When the Inch turned.

[Ironwoodsmith]

As I only recently became aware, in 1959 the USA adopted a new inch standard. It became equivalent to exactly 2.54 centimeters. Before that it was defined as 1 meter = 39.37 inches (exactly) which equated to an inch being equal to 2.05400050800101600203200406400813 mm.

My question is how this affected machining at the time. Ror instance is a collet form 1955 obsolete after 1959. Did the two systems live side by side for a period of time?

What about the measuring instruments as well? Did they need to adjust their standards?

Is it such a small number that it was just ignored?

Did it have any effect at all?

Things must be slow around here. What do you think?

[danlb]

I think you added a 0 after the decimal. It should be 100 / 39.37 == 2.5400050800101600203200406400812801625603

That's such a small amount that it would never be noticed in tools.

Dan

[Mister ED]

I think you added a 0 after the decimal. It should be 100 / 39.37 == 2.5400050800101600203200406400812801625603

That's such a small amount that it would never be noticed in tools.

Dan

I guess more importantly, did it impact the lowly Inchworm??

Sorry couldn't resist. ;-)

[boslab]

I think it became the nearly inchworm.
However the old boy got bigger according to the alternate news
Mark

[MattiJ]

No practical effect on machining, but US land surveyors continue to use old inch even today!

[Ironwoodsmith]

No practical effect on machining, but US land surveyors continue to use old inch even today!

I guess that would have to be the case!

And I supposed that would be important in a moon shot as well.

So how many millionths are we talking about in 1 inch?

[J Tiers]

Just count decimal places..... 5 millionths per inch at 2.54000508.

About 13 feet in 500 miles.

In one mile, not quite 3/8 of an inch.

Per 100 feet, about 6 thou. Not much to worry about for surveying the property.

[darryl]

I was going to say about 2 millionths of an inch- not very much anyway.

[lakeside53]

Another reason to just use metric

[John Stevenson]

We had to recalibrate our welding machines.

[Paul Alciatore]

Wikipedia has some facts on this. The change from 39.37 inches per meter to 25.4 mm per inch took several years internationally, but occurred in the US in 1933.

The difference is indeed only one part in about 2 parts per million so a 500,000 inch distance would be different by one inch. OTOH, I doubt that many people here or in almost any machine shop would ever notice the difference. a 50 inch long part would be only 0.0001" different. How many shops have the equipment to measure to even +/- 0.001" over that distance? Not mine, for sure.

Perhaps people like Starrett who make micrometers made some adjustments, I don't know. Jo block makers surely took this into account for their in-house standards.

In short, your 1930 Micrometer should be A-OK. There are other things to worry about.

[Paul Alciatore]

Forget your your sense of superiority, the length of the meter was originally defined as 1/10,000,000 of the distance from the North Pole to the equator. Talk about an awkward standard. It has also been adjusted over the years. And when they did so, they compared it to the various national inch/yard standards as well as the existing meter standards. Fact was, they had all either been made a bit different or they had changed by different amounts over the years since they were first made.

No telling when the meter will be changed again. It all depends on what problems they find in the 9th or 10th or even the 20th decimal place. At present I believe it is currently defined in terms of the distance that light travels in a small fraction of a second. But even that has some variations.

Science stands still for no man nor for any measurement system.

Another reason to just use metric

[Forrest Addy]

I go a problem with the metric system as applied here in the US. Paint brushes, these days, are sold in millimeters but paint is still sold by quarts and gallons. Does that mean if you expect coverage of 250 sq ft per gallon you have to measure the work in square meters? If you do will the coverage come out even? 250 square feet = 52.511 square meters; how do I roll the paint thicker to roll the paint to hit 52 square meters? Is there a penalty if a fraction is left over? Gawd! I'm confused.

Seriously, I don't see the point of this discussion outside of scratching the itch of curiosity. The difference between the two conversion factors is couple parts per million. Six significant figure linear measurement is the realm of the hard core metrologists and the people in NIST who sweat physical standards.

Closest I ever worked to in my life was five using the best equipment in the shop and that was a real nightmare where perfectly reasonable people took forever to deliberate every last potential source of error and their probable effect. Never again.

[Georgineer]

... It should be 100 / 39.37 == 2.5400050800101600203200406400812801625603 ...

Dan

To put it in perspective, anything after the sixth decimal place is smaller than the wavelength of light.

George

[MattiJ]

Forget your your sense of superiority, the length of the meter was originally defined as 1/10,000,000 of the distance from the North Pole to the equator. Talk about an awkward standard. It has also been adjusted over the years. And when they did so, they compared it to the various national inch/yard standards as well as the existing meter standards. Fact was, they had all either been made a bit different or they had changed by different amounts over the years since they were first made.

No telling when the meter will be changed again. It all depends on what problems they find in the 9th or 10th or even the 20th decimal place. At present I believe it is currently defined in terms of the distance that light travels in a small fraction of a second. But even that has some variations.

Science stands still for no man nor for any measurement system.

There is going to be some "major" chances in year or two in the definitions of mass, voltage, temperature and some other units.

In some fields you even have to specify wich definition you are referring to: IPTS-68 and ITS-90 temperature scales are radically different if you work on temperature calibration lab...