[broby]

Hi All,
I am in need for thread sizing for both Internal and External threads for a 2"-12TPI BSW (55deg) Medium Class fit thread.
I have an Australian Std's book that supposedly shows how to calculate the required min/max dims of the threads, but for the life of me, I am stuffed if I can make sense of this damn book!
I have in the past used similar books to calculate specs for both Metric and UN thread forms but this damn BSW crap is very confusing.
Does anyone have a set of calcs that I could use to get the sizes I need? (or even the actual answer???).
Thanks in advance.
Brian.:drowning:

[Kiwi]

....2"-12TPI BSW .......Brian.:drowning:

Is this 1/2" Diameter?

[broby]

Is this 1/2" Diameter?

Nope! NOT 1/2" It is a non-std pitch 2" (TWO Inch OD) 12 TPI thread.
I require the usual manufacturing information such as:
Medium Class Fit.
Major Diam (min/max)
Pitch Diam (min/max)
Minor Diam

For both Internal and External threads.

Thanks

[neilw20]

Page 1857-1859
Page offset in this file is 137.

[broby]

arrrghh! These calcs are driving me nucking futs!
What a crazy arse system!

[neilw20]

What did you expect. It is BRITISH subSTANDARD what was his name.

Anyway...
The standard specifies a 55° thread angle and a thread depth of 0.640327p and a radius of 0.137329p, where p is the pitch.

Those formulas include D for diameter, so as you have a non standard diameter, it will drive nucking futs.:argue:

You need to specify the length of engagement in the formulas.
What is the length of engagement?

And why did they use 55° ? Crazy 1850's stuff. BA (which is really metric) uses 47.5°

[broby]

(chair)LOL hmmm I so agree Neil!
There is quite a few formulae to work thru, then on the next page it states, oh look BTW there is also an allowance that you need to take into account, but hey! only if the thread is within a certain size range, then do this or do that!
Put your left hand in and shake it all about!
Talk about the hokey pokey!
I have a listing of threads (along with appropriate tolerances) for std sizes from 1/8"-40 thru to 6"-2.5 but damn, only a bunch of hocus pocus for calculating anything non standard! grrrrrr
In the mean time, my spreadsheet is looking like a dogs breakfast with all sorts of formulae!

[neilw20]

Just plug the rules into a spread sheet. Think once. Dumb mode next time, then you will never know how it works next week.:idea:

[broby]

Hmm, just exactly what I am trying to achieve!

My spreadsheet so far covers pretty all combinations of metric threads between M5 and M205, UN threads between 1/4" and 3".
I have used macros to quickly and easily select the required thread and either place the data on the clipboard, ready for pasting into other apps such as Solidworks or to create manufacturing data for "GO/NO GO" gauges.
Works a treat...
Now I am trying to get some formulae to calculate data for BSW threads, and I am having a horrible time trying to get to grips with this ridiculous antiquated system.
The Australian Standards 3501-1987 (Parallel Screw Threads of Whitworth Form...) is, like a lot of these types of books, very convoluted in the way it is written.
When I used the equivalent book for calculating the gauging practice for Metric and UN threads, at least they gave working examples! Did they do that this time? noooo that would be too damn easy!:tired:

[broby]

What a difference a sleep can do to the thought processes!
Took another look at the info this morning, and another paragraph of info became clear on it's intent and whoopee things fell into place!
Looks like my spreadsheet now accurately calculates sizes for the damn BSW system of threads.
Thanks for your piece of info Neil.
Turns out it was the same info that I already had, but worded slightly different.
Cheers
Brian.

[handlewanker]

Hi Broby, been cutting many odd threads over the years, and for 55 deg BSW threads I always used the formular... .64 X the pitch to get the depth of thread when screw cutting single point in the lathe.

12 TPI (threads per inch) is .08333" pitch.

Which gives you .08333 X .64 = .056" depth of thread, nearest "thou".

I think, without actually being certain, that 60 deg threads as for UNF, UNC and Metric, the formular is .68 X the pitch, needs to be confirmed.

Note that the pitch is the distance between threads, and the lead is the distance traveled by one revolution of the nut on the screw....single start means the lead is same as the pitch, and 2 start means the lead is twice the pitch etc, but the depth of thread is the same single or two start.

This will give you the exact depth of thread, 100% thread form for the screw, as if you were using a sharp pointed tool without end radius, and you ALWAYS cut the screw full thread depth, and make the nut to the fit required, eg 75% engagement, whatever.

You start the thread cutting with a 2.000" diam material size, and when finished cutting to depth the tops of the threads should end up in sharp points and get rounded off to the specified radius for the TPI (12 TPI) no matter what the diam of the screw, which REDUCES the OD of the thread diam by the amount the radius at the top is formed.

You must have a nut to try the thread fit, or else you'll have to use the three wires formular to cut the screw thread.

The same goes for a nut, you must have a screw (or thread gauge) to test the thread, and having a screw you can just cut the thread until it fits.

The Machinerys Handbook will give you a formular for the wire diam sizes, and size across the wires to cut the thread without having to use a nut at all, and you just have to make sure you round off the top of the thread when finished.

The radius top and bottom is irrelevant when using wires to test the thread as they measure the effective diam between the thread flanks, and I always made sure the tool point radius wasn't too rounded, but with the normal 75% thread engagement this is not an issue as the nut ID usually has flats and automaticall gives clearance to the bottom of the screw thread anyway.

If'n you remember those two simple depth of thread formulae you can cut any 55 or 60 deg thread, single point, without having to refer to a thread chart.

The hardest job is to cut the thread in a chuck back plate when the spindle nose has the thread you want to fit it to.

In that case you set up a thread caliper, (expensive), or make a dummy screw thread to the same size as the spindle end, using the three wire method directly from the spindle thread as a comparator....if'n the dummy screw thread fits, the chuck back plate will fit the spindle too.

BTW, if'n you want to use the three wire method for your 2 " thread, you'll need a 2"-3" micrometer (or a 50-75mm mike and convert to imperial) to measure across the wires with...... a digital or vernier caliper can not be used.

There are no other ways to accurately measure a screw thread, while in the lathe, than a thread caliper, gauge nut or three wires methods, out of the lathe it can be projected as well, and co-ordinate measuring machines are in fantasy land.....nuts can only be plug gauged.
Ian.

[handlewanker]

BTW Neilw20, Joseph Whitworth STANDARDISED the nut and bolt industry way back when, which previously was a random and personal preference "system" used by various engineers of the 1800's etc, and far from being a "sub standard" led the World in standardisation procedures.

Personally I prefer the Metric system both for measurement and thread range.

Almost all digital calipers and dial calipers are to .01mm resolution, which is approx 1/2 a thou, so when compared to an Imperial system it is more accurate because Imperialists only go down to .001".

Most Imperial micrometers also resolve to .001", (some to .0001"), and most metric micrometers resolve to 100th of a mm which is .01mm or half a thou about, again Metric is more accurate.

One day the Yanks will go Metric totally, and Napoleon can give himself a pat on the back.
Ian.

[RICHARD ZASTROW]

'wanker, we here in the USofA had a serious promoter of the metric system way 'fore Msr. Napoleon. His name was Thomas Jefferson. Tried real hard in the 1790's. However, we're mostly still on the #%&^ Imperial System.

Dick Z

p.s. Mr. Jefferson was also responsible for the cockamamie wine grading system in France.

[broby]

LOL whilst I appreciate the help... as stated previously, I have solved the question on how to Calculate the required manufacturing information I require for BSW threads.
It is a shame that the USofA is not also metricated, maybe that would have saved one of the last probes to Mars...? Didn't that get lost in space due to a failure to convert between Metric/Imperial by an engineer?
Makes you wonder why the Metric system (base 10) is not adopted world wide... after all we do have 10 fingers and not 12! (Well ok a very small number of people do, but they are the weird ones! ) Therefore counting in 10's would seem to be the natural order of things.
Thanks for your help anyway.
Cheers
Brian.

[handlewanker]

I have always been under the impressin that Nappy B invented the Metric system and made it the standard for the Frenchies.

At the same time, while Thomas J was deeply being involved with French politics regarding the Revolution era, it is little wonder that he recognised a better way to do things, but getting the message home would have been his real problem.....not many everyday people could read or write in the 1700's, so rejection of a system that requires a certain mathematical logical reasoning as opposed to the standard method of pacing out a piece of ground using the arms and legs as references is probable the preferred way for the people who have always done it that way and learned from their fathers and grandfathers.

How can anyone possibly relate to a mile being 1,760 yards and make calculations easily, and at the same time having 12 inches to a foot, 3 feet to a yard and dividing an inch into 64 parts to get a resolutuion of .015" approx.....must have used an advanced form of P&P calculator, (pencil and paper), to do the simplest calc.

The Brits only went Metric in about 1972 or so, so they could enter the European Common Market and flog their goods on the same platform, but the Yanks being a World power, didn't have to worry about Metrification advantages, that is not until the Chinese et al started to get big on the Manufacturing and world supply stage.

Now I see many USA people on these forums expressing their measurements in Metric dimensions.

I have "collected" over the years many drill bits, many imperial from many of the places I worked at, mostly from on the job acquisition, and the aspect of using the Metric system and imperial drill bits was frustrating, so I just converted the inch sizes to metric and had done with it.

Now a 1/2" drill is 12.72 MM, and so I am happy.

Once you get used to counting and calculating in 10's the rest is easy.

How can anyone imagine a 31/64" diam drill size, even decimilizing the fraction sizes isn't a solution, and to top it all....we have letter and number drills....aaagggghhhh.

Here's an example for common use.....you have a lathe with a leadscrew of 4mm pitch and want to cut a thread of 1mm pitch...calculation is pitch of thread to be cut divided by pitch of leadscrew......4 turns of the chuck and one turn of the leadscrew, any change wheels that fit in that "formular" 1:4 will do....simple, just don't disengage the leadscrew half nuts and expect to pick up the thread again.....I keep the leadscrew halfnuts clasped and run the lathe in reverse for each thread pass.
Ian.

[broby]

Now a 1/2" drill is 12.72 MM, and so I am happy.

OK... where did you learn to convert Imperial to Metric???

1/2" = 12.70mm NOT 12.72mm!!

Bugger all difference I know, but hey if you are fitting a bearing at that size you WILL have problems...

Once you get used to counting and calculating in 10's the rest is easy.

Aint that the truth there!

[handlewanker]

OOOOOOOHHHHHH, aint't you the pedantic one, I hesitate to say nit picker 'cos that's inflamatory.

I reckon if'n I could drill a hole using a 1/2" drill (12.7mm as the man said), and get a precision bearing fit to enable a .500" or 6.35mm bearing to press fit in the said hole, I'd be one gifted man......the round up sizes were for my collection of drills and when I'm at the lathe, Mill, whatever, that's a lot different...3 places it is for Imperial and 2 for Metric.

I don't think there's many people know what a tenth of a thou feels like, looks like yes...plenty drawings call for tenth limits, but working to tenths seperates the wankers from the workers, and when you can squeeze a micrometer down a couple of tenths it doesn't really matter in the end.....best most people can do is half a thou or .01mm.

It all comes down to feel, and when you work in a gauge room you know what tenths of a thou are, same for metric.
Ian.

[broby]

hahaha it was not so much a case of being pedantic as making sure that the great unwashed do not get confused...

[RICHARD ZASTROW]

Just to show my frustration about the Imperial system in the USA;

I just completed the design of a CNC gear hobber completely metric. The guy who machined the components converted all the dimensions to decimal inches. The kicker is he machined most of them on a Hermle CNC mill with a Heidenhain control. The thing works fine as a metric device just as all the rest of 'em.

Well, back to threads.LOL

Dick Z

[MetalZilla]

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