Metric Machinists

[rmm727]

Since there are a number of members here from other countries, I thought I'd ask...If you use the metric system and are a machinist, how far do you usually measure down to? In the US, I would normally measure to 1 thousandth of 1 inch (.001") with a dial caliper or 1 tenth of 1 thousandth (.0001") with a micrometer. How does that work in the metric system? What is the terminology? I see that the metric system uses a comma instead of a decimal point so in other words, how far to the right of the comma do you typically go and what are the terms for this?

Thanks,

-Ryan

 

[itsid]

Well I'm no machinist, but since I'm from germany we have rules for that..(as we have for EVERYTHING!)
depending on what you do and need...
symmetric or asymmetric tolerances, position or size (or both) roundness, straightness ...I could go on

but after all we follow the same IT grade you do
So IT7 is 35µm IT1 5µm.. IT10 140µm
1000µm = 1mm
1" = 25.4mm
1µm = 0.0000393700787 inches

'sid

 

[J_Walker]

basically the american way of machining is easier..........

 

[fowler]

Nope the imperial system is very backward and antiquated

Just ask all the old blokes around here who grew up on imperial and had to change to metric

Once they leart metric they found it way Easier

But because a lot of stuff was made in the US
All the manuals containing al the specs are imperial

So I have to work in both systems
Psi or kpi
Inches or cms

We have 0. Or a mill

35 thou = 0.889 mm

 

[machinist@large]

I can work with both systems; if I'm using electronic tools, (mostly at work ATM) I just select the system needed. If I'm using my personal analog stuff, all I need to know is that there are 25.4 mm's to an Imperial inch, and 1 mm is .03937 of an inch. The only other tools I need are a legal pad and a pocket calculator.

As far as the argument of which system is supposedly "Superior" as far as accuracy is concerned, I can find, measure, and slice, dice & cube the same part in either system equally well in both.

But that's just me.

 

[Felineman]

LOL I personally have 12 years machine shop and my father has 42 and I'll tell you we all use the Imperial system for the lathe and mill work and Metric for the measurements It depends what you have to make if its small its metric if its big its Imperial. There isn't any standard in the shop or any shop.

 

[exenos]

If its machining I use Imperial, but if I'm just measuring something I will use what ever side of the tape measure fits the measurement the easiest.

EDIT: I feel dumb, didn't read the op fully.

 

[rmm727]

Keep em coming. The previous posts have been helpful. So if you are running a lathe and where measuring in mm's, you could have 21.456mm or 21.4567mm if using a caliper or mic for measurement? (just example #'s) Main reason I asked this question to begin with was I was measuring the bolt pattern of a Honda pitbike clone engine. I got the pattern off of a website. The website said 45mm x 90mm. I measured something close. It ended up being 44mm x 90mm. If I reverse engineer something with an imperial measurement, often it will be close to a fractional measurement and I can just go with that. What do metric items end up being? Forgive me, I'm having a hard time trying to describe what I want to ask.

 

[landuse]

1mm out is quite a lot IMO

 

[itsid]

I think the real problem is, what you are measuring and how.

You somehow cannot measure inches and convert very well,
as long as you don't know what inches you're measuring.

imperial inches are defined at 25.4mm that's correct
but survey inches (US) are different than imperial inches.
So definition:
imperial inch: 254⁄10000 m (1" = 25.4 mm)
survey inch: 100⁄3937 m (1" = 25.400051 mm)
so you can be off the tolerances within just a few inches.
Even if you measure just inches but with two different systems.

So to measure metrics use a metric tool (caliper, tape, stick..whatever); especially on larger parts

'sid

 

[zbuck]

itsid, is not a survey inch used for land/carpentry measurements or metal fabrication? Therefore not pertaining to metric machining?

 

[itsid]

Well around here a cm is a cm, a mm is a mm and a km is a km..
and it's not different wheter you're a carpenter or a machinist.

While it's true that different tolerances are to be met, it's still the exact same nominal measurement.

I have not the slightest idea, if a carpenter has a different idea of an inch than a machinist in the U.S.;
how could I

But honestly if a metal fabricator uses a different inch than a machinist...
then please allow me to ask: "What the he!! is wrong with you people?"
No wonder 54 years of combined experience isn't enough to fit a 1" axle into a 1.2" hole

'sid

 

[fowler]

That was the whole reasn the metric system was invented

It's a standard
It's Set globally
All measurements that all relate to each other

U will also find that there's no longer a standard inch or yard
If u want the official measurement then u have to use the metric standard and take that as a percentage

 

[machinist@large]

I think the real problem is, what you are measuring and how.

You somehow cannot measure inches and convert very well,
as long as you don't know what inches you're measuring.

imperial inches are defined at 25.4mm that's correct
but survey inches (US) are different than imperial inches.
So definition:
imperial inch: 254⁄10000 m (1" = 25.4 mm)
survey inch: 100⁄3937 m (1" = 25.400051 mm)
so you can be off the tolerances within just a few inches.
Even if you measure just inches but with two different systems.

So to measure metrics use a metric tool (caliper, tape, stick..whatever); especially on larger parts

'sid

OK, we're getting really close to that slippery slope where we begin to argue for arguments sake, rather than the real world reason (I believe) this thread was started for.

'Sid, I've never heard of a survey inch before; I've always been taught that there were 25.4 mm to an inch, and a mm is .03937 of an inch.

1 / by 25.4= .039370079

1 / by .03937= 25.4000508

Real world #'s? I know there are many places where they measure, hit, and hold those types of tolerances; on the other hand, the ~ 95 to 99% of the shops like I've spent my my career in will never be able to afford the machines to manufacture parts to that close of tolerances, let alone be able to reliably and repeatably measure them.

Since you usually need some high $$$ equipment to measure that small (read lab grade), it got me wondering about how, when I first got out of school and into the trades I got my stapled to the wall for using the conversion factor .394 for a mm; just ran that one with the calculator too.

1 / by .0394= 25.38071066

That little mistake could cost you big time.

That was the whole reasn the metric system was invented

It's a standard
It's Set globally
All measurements that all relate to each other

U will also find that there's no longer a standard inch or yard
If u want the official measurement then u have to use the metric standard and take that as a percentage

Umm, no. The metric system has it's start in the late 1600's in France. The French revolution in the 1790's just pushed it along; and while the basic logic is sound, the politics behind it is full of skullduggery.

As for the standards of length (distance) and speed, I may be wrong, but I believe they are now referenced off of the specific wavelengths of a specific frequency of light, due to it's relative safety from human mistakes.....

 

[Felineman]

As I have stated I have 12 years and my father has 42 years of machine shop experience. Metric is good for some things and imperial is good for others bearings come in both. When using a lathe most read in both if you have digital read outs. The more common is 1/1000 inch by preference. But with the push of a button you can switch. My buggy I'm building is in metric so having to convert it would take forever and you would never get the measurements right. Hence I'm using metric, BUT I will be using Imperial when I have too. Metric is used in many places from automotive to architecture and has its place for being extremely accurate, BUT so can Imperial. Its more a what do you prefer then what is more common.

 

[J_Walker]

From my understanding.. imperial was kept around because when you got into the thousands + it was actually more accurate. I don't remember where I heard this, but I've always thought that's why its was so commonly still used in machine shops..?

again, I don't got much to offer here..

no matter how it gets done, as long as it gets done, and done right.

 

[machinist@large]

Like I said above, I can find, measure, slice, dice & cube any part with equal accuracy in either system. And while 'Sid rightly brought up an accuracy issue/ discrepancy issue with the mathematics (which I added to), if you're worried about the crank bearing dimensions in your lawn mower/ motor cycle/ automobile, those are the ~5 to1 % of applications I was referring to when it came to real world measurement....

 

[fowler]

I don't see this going anywhere

Buy to was wondering today having just got our first rain in more than 3 months

What is rain measured in over there?

I got 5 1/2 mm today which isnt bad
Good for the garden

 

[itsid]

As for the standards of length (distance) and speed, I may be wrong, but I believe they are now referenced off of the specific wavelengths of a specific frequency of light, due to it's relative safety from human mistakes.....

yes, there are several definitions by now.
the 1650763.73 mutliple of the wavelength generated when the Kr Nuclide changes it's state from 5D5 to 2P10 in a vacuum..
*cough* okay why not

A little more user friendly is the other definition:
the distance light travels in 1/299792458 seconds.

But I really love the original idea of the meter

Since all countries had their own system of measurements, based on several stupid things (like the width of king Henry's thumb)
The idea was to divide the meridian arc
(expected to be equal around the world) into ten million pieces.
To have a length measurement that is independend from it's origin, kings and politics; and thus perfect for international trade.
Same goes for the gramm,
One kilogramm is the weight of one litre of water (@4°C),
one litre is a 10cmx10cmx10cm cube.
And since needed the celsius scale is also easy to check with water.
it freezes at 0°C and boils at 100° C

By now we know the french were off by a bit.
But still, if you assume a meridian arcs length (travel distance) from the north pole to the equator of 10000km you're still very close to what it is. (for Paris it's 10002km)

simple to convert length to mass, easy to scale up and down;
perfect for international acceptance.

The best idea the french ever had IMHO

'sid