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Metric vs Imperial - Practical or Traditional?

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John Stevenson14/11/2009 01:28:43
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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 stephens14/11/2009 01:55:58
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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


mgj14/11/2009 10:06:44
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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 C15/11/2009 09:30:01
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Wee bit off target,just been watching Top Gear,Clarkson driving a BMW Z4 140mph,program UK car German ?mph.IAN S C
Ian Abbott15/11/2009 22:37:35
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The boss and I were rolling off the couch watching the Stig chauffeur James May.
 
Ian 
Ian S C16/11/2009 00:30:56
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Ah yes,but mph?Ian s c
Bowber16/11/2009 21:44:27
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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

Circlip17/11/2009 08:34:14
1723 forum posts
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.
Bowber17/11/2009 20:41:12
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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
keithmart17/11/2009 21:00:27
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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 Jones05/10/2010 14:26:44
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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 stephens05/10/2010 15:59:32
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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 Garnett05/10/2010 19:01:24
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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...
ady05/10/2010 22:17:35
612 forum posts
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Just use what you're happy with. I use both.
 
Do you want salt and vinegar or salt and sauce on your chips sir?
Sam Stones05/10/2010 22:28:08
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I’m not sure where this came from, or if it even fits into this Metric versus Imperial posting, but I was told a story about a really old toolmaker.

He was heard muttering words along the lines of :-

"I don’t why you lot bother with all these plus and minus tolerance things. When I was a lad, we used to get it right!"

Sam

Terryd05/10/2010 22:35:27
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Posted by its-smee on 17/08/2009 08:38:00:
 
Some talk of millimeters and some of kilogrammes,
and some of decilitres to measure beer and drams,
but I'm a british workman,too old to go to school,
so by pounds i'll eat,and by quarts I'll drink,and
I'll work by my three-foot rule
                                ( Rankine 1874 )
 Rankine said this because this as this was the date that the British government decided to adopt metrication because of the superiority of the system.  However several wars got in the way and the decision was not implemented until the 1960's. 
 
The arguments rather remind me of the argument against metrication by the Tory MP who said that the kilogramme was too heavy for British housewives to carry!
 
By the way, as a matter of interest, the Florin (2 shilling piece - 10 p piece) was introduced in the 1840s as the first step in decimalisation of our currency.  Of course British inertia won the day and we still suffer from the mixed up ancient continental/French, old system of pounds and ounces and pounds sterling ( i.e. Livres in French - hence the modified L or £ for our pound and lbs for pounds weight,)
 
Try dividing inches by 5 instead of 3 and you'll also see a problem.
 
Regards
 

Andrew Johnston05/10/2010 23:12:13
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Not only that but the US and English inches were different until the late 1950s.
 
Regards,
 
Andrew
Nicholas Farr05/10/2010 23:51:53
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Hi, I have no problem using imperial or metric, I use them both. This argument has and will rage on and on. What if some clever dick finds a more effective and efficient unit in the future, will we stick in the mud for metrication. Imperial units don't aways suit everything, niether does Metric. Standards are certainly efficient, but do they always need to all be the same. Imagine if people were all conceved by the same efficient standed, well! you might not know who is you or me. Just a thought.
Regards Nick.
DMB06/10/2010 08:55:44
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Hi all,
Here we are again, arguing about Imp/Metric. Metric has a number of very useful benefits, like multiplying/dividing can easily be done by shifting the decimal point in head or on paper, no need for a calculator. Just try doing similar calcs. with thous-a thousand to the inch then we go to 12 inches to the foot, 3 feet in a yard, a cock-eyed 1760 yards to a mile. Now add in all the other daft  measurements, many still being used , like chains (22 yards?) used by our Railways. Have you forgotten Rods, Poles, Perches, Gills, Ounces, Pounds, Hundredweights and Pennyweights, Quarters and Tons, Nautical Miles, Fathoms, 4 Quarts to 1 Pint, 8 Pints to a Gallon and so the rubbish goes on and on.
BTW, I am British, schooled in England and taught both systmes at school.
In a nutshell. the problem is either one of being to VISUALISE how far is a Kilometre rather than half a mile or thous against tenths or hundredths of a millimetre or half a litre versus a pint or old codgers RESISTANCE to change.
 
THIS IS HOW I GET AROUND THE PROBLEM
 
Iam building a loco to Imperial dimensions so I use the Imperial edge of my steel rule and my Imperail mike, electronic calipers set to Imperial measurements.
Suppose I need to know how long a screw or rivet is needed to fix together say running board to valance angle and one piece of metal is in Imp. and the other in Metric. Ju st measure thickness of both in same units then calculate length of rivet/screw in same units. I dont have a problem with something like that. 
DMB06/10/2010 09:04:32
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Hullo Terryd,
Just read your post about the origins of our currency. I think you may be wrong. As I understand it, Pounds, Shillings and Pence or L, s, d, came from the Romans and Latin words, Libra, Solidus and Denarius. Still alousy method of measuring value.
John 
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