measurement language

[yobbo89]

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

 

[JimDawson]

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 )

 

[RJSakowski]

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.

 

[yobbo89]

great idea, i might print out some charts!

 

[Canus]

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.

 

[Flyinfool]

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.

 

[matthewsx]

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

 

[savarin]

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.

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[Mitch Alsup]

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

 

[Hawkeye]

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.