Homebrew PCBs

"Mil" isn't used by machinists (here in the U.S.).  Our basis is the 
thousandth of an inch .001 and of course the Inch unit.

So, our references follow those two terms.  For example, the distance 
of 12 13/16"

A machinist might say that as 12 inches and 812 and a half thousandths.

12 13/16 = 12.81235

The 12 inches is obvious
the 812 refers to the number of thousandths
the 1/2 refers to 1/2 of a thousandth = .0005

If you hear a machinist say something like "It 3 tenths off".  He 
isn't referrening to tenths of an inch, it's tenths of a thousandth 
= .0001"

Very few machine shops have the real ability to work to any greater 
precision than a tenth of a thousandth .0001".  However, there are 
rare cases when you get into the millionths of an inch.  Tolerances in 
this range are usually found in electronics and optics.  I personally 
have only had exposure to millionths of an inch when working with a 
company that produces optics for spy cameras (satellites).  But again, 
we would say a "Mil" in that reference, it would be said as for 
exampe: Twenty millionths of an inch

The crazy terminology of each industry, it keeps everyone confused ;-)

Chris




> > 
> > contrary to what is implied in the term "mil",
> > 1 mil = 0.001 inch
> ...
> 
> 
> Actually, the term "mil" _does_ make sense.  Americans assume it's 
supposed
> to make sense in English;  "mille" means thousand in French.  But I 
wouldn't
> be surprised if machinists used that term for some other reason too.
> 
> Robert
> :)
>