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Metric vs Imperial - Practical or Traditional?

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David Clark 111/11/2009 09:14:19
3357 forum posts
112 photos
10 articles
Hi There
Metric and imperial will continue to be used side by side for many years to come.
Anyone who can build a locomotive or similar and get it working should have no trouble converting between the two systems.
regards David
John Wood111/11/2009 09:47:37
116 forum posts
Well said David, I think that about sums up the debate. I shall certainly avoid any comment but will just keep my head down and enjoy the versatility of using both systems when required in my own workshop.
Keep turning
DMB11/11/2009 11:58:02
1222 forum posts
1 photos
I also suffered being taught irrelevant historical measurments like rods poles perches.
Also, decimetres hectometres and so on. Then there was our historic money  with farthings threpenny-joeies, guineas, tanners and so on.
How much easier is the new currency to add, divide and multiply.
The same goes for metric e.g.,  we seem to be using only mm, cm and M, having apparently cut out all the in - between ones like decimetres. Pity 600mm has to be used for kitchen cupboards instead of .6M  Its all about visualising, like Ian said 18,288mm. Why not  make it simpler by saying 18.288M then mentally add on another 10% (1 .8) and you have near enough, 20 (yards!) I am making a Harold Hall design to his metric drawing which uses piece of metal 70 mm long; no prob. visualising this. Equally, I can look for a piece of stock thats around 3 ins. long for it. Most people have digi-calipers and/or DROs, so whats prob? Just seems like something to moan about.

Edited By John Coleman 1 on 11/11/2009 12:00:07

Martin W11/11/2009 12:00:39
908 forum posts
30 photos
Hi All
I think what this extensive thread shows that we are all capable of handling numbers and measurements whatever their origin but the real problem is CHANGE. As a species it seems that we humans love what we get accustomed to, even if we dislike it at the time, and resist changes to that system.
In the computer world there are/were several ways of expressing numbers, binary, octal and hexadecimal just to mention three, oops here come the thirds again, but there was little argument about using them as they were, relatively speaking, new standards and in them days people were used to them all.
I reckon variety is the spice of life and this thread has surely shown that, so not sure which is best then press the conversion button on your vernier/calculator and use the easiest for you. What the hell it's only how far two points are apart physically.
Peter G. Shaw11/11/2009 13:01:50
1359 forum posts
44 photos
I too was brought up on imperial. Indeed when studying Telecomms Principles we were still using cgs units which eventually became mks.
For myself, I decided one year that a kitchen cabinet I was making would be done completely in metric - which was how I changed over as it were. Later, I bought an imperial micrometer (for use on a Maxi engine). Then later again when I got involved in engineering I decided that I would only use metric equipment.
Ok, I do have imperial drills, but have decided to go totally metric so as they break/wear out, they will not be replaced.
These days, I now automatically think mostly in metric, but can still use imperial - indeed, if measuring something I use whichever system has the nearest graduation mark.
Like other people, I don't see it as a problem, after all conversion is easy. What I would suggest though, is that anyone starting from new in this hobby possibly needs to consider going totally metric as certainly in the UK, everything else is slowly going that way.
Finally, a comical story.
About 15 years ago I wanted to build a porch and so I contacted the local planning department. Very helpful except And she kept saying it. Fair enough, I was quite happy and it was easy for me to do.
Then, one day, whilst discussing something, it happened!
LP: "Of course, we're not really bothered about the odd inch or two."
Bit of quiet, then-
ME: "Er, excuse me. What are these inches you have just mentioned?"
Dead Silence, then I slowly burst out laughing.
After that, we got on just fine.
Peter G. Shaw
Vapeur8911/11/2009 13:02:10
18 forum posts

I’m on the metric side of the hobby and I use either metric or imperial drawings or kits.  I have no problems with that. It does’nt take very long to convert from imperial to metric. The reverse is not true if you have to work with imperial fractional. You can download a free professional unit converter at

 As to what represents a distance... it’s not easier to figure 1 53/64’’ than 46,434 mm.

 Many times I order stock bars from bristish suppliers as it is a cheaper than in France.

To deal with imperial drawings I use exclusively metric holes and metric thread and I convert distances in mm with three decimals.

In fact before making chips we have to  study and dig into the drawing to understand or to adapt the stock bar we have on hand or to deal with a limited set of tools. So converting is a good starter.

The strengh of SI unit system clearly appears when you have to work with complex physics formulas, but as long as only length is concerned why worry about that?

Ian Abbott11/11/2009 19:40:00
279 forum posts
21 photos
Thinking back to the old pounds, shillings and pence etc., we used to make money out of American tourists who couldn't handle the exchange.  I thought that those days had passed, but when I was living on the West Coast of Canada, we had a lot of American tourists and they couldn't do the conversion from US to Canadian dollars either.
Now this seems strange, as it's just a matter of percentage, metric again, but on thinking about it, the problem, it's that the school system there in the US, as over here in the UK and in Canada is now crap.
How is it that we were thrown out of school aged 15 and on to college, with a complete knowledge of ten different types of measurement and a wide vocabulary, but in the current western "civilization", schools turn out young adults at 18 who can't spell (even with a spellcheck) and can barely count to ten?
Feeling curmudgeonly tonight.
Circlip11/11/2009 19:54:57
1426 forum posts
You dissin the well good bro's??   Innit
Ian Abbott11/11/2009 20:26:16
279 forum posts
21 photos
Sorry, it's just too easy, like making fun of Gordon Brown.....
Still curmudgeonly.
Martin W11/11/2009 22:16:36
908 forum posts
30 photos
Just keep the curmudgeonly hat on cause its makes good reading and raises a smile.
Tony Martyr13/11/2009 17:46:02
209 forum posts
41 photos
I have lived in both New Zealand and Fiji for the years during which they transferred from imperial to metric systems. Both countries took just 12 months and did the job to the satisfaction of the general population by an extensive publication campaign. I remember local discussion in Fiji about the metric equivalent of a 'string of mud-crabs' but common sense prevailed and in the absense of an SI unit the local market measurement stayed.
So I was very shocked when returning to the UK in 1980 to find weather forecasts being given in Fahrenheit and Celsius.
I explained to an French colleague that some folk in England believe that if we adopted the metric system then Napoleon would have won !
Any engineer knows how derived units in dynamics, thermodynamics and power are so much easier using SI units.
The model engineering fraternity do have to deal with the problem of their archived designs and the heritage of old documentation but it was no different in New Zealand where I simply converted my drawings to metric dimensions.
Imperial (the clue is in the anachronistic name) will die out with my generation except for a few die-hards who presently measure their gardens in chains.
Tony M
mgj13/11/2009 18:37:37
1017 forum posts
14 photos
I must admit, I prefer units which actually change the figures - much easier when you do your bit of dead reckoning to check. I tend to use metric because one sort of has to, but I frequently revert to imperial to do a check if its important.
These days with  decent (programmable) calculators it makes no difference what figure you feed in and what figure you multiply by.
Ask Nasa and the designers of the Hubble telescope. Wasn't it they who did so well in metric units. Dropped a zero or some such? Some trivial deal in space that needed a Shuttle journey to fix.
So no, they are not easier - its just what you get used to.
In fact almost the only pitfall for the imperial user to remeber is to stick in lbs FORCE and not lbs. Other than that is unit/ unit (pressure time distance. Makes no difference if its .001 or .025 does it? Just a number in a calculator panel.
chris stephens13/11/2009 18:55:00
1049 forum posts
1 photos
Hi Tony,
I think you are missing the point. SI units are fine for Pro engineers, but the majority of people in this country, the ones who matter, like to use units they are comfortable with.
It's like an alleged intelligent philosopher saying on TV  he can't understand why people still like and continue to built  Tudor-bethan houses, when they could have modern homes.  Well a chap wants to feel comfortable and safe in his home and he associates Tudor-bethan with good old fashioned values, comfort and security. Just like the feeling you get from pounds, feet and Pints, and Miles are still the standard for distance. Racing, for example, would not be the same if they raced the last 201.1680metres instead of the last Furlong. Tradition counts for something to most people. Stop complaining, any engineer worth the name should be able to work in any unit, they are after all just numbers and have no magical properties. Unless you believe in numerology, in which case you are forgiven.

Oh and you should really call Celsius, Centigrade. Anders Celsius invented the centigrade scale i.e. dividing freezing to boiling point of water into 100 units, but he had boiling at zero and freezing at 100. As we use it the other way round it cannot really be called his system. Discuss! for 10% of your marks.

How dare you call Imperial, anachronistic.  It is because we have lost our Imperial ways and have no clear direction to follow that we are now floundering in the mire. We should be proud of our, unfortunately no longer, great Past. Not made to feel guilty of it, as the politically correct apologists would have it.
Discuss also! (but not here, this is an engineering site)
All the best and keep smiling through just like we used to do etc. etc.
chris stephens

John Stevenson13/11/2009 21:02:34
5068 forum posts
3 photos
I have to work in both so what with modern DRO's, calculators and such like it's not that hard.
We could go on for ages about this Imperial v metric but what has been made will never change and some have to work with it so have to be conversant with all systems.
One thing I will say is that metric is a lot easier on the maths. This was brought home the other week when I was sent a copy of an old engineering drawing and one measurement, a turned diameter was listed as 25/32"
Now not being mentally conversant with all the 32th's of an inch I reached for my laminated conversant chart on the back of the bench, couple of minutes later found it right at the bottom of a pile of drawing, sweet wrappers and tool porn magazines, made the conversion on the drawing to 0.78125" and carried on.
Later it hit me that the chart was at the bottom of the pile because most of my recent work has all been modern metric based work.
Think on this, in Imperial there are two methods of measurement, fractions and decimal, most machines use decimal but not all drawings do hence having to convert 25/32" to decimal inches.
In metric there are only decimal measurements and conversion charts are not needed. Probably the second most use of charts is for drilling and tapping. I have no idea what any of the larger tapping drill sizes are without a chart but metric can be done in your head, picking a weird one that's probably not in any charts, 37mm x 2mm pitch, subtract the pitch from the diameter and that's your size, in this case 35mm.
Works for any metric sized thread regardless.
John S.
mgj13/11/2009 21:50:00
1017 forum posts
14 photos
In metric there are only decimal conversions?
Try changing from kilos to newtons.
Machine tools with 3 mm pitches on the feed screws, or 6mm on diameter.
One of the wonderful things Her Majesty gave me (other than her autograph at the top left of a bit of paper) was a  book of common conversion factors and useful equations.
Coefficient of kinematic viscosity - is that a nice neat decimal. One of the nuts and bolts of every flow engineer - the Reynolds Number
The force of air/steam on a plate. 1/2 rho U^2 s - that will be a nice neat decimal/
You want to transpose a sine wave from time to space and have to do a Fourier transform on it.  Or even a fast Fourier transform.  - thats a nice neat decimal?
You want to work out accelerations in a spring system, and they are to be nice neat decimals? Nor will two of your building blocks - g at 9.81m/s/s, and Youngs modulus (stress over strain as a ratio) for for your material.
You start working out temperatures in Degrees K - that doesn't start as a nice neat decimal. in fact most temperatures are quoted in dec C, and most engineers have to work in deg K
Gas constants- are they nice neat decimals.
I'm sorry, but its cobblers.
There is no such uniformity in the natural world. Engineers never worked in fractions anyway. Sometimes old fashioned dimensions were quoted in fractions - but thats not engineering. Thats carpentry, and even at that level, many machine tools have leadscrews of 3mm pitch graduated in .04 mm gradations.
As for pitches - I don't need to work them out. I know them, and the common ones I use I know what the additional infeed is when my toolpost is inclined at 27.5 deg. Your metric flank angle is 60deg. How much more decimal is that than the 60deg  of the UNF UNC imperial ANSI system, or the 55deg of the Whit system.
with decimal, a particular diameter can have metric fine, metric coarse, and some dias even have 3 or 4 associated pitches. At least with imerial 3/8 Whit is 3/8 whit, and a darn sight stronger than its metric equivalent.
OMIGOD, we now have metric aircraft pitches because the metric theadform is NBG, and different metric bolt heads, and we still need to convert at 127/60 or whatever it is, (see chart I don't need) because metric gas doesn't exist.
Oh hell- metric pitches even metric coarse are NBG in aluminium under severe load.

Edited By meyrick griffith-jones on 13/11/2009 21:50:32

chris stephens13/11/2009 22:44:02
1049 forum posts
1 photos
Hi John and Meyrick,
If imperial fractions were the only imperial measurements, I would be working wood instead of metal as you suggest. Luckily there are decimal inches, which  are as easy to work with as Metric decimals. I wonder if there had been no Imperial fractions, only decimal, whether metric would ever have stood a chance?
Of course we are all going to go Metric eventually, but as we have such a rich past in Imperial, it would be a shame if a  decree come down from our "elected" leaders to prevent making Imperial sized spares to keep our history live. Not that our leaders, have much interest in History, so they don't stand much chance of learning from it. Hey-Ho!
The other John Stephens'son
chris stephens

John Stevenson13/11/2009 23:22:58
5068 forum posts
3 photos
"In metric there are only decimal conversions?"

Where did i say that ? I said measurement.
Take that 25/32" which equates to 0.78125 which also equates to 19.84375mm

Now tell me how else can this be written ?

I know not everything works out like steam pressures but none of my machines work in steam pressure or temperature.

"Engineers never worked in fractions anyway. Sometimes old fashioned dimensions were quoted in fractions - but thats not engineering. Thats carpentry,"
Thanks for that, made my day because I can't stop laughing. Didn't know Edgar T Westbury was a carpenter and the Rolls Royce Merlin engine as made out of wood.
John S.
Frank Dolman13/11/2009 23:42:34
106 forum posts

    When I was being taught, or attempts were being made in that direction, both
  English and French units were used.  For force, the English unit was the poundal
  and the French unit was the dyne.
    Lots of chaps have mentioned lbs force but the poor old poundal seems to have
  been forgotten.  I have to admit that an awful lot of pressures were always given
  in psi or psig, poundals being universal only in dynamics but this is no excuse
  for misrepresenting the case.
    Don't forget the poundal Mum!
John Stevenson14/11/2009 00:16:51
5068 forum posts
3 photos
I think we are getting off the beaten track here.
The OP was about measurement as in metric / imperial and nothing to do with pounds, Dyne's, sine waves or Fourier's.
can't see what horses have to do with this anyway.
John S.
mgj14/11/2009 00:32:55
1017 forum posts
14 photos
Ok - well machining, as opposed to engineering.
You want to work out anything going round in circles- you use radians, and there ain't much decimal about a radian. Come to think about it, there's not much decimal about a degree, and the grad hardly caught on.
Don't forget the slug (along with dynes ergs and) neither. And the kilopond or somesuch.
Lbsf/ the way. The g bit appies to either side of the argument, gauge or absolute.  (bar or PSI)
Then when they did try to rationalise, the measure of force n/sq m the Pascal was too small to be useful, so they had to use the megaPascal, or n/sqmm. And unfortunately that didn't fit the bar because you have 100 millibar per Pascal And of course to convert from 1bar (atmospheric pressure), you actually have to convert by 1.013 to get to a standard atmosphere.
Brill the metric system - its just so logical
All that has happened is that those involved in machining, as opposed to the engineering behind the design, have latched onto one tiny bit of the metric system, and said how useful it is. As if some carpenter didn't know precisely which drill to use for a No8 screw, or which letter drills to use for a tapping or clearance size.  And in exactly the same way, most of us know perfectly well what size the common fractions are.50625 is 9/16.  a 64th is about .015, a 32nd is about 32 thou. Whats the problem?

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