Metric / Imperial conversions

[David Morrow]

In "Steam and Stirling" there is an interesting article on page 39 about converting plans from Imperial to Metric or vv. He gives an example of a Dutch builder who simplifies the conversion by using 1/32 = 1mm ( instead of 1/25.4 = 1mm). This gives the builder an engine that is about 20% bigger. I would like to build one of Jan Ridder's creations but convert on the basis of 1mm = 1/16 which would result in a somewhat larger engine. Maybe 1mm = 1/32th inch for a smaller engine ?

I realize that not everything will convert perfectly but many people don't always build exactly to plan anyway.

Any thoughts ?

 

[rake60]

When I'm converting Imperial to Metric I multiply by 25.4
Metric to Imperial I multiply by .03937

It's not perfect to an infinate number but it is close enough
for anything I can see on a mic!

Rick

 

[David Morrow]

When I'm converting Imperial to Metric I multiply by 25.4
Metric to Imperial I multiply by .03937

It's not perfect to an infinate number but it is close enough
for anything I can see on a mic!

Rick

Yes, but the use of 1mm -> 1/32 in. makes the need for a calculator or conversion chart all but unnecessary. And, much more common fractional & decimal results.

 

[greenie]

OK, here's another way of doing the conversion and it's real easy too, it is not 100 % dead nuts spot on, but it's that close it actually works.


0.040" = 1.00 mm

0.020" = 0.50 mm

0.010" = 0.25 mm

0.004" = 0.10 mm

0.002" = 0.05 mm


It's so quick and easy to do any converts from either Mongrel to Metric, or the other way round, it gets you that close, then it's out with the appropriate micrometer and you can then get it dead nuts spot on.

The machinery I work with, lathe and mill, are all graduated in Metric and to get it converted to Mongrel, well it's just a "chunk of urine" doin' it this way

OK, the purist are gunna shoot this lot down, just 'cause it aint 100% dead nuts spot on, all I'm sayin' is, give it a go and see how easy it is to do the converts.

regards greenie

 

[John S]

I work on a wide variety of parts, some going back well over a 100 years and in that time frame i come across imperial metric and where threads are concerned there is no standard at all.
The metric of 100 years ago it's the metric we know today.
There was a lot of kludging during wartime years to try to achieve a standard that met the axis powers requirements.

We also had a big transition period in manufacturing when we swapped from imperial to metric in the 1970's something the US hasn't had to do yet and when they do [ or if ? ] that will also be different as we didn't have the range of CNC machines that are available now and had to rely on manual machines with limitations.

A classic example of this was Wadkin, the large woodworking machine makers. When they changed over from imperial bearing to metric it was a simple design change to alter the print from say 1.250" to 30.00mm where it got sticky was that 1.250" x 16 thread nut that held the bearing on wouldn't fit, the lathes of the day wouldn't do metric without a lot of shagging about so the designer in his infinite wisdom specified 30.00mm x 16.
Not a problem to the turner as he had to screw cut it anyway.

Result is now we have a shed load of machines, still in everyday production with these bastard threads. Loose a nut for a spindle moulder and you are stuffed. Wadkin's only stock the newer 30.00 x 1.5

I work with what Greenie has said and round up until I get close. Greenies way is only like using verniers to rough out and a mic to finish off. It's well worth learning or even making a cheat sheet up to go above the lathe.

Not wanting to get into the imperial / metric debate as everyone has fixed views but there is one big difference.
Imperial has two methods of measurement, metric only has one.

In metric you get something like 23.81 there is no other way to show this.

In imperial you have decimal and fraction.

The other day I had to do a 3/16" keyway 1 - 1/8" long. Set up on the will plunged down set the DRO and I now have to go 1-1/8" minus 3/16" in distance so working in my head that 15 /16" ??? the DRO don't read in 16ths of an inch.

If that had been a metric keyway it would have been 28mm minus 5 mm so cut along 23mm

.

 

[Circlip]

Trouble is John, we never did TOTALLY go Metric did we ? Not like they had to in OZ. Said before component leg pitching was dictated by the LOTF in the electronics industry for IC's and Processors etc., don't know if they still are but it made laying out PCB's interesting using bisexual measurements n the panels.
Dave, do the conversions LOGICALLY, if a shaft is called up as 1/16" (1.62mm) and you're using Metric material YOU choose, - 1.5 or 2mm dia, just make sure that the corresponding part is the same. If a block is called up as 1" square material you ain't going to get a piece of 30mm square and machine it down to 25.4mm. Every fortnight I used to spend a couple of hours converting photocopy's of the next installment from ME of an imperially dimensioned twin steam engine cos of my metric machines. :wall:
Regards Ian

 

[David Morrow]

<snip> Every fortnight I used to spend a couple of hours converting photocopy's of the next installment from ME of an imperially dimensioned twin steam engine cos of my metric machines. :wall:
Regards Ian

Is "fortnight" Imperial or Metric ?

 

[mklotz]

There's a simple trick for mentally converting fractions to metric. It relies on the fact that 256 is very close to 10*25.4 = 254.

Keep doubling numerator and denominator until the denominator is 256. Then the numerator divided by ten is the equivalent in mm with an error of only 0.78%.

13/64 = 26/128 = 52/256 -> 52/10 -> 5.2 mm (correct value = 5.159375 mm)

 

[John S]

Is "fortnight" Imperial or Metric ?

True story, When we went metric in the 70's we had to hand in all our money and get it changed to new currency.
As you would expect there was load of bitching about this and at the time I worked for a haulage company, one of my jobs was to go down the row of trucks waiting to diesel up at night and check for defects, check lights and fit new bulbs etc

Invariable you would have a crack with the drivers and at this time it was all about the change over to decimalisation.
I started telling them that if they thought the currency was a lot to take in just wait until we went on decimal hours, 10 hors am, 10 hours pm.

Straight away they clocked onto the fact they would be out of pocket, next thing they had a meeting and went straight into the office to get it sorted ;D

Of course the office hadn't a clue what they were talking about .............. ???

 

[baldrocker]

John Stevenson youre a nasty nasty man :big:

 

[Kludge]

John Stevenson youre a nasty nasty man :big:

YEs. Yes, he is. Admirable trait, isn't it. ;D

Best regards,

Kludge

 

[David Morrow]

OK, here's another way of doing the conversion and it's real easy too, it is not 100 % dead nuts spot on, but it's that close it actually works.
0.040" = 1.00 mm
0.020" = 0.50 mm
0.010" = 0.25 mm
0.004" = 0.10 mm
0.002" = 0.05 mm
It's so quick and easy to do any converts from either Mongrel to Metric, or the other way round, it gets you that close, then it's out with the appropriate micrometer and you can then get it dead nuts spot on.

The machinery I work with, lathe and mill, are all graduated in Metric and to get it converted to Mongrel, well it's just a "chunk of urine" doin' it this way

OK, the purist are gunna shoot this lot down, just 'cause it aint 100% dead nuts spot on, all I'm sayin' is, give it a go and see how easy it is to do the converts.

regards greenie

At first I wasn't sure where you came up with your metric/imperial conversions but I made up a table on Excel using 1mm = 1/25 inch ( instead of 1/25.4) and your numbers all showed up. All of the imperial equivalents come out to nice clean numbers to 2 decimal places. This looks like the way to go.

BTW, normally I create conversion tables on Excel and then print them for hanging up on the wall by the mill.

 

[rake60]

If you look at the top left of this web page you will see a Unit Converter under the Menu.

There are many ways of doing the approximate conversions.
You have a perfect easy to use converter in front of you right now.
We placed it there to be used.

Rick

 

[mklotz]

What is it with you guys? Don't you have a calculator lying on your workbench?

Want the decimal equivalent of 39/64? Just divide 39 by 64

Want to convert 6 mm to inches? Just divide 6 by 25.4

Want to convert 3.5 inches to mm? Just multiply 3.5 by 25.4

 

[David Morrow]

What is it with you guys? Don't you have a calculator lying on your workbench?

Want the decimal equivalent of 39/64? Just divide 39 by 64

Want to convert 6 mm to inches? Just divide 6 by 25.4

Want to convert 3.5 inches to mm? Just multiply 3.5 by 25.4

As opposed to just looking up at a set of tables grabbing a nicely rounded number? Not poking around at a calculator and getting long strings of odd digits? What's wrong with simplicity? The side benefit is it generally leads to fewer errors. Why make a task more difficult ?

 

[David Morrow]

I made up a table in Excel for both the simpler 1:25 conversion as well as the 1:25.4 conversion. It's a 1 page pdf document which you can print and hang up in your shop for quick & easy reference.

http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item45

 

[MadKad]

This is me and what i do, as I work online if I am not in my shed I do all my working out next to my computer also and I just search google for any working out (waights, sizes etc)

[ame]http://www.google.co.uk/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-GB%3Aofficial&q=3.5+inches+to+mm&btnG=Search&meta=cr%3DcountryUK%7CcountryGB[/ame]

 

[chucketn]

I found a great comparison of imperial and metric bolt/screw sizes here:

http://www.physics.utoronto.ca/~astummer/pub/mirror/Docs/Engineering/Bolt%20sizes%20and%20dimensions.pdf

I copied the chart to Excel and made a printable version for the shop. The web page is helpful but a lot of flaffing about to get the info into Excel and print it. The format of the webpage doesn't favor printing directly.

Chuck in E.TN

 

[mike os]

Why make a task more difficult ?

I take it you are not an engineer then? ;D ;D ;D ;D

 

[Tin Falcon]

Dave :I read the same page in steam and sterling and I agree it makes a lot of sense while not a true conversion it allows for a translation for imperial to metric materials and visa versa . And if someone wants to do a true conversion or an approximate one that is ok too .
There are lots of ways to skin the proverbial cat or as i sometimes say remove the fur of a feline.
Machining is of of those things ask 8 guys the best way to perform a task and get 10 different answers.
have fun make parts .
Tin