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12-21-2008, 01:07 AM   #1
Debian

Join Date: Aug 2008
Posts: 37
metric, imperial, whitworth - could You explain?

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

12-21-2008, 03:32 AM   #2
shred

Join Date: Jul 2007
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Re: metric, imperial, whitworth - could You explain?

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

12-21-2008, 05:05 AM   #3
Marinesteam

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Re: metric, imperial, whitworth - could You explain?

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

12-21-2008, 05:20 AM   #4
tmuir

Join Date: Oct 2007
Posts: 888
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Re: metric, imperial, whitworth - could You explain?

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.

Attached Files
 BSW Chart.xls (22.0 KB, 326 views) ISO tapping Chart.xls (28.0 KB, 255 views)

12-21-2008, 09:29 AM   #5
Bluechip

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Re: metric, imperial, whitworth - could You explain?

Hi Debian

Confuse yourself even more

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

dave

12-21-2008, 01:49 PM   #6
Loose nut

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Re: metric, imperial, whitworth - could You explain?

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.
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12-21-2008, 02:28 PM   #7
Kludge
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Re: metric, imperial, whitworth - could You explain?

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

12-21-2008, 05:51 PM   #8
Mainer
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Re: metric, imperial, whitworth - could You explain?

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

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.

12-21-2008, 06:13 PM   #9
mklotz

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Re: metric, imperial, whitworth - could You explain?

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.
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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12-21-2008, 06:30 PM   #10
ronm

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Re: metric, imperial, whitworth - could You explain?

Quote:
 Originally Posted by Mainer 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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