# Converting metric prints to imperial



## GKNIPP (Feb 24, 2013)

I noticed in one of the threads in this section that there is a metric conversion table posted somewhere on HMEM, however, was unable to locate it.  I would be able to easily accomplish this task via long hand arithmatic but felt the conversion chart on HMEM would be more expedient.

I would  appreciate anyones help in finding this link as I have obtained a set of  BONZER hit-n-miss engine plans drawn in metric dimensions and need to convert them to imperial.

Thank you in advance for your help.

Greg


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## GWRdriver (Feb 24, 2013)

Greg,
The easiest way for me to become handy with Metric was to memorize the number *.03937*.  Some folks do it the other way around and memorize *25.4*.

1.000" = 25.4mm
1.000" ÷ 25.4 = .03937 = 1mm

Just multiply by .03937, or or divide using 25.4, and you will have the Imperial inch equivalent.    A small calculator will be a help.

For example . . . if a dimension is 77.25mm, multiply that by .03937 and the result is 3.0413".  Divide by 25.4 and the result is 3.0413".


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## basement_guy (Feb 25, 2013)

I converted some plans from inch to metric and found this list very useful.  
http://mdmetric.com/tech/cvtcht.htm


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## Woodster (Feb 25, 2013)

A word of caution!! To divide by 25.4 is EXACT, to multiply by .03937 is APPROXIMATE.


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## Ken I (Feb 25, 2013)

A further word of caution in that dividing by 25.4 is going to give you some really odd numbers - you then need to round them up or down to the nearest fractional or decimal equivalent (plus the same for matching parts) as well as the closest threads etc.

Otherwise just execute it in metric.

Since I work in metric and all my taps, reamers etc are metric my problem is usually the other way around - typically I redraw the imperial plans into the nearest metric.

Metricating / Imperialising is not as simple as just multiplying by some constant.

There are some other threads on this site giving some rules of thumb guidance for this.

Ken


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## trumpy81 (Feb 25, 2013)

GKNIPP said:


> I noticed in one of the threads in this section that there is a metric conversion table posted somewhere on HMEM, however, was unable to locate it.  I would be able to easily accomplish this task via long hand arithmatic but felt the conversion chart on HMEM would be more expedient.
> 
> I would  appreciate anyones help in finding this link as I have obtained a set of  BONZER hit-n-miss engine plans drawn in metric dimensions and need to convert them to imperial.
> 
> ...



Do you use CAD or 3D modelling at all?

If you 3D model you can make the conversion in any way you wish. Rounding up or down as you go and you have the benefit of checking clearances and looking for obstructions along the way too.

It would not be the first time I have discovered some anomalies between the model and the drawings which would save a lot of wasted time and/or material.

Just a thought.


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## kvom (Feb 25, 2013)

When I built Bogstandard's paddleduck engine, which is in metric, I just multiplied all the mm dimensions by .059 (was building at 1.5x).  Usually I would just write the inch dimensions on the plans, or do the calculation directly at the mill.

The only non-conversion needed is deciding on fastener sizes.  These are typically not that critical, so the nearest size or the most convenient works for me.


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## GWRdriver (Feb 25, 2013)

Woodster said:


> [snip].03937 is APPROXIMATE.


Yes, the full multiplier is .0393700787401575.   Perhaps it would a good idea, in the interest of absolute correctness, if all of us carried our drawing dimensions to 16 decimal places, otherwise disastrous dimensional consequences could possibly occur.


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## Woodster (Feb 25, 2013)

GWRdriver said:


> Yes, the full multiplier is .0393700787401575.   Perhaps it would a good idea, in the interest of absolute correctness, if all of us carried our drawing dimensions to 16 decimal places, otherwise disastrous dimensional consequences could possibly occur.


 Reminds me of a story i read years back where the powers that be decided that for all engineering works, to make the maths of it easier, Pi should be 4!!!
I was always taught that precision engineering is precise therefore you should always aim to be as accurate as your equipment allows.


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## Maryak (Feb 25, 2013)

The easy way to convert imperial to metric is to use 24mm = 1in.

This gives a slightly smaller model in metric than the imperial original but most standard imperial sizes convert to standard metric sizes e.g. 1/4" = 6mm.

Try it and you may be pleasantly surprised at the result.

Best Regards
Bob


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## GKNIPP (Feb 26, 2013)

Thanks for all of your input.  I am simply  taking the metric dimension, in miilimeters, and multiplying it by .03937 which gets me 4+ place decimals which I feel are more than satisfactory for the task at hand.  Yes, i am changing material thickness, shaft diameters and screw threads to  nominal imperial sizes and adjusting others where needed.

Basically, I am redrawing sections of the plan, longhand, so I can insure center distances and the such.

Greg


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## nemt (Feb 26, 2013)

Beiing brought up with the metric system,
I convert most drawings from inch to mm by taking 32 mm for the inch. Model becom about 25% bigger. Since I have rather sturdy machines, it is no problem and it works for me.
Just busy building Hit and Mis from David Kersel. First I draw all parts in Solidworks and use the drawings in the workshop.
Nemt


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## mygrizzly1022 (Feb 26, 2013)

Hi

There is a unit converter at this link.  Which you might find helpful, also included is a scientific calculator.  This link was suggested by another poster and I find it extremely helpful.  See below

http://www.eeweb.com/toolbox/calculator

Regards .....bert


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## blighty (Feb 26, 2013)

get yours self a calculator.......

take any metric number on your drawing  and divide by 25.4 you will then get the decimal equivalent in inches. round up or down to 3 decimal places.

e.g 12.5mm / 25.4 = 0.492125984251 bla bla bla make your bit 0.492"

don't worry about the 0.0001 your part will just be 2.5 microns to small.


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## Mbusha (Feb 26, 2013)

Woodster said:


> A word of caution!! To divide by 25.4 is EXACT, to multiply by .03937 is APPROXIMATE.



Thank you! Any conversion other than 25.4 is nonsense. I am new to this forum and home model machining but metrology is my life. 

Don't get me started. :rant::rant::rant::rant::rant::rant:


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## littlelocos (Feb 26, 2013)

Hello,
Another method I've seen (in one of Kozo Hiraoka's books on logging locomotive construction) was to use 25.6 instead of 25.4.  The idea here is that fractional-inch stock can be more easily substituted.  8mm goes to 0.3125" or 5/16".  100mm yields 3.90625 or 3-29/32, odd, but familiar to those of us using the IPS system.  

The other way around 1/25.6 = 0.0390625 exactly, so a 2" dimension becomes 51.20mm, etc.

Enjoy,
Todd.

p.s.  Mbusha is right..... Use 25.4 ALWAYS if your intent is to make a direct conversion.  The inch was redefined as 25.4mm a long time ago.  Some history can be found here:  http://en.wikipedia.org/wiki/Inch


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## GKNIPP (Feb 26, 2013)

I am not sure what you guys mean, but if I multiply the metric dimension by the .03937 or divide by the 25.4 I get the same answer out 5 or 6 places ??  Not sure why one hold more merit than the other, especially in context I am using them, model engines.    Either way I would simply round the answer  below to 1.417, however, if the 4th place decimal was greater than 5 I would go to 1.418.  Rarely have I needed to utilize or seen 4 place dimensions even used on model engines, much less going out 5, or more.  Now in other much more critical applications maybe.

36mm X .03937= 1.41732

36mm / 25.4 = 1.41732

Greg


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## Mbusha (Feb 26, 2013)

Greg, I was going to reply, but I split microns for a living. So I guess this is not the place. Carry on and I'll MYOB and make another Martini.


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## littlelocos (Feb 26, 2013)

Mbusha said:


> Greg, I was going to reply, but I split microns for a living. So I guess this is not the place. Carry on and I'll MYOB and make another Martini.


 

Ditto,  I work for NIST (and am off duty).
www.nist.gov
Carry on,
Todd.


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## GKNIPP (Feb 26, 2013)

I appreciate your input and value it.  I apologize if I came across incorrectly but I didn't intend to.  I can understand where you are coming from especially when you work with such tiny increments.  I have the same problem at times being a toolmaker as I work in tolerances of +/- .0002  very frequently so when I do get a dimension at  +/- .005 I feel like I have a mile to work with.  I find it hard, at times, to adjust myself.  BTW, have a martini for me as well!!!!!  LOL!!!!!

Greg


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## Mbusha (Feb 26, 2013)

GKNIPP said:


> I appreciate your input and value it.  I apologize if I came across incorrectly but I didn't intend to.  I can understand where you are coming from especially when you work with such tiny increments.  I have the same problem at times being a toolmaker as I work in tolerances of +/- .0002  very frequently so when I do get a dimension at  +/- .005 I feel like I have a mile to work with.  I find it hard, at times, to adjust myself.  BTW, have a martini for me as well!!!!!  LOL!!!!!
> 
> Greg



Greg,

Apology is not necessary, I am just nuts. 

All the talk about conversions causes nothing but problems in my world. The next common step in the conversation is the rounding problem. There are countless contradictory standards clouding the whole issue. IEEE Standards, would have us round to the nearest even integer. ASME Y 14.5 always round in, so as not to violate a tolerance zone. 

This all has not so much to do with trying to build an engine with my Grizzly mill  (where I hold .001" on a good day) but makes me crazy nevertheless.

So here I sit, having my second Cocktail, and bothering friends on the forum.

Sorry


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## blighty (Feb 26, 2013)

GKNIPP said:


> I appreciate your input and value it.  I apologize if I came across incorrectly but I didn't intend to.  I can understand where you are coming from especially when you work with such tiny increments.  I have the same problem at times being a toolmaker as I work in tolerances of +/- .0002  very frequently so when I do get a dimension at  +/- .005 I feel like I have a mile to work with.  I find it hard, at times, to adjust myself.  BTW, have a martini for me as well!!!!!  LOL!!!!!
> 
> Greg



im with your there..... i was a toolmaker once and mainly dealt with imperial and now its more metric. but i tend to have a small strop when i measure something  that comes out at 24.97mm when it should of been 25.00mm. to me i see the 3 but realy its just .001" which is bug all in typical garden shed enginerring.

how i see it...... tolerances are put on a job for a specific reason. there not put there to boost the ego of a engineer that can just give it the old "i'm way better than you bla blaa cos i can work to blaa blaa" (not making a dig at the micron guy's above )

as for the 25.4 or .03937 debate.... i have never heard of using .03937 as a conversion.  that doesn't mean to say its wrong, but as other have said 25.4 seems to be righter.


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## AussieJimG (Mar 6, 2013)

Download Alibre, feed it the Alibre files that I sent and tell it to print them with all dimensions in inches to 3 decimal places. Nag Nag Nag.

Jim


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