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Help with ISO Metric thread form specifications
« on: December 29, 2009, 09:38:15 AM »
I wonder if anyone can help me with a reference to the complete iso metric thread definitions please?  I have the references to the ISO documents themselves but they are rather expensive!  (BTW, being in the mobile phones business I can't understand why I can download the complete set of latest specs for the 3G mobile phone system for nothing but have to pay $35 for a 20 year old document specifying screw threads - seems crazy to me.)

There are lots of thread tables on the internet but they all define the thread in terms of the major and minor diameters, and the major diameter is usually about 98 - 99% of the nominal thread diameter (e.g. for M12x1.75 it is 11.834 mm which is 98.6%).  There doesn't seem to be an exact definition that I can find of how you calculate the major dia. from the nominal diameter.  To make matters worse, the formulae for the thread dimensions (e.g. in Wikipedia) don't match the tabulated values!

The reason for this request is that I am having difficulty cutting threads that work using the thread wizard which asks you to define the start and finish diameters - I assume the start diameter is the Major diameter?  Using a pointed tool rather than a correctly rounded one, the in-feed is theoretically 7/8ths of the thread height which is itself 0.866 times the pitch; but using this value the crests get slightly turned away and the thread is too small for the nut.  (The tool is a commercial brazed thread tool but I do need to check the tip angle - but the tip has negligible radius.)


Thanks in anticipation,

John.


Offline RICH

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Re: Help with ISO Metric thread form specifications
« Reply #1 on: December 29, 2009, 12:12:07 PM »
John,
Unfortunately if you want an Iso standard you need to pay for it unless you are a member of the orginization.  Even the public library has to pay for them.That's how those orginizations support themselves. I no longer have access to them. A set of all of the standard  books will fill a library.
Most of the info in the Threading on The lathe write up is for imperial threads but you may want to do a quick read of it. If you have a machinist handbook you can find what you are after.

The reason it is all so confusing is that everything is based on tolerance that is related to some class of thread and profile and how it's will be applied to what you are going to do with the threaded piece ( ie. maybe you are going to plate the thread ).

One of the things that screws up a thread is not accounting for the cutter tip radius. I usualy do a light finish cut with the threading tool, measure the turned diameter, then back the tool off the piece by the tip radius ( which i know or have measured ), set the major diameter to it max allowable , and base the depth of cut on .866P. Tip radius, even a sharply ground V is not truely "zero".
If all was perfect, the lathe system, the cutting action, setting of the tool, stock support, etc.....you should be able to be in spec.
But in reality "perfect" never happens, so recomend you try experiment some for experience.
RICH
Re: Help with ISO Metric thread form specifications
« Reply #2 on: December 29, 2009, 12:39:51 PM »
Sorry I posted....must refrain.... right RICH?

Offline RICH

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Re: Help with ISO Metric thread form specifications
« Reply #3 on: December 29, 2009, 04:48:16 PM »
BClemens,
"Sorry I posted....must refrain.... right RICH?"

No.... i was just too lazy to look up stuff in the Machinist Handbook and i don't do much metric threading. Thus didn't want to miss inform you.  Some folks are happy that the nut goes on the thread tight, loose, or whatever!
The metric thread for a M profile is like an American National Standard thread for a sharp V but the tolerances are desgnated differently and of course the pitch and such are different.
certainly don't know them off the top of my head.
RICH
Re: Help with ISO Metric thread form specifications
« Reply #4 on: December 29, 2009, 05:25:41 PM »
Thanks Rich...I just discovered that my local library (Cambridge UK) has all the BSI specs accessible on-line from home and I can save them (but not print) as a library member!  Glad I renewed my membership now (and it's free!).  Now to get puzzling...

BClemens...didn't see your post...?

Thanks all,

John.
Re: Help with ISO Metric thread form specifications
« Reply #5 on: December 29, 2009, 06:52:20 PM »
No Rich, I know you're the expert. The basic point of my post was that the ISO standard is strictly based on the pitch of the thread on-which all other specific dimensions are based exactly. The rest of the dimensions are similar to USS, SAE including BA in their derivation of tolerances. The ISO is not a 'super' form. It is based on easily calculated inside and outside dimension factors and the tolerances for standard fit are within anyone's scope, with a calculator and a basic understanding of thread forms. These forms and their derivation are available within the text and diagrams of a Machinery's Handbook and other quality publications as well. ISO threads and their tolerance standards make more sense to calculate that any standard thread form - IMO.

The most basic point of my post is that although the internet is indispensable for information, a seeker must take into consideration the fact that there are many individuals and institutions that will attempt to charge for what is basic information for the benefit of universal use and understanding. Threading nor threads as a group of collective sciences are not propriety nor can anyone claim them as such for their own personal benefit. The proof of this is the fact that without published standards, the use of a special form of threads would deem a product obsolete in a short period of  time. Even extreme high pressure fitting thread standards and dimensions are published and available for basic public use. There are also US Federal Laws that prevent the use of a form of commonly used and accepted sciences becoming proprietary by their modification from a set of established standards - that was when our government was 'for the people' so that may change in the near future.....

Bill C.

Offline RICH

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Re: Help with ISO Metric thread form specifications
« Reply #6 on: December 29, 2009, 07:57:25 PM »
"No Rich, I know you're the expert."

You must must have me confused with another Rich.
My favorite saying at work was that you never know how stupid you are until you
enter anothers expertise or knowledge base!

Bill, do you have a link to where standards can be downloaded or viewed by the public for free?


RICH
Re: Help with ISO Metric thread form specifications
« Reply #7 on: December 30, 2009, 08:41:04 AM »
Rich,

The 724, 261, and 262 standards are all that are needed and are readily available on numerous sites. The tolerance standards 965 1-6 are where the ISO and industry have come to blows. The ISO claims:  "The foremost aim of the International Organization for Standardization is to facilitate the exchange of goods and services through the elimination of technical barriers to trade." But in reality, the organization wishes to totally control and change all standards worldwide and basically roll over the current and past industrial standards that were established in the USA (the most industrial country on the planet). The US Government attempted to force companys in the US to change over to the metric system and in doing so, companys sold their 'outdated' thread tooling as scrap. Now, you purchase a product from Taiwan and China, they are using the 'scrap' US tooling to manufacture goods sold in the US with SAE and USS threads. The Government did it again and, the nations that advocate the use of ISO thread forms claim that the US attempted to undermine the ISO by 'dumping' US standard thread tooling on the world!

 ISO 9000 is another area that we are being brought into alignment with the world.

Thread tolerances are the same whether they are ISO, USS, SAE, Witworth (angle difference but trigonometry works at 55 degrees too) and they are all figured the same for 'nuts and bolts' types of threads. The 965 1-6 are not needed as a prime part of their ISO threads

A site of interest: http://www.gewinde-normen.de/en/index.html

Bill C.