Yeah, O'l Metric System and catch up Everyone else, Please.

[J Tiers]

1 mm = 0.03937007874015748031496062992126", many home shop machinists find this daunting.

I found that even more professional machinists found it daunting.... some have even said things like "you can't measure that in millimeters".

I DO think you would find it MUCH more comfortable over on PM, where you can slam "home shop" folks as much as you want without anyone thinking badly of you. Of course, you will find some people who are on this forum are over there also, even lowly and despised "home shop" people who everyone knows have to look up with a telescope to see whale crap.

[Black Forest]

All are fractions, 1/8" is also expressed as125/1000".
Using a common denominator is easier.

10.20 mm is also expressed as 10 1/5 mm

10.2mm I immediately thought of the drill size for a 12mm standard thread which is 1.75mm. That is the beauty of metric when it comes to drilling and tapping. You just subtract the thread pitch from the outside diameter and you have your drill size. I live in the civilized world and I have never seen in Germany 10.2mm expressed as 10 1/5mm

[Black Forest]

There are four main reasons I will probably not move back to USA. 3 phase in every home, the metric system, walk-in refrigerators and unlimited hot water!

[Galaxie]

I dont want a Fraction when dealing with numbers. I want exacts. The Metric system gives you that.

JRouche,
Forgive me, but I don't understand this part. Fractions are exact. Take this example:

1/3 = 0.33333... it goes on like that with threes forever.

In that case the fraction is exact and the decimal is an approximation.

[Galaxie]

My only real beef with the SI system is that they chose to define their threads by the pitch instead of by threads per distance. Seems to me that choice makes it a bit more complicated for manual lathes & thread dials. Am I off base there?

[old mart]

My only real beef with the SI system is that they chose to define their threads by the pitch instead of by threads per distance. Seems to me that choice makes it a bit more complicated for manual lathes & thread dials. Am I off base there?

I had never noticed that difference until you mentioned it, but both systems seem totally natural to me.

Imagine, that 6mm thread has 1000 tpm.

[plunger]

I live in an uncivilised world at the bottom of Africa. Thank God we use the metric system

[Bented]

All lathes produce threads by the "lead" of the helix desired, including multiple start threads.

A 1 mm 60 Deg, V thread is .03937 lead per revolution, a 32 TPI 60 Deg. V thread is .03125 lead per revolution, the difference is small but will not work together.
I fully understand this, one system is not superior however.
If you have a machine that will only produce threads using one system this is limiting.
More modern machines produce threads with the required lead, you may produce inch or metric products at will.

There is nothing to stop you from making parts that are 3/8 Dia-15 15/16 threads per inch should you choose to do so.
As much fun as this may be (-:

[mickeyf]

Originally posted by J Tiers View Post


Not every number is a fraction. There are numbers that can be expressed as a fraction, and ones that cannot. All of them can be expressed as decimals.




Only integers are not fractions
For practical purposes in machining all numbers expressed in decimal form are fractions with a common denominator.

.001/1000" is a fraction no?
1' = 1000/1000"
1 mm = 1000/1000 mm
100 people in a single room = 100/100 people there, if one leaves you now have 99/100 people.

Beat me to it.
1/1 is a fraction. A silly, unnecessarily complicated fraction perhaps, but the value it represents can certanly be expressed as a fraction.

All of them can be expressed as decimals

Ok, show me pi as a decimal. Get back to me when you're done. I'll wait here!

Seriously, in practice it should not be a big deal either way. I use both. I do not own any metric drills, but I have made a chart that shows me which number, letter or fractional bits are "close enough" when I need a hole for a metric tap.

[Rich Carlstedt]

There are many inequities in the world, none are perfect....some may think "Their" system is better...but it isn't
It is a reflection of the work that one does , and what fits "Your" needs in that work.
Take a Crayon maker, and a Portrait Painter , they both use colors , but to the crayon Guy, a Red may have 2 or 3 shades, while the Portrait painter
will have 1,000's of Red in his repertoire .. Who is right ? Neither !
They reflect what they EACH need !
How about Jewelers , They use carots, not micrograms or pounds and even their "Ounces" don't match the rest of us.
And why do machinists whether Metric OR Imperial not use "Radians" ? and instead specify Degrees !
And if METRIC was perfect, the circle would be divided by 100, and not 360 units ..
The only thing they have in common is the number of letters--Meters and Inches - both have 6 letters ...HOORAY, there is some commonality !

The above may be tongue in cheek , so to speak , but really nothing is perfect.
Like many who have posted here, I have worked with Metric and Imperial and even been hired due to my ability to equally use both systems
You might say I am color blind to variations . What your personal opinions are , reflect the type of work you want to do, not what is available
If you think this subject is personal , lets talk about women......err no that is another argument

Rich

[mklotz]

...How about Jewelers , They use carots, not micrograms or pounds and even their "Ounces" don't match the rest of us....

It's spelled "carat" and, just so all the jewelers use the same weight, it's standardized to be 200 milligrams since 1907

https://en.wikipedia.org/wiki/Carat_(mass)

Many of these other wacky units are standardized in metric terms when used in commerce.

[Rich Carlstedt]

Right Marv , and lets not forget with Jewelers here in the USA and GB that
"Karats measure the parts per 24, so that 18 karat = ¹⁸⁄₂₄ = 75% and 24 karat gold is considered 100% gold.^"
Now the Europeans call 18 karat 75 % and "I" think that is the better way
So in other words, Carat is Metric and Karat is fractional and both used in the same business
But I am not a jeweler either, so-----
guys it is not unusual to be Bi-Math Enabled

Rich

[JRouche]

JRouche,
Forgive me, but I don't understand this part. Fractions are exact. Take this example:

1/3 = 0.33333... it goes on like that with threes forever.

In that case the fraction is exact and the decimal is an approximation.

how is dropping 3s forever exact?

You are dropping threes looking for the two then one then zero forever. Not exact. For that matter no system is. Wait? Metric. When you start with zeros (base 10) you can get back to zero. With the fraction/decimal system we start with one and one is not a good starting point. Zero is. JR

[tom_d]

Well WRT (with respect to) the use of the term "Metric System" on all the computer forums I've read the "intent" is the SI bull**** that I was referring to. :-)
There are even more than two "metric systems" the Japanese and German usage both and maybe others have various versions that don't follow the SI exactly.

...lew...

Wasn't it the Germans who wanted to cut up a full circle as 400 degrees?

[Paul Alciatore]

That is not "daunting". It is just too much. After the first five places, which are followed by two zeros, it has no real meaning in your shop or mine. Or even in any but the absolutely most accurate ones. Even the wavelength of visible light (around 500 nm), which is a dimension that optical shops work to, even to a fraction thereof (1/10 wave), is covered by the first 8 or 9 decimal places of your example. I doubt that you or anyone else here will ever work to any greater accuracy than that. And in truth, most machining work is only done to a thousandth of an inch (0.025 mm) or so.

And no matter how many decimal places you add to that number, you will never reach the end. It goes on forever. But, SO WHAT?

But you do not even need to use that number in any of your shop calculations. One inch is DEFINED as EXACTLY 25.4 mm and there are no more, non zero, places to that conversion number. 25.4 IS THE EXACT CONVERSION, BY DEFINITION. And 25.4 is really easy to remember, even my memory-challenged brain can and does remember it. Conversions with it are dead simple:

To convert inches to mm, just multiply by 25.4.
and
To convert mm to inches DIVIDE by 25.4.

It could not be any easier. Nor any more accurate, either in theory or in practice. If you were asked what is a seventh of an inch (or meter or mm) would you multiply by 0.142857143......? That is another infinitely long decimal number. But would you use it to find one seventh of something? Well I certainly would not. I would just DIVIDE by 7. That's the easy way and so is dividing by 25.4 instead of multiplying by the infinite decimal that you cite.

I am sorry, but citing the hard way to do something is not a valid argument against it. Another example; you could build an automobile piece-by-piece. There is an old song about that. You could even make the pieces as you go. But few ever do it that way. Most of us just buy an already built car, either new or used. We do it the easy way or, at least easier one.

1 mm = 0.03937007874015748031496062992126", many home shop machinists find this daunting.