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Awesome. What a concise and detailed rundown. Not only the how- but the *why* a great 20th-century definition of standards finally settled into its final form.

Why 25.4mm instead of 25.0? Well, both the inch and the meter were already established when today's standard was adopted in the 1950s and the 1960s. Both had gone through several refinements before that point and more problematic was the fact that different countries had slightly different standards, at least for the inch. When the 25.4mm factor was adopted as the exact conversion, that distance for the inch was 1.7 millionths longer than the existing imperial inch and 2 millionths shorter than the existing US inch. I believe other countries also had differences. In short, it was a compromise and, being only a three digit number, it was a very convenient one. Two parts in a million was almost a negligible difference for almost all practical purposes.

If the inch had been redefined as 25mm at that time, then that would have been a difference of almost 1.7% and that would have been noticeable on almost every ruler and other measuring device that used inches. Virtually every one of them in the world would have been instantly inaccurate and would need to be replaced. Just for one example, a one foot ruler would be off by about 3/16". Even a first grade school child would notice. The confusion in the world of engineering would be enormous.

But, as I said above, the 25.4mm figure was only off by millionths of an inch. So, a 100 foot tape measure would only be off by 0.0002". For all practical purposes every existing measuring device could be used with no worry about it's accuracy. Only the most demanding measurements in physics and astronomy would need to ever consider the differences between the old standards and the new one.

So the 25.4mm figure was a magnificently practical compromise. I was getting my degree in physics (the science of measurement) in the early 60s and there was absolutely no reason to correct for the change in any of the experiments or calculations that I did in that program. The blueprints for bridges and buildings and automobiles and everything else were just as good after the change as before. And the same measurement devices were still used with no problems.

Send an email:

I use a Sherline electronic rotary table for cutting gears. One push of the button and you are at the next cut.
So very simple, but somewhat expensive solution.
Rick

"'Jon Woellhaf' jonwoellhaf@..." writes:

Which begs the question, “Why did they decide to define the millimeter
to be exactly 1/25.4 of an inch? Why not 1/25th?”

Well, the inch was defined by some ancient king's finger, and the meter
by some random planet's circumference... so go blame them :-)

And BTW, the inch is currently defined as 25.4mm, not the other way
around. Go figure.

Also BTW the meter is now defined in terms of the speed of light.
Slightly less random, but just as arbitrary :-)

"Edward Draper eddie.draper@..." writes:

I have a question for those who use changewheels: 25.4 x 5 127.0, 5
being the first multiplier that results in a whole number of teeth, so
does a wisely deployed 127 tooth gear provide EXACT metric pitches?

Yes. A 127-to-something conversion essentially converts your leadscrew
from "some imperial pitch" to "some metric pitch" (because 127mm is
exactly 5in, so whole numbers :). You still have the complexity of
*what* pitch, of course :-)

(note: it doesn't convert the physical TPI of the leadscrew, hence the
threading dial issues mentioned before)

Another alternative is to use an IN/METRIC thread dial (METRADIAL)

"'m. allan noah' kitno455@..." writes:

The imperial thread dial is useless with metric threads. You either
need to leave the halfnut engaged, or use the technique half-way down
this page: http://conradhoffman.com/metricthreading.htm

Well, not *totally* useless, as you need to have it to use that
technique (which I use). You just have to use it differently.

Also, that technique in video form:
https://www.youtube.com/watch?v=HXt4TWa382Q

You could also combine it with inverse threading so you don't have to
disengage in a panic: https://www.youtube.com/watch?v=Z-dqOi_z5bk

"Kelly Howerton kellyhowerton@..." writes:

I have a set of metric gears for my 9 but have yet to get counter to
sync with them just have to set each cut into the thread. Any simple
answers? Thanks

I had a similar problem with my model 34 and ended up 3d-printing that
part: http://www.delorie.com/photos/southbend-lathe/img_3133.html

(of course, I 3d-printed the metric gears too :)

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