stupid question:  What's the difference between mil and mm?

dingebre wrote on Sat, 15 May 2010 14:06

Hi James,

 

Coming from an education in mechanical engineering and physics, the

terms "mil" and "thou" are completely unambigous and equivalent. That

said, I think in the world of PCB layout, "mil", "thou", and "mm" do get

confusing, especially to one with little or no machining background or

mechancial engineering studies.

 

Form my experience, "thou" is more arcane and "mil" is more commonly

used. Either works.

Maybe there is a cultural thing here too.  First, I am not a mechanical

engineer so I don't have any background in using this term in that context.

Second, I grew up in Canada as the first generation that only learned

metric and not imperial units.  Thirdly I grew up on a farm so we weren't

often concerned with things that were so small--if you couldn't fix it with

a hammer, some binder twine, and a role of duct tape it wouldn't last very

long

In any case, I have heard mil used as a short form for both "millimeter"

and "mm" within about 15 minutes in a high end PCB design class by an

American (who has been educated in imperial units)!  This person is very

well known and well respected in the PCB design world.

The reason I like "thou" is because it is arcane and no one uses it any

more for anything.  So I can define that in my organisation as 1/1000 of an

inch.  I realise it could be confusing but for us it is not.  I have yet to

hear a better alternative that isn't confusing.  I'm open to it though

since I like unambiguous.

Quote:

A curse on whoever set up English units!

Agreed.  I suspect most of those units were devised in the late evening

inside English pubs.

--

James Morrison  ~~~  Stratford Digital

email:  james@eaglecentral.ca

web: http://www.eaglecentral.ca

Specializing in CadSoft EAGLE

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--

Web access to CadSoft support forums at www.eaglecentral.ca.  Where the CadSoft EAGLE community meets.

dingebre wrote on Sat, 15 May 2010 14:06

Coming from an education in mechanical engineering and physics, the

terms "mil" and "thou" are completely unambigous and equivalent. That

said, I think in the world of PCB layout, "mil", "thou", and "mm" do get

confusing, especially to one with little or no machining background or

mechancial engineering studies.

I don't have a mechanical engineering background, and to me "mil" is

unambiguous.  That term has been around for a long time.  Remember when

real to real audio tape was rated in mil thickness?

Quote:

Form my experience, "thou" is more arcane and "mil" is more commonly

used.

"Thou" seems to be something mechanical people use.  For PC boards, its

always mil, inch, or millimeter.  Lately more parts are defined in mm.

Although sometimes you see a datasheet insist on all mm and then everything

is cumbersomely a multiple of 2.54mm.  C'mon guys, if your pin pitch is

really .1 inch, don't pretend it's in mm.

--

Web access to CadSoft support forums at www.eaglecentral.ca.  Where the CadSoft EAGLE community meets.

There seems to be a cultural difference indeed. Sticking to length

measurements only:

I have heard Australians talk about mils, and it took me a while to

discover that they were talking about millimeters. I also heard British

engineers refer to millimeter as "mil".

Both referred to 1/1000 inch as "thou", probably to avoid confusion...

In Norway and Sweden there is also a mil, but that one is equal to 10

kilometers and not very usable in PCB design ;).

In my neck of the woods, "mil" stands (rather unambiguously) for 1/1000

inch, or 0.0254 millimeter. Then again, I grew up with metric units...

Those readers interested in the the precise(?) what, where, why and how

could look (but probably already have looked) here

http://www.answers.com/topic/mil or here

http://dictionary.reference.com/browse/mil

Have fun (and always check your measurements).

--

Web access to CadSoft support forums at www.eaglecentral.ca.  Where the CadSoft EAGLE community meets.

On 5/15/2010 12:06 PM, David Ingebretsen wrote:

Hi James,

>

Coming from an education in mechanical engineering and physics, the terms

"mil" and "thou" are completely unambigous and equivalent. That said, I

think in the world of PCB layout, "mil", "thou", and "mm" do get confusing,

especially to one with little or no machining background or mechancial

engineering studies.

>

Form my experience, "thou" is more arcane and "mil" is more commonly used.

Either works.

A curse on whoever set up English units!

>

David

>

I think we are missing the real point here.  When is the world going to

finally go metric and completely drop the imperial system "FOREVER".

Then the use of "Mil" will become more obvious.  Lets face it; doing PCB

layouts with both metric & imperial components can be a pain.  How many

Mars probes do we have to loose.  How many 100 MPH speeding tickets are

we going to get.  The metric system is clear, (based on 10) and can be

checked back to a couple of standards.  I believe the imperial system,

based on the inch, was the width of a kings thumb pressed up against

glass.  And by "King" I don't mean Elvis or the Burger King guy.

John