Thread pitch math? (AKA...can I cut this Metric thread on my SAE only lathe? - Page 3
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  1. #41
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    Nov 2003
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    This is one of those situations where errors in the parts per million will make a difference. It is true that the official definition of the inch in the US as well as in most countries is exactly 25.4mm. This is by international agreement and a common definition is really needed for international commerce to take place.

    But surveys and maps of the land date back to far earlier years and at that time, the inch was defined differently. We can not expect the owners of a 50 or 100 story sky scraper to move it over by a half inch due to a change in the official definition of the inch. This would also apply to ordinary property owners who are allowed to build up to the boundary line of their property. They purchased the property and built the buildings or other structures in good faith using the standards of that day. We must respect those good faith transactions. So it is necessary to either use the older definition of the inch or make appropriate changes to the boundary lines as they would be defined by the new unit. They choose to continue using the older definition of the inch and I can't say that I blame them.

    But this continued use is a necessary compromise and does not, in any way, constitute an official, present day standard.

    I would not be surprised to find other areas where people have been forced to continue to use older standards.

    Note: For what it is worth, a two part per million difference would amount to a difference of one inch in about 7.89 miles. That could be an important difference even within the confines of a single, small city.

    EDIT: I do not think that this two parts per million will make any difference in any of our shops or in any work on hand weapons like pistols or rifles. My best reference is a set of shop blocks and most of them have far larger errors than that. So if you are working on something that is only a few feet long in it's greatest dimension, you can use whichever definition of the inch that you want. You probably don't even have any means of determining which version you used for any dimension that you may machine. But, if you do have that ability, then you probably know it and know better than I which jobs it may apply to and which it does not. I will not lose any sleep over it and neither should most of you.

    Quote Originally Posted by MattiJ View Post
    Way off-topic but United States still uses two different definitions for inch.
    the "metric inch" and US Survey inches&feet. Survey inch is some 2 parts per million different..

    "When international measure was introduced in the English-speaking countries, the basic geodetic datum in North America was the North American Datum of 1927 (NAD27), which had been constructed by triangulation based on the definition of the foot in the Mendenhall Order of 1893, that is 1 foot =
    1200/3937 meters: this definition was retained for data derived from NAD27, but renamed the US survey foot to distinguish it from the international foot.[2] For most applications, the difference between the two definitions is insignificant – one international foot is exactly 0.999998 of a US survey foot, for a difference of about
    1/8 inch (3 mm) per mile – but it affects the definition of the State Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles.[10]

    The NAD27 was replaced in the 1980s by the North American Datum of 1983 (NAD83), which is defined in meters. The SPCSs were also updated, but the National Geodetic Survey left the decision of which (if any) definition of the foot to use to the individual states. All SPCSs are defined in meters, but seven states also have SPCSs defined in US survey feet and an eighth state in international feet: the other 42 states use only meter-based SPCSs.[10]

    State legislation is also important for determining the conversion factor to be used for everyday land surveying and real estate transactions, although the difference (2 ppm) is of no practical significance given the precision of normal surveying measurements over short distances (usually much less than a mile). Twenty-four states have legislated that surveying measures should be based on the US survey foot, eight have legislated that they be made on the basis of the international foot, and eighteen have not specified the conversion factor from metric units"
    United States customary units - Wikipedia

    LexD's piece of old text is probably from between 1944 and 1959 as it mentions British standard from 1944 but talks also about old US inch that was used until 1959 "metrification"

  2. #42
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    Feb 2014
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    Quote Originally Posted by EPAIII View Post

    Necro post revived by Samantha the Spammer...

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