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Arbo
10-07-2005, 09:44 AM
When converting SAE to Metric or vise-versa, does 2.54 cm EXACTLY equal one inch when we are talking in thousandths? Thanks.

BillH
10-07-2005, 09:56 AM
Yes, im pretty sure.

JPR
10-07-2005, 10:00 AM

SGW
10-07-2005, 10:41 AM
Yes. That was established by international decree around 1961.

Prior to that, the English had a definition that was ever-so-slightly smaller (or maybe it was larger), the Canadians, had the 2.54cm=1" exactly definition, and the US had a definition that was ever-so-slighly larger (or maybe it was smaller). They all got together and agreed to use the Canadian definition, 2.5400000..... cm = 1", exactly.

Swarf&Sparks
10-07-2005, 10:56 AM
to the best of my knowledge, it is 25.40mm = 1"

Swarf&Sparks
10-07-2005, 10:58 AM
and another extremely useful converter, versaverter. Freebie from

http://www.pawprint.net/vv/

Converts damn near anything http://bbs.homeshopmachinist.net//biggrin.gif

Evan
10-07-2005, 11:05 AM
Which means that if you have an old inch ruler it is off slightly. It depends on where you are. Up to 1959 the UK inch measured 2.53998 cm while the US inch was 2.540005 cm. The inch is and was defined by the meter so really we all use metric.

Here is a fun site:

http://www.weights-and-measures.com/index.html

[This message has been edited by Evan (edited 10-07-2005).]

Paul Alciatore
10-07-2005, 11:58 AM
Evan,

Older rulers are just as accurate as present ones, at least as far as the standards they were based on. You are talking about millionths of a centimeter for gosh sakes. That's about one third smaller than millionths of an inch. Even grade 2 gauge blocks have a tolerance range of 0.000018 cm.

Don't get the newbees worried about their older rulers or even their older mikes.

But yes, since '61 the conversion has been exact. Even before then, I doubt that anyone here ever had a situation where it made any real difference.

Paul A.

Evan
10-07-2005, 12:47 PM
Spoilsport http://bbs.homeshopmachinist.net//biggrin.gif

andy_b
10-07-2005, 12:54 PM
<font face="Verdana, Arial" size="2">Originally posted by Paul Alciatore:

But yes, since '61 the conversion has been exact. Even before then, I doubt that anyone here ever had a situation where it made any real difference.

Paul A.</font>

unless you were launching a probe to Mars. oh wait, in that case they forgot to even do the conversion.

andy b.

Yankee1
10-07-2005, 01:49 PM
.001"=.0254mm

John Garner
10-07-2005, 04:12 PM
Back before the US, Canada, and Great Britain agreed to a common standard length for the Inch, the US Inch was officially defined as 1/36 US Yard, not directly linked to the length of the Meter. But since the US Yard at that time WAS officially linked to the Meter, being defined as 36 / 39.37 Meter, a bit of simple arithmetic showed the US Inch to be 1 / 39.37 Meter, which made the US Inch 25.4000508 millimeter long.

The post-1959 Inch is two parts-per-million shorter than the old US Inch, which is now obsolete in all fields except land surveying, where it remains as a rarely-encountered-or-used subdivision of the US Survey Foot. The US Survey Foot, defined as 12 / 39.37 Meter, was introduced in 1959 to prevent "muddying" high-order surveys with New Foot / Old Foot differences.

sauer38h
10-07-2005, 11:51 PM
As long as the Pedantic Squad is saddled up and riding, I might note that ANY coherent measurement system is a "metric" system. "The" metric system is SI metric, or Système International d'Unitès, a development of the older MKS (meter/kilogram/second) system. The even older CGS (centimeter/gram/second) system seems to be deader than disco.

The really important units (such as the Unified thread system) were settled between the civilized countries - the US, Canada, Britain, and Australia - in 1948, when France and Germany were too far out of things to make the usual pig's breakfast of it all. In that era, French, German, and Swiss "metric" systems were not particularly well standardized.

The only thing which puzzles me about the old inch is, why were those old wood school rulers - the ones with the metal edges, good for whacking things with - always made of beech? Perfectly good wood, but not obviously superior to birch or hickory or ash or maple or whatever. I still have some from 40+ years ago - damn things just won't die.

david_r
10-08-2005, 12:21 AM
sauer,
I suspect for the same reasons that traditional planes had a beech sole. Low movement and high durability with the radial rays providing end grain endurance no matter how it is sawn.

sauer38h
10-08-2005, 01:08 PM
Was that beech or hornbeam on planes?

Swarf&Sparks
10-08-2005, 01:39 PM
or if you get real lucky, lignum vitae http://bbs.homeshopmachinist.net//biggrin.gif

Norman Atkinson
10-08-2005, 03:32 PM
Or African Blackwood or Rosewood or if your really into things - brass and ivory.

To think that the younger generation have to use plastic. Clear might be OK- but I got a yellow one. These little fellows seem to get everywhere.

And then there are " Guessing Sticks"-
for the Harry Potters of Yesteryear?
Magic things.

Norm