# measurement language



## yobbo89 (Aug 9, 2019)

Hi all ,new here,i did a introduction post the other day , pretty much long time boilermaking, astronomy hobbyist, picked up lathe recently, interested in gear making/hobbing and threading.

I see a lot of talk about thou , thousands of an inch giving that there are charts available ,as i hobbyist in the metric system primarily ,do i even need to learn thousands ?

even thousands can be converted to a decimal in inches ?.

can i learn without this system, are there books available learning channels?

I inserted some pictures in metric mm measurements, this was my backplate i had to machine down to fit the new 4 jaw chuck, what is the clearance in thousands between the two ? is this a good fit for a back plate haha ?

google says 0.01mm = (0.3937008 thousands)?

i also founds this chart online that says 1 thou = 0.0254 mm.. confusing..

edit: i got it i think, i'm at almost 1/2 a thousands with the fit ? 0.5 thousands


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## JimDawson (Aug 9, 2019)

Great questions.  If you normally work in metric units then working in Imperial units is not really necessary.  A ''thou'' is just simply 0.001 inch.  It is normally the base unit that machinists work with in the USA.  0.001 inch = ~0.025 mm, for most work 0.02mm is close enough (what's 5 microns among friends   )


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## RJSakowski (Aug 9, 2019)

I work with both the Imperial aqnd metric system although most of my machining is in the Imperial system.  

There are just two numbers that you need to know.  There are exactly 25.4mm in an inch and going in reverse, .039370078  (usually rounded off to .03937) inches in a mm.  I keep a calculator handy for critical measurements but for  back of the envelope conversions, 25 mm/inch and .040 inches/mm work.  Converting from inches to thousandths just involves moving the decimal point three places to the right as does converting mm to microns. 

A trick that I use when machining and a calculator isn't at hand is to set my digital calipers at the dimension I want to convert and press the in/mm button.  For converting fractional inch measurements, as in drill sizes, I have several wall charts which give equivalent decimal inches and mm.


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## yobbo89 (Aug 9, 2019)

great idea, i might print out some charts!


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## Canus (Aug 9, 2019)

You are saying "thousands" when you mean "thousandths".  As Jim Dawson said 0.001 inch is 1 thousandth of an inch.  A good example would be as follows.  3.649" is 3 inches, 6 tenths of an inch, 4 hundredths of an inch, and 9 thousandths of an inch.  Correctly stated would be 3 inches 649 thousandths.


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## Flyinfool (Aug 9, 2019)

The bottom line is, are the graduations on your machine and measuring tools in imperial or metric. That will dictate the system you will want to use. If you are working from plans in the wrong system, it is best to sit down with a calculator and do all of the conversions of all the dimensions before you even start cutting metal.


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## matthewsx (Aug 9, 2019)

yobbo89 said:


> great idea, i might print out some charts!


Wall charts are the thing. Check withy your local tooling supply as they often give nice ones away free. 

And welcome
John


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## savarin (Aug 9, 2019)

Use a converter or digital calipers calipers to convert from one system to the other.
I use mainly metric and only use imperial when I have to.
I got so fed up with the 1.25 mm (approx) travel per turn on my cross slide dial that I made an M10x1 screw and nut along with a 100 divisions dial so its all metric now and easy to work out .
I say stick with what you are used to as theres less chance of making mistakes.
I'm also into astronomy and telescope making.








						80mm long focal length refractor
					

I came into possession of a matched pair of Jaeger 83mm dia lens of f15 focal length. Although a focal length this long is somewhat against current practice I thought it might make a good project. As this project is a long one I thought rather than just put the odd bit in the “what did you do in...




					www.hobby-machinist.com
				











						The Giant Binocular
					

Its time to re-visit the binocular. This is roughly what I want.   I started these over 5 years ago but got side tracked building recumbent bicycles. Now I've almost finished the refractor I've started back on these. This is being built entirely from scratch including grinding and polishing the...




					www.hobby-machinist.com


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## Mitch Alsup (Aug 9, 2019)

A thou is a way of expressing Imperial measurements in a decimal system.
{The "preferred" method of expressing Imperial measurements is fractions:: 5/64ths}

For almost any amateur use 1mm = 40 thou
For almost any amateur use 25.4 mm = 1000 thou


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## Hawkeye (Aug 9, 2019)

My main lathe has metric dials, while all my other machines are Imperial. The great equalizer is the modern Digital Readout (DRO). They all have a button that lets you switch between both systems in the middle of an operation.

If you do your own designs, you can use whichever system you like. If you're working off someone else's drawings, it may be worth knowing both.


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## yobbo89 (Aug 10, 2019)

savarin said:


> Use a converter or digital calipers calipers to convert from one system to the other.
> I use mainly metric and only use imperial when I have to.
> I got so fed up with the 1.25 mm (approx) travel per turn on my cross slide dial that I made an M10x1 screw and nut along with a 100 divisions dial so its all metric now and easy to work out .
> I say stick with what you are used to as theres less chance of making mistakes.
> ...



very nice!, i went through and read  both the threads!


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## yobbo89 (Aug 10, 2019)

i might be already won over, i've already have the units stuck in my head, 500 thou is 12.7 mm, i guess it makes me sound cool as if i know fine measurements and skilled at machining


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## yobbo89 (Aug 10, 2019)

JimDawson said:


> Great questions.  If you normally work in metric units then working in Imperial units is not really necessary.  A ''thou'' is just simply 0.001 inch.  It is normally the base unit that machinists work with in the USA.  0.001 inch = ~0.025 mm, for most work 0.02mm is close enough (what's 5 microns among friends   )



so what the thou system has introduced is an easier language for fine measurements , can it be applied to the metric system ? instead of saying 0.002 mm can we just say 2 thou in metric ? or is that too taboo ?


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## JimDawson (Aug 10, 2019)

yobbo89 said:


> so what the thou system has introduced is an easier language for fine measurements , can it be applied to the metric system ? instead of saying 0.002 mm can we just say 2 thou in metric ? or is that too taboo ?



In the metric language 0.002mm would normally be said as ''2 microns''.  That measurement is really too small to accurately measure in most home shops.  0.002mm = 2 millionths of a meter, or about 80 millionths of an inch, 0.00008 inch.  Small changes in temperature would affect the measurement.  The scales on my mill and lathe have 1 micron resolution, but I have no belief that they actually will measure to that accuracy.  Resolution and accuracy are two different things.


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## RJSakowski (Aug 10, 2019)

The term "thou" is a colloquial shorthand for a thousandth of an inch. On the other hand, "mil" is an official unit of measurement for a thousandth of an inch.  It's most frequent use is for the measure of the thickness of plastic sheeting.  Unfortunately, it is also used to designate a ml and, in some locations, a mm.  It also is used to designate 0.1% as in a mil rate and can mean 1,000 as in a mil = $1,000 dollars.  To add to the confusion, it is colloquially used to describe a million as in $1,000,000. (I prefer megabucks myself)  The mil is also a unit of angular measure, short for milliradian, and equal to 1/6400 of a circle or .05625 degrees or approximately 1/1000 of a radian(57.29577951.... degrees). Primary usage is by military.


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## JPMacG (Aug 10, 2019)

This might be regional, I don't know.  Around here we talk of "mils".  A mil is .001 inch.   Maybe it is short for milli-inch.

Oops, I missed RJ's post.   Never mind..........


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## RJSakowski (Aug 10, 2019)

Wire is often measured in mils.  Another wire measure is circular mils.  It is the square of the diameter in mils and is proportional to the current carrying ability of the wire and inversely proportional to the resistance, both of importance in electrical work.


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## NortonDommi (Aug 11, 2019)

Growing up with Imperial and having the country switch to metric while the bulk of everything was still Imperial swapping from one to the other is just practice.  Plenty of conversion charts available for anything to anything just start collecting and save to memory the most often used conversions in your calculator or else buy a calculator with a conversion function.
  If you normally work in metric and need to work off an Imperial drawing just copy and write all the conversions next to the original dimensions.
  As I frequently swop from one to the other I find paper on a clipboard and a sharp pencil along with a calculator are invaluable.
  Just be sure that you know 100% what system you are using and try and stick to just that system on any given project.  Mix Apples with Apples and Oranges with Oranges.


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## yobbo89 (Aug 11, 2019)

RJSakowski said:


> (I prefer megabucks myself)



hahaha, i like that one too!


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## yobbo89 (Aug 11, 2019)

In my trade we often see pipe listed as 25 nb 50nb 100 nb ect..usually  get the young apprentice to tell the boss that he ordered in 33mm pipe instead of 25mm!. ahah ..nominal bore should be the internal measurement .


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