# Metric/imperial

Actually the length of an "inch" is tied to the metric system. There is a standard "meter" which is also just another random length chosen by someone. Actually it was supposed to be 1/10,000,000 of the distace from the pole to the equator, but somebody miscalculated. Later the meter became defined by a length of a platinum irridium bar kept stashed away in Paris. The problem with that length "standard" is that it can vary and also the bar could wear down. Since the meter is the basis of all mechanical measurement it can't be measured by mechanical means, at least not accurately, so the standard meter today after several other standards were adopted and discarded is defined as the distance traveled by light in vacuum during 1/299,792,458 of a second.
The standard length of an inch is defined as 25.4 millimeters. So the calibration of every "inch" mesuring instrument from gage blocks to micrometers is tied to the meter.
That being said, the metric system has it faults. The threaded fastener system is horrible. The scale is not "human". and fractional portions are part of every day life. Rarely does everything get divided by tens. In the case of machining parts, neither system is more "accurate" than the other.
The reason the US remains for the most part on the "inch" or "imperial" system is that we don't really have to change, so we don't. Despite all of the moaning about losing our industrial base, our industrial output is the largest in the world. The last time I checked we were still the largest free market in the world. We were also the world's largest exporter of goods and services. So if other countries want the US consumer to be their customers, then they need to deal with our quirks. If our position were reversed and we were an up and coming industrial nation then there would be more incentive to change.
It comes down to the cost and inconvenience of change. Would we export more goods if we were "metric"? Maybe. But would that new income pay for the cost to convert? Doubtful.
As Anthony's comments show it's really no big deal for a US manufacturer to work in metric. His company has to in order to comete on a global basis, so they do. If there were no reason to use millimeters instead of inches, his company likely wouldn't.
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Dan

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Actually, we would be metric. As a global company, it would be a nightmare to collaborate within the company if one was using imperial. Being european parented just absolutely ensures that we would be metric.
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Anthony

You can't 'idiot proof' anything....every time you try, they just make
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D Murphy wrote:

Need some historic lessons? It initaly was *not* tied to the metric system. It was the thickness of someones thump (after having sex, in the morning, how many ale/beer?). The conversion was initially 24.39997xxx mm. That changed around the 70's. Then it was defined to be exactly 25.4mm.

It is, no doubt. But there is just *one* unit.

That doesn't matter. The difference is, that the reference was not to some person -moi le roi- but something that seemed to be quite reproducable at that time. Their decision was to select something that was more "democratic" than the former definitions. The length itself just had to be somehow handy.

Wrong. The initial meter was the one in Paris. They invested a lot of time and effort to find a stable alloy. Lots of investigation and the highest obtainable accuracy at that time.

They knew that, and used the reference as little as possible. They developed the alloy for that purpose.

That strange number is a stable reference, to the knowledge of our time. They couldn't count waves at that time.

That happened later. There was a time when we here in Bavaria (Germany) had our own inch that was just a tad off the imperial inch.

What!?
Does it have to be? Do seconds have to be "human"? Like Volts, Amperes, Pasqual, Liters, Kilograms... ?

Just because you are using a fractional system. I can well live with the metric system and "limited" fractions. I don't need 1/3 of a meter. And if, I pick 300mm or 350mm. That doesn't hurt me at all.

You are mixing things. One foot is how many inches? That question doesn't exist in the metric system.

Nobody said that! And if, he must be stupid.

Oh. Now that is an argument. You don't have to change? How well would you sell cars to the rest of the world (see wikipedia for the remaining countries, but don't get shocked) when they still would use imperial screws?

Your last reality check was when?

goods just for the US-market in the imperial system? LOL!!! You really got to wake up.

So why do you insist on being one of the last, and absolutely the last one of the "civilized nations"?
Nick
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Nick Mueller wrote:

So how many cars are produced in the US these days are using inch fasteners?. A mate who has a big hangup on this and British industrial, particularly automotive, decline because it didn't metricate and was hampered by selling a product with inch fasteners or at least inch hex fasteners into a metric market. Some specialty markets can get away with it, IIRC the big Renault with the Mack engine, cost of a socket set is cheap by comparison.

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Actually no i don't need a history lesson. But I wasn't talking history in this case. I was merely pointing out that the standard "inch" is in fact tied to the the standard meter.

Obviously.
How do you figure I'm wrong? I was being a little tongue-in-cheek but trust me I know what a gage block is, I know why they chose platinum irridium, and I also know why it's a very poor idea to use a mechanical device as the standard for length for all other mechanical measuring devices.
Or are you picking nits with the term "later"? The length was defined first, then the master reference (Platinum Irridium gage block) was produced after. In English, that would be considered "later".

Didn't make the problem go away, hence the change coming below...

Well, that was the whole problem with the early industrial revolution. Interchangability. Hence the drive for standardization.

'Tis true. Not nearly enough sizes and pitches for us finicky Yankee bastards.

It doesn't have to be. I'm just pointing out that it isn't. I find it fairly easy to pace off something in fett, since I came equipped with two of them.

Sure it does. Hence the prefixes deci, centi, milli, etc...

So you haven't met Cliff?

Hmph, the rest of the world seems to have no problem selling us imperial fasteners. They mark their exported food packages in ounces, pounds, etc.. But somehow we are too stupid to figure out their system? Yeah, right.

Feel free to support your bias with some actual numbers. Here's a little help, we exported over \$1 trillion dollars worth of goods last year. Let me know which country exported more than that.

What makes you think that I think that? Sheesh. But if you want to sell products here in the largest consumer market in the world, then you will need to abide by the consumers demands, as well as relevent laws.
Oh and by the by, the last time I checked the speedometer in a BMW, Mercedes, or Volkswagon sold here, it was graduated in miles per hour. So while I don't think that Europeans are making goods "just" for the US market, they do need to adapt them to be able to sell them here.
I've also noticed that Sandvik, Kyocera, Mitsubishi, Iscar, etc.. sell carbide inserts that are in "inch" dimensions, even in their own countries. The tool shanks sold here. 3/8", 1/2", 5/8", 3/4", 1" are all available from manufacturers located in "metric" counties.

Nobody is "insisting" on anything. As I pointed out there really isn't a huge impetus for change here. Foreigners that want access to our market need to make some adjustments, just as we adjust to theirs when need be. Seriously , if the US had lagged in indusrialization rather than being one of the leaders it would be a different story. Since that wasn't the case, and we were the only undamaged country after WWII, we just cranked out the stuff the world wanted, cars, steel, medicine, machinery, etc.. We didn't need to worry about competing with Europe, we were selling her the stuff with which to rebuild and join in on the trade.
The problem is some fools beleive that that brief moment in time is how the US should remain. That is, the only manufacturer to the world. That we should never have competition, nor we should never lose a single job to a foreign country. That we should not have to compete nor should we ever lose. Then there are the idiots on your side of the pond that look at every loss as evidence the US is finished. Ignoring the reality that for every industry and job lost, better ones have filled the gap.
All of this competition has resulted in the average man in an industrialized nation living a lifestyle that was unobtainable to the wealthiest monarchs a mere 150 years ago. And we are paying WalMart prices for it.
Things could be a whole lot worse.
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Dan

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Germany
Even your own CIA says that. But don't let facts get in the way of the argument.
https://cia.gov/library/publications/the-world-factbook/rankorder/2078rank.html

No, the USA consumer accepts the quirks of China.
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Seems the commerce dept. knows something the CIA doesn't
<http://www.commerce.gov/02-13-07%20Export%20Fact%20Sheet.pdf

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Dan

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D Murphy wrote:

Ack. It now is tied to the meter. Isn't that frightening for you? ;-)

What was wrong is that the meter was defined by the block in Paris not by some -more or less- wrong math. And what do you suggest they should have used as reference at that time? Could they come up with something more stable?

wrong with calculating the equatorial length.

Now I need a suggestion what they could have made better at that time.

What sizes are you missing?

So that wasn't an argument? ;-)

The prefixes don't make a new unit. They are just handy factors. Placeholders for zeroes.

Should I? :-)))

Would be interesting to see wether more metric than imperial fasteners are produced worldwide and how that influences their price.

I hope these numbers don't shock you: <http://en.wikipedia.org/wiki/List_of_countries_by_exports They look even worse if you see them relative to the population.

And all screws were imperial? :-)

They do have to follow local laws or facts. Like crash-test, quality of gas, ... and the unit on the speedometer.

Did you actually measure them? They sound too much like 10mm, 12mm, 16mm, 20mm and 25mm. Exactly the sizes of my ER25 collets (except the 25mm).
Anyhow, if you wouldn't ask them for these "odd sizes", they wouldn't exist. It's you who has to pay for special runs. Not that relevant in high-volume goods.

OK.
Nick
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You mean "thumb"?

Well ... that depends. From the 43rd edition (1961-62) of the _Handbook of Chemistry and Physics_ by the Chemical Rubber Publishing Company,
For the US:    1 inch = 25.40005 mm For England:    1 inch = 25.39998 mm For France:    1 inch*= 27.0700 mm (called the "Ponce")

IIRC, I found the "new" definition in the 44th edition (1962-63), so it was apparently a bit before the 70s. :-)

And looking at the three different sizes of the "inch" between the three countries covered by the tables, that unification was a significant benefit of the metric system -- even when the residents do not directly use the metric units. I presume that the French Ponce was already obsolete at the time of publication, but the differing British and US inches *were* certainly in use.
At that time, it was the scientific community which used the metric system.

And a length unit based on the reach of a typical arm was more handy than the somewhat longer meter for those who did not have measuring tools handy at all times.
[ ... ]

I have to agree -- the additional complexity of a threading dial on a metric lathe is one example. The lack of a number of intermediate sizes in screw diameters is another. It is easier to scale the screws to the task with number-sized screws (which granted are mostly not reasonable fractions of inches, so they could just as well be specified in mm -- but the corresponding thread pitches do happen to be purely fractional inch sizes.

But the ability to get an approximate length by extending one hand to the side while holding the other end near ones nose is a benefit when serious precision is not called for.

Some of the finer pitches in metric screws are not exactly logical sizes -- 0.45mm on a 2.5mm screw, IIRC serves as an example.
[ ... ]

Of course 0.01mm offers somewhat more resolution than 0.001". But most of the metric mechanical micrometers which I have stop at 0.02mm steps -- pretty close to 0.001".
Enjoy,         DoN.
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DoN. Nichols wrote:

Yes.
Just a few ones. Even in Germany -centuries ago- there existed a lot of different units for whatever.

OK, I wasn't sure about the date. I can well accept the 60s.

My arm? I span nearly 2m with my arms, so the meter is natural. My steps are -if I make them a bit bigger- 1m. So *that* is natural. :-) No, I don't think discussing about what is "natural" and wether it helps nowadays is worth continuing.

Complex? First of all, I don't use it. Second, it doesn't look that complex if you have a metric leadscrew. I could use your argument with my metric lathe that making imperial screws is a pain, because I have to change so many gears.

So I don't know what sizes you have. But I never missed one. On the small sizes (between M1 and M2) there are some sizes in between.

Why is that easier? If a M5 is to small, I pick a M6, then an M8, then M10. Oh, remembering what sizes exist? If *that* is a challenge for you, ... :-))

1m. That is one meter. No really, it's not an argument. You'll find good numbers with the metric system too. A thump is 2.5cm, that's a forth of 1dm. Handy, isn't it? :-)

It's OK if it isn't logical. It just has to work. There is an optimum relation between diameter and pitch. All standard screws are close to it. So they had to use a 0.45 pitch for M2.5

Ah, there is nor argument for one or the other system. Accuracy has nothing to do with the system used. You could etch ICs with lightyears if you want and don't get lost in zeroes.
Nick
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[ ... ]

O.K. You may be a bit larger than the average person was when those units were formed. :-)

:-)
You don't use the threading dial? That may explain it -- but it suggests that you either have a CNC lathe, one with additional features to automate that, or don't cut many threads on the lathe.
On the imperial lathes, cutting imperial threads using an imperial leadscrew the threading dial is mounted on the carriage and meshing with the leadscrew. It has some numbered positions, and some positions half-way between the numbers. With even numbered threads you can close the half-nuts at any mark, numbered or not, and be on track for the next pass. For odd numbered threads, you select only the numbered positions, and for the rare fractional threads (like 11-1/2 TPI, you simply use the same number each time. If this is too much trouble to remember, you can *always* close on the same number -- it is just a little slower because you have to wait longer.
However, when cutting metric threads on a metric lathe with a metric leadscrew -- a single pick-up gear for the threading dial won't work. They have four separate gears, which require loosening the threading dial, tilting it out of mesh, sliding up or down until the proper gear is in place (determined by either looking up in the manual to the lathe, or squinting and looking it up on the threading data plate -- if it is still legible), and tilt it back into mesh and lock it down. Only then can you start cutting threads. (Obviously, with CNC this all goes away, as it does with die heads on a turret lathe -- which you don't often find in a hobby machine shop.) Just one more operation to introduce possible error into the job. It is *this* additional complexity to cutting metric threads on a metric machine which I am talking about.
I cut my metric threads on a small CNC machine, so I don't have to deal with a threading dial there. So far I have not had to set up the transposing gears on the bigger machine.

Sometimes it is a tradeoff between screw strength (diameter) and the amount of material available for the screw to pass through. The more common imperial threads offer more intermediate steps for these situations. For most things, the standard metric sizes will work fine. But if you must stay metric, and hit one of those situations, it calls for a special thread which will cost you more. And -- it will offend your customers when they need a replacement screw and it has to be ordered from the factory at a high price. (Granted -- proper design should have avoided this problem -- but sometimes things are made as a retrofit, where the envelope of the product is defined by where it must fit, and not by what produces a strong enough design with standard screws.

Hardly -- but finding something like a M3.5 when a M3 is too weak, and a M4 results in too large a hole in the other piece, or threads close to breaking out the side of the threaded part.

O.K. I'll accept that. I really would not care if the foot, the yard and the mile went away as long as they left the inch. That would eliminate some of the complexity of the Imperial system which you are complaining about. When *I* am working in feet, it is usually something like the length of an extension cord, which does not have to be that precise as long as it is long enough.

Well ... most metric things try to avoid fractional sizes, thread diameter or thread pitch. this one is an exception.

O.K. The only place in the imperial screws (that I have experienced) where the design is really insane is the 6-32. It is too coarse a thread for its diameter, and thus weak -- resulting in more broken taps than any size larger or smaller. :-) Someone fell in love with the 32 TPI setting on their lathe, and kept it too long. 8-32 and 10-32 are quite reasonable threads, but 6-32 is not.

I never said it was. I was simply replying to the suggested claim that Imperial was more accurate than metric, and showing that it was not. Stop fighting me when I am arguing on your side. :-) I am simply discussing all points of the subject, not fighting for one side or the other. Each has its good points (based on the implementation, not the theory).

Of course. I never said that it did.
Enjoy,         DoN.
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DoN. Nichols wrote:

Admittedly, not too often to get bored with using the reverse for the next pass. On my small lathe, I have a VFD and crank it up to come to the start of the thread.

OK, it's a bit more complicated with metric. Three gears on the thread dial (28, 30, 32 IIRC) and only selected positions like 1, 4, 7 etc.

Accepted! :-)

Just pick a higher grade. :-) Standard is 8.8.

Picking a special thread (whatever that is, no one would make a M 7.2 just for that) is not the solution.
OK, lets see: M 1.6 M 2 M 2.5 M 3 : 4020 N M 3.5 : 5420 N M 4 : 7020 N M 5 : 11350 N M 6 : 16100 N
[M 3.5 should be avoided; I have no values below M 3] These are the minimum loads 'till failure with grade 8.8 screws. I mean, few are aware how much a M 3 can hold. Furthermore, the sizes increase in a geometric sequence (rounded). It is roughly 25% to the next bigger size. Do you want a finer granularity?

That's why the M 3.5 exists, but should be avoided. The step from M 3 to M 4 is 33%. Seems they realized that and added the M 3.5 for you. :-)

Then if you would quit the fractions, we are closer. What remains now is the conversion factor. No problem that much. But the much bigger problem is that there are two standards worldwide. The US of A is not an island. Double stock for screws, tools, .... And it also hurts *your* business. It was just yesterday when someone in a forum posted a link to a US-CNC mill (a smaller one \$6k range). "Oh, imperial! Forget it!".

OK. I had to repeat that.

Sorry! :-) But it's no argument for any side. It's an "argument" that reveals the ignorance. NO, you did *not* say that!
Nick
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DoN. Nichols wrote:

If you are using a scale, the inch scale can be read to a finer dimension than a metric scale.
You can easily read a 64ths scale which is a finer dimension than a 1/2 mm scale. Anything finer than a half mm scale will be almost impossible to read.
But then who uses scales any more, with DRO's.
John
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snip------

I do, for one. I have never, in my life, used a DRO-------and don't see the need for one. Learned how to run machine tools when they were not in existence. I still harp on the use of scales, which usually save you from your own stupidity, assuming you use them.
Harold
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Nick Mueller wrote:

I think Nick needs a "history lesson". It was near the beginning of WW II (not sure of the exact date). ...lew...
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Lew Hartswick wrote:

Here its mentioned as July 1959 http://www.ex.ac.uk/cimt/dicunit/dicunit.htm
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Correction http://www.ex.ac.uk/cimt/dictunit/dictunit.htm
David Billington wrote:

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All this is confusing. A pint's a pound the world around. They'll have to pry my fractional drills from my cold, dead hands.
David Billington wrote:

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A British pint is 20 fluid ounces, whereas the US is 16 fluid ounces. The US fluid oz is slightly larger IIRC by about 4% so someones pint is not a pound. Of course a British pint is 568ml so a pint of water will weigh 568grams or 1.25 lbs, so it must be the US pint that is a pound of water.
Louis Ohland wrote:

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David Billington wrote:

I think he ment a pound sterling.
John