# metric, imperial, whitworth - could You explain?



## Debian (Dec 20, 2008)

I'm searching to understand the differences between those systems, for sure we use metric but I still have not so clear what do You use in England, what in Usa or Australia.
I refer to threading system. Which is the difference between imperial and whitworth?
I know in metric we call a M4 a screw with nominal diameter 4mm and pitch 0,7mm (big pitch, most used) or we call a M4x0.5 a fine pitch screw with a nominal diameter of 4mm and a pitch of 0,5mm.
Now I read on wikipedia that, in USA, is used a system with couple of number, such 4-40, 6-32,... that belong to a logaritmic scale. So, if I have to convert a 6-32 screw in imperial i need to use the formula: dia = (#N x .013") + .060" and, after about 5 minutes, I know a 6-32 it's about a 3,5mm nom.diam. with 32 pitch per inch. Is this system called the imperial or whitworth?
Boh... I have a really big confusion in my head. I hope someone here could explain to me where is the secret, and what You usually use on your plan! 
All the best, and a good sunday to everyone,

Paolo


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## shred (Dec 20, 2008)

Whitworth to my knowledge is almost entirely an old British thread form. Never took off in the US. We use the "numbered sizes" of #000,00,0,1,2,3,4,5,6,8,10,12 threads but the trick is most all of us just look them up in a book or table when we need to know what they actually measure. It can be a nuisance at times


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## Marinesteam (Dec 21, 2008)

I don't have my reference materials at home with me so I'm trying this from memory. Someone feel free to correct me if I don't get all the details right.

Whitworth is an imperial (inch based) measurement thread using a 55 degree thread form.

Unified Thread Standard is an inch based measurement thread using a 60 degree thread form. Pitch is measured in threads per inch. Used in the US.

ISO metric standard is also a 60 degree thread form, but using nominal diameters in millimeters and pitch is measured directly from peak to the adjacent peak (or valley to valley)


Hope this helps

Ken


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## tmuir (Dec 21, 2008)

I use a mixture of BSW, BSF, ISO and BA threads depending on what I'm doing.
My personal preference is the metric ISO but if I'm fixing something I'm stuck with what was used in the first place.

Not sure if this will be of any use to you but here are a couple of charts I made up for ISO threads and BSW threads.



View attachment BSW Chart.xls


View attachment ISO tapping Chart.xls


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## Bluechip (Dec 21, 2008)

Hi Debian

Confuse yourself even more  

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

Even this is not complete, I know of some more !!

dave


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## Loose nut (Dec 21, 2008)

The simplest way to look at at it, is that those people that are still using the Imperial systems are freedom loving people who are resisting a international conspiracy of metric world domination, next thing you'll know they will want everyone to give up their guns. Is there no end to the insanity.

'gunna hear it now. :big:


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## Kludge (Dec 21, 2008)

We can blame our esteemed British cousins for all the thread names starting with B, ME and Whitworth while we in the US expanded on the number sized screws with thread counts that go past the accepted UNC and UNF ones as well as course and fine fractional sizes. Metric (now called ISO) was designed by committee and has its own variations away from the "standards". Even pipe threads aren't safe since there are both tapered and straight ones.

What all this adds up to is a uniform lack of uniformity which makes life ever so interesting but allows us some freedom as to what systems to use - or mix together - on our various projects.

What it comes down to is to use whatever system you're comfortable with. The rest can follow along or convert as is their preference.

Best regards,

Kludge


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## Mainer (Dec 21, 2008)

I'll attempt to add to the confusion.... ;D

Before the British Empire crumbled, the Brits used the Whitworth thread form, 55 degree included angle with rounded crest and root, I believe for threads 1/4" dia. and larger. The threads are given as threads per inch (1 divided by the pitch). Metric, on the other hand, specifies threads by the pitch (0.5 mm pitch, or whatever).

For smaller, down to "really, really tiny," the Brits used BA, which has a 49 degree included angle and is a geometric progression of numbered sizes from 0BA (about 3/16" dia.) down to at least 16BA, I think, which is starting to get watchmaker-size.

In the US threads are also specified as threads per inch, not by the pitch. Included angle for all threads (or at least for the most widely-used thread systems) is 60 degrees, same as metric, with a flattened crest. For 1/4" and larger, a thread would be given as diameter and tpi, for example 1/4-20 is 1/4" dia. and 20 threads per inch.

For under 1/4", the number-size screws are used. Formerly the range of sizes was more extensive, up to at least #20 (which overlapped with the fractional sizes), but now the number sizes typically go from #12 (about 0.216" dia., if memory serves) down to #0000 or other insanely small sizes. These are spec'd by the number and tpi, for example #10-32, a #10 screw with 32 threads per inch. You can calculate the diameter by that formula you looked up, or as somebody else said, look up the values in a table if you need to know.

There are a coarse thread series, a fine thread series, an extra-fine series, and specials. 

All this stuff is explained in overwhelming detail in Machinery's Handbook.


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## mklotz (Dec 21, 2008)

Mainer,

I think that BA threads had a 47.5 degree angle, not 49.

From my shopnotes, I have the following:

Full series runs from 0 to 22.
Thread angle = 47.5 deg.
Crest and root radii = 0.18p (p=pitch)
Pitches are calculated as p(mm) = 0.9^n (n=number of thread in BA series)
Diameters are calculated as D(mm) = 6p^1.2


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## ronm (Dec 21, 2008)

Mainer  said:
			
		

> All this stuff is explained in overwhelming detail in Machinery's Handbook.



Exactly-I'm surprised this resource isn't mentioned more on here. Nobody who is or aspires to be a machinist should be without one-talk about information overload...266 pages on threads...


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## mklotz (Dec 21, 2008)

A new MH is a bit pricey but I've picked up older ones at library used book sales for prices ranging from $1 to $5. 

The older ones are just fine for amateurs like us working with machinery that is outmoded by industrial standards. All the stuff we need was in the edition that Noah carried on the ark.


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## Mainer (Dec 21, 2008)

You're right on the BA angle. My memory ain't what it was...if it ever was....

Also on the older editions of MH being just as good, if not better, for most of us. I figure I'm working at about a 1953-technology level in my basement, except for my DRO, so a 1950s edition of MH is just fine.


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## Maryak (Dec 21, 2008)

ronm  said:
			
		

> Nobody who is or aspires to be a machinist should be without one



Some of us aspire to going to the library to keep our pensions intact 
	

	
	
		
		

		
			





Best Regards
Bob


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## shred (Dec 21, 2008)

2nd the Machinery Handbook, I have two (both under $10 used, one amusingly marked down many, many times since the used book store saw the latest edition was $169.95 and wondered why this one didn't sell), but sometimes digging the info out can be tedious. Is it commonly available outside the US? I seem to remember Bogs had a much smaller book of key information.

Here's a site I just ran across looking for another site on thread identification. It's a nice cross-reference between the sizes and answers 'what thread izzat?' fairly well

http://www.metricmcc.com/catalog/Ch10/10-1012.pdf

Check out the rest of the pages in that chapter as well.


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## artrans (Dec 21, 2008)

I am glad thats resolved :big: :big: :big: :big:


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## Debian (Dec 21, 2008)

Well, in Italy we'd say: "se non altro ci avete provato" that should sounds like "at least, You tried".
I'll use my Christmas holiday to to take a careful look at all Your kind answer, right now I feel a little bit more confused than yesterday, but, for sure, with much more informations and references than yesterday!!!
A fact is assured, when I'll finally start my first project on my very first set of plans... I'll refer to all of You to ask help on translating plan's screw informations!
Thanks to all and good night, 

Paolo

p.s.: I'd dream to take a plane on 16 jan to come to visit the cabin fever at York, Pennsylvania, but maybe it would be too expensive. Could You please indicate to me another great model engine show nearest to me? maybe in england?


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## Kludge (Dec 21, 2008)

A digital copy of the 5th edition (1915) is available on google books at http://books.google.com/books?id=VkEYAAAAYAAJ&pg=RA1-PA1220&dq=machinery+handbook+date:0-1920&lr=&num=100&as_brr=1&as_pt=ALLTYPES&ei=995OSa-WJ4_GlQTZoK3vBg#PPA1913,M1 

I have a bookmark for google books for the time period of 0 to 1920 that has provided me with a great number of cool texts. All I do is go into advanced search and add key words (or authors) as needed. I also love searching pre-1920 patents since there's a wealth of ideas there that never quite made prime time.

I haven't looked at Gutenberg and a few other freebie sites for a while but they've come up with some interesting texts as well.

Best regards,

Kludge


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## ronm (Dec 21, 2008)

I have the 24th edition, it has some of that "CNC" junk that I'll never use, this ol' dog's too old for that trick, but my youngest kid is into it...I got it from MSC on a year-end special, seems like it was only about $30 back in the 90's...I undertook to read it from cover to cover, don't think I made it... ;D Makes War & Peace look like a comic book...


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## Kermit (Dec 24, 2008)

Here is one thousand words worth







From pre-1920's,
Kermit


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## bentprop (Dec 27, 2008)

Fwiw,the english BA thread,so beloved of "model engineers",was often used on electrical items.
I seem to remember reading somewhere that BA is actually based on an old metric system.
Then there's also the ME(model engineering) thread,all in 32 or 40 tpi.
I tend to use 26tpi for a lot of fittings,which however is not the done thing,apparently.But when you're making something for your own use,you can take any thread you want.Then 80 years later,some model machinist will pick up your item at a flea market,and the threads will confuse the %$$% out of him.Priceless! :big:
Whitworth,or BSW,was used a lot in older british cars.But just to make life interesting,they could have UNF on the same car as well,plus BA on small fittings.
Be glad you're not in the rag trade.They used to have a measurement called the "El".This was the distance from the shoulder to the wrist.Tough luck if you got a scrawny little runt of a salesman ;D


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## S.D.L. (Dec 27, 2008)

Debian  said:
			
		

> p.s.: I'd dream to take a plane on 16 jan to come to visit the cabin fever at York, Pennsylvania, but maybe it would be too expensive. Could You please indicate to me another great model engine show nearest to me? maybe in england?



One coming up in January, see here

http://www.meridienneexhibitions.co.uk/our-events-detail.php?id=0000000002

One of the best located north London, The best is in April or May at Harrogate 

Here

http://www.theeventsoffice.co.uk/events.shtml

Steve Larner


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## deverett (Dec 28, 2008)

In the Downloads section, I have put a thread equivalent file which can be used to compare British, American and Metric threads.

I hope it will help at least one of the members.

Dave
The Emerald Isle


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## scoop (Dec 28, 2008)

Bentprop admits to using 26 t.p.i threads now and again.This is probably the old British brass thread which uses 26 t.p.i for all diameters of thread.Anybody else out there still using this thread system on a regular basis?


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## mklotz (Dec 28, 2008)

steamdave  said:
			
		

> In the Downloads section, I have put a thread equivalent file which can be used to compare British, American and Metric threads.
> 
> I hope it will help at least one of the members.



Dave,

Very nicely done. I've saved a copy.

Two minor nits...

You have the tap drill diameter units listed as "(thou)" although the value given is really inches.

It would be helpful to have the pitch of the individual BA sizes listed as you've done with BSW, BSF, etc.. - 0.9^N mm IIRC.


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## Maryak (Dec 28, 2008)

scoop  said:
			
		

> Bentprop admits to using 26 t.p.i threads now and again.This is probably the old British brass thread which uses 26 t.p.i for all diameters of thread.Anybody else out there still using this thread system on a regular basis?



I think there's a 26 tpi thread system known as British Cycle or maybe Bicycle threads which can still be found as a boxed set. Maybe our UK colleagues can advise.

Best Regards
Bob


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## deverett (Dec 28, 2008)

scoop  said:
			
		

> Bentprop admits to using 26 t.p.i threads now and again.This is probably the old British brass thread which uses 26 t.p.i for all diameters of thread.Anybody else out there still using this thread system on a regular basis?



Yes, I admit to using it and other constant pitch threads on a regular basis.

The old British Model Engineer thread series used a constant pitch; the pitch depending upon the diameter. 

The series consisted of 26 tpi, 32 tpi, 40 tpi and (rarely) 60 tpi.

The 26 tpi series is the same as the BSB (British Standard Brass) threads, and go from 1/8" up to 5/8".
32 tpi is partly BSW (British Standard Whitworth) and partly BSF (British Standard Fine). and goes from 1/8" up to 1/2".
The 40 tpi series is, again, partly BSW and BSF and normally goes from 1/8" up to 1/4".

These threads were developed pre WW2 when model engineering in the UK was geared towards 2-1/2" gauge and below railways where the normal range of commercial threads were deemed unsuitable.

BA threads are a different series, and were commonly used in the electrical industry.


Marv. The chart was drawn up just so that I could have a quick comparison between the various threads specified on drawings, particularly in my case, US to UK as most of my threading tackle is the old Imperials stuff, although I am widening my base reluctantly - cost, my boy.

Dave
The Emerald Isle


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## d-m (Dec 30, 2008)

Vary intresting to run across this thread, yesterday as I was working on putting the new front brake hoses in my 62 Austin Mini I could not find the nuts to hold them to the sub frame. With out thinking off I went to the parts store for some nuts. (thinking that it's all us fine thread LOL and not taking the time to count the thread) It turns out to be 26tpi ? Luckily Mini Spairs in the UK has them. Pic explanes why I haven't been posting in a wile lol it is keeping me vary busy and the wife wants it done by spring.
Dave


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## baldrocker (Dec 30, 2008)

Wots AF then
BR


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## Kermit (Dec 30, 2008)

http://www.sizes.com/tools/thread_whitworth.htm


neat website that one, 
Kermit


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## Maryak (Dec 30, 2008)

I think 
	

	
	
		
		

		
		
	


	




 AF is/was American Fine, similar to UNF. Also the term was often used to refer to OE spanners eg 9/16" AF (across the flats).

Oh what a tangled web we weave  

Best Regards
Bob


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## baldrocker (Dec 30, 2008)

> Also the term was often used to refer to OE spanners


Maryak
You mean those that fit sizes unknown to man. :
BR


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## Maryak (Dec 30, 2008)

baldrocker  said:
			
		

> Maryak
> You mean those that fit sizes unknown to man. :
> BR



Yep, them's the one maybe AF should be Alien Fit :

Best Regards
Bob


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