens
Ian S C24/11/2011 13:33:46
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Hansrudolf, you could well be correct, a few years back I did some work on a 1934 Lanz Bulldog tractor, it had Whitworth threads, a few years later some parts were required for a newer tractor(1937/8), and I found that all the threads were metric, luckily I went and looked at the tractor before starting work on it, or I would have just made the bits as I had with the older one (someone blamed a bloke called Adolf). Apparently Germany up until the mid 30s used imperial threads, because the French used the metric. Ian S C
Terryd24/11/2011 16:37:47
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1946 forum posts
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Posted by chris stephens on 24/11/2011 12:50:20:
Hi Terry,
"to find the pitch of the next size down multiply the pitch by 1.11r. E.g the pitch of 0BA is 1mm and the pitch of 1BA is 1.11mm, 2BA pitch = 1.111x1.11 = 1.234, etc down to 22BA"
Foolish me had always thought that the pitch of BA threads gets smaller as the diameter reduces and the number increases. Are you sure you are correct by saying that it increases? No wait, I was not wrong, each subsequent thread is 90% of the next larger. 1mm(0BA), 0.90mm(1BA), 0.81mm(2BA) etc.
 
I guess it goes to show that you have to be senior, to have a senior moment.
chriStephens
 
Hi Chris,
 
I don't have senior moments any longer, Just alzheimer episodes . They soon pass though. thanks for pointing out my error, it's a very long time since I was at Tech.   I really, really must get out more ,

Best regards

Terry

Edited By Terryd on 24/11/2011 16:40:19

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