[Geof]

1" = 25.4007mm

No it doesn't; 1 inch is defined as 25.4 mm exactly.

edit
Machinery's Handbook, Twenty-Sixth Edition; page 2525, Table 1. Metric Conversion Factors
inch multiply by 2.54 to obtain centimeter (cm)

The 2.54 has a lowercase superscript a.
On page 2531 at the end of the table the footnote for a reads: "The figure is exact."

[martinw]

[QUOTE=Geof]No it doesn't; 1 inch is defined as 25.4 mm exactly.

"[/QUOTE

Dear Geof,

Thank-you for that. For a few minutes I genuinely had doubts about the conversion I have used for decades.

Best wishes

Martin

[bigz1]

Sorry 1" = 25.407mm(heavy handed on the 0)

I guess we will have to disagree but a quick search of the internet and a look in my 'Colins' dictionary confirmed this figure(Although I quite regularly use 25.4mm myself, as I do not require to be more precise). I also believe cm are not a scientific measure unlike mm.

http://www.oldengine.org/members/die...bles/comp1.htm

It reminds me of a story I heard about the French wanting to use the latest technology to re-weigh the original Kilo. The problem was how to clean the dust of it without scrapping off atoms from the Kilo thus effecting the re-weigh results.

[Geof]

I guess we will have to disagree but a quick search of the internet and a look in my 'Colins' dictionary confirmed this figure(Although I quite regularly use 25.4mm myself, as I do not require to be more precise). I also believe cm are not a scientific measure unlike mm.

In other words you disagree with Machinery's Handbook.

[Jason Marsha]

I am not sure where on the the net this 1 inch = 25.407mm but the closest my searches have come to this figure is 25.4000508001 and this is due to rounding errors.
So as far as I am concerned 1 inch = 25.4mm.

Jason

[martinw]

I also believe cm are not a scientific measure unlike mm.

[.

Dear bigz1,

The SI system may use metres, centimetres, or millimetres. I think, through the haze of memory, that it uses metres (I may be wrong).

What is absolutely certain is that one metre is the same as 100 centimetres and a thousand millimetres. Anybody who attempts to say otherwise is in need of a brief lesson in powers of ten.

Best wishes,

Martin

[unterhaus]

there was a change in the definition of an inch to correspond to 25.4mm. A quick google search didn't reveal when that change was made.

[Jason Marsha]

Well said Martin.

Searching further, under the SI system of units: 1 inch = 2.54cm= 25.4mm.
The above figures are exact.

Jason

[Mcgyver]

never realized it was contentious, curious so i hit the search bar

ISO page:

http://standards.iso.org/ittf/Public...t/annex_a.html

defines exactly 25.4

US & Commonwealth went to this standard '58 according to wiki

so if the dang parts don't find, it must be cuz its an old micrometer you're using

[Geof]

....The SI system may use metres, centimetres, or millimetres. I think, through the haze of memory, that it uses metres (I may be wrong)....Martin

The SI system now is mks ; metre, kilogram, second; for distance, mass and time with the Newton used for force.

Quite a while ago before metric became as widespread and places like the UK still based their measure on a king's thumb width the SI system was cgs for centimeter, gram, second and the unit of force was the dyne.

The US standardized linear measure on the metric system by an act of Congress in the late 19th century when the yard was defined by reference to the metre.

In 1958 the yard - metre relationship was made international and sometime around then; or at least between the 19th century act and 1958 there was some question as to whether the US standard yard was the correct length. It might have been out by 0.0001" or something.

The 1" = 2.54 cm exactly has been in effect since 1958.

And it all is referenced back to kazillions of wavelengths of the light emitted by a cesium atom going through a particular energy transition which I am too lazy to research in detail.

And before that it was referenced to the distance between the pole and the equator on a line passing through Paris or some other place in France. The metre was intended to be one ten millionth of this distance. Unfortunately they got it wrong so for many years the standard for the metre was a platinum bar with a couple of scratches in it.

But then again maybe I have it all wrong.

edit:
Sorry my chauvinism is showing. The 1958 standardization was not truly international it only included the US and the Commonwealth of Nations (which was pretty darn close to international back then).

[martinw]

Searching further, under the SI system of units: 1 inch = 2.54cm= 25.4mm.
The above figures are exact.

Jason

Dear Jason,

Thank-you.

Best wishes,

Martin

[martinw]

But then again maybe I have it all wrong.

Dear Geof,

As long as one inch equals 25.4 millimetres, I really do not care!

Thanks

Best wishes,

Martin

[HayTay]

Grabbed my digital calipers and dialed in 1.000 in. Then I pressed the mm/in selection button and, voilà, out popped 25.40mm. Re-pressed the mm/in selection button and, lo and behold, back to 1.000 in. Pressed the mm/in converter button and, what do you know, still 25.40 mm. Snuck up on the conversion button and...


It works , give it a try ,

HayTay

[the4thseal]

I though the whole idea behind the metric system was that we are too stupid to deal with fractions. As if it was too hard to get the brain around. Though last time I checked .4 of a mm is a fraction, I have yet to hear anyone use the correct whole unit term for portions of a millimeter. Micrometer, Nanometer ect. I hear people say a tenth of a millimeter and so on. When I check my calipers I get 25.4 for the inch, I have no problem with this, but is not 4/10ths a fraction? I actually do not care, I just get a little steamed over the idea of one system being better. I guess what ever it takes to feel good about yourself in the morning, if using the metric system does it for you, than great. :cheers:

Just some thoughts at o'dark thirty after tool long at cad
:cheers:

[dertsap]

im still trying to figure out how to punch in fractions in mastercam

[Jason Marsha]

I though the whole idea behind the metric system was that we are too stupid to deal with fractions. As if it was too hard to get the brain around. Though last time I checked .4 of a mm is a fraction, I have yet to hear anyone use the correct whole unit term for portions of a millimeter. Micrometer, Nanometer ect. I hear people say a tenth of a millimeter and so on. When I check my calipers I get 25.4 for the inch, I have no problem with this, but is not 4/10ths a fraction? I actually do not care, I just get a little steamed over the idea of one system being better. I guess what ever it takes to feel good about yourself in the morning, if using the metric system does it for you, than great. :cheers:

Just some thoughts at o'dark thirty after tool long at cad
:cheers:

I think the elimination of fractions is between the different measurements of the metric system ie 1m = 100cm = 1000mm and not between the imperial and metric system, everything relates in powers on ten. If we had 367mm, then that is equal to 36.7cm which is equal to 0.367m.

Fractions can never be eliminated simply because its number, 1/2" is represented as 0.5" in decimal form, 1/3" is represented as 0.3333 with the 3 repeating.

Its not that one system may be better but everyone needed to come to a standard format and the metric system was defined.
Use the system that you are comfortable with, I use both and never had problems.

Jason

[NC Cams]

This debate on metrics clearly goes back a ways:

http://lamar.colostate.edu/~hillger/dates.htm

Even the "official" conversion changed and the "correct" one could be "incorrect" depending on the time frame the conversion was done.

Moroever, the conversion from inches to mm and vice versa is NOT universally accepted at 25.4 or the inverse there of. Witness the following:

www.nist.gov/metric

www.simetric.co.uk/

If you delve deeper into the "metric conversion standards" search on Google, you will surely find different factors depending on where you go and which conversion they used at the time.

The difference between 25.4 and 25.39 is relatively of little consequence in one or two inches. However, as one member pointed on on PCB layouts, the addition of the "error" as you get to longer and longer conversion lengths becomes substantial.

Remedy (there is no solution) that works for us is to ask your client what factor they use when converting metric to imperial and vice versa. If it is 39.36 or 39.37 or 25.4 or 25.39, it is at least consistant with what THEY are expecting.

Then go off on your merry way and make it using the appropriate MIL standards that we all need to follow with your metric or imperial scales:

MIL- TFP 4.1 (Make It Like The Frigging Print For Once).

[Jason Marsha]

After looking at more sites on conversion I am see that persons doing conversion are rounding off quite a few decimal places. Some sites have 1mm = 0.03937 inches which causes a problem when reconverting to mm.

If 25.4mm is defined as 1 inch

then this would imply that 1 mm = 1/25.4 inches,
so we get 1mm = 0.03937007874015748031496062992126 inches.

If we take the reciprocal of 0.03937007874015748031496062992126 we get 25.4 .

If you do not round off you will not get an error.

Since I am using the microsoft calculator on my computer I am limited by the number of decimal places that it can display.



Jason

[Geof]

After looking at more sites on conversion I am see that persons doing conversion are rounding off quite a few decimal places. Some sites have 1mm = 0.03937 inches which causes a problem when reconverting to mm.

If 25.4mm is defined as 1 inch

then this would imply that 1 mm = 1/25.4 inches,
so we get 1mm = 0.03937007874015748031496062992126 inches.

If we take the reciprocal of 0.03937007874015748031496062992126 we get 25.4 .

If you do not round off you will not get an error.

Jason

Jason, stop it you are being too logical and rational, tha's no fun, surely it is much more fun to needle the stick-in-the-muds who find a need to attack metric or manufacture a controversy. I live in Canada which started out to convert officially to metric but stalled partway. There was a sector of the population who predated the introduction of metric by many years and represented a large voting block and one government a few years ago repealed the legislation for the final step as a vote buying gimmick. (I suppose I should put the 'In My Humble Opinion' after that.) A lot of the posts in this thread a somewhat reminiscent of the types of arguments put up against metric back then.

When you are familiar with it Metric is easier to use it alone than inches and decimal units based on the 1/2, 1/4 etc series of fractions but it is an absolute pain switching between the two systems sometimes when you also have to take into account tolerances; quick; what is 0.875" =/-0.002" in metric? This is where the opposition stems from I think.

It is amusing how vehement people get in resisting something they are unfamiliar with or which they perceive is being imposed on them from outside. I have met a few software people who are perfectly happy doing mental conversions between binary and base ten and back and forth between these and hex and bcd but who almost foamed at the mouth at having to convert from inches to mm.

[sdantonio]

After looking at more sites on conversion I am see that persons doing conversion are rounding off quite a few decimal places. Some sites have 1mm = 0.03937 inches which causes a problem when reconverting to mm.

If 25.4mm is defined as 1 inch

then this would imply that 1 mm = 1/25.4 inches,
so we get 1mm = 0.03937007874015748031496062992126 inches.

If we take the reciprocal of 0.03937007874015748031496062992126 we get 25.4 .

If you do not round off you will not get an error.

Since I am using the microsoft calculator on my computer I am limited by the number of decimal places that it can display.



Jason

This is all true if you hold an infinite number of decimal places during your calculations. In reality the mantissa of most calculators is only 8, after which roundoff errors do occure. I had an example I used to teach my students, and I can't remember it off the top of my head. (If I get really ambitious I'll look it up. But I doubt I'll get that ambitious.)

But it was something like doing the same calculation series and in one case rounding to 1 decimal place each set of the series while in the second case rounding to three during the course of the series of calculations. One answer turned out to be almost double the other. So roundoff can make a difference.

But, for 99% of what were doing here, you are right.