Metric vs Imperial – Practical or Traditional?

[Martin WParticipant @martinw]

Ian

 

Just keep the curmudgeonly hat on cause its makes good reading and raises a smile.

 

Cheers

 

Martin

[Tony MartyrParticipant @tonymartyr14488]

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

[mgjParticipant @mgj]

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 stephensParticipant @chrisstephens63393]

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 Stevenson 1Participant @johnstevenson1]

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.

[mgjParticipant @mgj]

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.

 

Decimal!

 

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.

 

Decimal!

 

 

 

 

 

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

[chris stephensParticipant @chrisstephens63393]

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 Stevenson 1Participant @johnstevenson1]

“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 DolmanParticipant @frankdolman72357]

    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 Stevenson 1Participant @johnstevenson1]

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.

[mgjParticipant @mgj]

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/psi.by 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?

 

[John Stevenson 1Participant @johnstevenson1]

Posted by meyrick griffith-jones on 14/11/2009 00:32:55:

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.

 

 OK machining,

 

Is this a dick slapping contest Eton v Harrow ? Well I went to Oxford, delivered 28 tonnes of bricks.

 

In the time I have been in this trade I must has turned at least 6 circles, 9 diameters, 11 radius and a few peripheries.

 

 

never harmed one radian in all that time, in fact never seen a radian on any drawing i have been given and believe me I’ve seen some crap. Most of the first release drawings most universities put out are unmachinable, even they don’t use radians.

 

Slugs are a different matter those little blue pellets soon take care of those.

 

 

I supply support on about 3 CNC forums’ first thing I ask when dealing with a practical machining problem is what units do you work in? This is so i can speak in the same units they are used to dealing with.

 

Most US users say imperial although i have noticed a very small influx to metric, Canada and Australia is usually metric and probably 75% of the UK users work in metric.

 

 

As more users come into this hobby it will become more common because they have never used Imperial. Talking to Ketan at Arc he says he sells three times as many metric machines as imperial even though he keeps both.

 

I don’t push any system, I leave it to whatever the user feels most familiar with and I can understand someone with all imperial machines keeping to that method of measurement as it does make sense to an extent although my Bridgy is an all imperial machine and I work mainly in metric but the DRO’s take care of that.

 

John S.

 

[chris stephensParticipant @chrisstephens63393]

Shame on you, namesake,

Those little blue pellets poison the hedgehogs which would, if left to their own devices, would eat your slugs. That’s called counter-productive, where I come from.

I went to Oxford too, delivered a marine diesel, I had rebuilt, to a canal boat hire co. Almost delivered it in the boot of a Sir Henry’s finest, but that’s another story.

As I have said before, an inch is an inch, whether you call it 25.4mm or not. Just made up numbers, that’s all.

the other John Stephens’son

[mgjParticipant @mgj]

Radians – – anything with angular momentum or velocity, you work in degrees, you will get the wrong answer. Metric or imperial you work in radians per second, or you’ll come badly unstuck – unless as a (metric) user you like to convert by 57.297.

 

Machinists can use any system they choose, and convert very easily with calculators and DRO – why only last night I machined a bore fore an imperial sleeve 1 1/8 nominal (fractions while we remember are only for ruler lengths and carpenters?even on GHTs drawings)1 .1249 true, with a bore to fit, machined to  1.251 (for the Loctite). On a metric lathe that’s simply 31.78mm. As Chris says – just a figure, and on this we have no disagreement at all. (I love the DRO on my mill, and I shall get another for the big lathe at MEX)

 

But if you are doing engineering calculations the metric system is no more rational, or decimal than imperial,because the physical world isn’t neatly decimal.

 

I am surprised that you have never worked in radians BTW. You are an engineer and you need to know the deceleration of a disc (brake), or the momentum of a flywheel, or the force required of an electric motor to accelerate a machine up to speed, or the  Phase angle of any sine wave – Sin omega T. where t is seconds and omega is in radians.

 

Force  = mass by acceleration? Well if its going round, that acceleration or velocity is in radians/sec.

 

The metric system isn’t just a matter of a few screw threads, and the odd milimeter, and nor is the imperial system a matter of a few conventions on a drawing, with which people have now become unfamiliar.

Edited By meyrick griffith-jones on 14/11/2009 10:08:18

[Ian S CParticipant @iansc]

Wee bit off target,just been watching Top Gear,Clarkson driving a BMW Z4 140mph,program UK car German ?mph.IAN S C

[Ian AbbottParticipant @ianabbott31222]

The boss and I were rolling off the couch watching the Stig chauffeur James May.

 

Ian 

[Ian S CParticipant @iansc]

Ah yes,but mph?Ian s c

[BowberParticipant @bowber]

I use metric mainly but I’m also a self employed sign maker and commonly have to measure stuff while up a ladder so remembering whole numbers is much easier so I may measure something as 3ft X 500mm, just easy to remember.

Engineering wise I use metric for everything, and as for remembering that 17/32 is 0.53125 no thanks.

As for Radians, well I used them at collage, I may have even used them in calculations but I’ve never seen them on a drawing and that is what this discussion started as wasn’t it?

The US train industry used mm as their tolorance was 1mm, this stopped miss reading any decimal points.

At the end of the day it’s just what your used to.

 

Steve

 

P.S. I do think that all drawings should be decimal, be they metric or imperial.

Edited By Bowber on 16/11/2009 21:49:17

[CirclipParticipant @circlip]

Sorry to disagree Bowber but decimals with everything gives some the problem of trying to chase “Thous” that aren’t neccessary in lots of cases. 1.000″ ?? instead of 1″ bar or plate?  It’s one of the ills of the cad programmes (programs) if the “designer” isn’t conversant with the engineering required and sadly with the advent of cheap drawing programmes EVERYONE and their dogs are DESIGNERS.

 

 “Six munce ago I cutent even spell ingineer an now I are one”

 

   Regards  Ian.

[BowberParticipant @bowber]

I set my dimenions to remove trailing or leading zero’s and “chasing thous” is only a case of sticking to the tolerance.

So if I dimension something that’s 25.00mm it shows as 25mm and if the part only calls for 0.1 tolerance then that would be set in the dimension settings so a part measuring 25.01 in cad will be dimensioned as 25mm.

I still think drawings should be decimal, I don’t have any measuring equipment that measures in fractions their all decimal so one less calculation to make (or looking at a chart)

 

Steve

[keithmartParticipant @keithmart]

Just a little caveat, In the ’80s I worked with an apprentice training school, and we taught both imperial and metric. The schools were only teaching metric.

I remember one apprentice saying to me ‘There are an awfull lot of thou’s in a centimetre aren’t there

Keith

Leeds UK

Edited By keithmart on 17/11/2009 21:01:17

[Howard JonesParticipant @howardjones35282]

I grew up metric.

years involved in aviation made me footric.

in model engineering I use the dimension with the simplest number.

however I’ve become convinced that we need to keep both systems (or if you use BA, all three)

I’ve never seen the metric tapping drills ever in the shops.

most metric tapping drills are the nearest imperial equivalent.

in drilling holes I use a combination of number drills, imperial 64ths sizes and metric.

 

all metric rulers with just millimeters on them are useless! there must be nearly a foot between graduations.

 

no, keep all the standards. they all miss out somewhere and by combining all of them you can get the job done.

[chris stephensParticipant @chrisstephens63393]

Hi Howard,

You have been trawling the archives, haven’t you. 

 

Can’t help but agree with you. One shouldn’t be restricted by dogma, use whichever is most appropriate be it inches, metres, King Tut’s toes, gnats foreskins, or the shallowness of a politician’s promise. The physical size of something does not change, no matter what units you measure it in. (That is not necessarily true at the Quantum level, where just observing something can change it, but this need not concern us Model Engineers  overly, unless of course Angstroms are your favoured units.)

chriStephens 

[Steve GarnettParticipant @stevegarnett62550]

Well, we don’t measure much directly in Angstroms, but we have to build things which are stable to within parts of an Angstrom for an instrument that does, otherwise you can’t count the interference fringes accurately, or determine drift.

 

But Angstroms are metric anyway…

[adyParticipant @ady]

Just use what you’re happy with. I use both.

 

Do you want salt and vinegar or salt and sauce on your chips sir?

[Author]

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