Metric or Decimal..accuracy?

I recall a thread sometime ago, that in normal usage, US decimal measurement was more "accurate" and lead to tighter tolerences under
normal machining usage.
The subject came up on another group, and I couldnt find the thread on google so Ill ask here.
As I recall, the example was made of an engine being made by US and Euro manufactures (aircraft?) and the US engine was "tighter" than the Euro one, and as I recall, perhaps incorrectly, it was due to rounding or the smaller incriments normally used in US decimal system
Im prepared to be wrong on this one.
Any comments?
Gunner
"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner
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I work in both metric and imperial (inch) on a regular basis and I would have to say there is no difference in the accuracy or repeatability that can be achieved.
I've heard similar to what you're saying and I can see where that assumption might come from. A 0-1" micrometer is probably the most used precision measuring device. With a conventional micrometer it is possible to read .0001" off of the thimble. Reading .001mm off of a conventional micrometer is not practical as the increment is too small. Most metric mikes that are the equivalent to a "tenth's" mike use .002mm as the smallest increment.
Even at .002mm it can be hard to read, especially on a skinny thimbled mike like a Starrett. In the end, if you really need to measure to .0001" or .002mm a conventional micrometer is not even close to being the best way to measure. But you can see where someone might say the .001mm (1 micron) is too small of an increment to be practical. They would be wrong, but you can see where the assumption comes from.
Nowadays the micrometer being used is likely to be digital anyway and will generally offer the same accuracy in either inch or metric mode.
In terms of tolerance, is there really any significant difference between a +.000/-.0003" and a plus nothing minus 8 micron tolerance? If the 15 millionth of an inch difference was significant enough the tolerance could always be specified as plus nothing minus 7.6 microns.
In a CNC shop environment there is no significant advantage of one system over the other in terms of accuracy.
As far as which system is better in general, there are trade-offs to each system. The imperial system has been around longer and evolved with civilization. So it's based on a human scale for the most part. The metric system was developed based on the need to have common transferable units in science. So the scale isn't always convenient but the system is uniform and simple. IOW, you can easily figure out how many microns are in a kilometer. Try figuring out how many ten thousandths of an inch are in a mile.
I like the the system of fits that is employed in the metric system. If you have a shaft that needs a certain fit into a hole there are simple standards for that in the metric system. On a drawing you simply give the dimension and the fit. For example 5mm h8 or 45mm h8. The actual tolerance is larger for the larger diameter as it should be. And it's all standardized.
The downside to metric is the lousy screw thread system it uses. More often than not the "perfect" solution doesn't exist, so a designer ends up making some sort of compromise, as opposed to the inch system which has far more options that are "standards"
As far as scale goes, when dealing with a small dimension, metric is easier as you are usually dealing with whole numbers rather than decimal amounts. Metric gets out of hand in a hurry as the parts become large. So neither system makes the math easier all the way around.
In terms of quality of product produced, be it a jet engine or a car, that has more to do with the abilities and discipline of the respective companies.
And of course the price. There are no free lunches.
--

Dan

Scopulus est usquequaque nefas
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That's a real necessary conversion, it comes up all the time.
quick- what's a third of a meter?
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My bud that does TiN coatings measures vapor deposition in furlongs/fortnight.
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the system is uniform and simple. IOW, you can easily figure out how

33.3 centimeters. No calculator required.
Quick - you build a tank 1 yard by 1 yard by 1 yard. How many gallons does it hold?
Give me the distance between two points. You can't because I didn't tell you whether I wanted the answer in decimal inches, fractional inches, furlongs, miles (statute or nautical), hands, etc.
Give me the volume of an object. You can't because I didn't tell you whether I wanted the answer in cubic inches, cubic feet, gallons (standard or imperial), barrels (several standard sizes), kegs (also several standard sizes), bushels, etc.
Same with weight.
As soon as I get the flux capacitor fixed in my DeLorean I'm going back 200 years to beat the hell out of every member of Congress who says he is going to vote to keep us on the English system of measurements.
Steve.
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I appreciate your 'zeal' but it seems you lost it a bit.
The distance between two stationary points is a constant, regardless of what unit of measure you use ????
A mile, there are different kinds, as opposed to a kilometer is a kilometer is .....

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You missed the point.
If you want to know how big my horse is the "constant distance" is 15-2. You're kinda' screwed if you don't know "horse" measurements, aren't you?
The metric system has one unit for length, volume and weight and those units are related. The metric system is FAR superior.
Steve.
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SteveF wrote:

Once again: Time for truth, not rumor. Dispite the oft stated claim that only the US, Liberia and Myanmar (Burma to older readers) have not officially adopted the Metric System, the Metric System (later SI) has been the offical standard in the US for over an century.
History time: 1795 France adopts and make compulsory the use of the Metric System. A set of standard meter and kilogram prototypes are sent to the US, but the ship is lost at sea. 1812 France suspend the compulsory use. 1840 France reinstates the compulsory use. 1864 GB permits the use in contracts and scientific use, but forbids its use in retail sales. 1866 US permits the use in contracts and retail sales, and mandates that it is an acceptable system, forbiddinG any state from banninG its use. 1889 The US recieves a prototype meter and kilogram. 1893 By the Mendenhall Order, the US offically went on the metric system, with all measurements be defined in reference to the standard metric meter and kilogram held in Washington. D.C.. Any system could be used, but all measurements would be defined in metric terms. 1897 GB finally permitted the use of metric measurements in retail sales. 1958 A conference of English-speaking nations agreed to unify their standards of length and mass, and define them in terms of metric measures. The American yard was shortened and the imperial yard was lengthened as a result. 1965 GB decides to go metric "over the next 10 years".
Both the US and GB have used voluntary rather than compulsory adoption of the metric system. Both countries mandated that the system be taught in the schools in 1974. (Me? I was taught it in school in the US in 1964, but I went to a good school.)
1975 The Metric Conversion Act of 1975 (Public Law 94-168) passed by the US Congress. The Metric Act established the U.S. Metric Board to coordinate and plan the increasing use and voluntary conversion to the metric system. 1988 The metric system is designated the preferred system in the US. 1991 All US federal agencies and executive departments are mandated to use SI. 1994 All commercial consumer packaging in the US must use both SI and inch-pound labeling. 1995 GB mandates SI lableling on consumer goods, but permits dual labeling with inch-pound.
David J. Hughes
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Thanks for posting this chart! I always woundered when the inch changed from 25.39997 mm to 25.4 mm
Nick
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Nick Mller wrote:

That would be the Imperial inch. The American inch was 25.40005 mm. The American yard was defined as 3600/3937 m by The Mendenhall Order 1893.
In 1959 it was changed again, to the exact value of .9144 mm. The conference David J. Hughes mentions must have been held the year before the actual legislation.
A little known fact (one of the very few things I have managed to remember from going to machinist's school for a short while a long time ago):
Long before the American inch was changed to the current standard of exactly 25.4 mm, the actual inch used in American industry was really the same as that, because the first set of measuring blocks delivered to Ford Industries (by the industry of my famous countryman C.E. Johansson) was made to that standard. This was probably due to someone making a mistake about the true size of an American inch. Ever since then, all measure blocks sold in America have been made to the modern standard of exactly 25.4 mm to an inch. When the mistake was discovered it was deemed unwise to suddenly change back to the real standard now that all the American industries were using the block standard.
I'm not sure about the British industrial inch of the time, but I guess they would get the same kind of measuring blocks as the Americans, only with the word "Imperial" stamped on the box. Anybody know for sure?
S.
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Interesting again! Thanks for that info.
Nick
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Apparently no accuracy either.

Explain the point you're trying to make. You can't because you don't have one.

You really are stupid.

I forgot. Weight changes depending on the unit you use, and in your broken logic length and volume also change depending on the units you use.
Good one.
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On Wed, 5 Jul 2006 05:41:12 +0000 (UTC), Cydrome Leader

I guess that his point is that U.S. customary system is not and SI is a coherent system of units.
-- Sincerely yours Zatoichi
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And I'm sure you think that an ounce of lead weighs the same as an ounce of gold.
Steve.
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It does, 1oz of lead weighs exactly the same as 1oz of gold. 1oz troy of lead weighs the same as 1oz troy of gold. Are you saying that lead can't be weighed in units of 31.21gm (1oz troy)
--
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out it's a 900lb gorilla with a flashlight!!
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Depends... Weigh them both under the same system, and they'll weigh the same. Weigh the gold under its usual "troy" system, and the lead under its usual auvudipous (I bet a nickel I mangled the spelling beyond any hope) system, and they'll be substantially different.
As far as the original metric/decimal accuracy question... Measure it in metric, then convert it to inches, and you can bet on there being at least SOME error, no matter how many decimal places you take it to, since a "used" (as opposed to "The math says...") metric/inch conversion is always going to be an approximation. For quite a few, there's a "close enough" approximation - Think "half inch" and "13 millimeters" - They're not *EXACTLY* the same, but for purposes of wrenching on a bolt, they're close enough that the discrepancy doesn't matter. Get into a situation where there are enough digits to the right of the decimal, and suddenly you find out that the fit of the half inch wrench you're using on that 13mm bolt has *ALL KINDS* of slop to it, even if you can't "see" it in the "real world".
It's all about tolerances... For "wrench on bolt", 13mm/half inch is "close enough". But if you turn a part to 0.500 inch when the sheet specs 13.0000mm, or vice-versa, you may find that your 0.0500 inch part doesn't fit into the 13.000mm hole, or the 13.000mm part rattles around in the 0.500 inch hole like a BB in a boxcar when it's supposed to be a tight press-fit.
Personally, I think it's always best to work in *ONE* system if at all possible. Mixing measuring systems is a recipe for a crashed Mars lander - As NASA so ably demonstrated a couple years back when they calibrated hardware in english, and wrote software in metric. (or was it the other way around? Either way, I think you get my point.)
(Hey, Gunner - I'm curious... WTF did this question have to do with misc.survivalism, other than perhaps inviting the whacko element that inhabits that group to cross-post into the machining/metalworking groups???)
--
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or the subject of the message doesn't contain the exact text "PopperAndShadow"
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There are all sorts of folks on MS..the vast majority being of sound mind. All manner of topics are discussed, including machining, etc etc on occasion.
I dont see any nut cases (other than myself of course) from MS posting on this thread. Do you?
On the other hand..I recognize several that are of sound mind, some of which are machinists, or scientists.
Gunner
"If thy pride is sorely vexed when others disparage your offering, be as lamb's wool is to cold rain and the Gore-tex of Odin's raiment is to gullshit in the gale, for thy angst shall vex them not at all. Yea, they shall scorn thee all the more. Rejoice in sharing what you have to share without expectation of adoration, knowing that sharing your treasure does not diminish your treasure but enriches it."
- Onni 1:33
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33.3333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333 centimeters.
--
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Now that's cute!
Bob Swinney
wrote:

33.3333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333
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snipped-for-privacy@spamblock.panix.com wrote:

33.3333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333
Ah, but try to find the .33 mark on your tape measure.
I do some recreational woodworking, and one thing that bugs me about metric is that it's difficult to break something into odd units. Sure, 10 is evenly divisible by 5 or 2, but a foot can be split up rather easily into 1/2, 1/3, 1/4, and 1/6 units. Again, as Dan said, it's part of the "human scale" that makes quick and dirty construction a bit easier.
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