#### Metric threading

If the change to a new standard was greater in the UK, is a different set of metric transposition change gears needed for very old imperial lathes?

I take it that the now internationally accepted is close to the post 1900 standard. This I take it means that those with lathes made in the early 1900 will not (from this) have trouble cutting threads to the current American or imperial standards?

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.

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.

Trying to reach Latheman...Fred

The (international) inch has been exactly 25.4 mm since July 1959. At this point in time the (international) yard was redefined as 0.9144 metre - until this time the ratio between the US yard and the metre was different to the ratio between the UK yard and the metre. For more information, see Engineering Metrologyby K J Hume (2 ed) Macdonald London 1967. The American inch changed by 2 millionths of an inch and the UK inch by 1.7 millionths of an inch. The international inch falls mid way between the old UK and US inch.

References:

- The Yard Unit of Length, Nature, Vol 200, No 4908 pp 730-732 23 Nov 1963
- The United Kingdom standards of the yard in terms, of the metre (British Applied Journal of Physics)

**From:**DJ Delorie dj@... [SOUTHBENDLATHE]

**Sent:**Monday, February 26, 2018 15:49

**To:**SOUTHBENDLATHE@...

**Subject:**Re: [SOUTHBENDLATHE] Metric threading

"'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
:-)

Which begs the question, “Why did they decide to define the millimeterWell, the inch was defined by some ancient king's finger, and the meter

to be exactly 1/25.4 of an inch? Why not 1/25th?”

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 :-)

**From:**DJ Delorie dj@... [SOUTHBENDLATHE]

**Sent:**Monday, February 26, 2018 15:06

**To:**SOUTHBENDLATHE@...

**Subject:**Re: [SOUTHBENDLATHE] Metric threading

"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)

I have a question for those who use changewheels: 25.4 x 5 127.0, 5Yes. A 127-to-something conversion essentially converts your leadscrew

being the first multiplier that results in a whole number of teeth, so

does a wisely deployed 127 tooth gear provide EXACT metric pitches?

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)

**From:**"DJ Delorie dj@... [SOUTHBENDLATHE]"

**To:**SOUTHBENDLATHE@...

**Sent:**Monday, 26 February 2018, 18:33

**Subject:**Re: [SOUTHBENDLATHE] Metric threading

"'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

"'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

The imperial thread dial is useless with metric threads. You eitherWell, not *totally* useless, as you need to have it to use that

need to leave the halfnut engaged, or use the technique half-way down

this page: http://conradhoffman.com/metricthreading.htm

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

I have a set of metric gears for my 9 but have yet to get counter toI had a similar problem with my model 34 and ended up 3d-printing that

sync with them just have to set each cut into the thread. Any simple

answers? Thanks

part: http://www.delorie.com/photos/southbend-lathe/img_3133.html

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

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