Why can't the US get serious about moving into the metric system?
We are one of only three countries in the entire world that does
not use it officially!
Back to the old chestnut again… The biggest problem with
changing to a new standard is that you never lose the old one - just
gain another to add to the list! In the past, the UK probably had
the most measurement standards of any country but, in its infinite
wisdom, our government declared in 1971 that we should change to the
metric standard as part of our ‘integration’ with the rest of Europe
(or, at least, the parts we consider to be part of the European
community) and made most of it mandatory in 1973. As a result of
this it is now very difficult to buy nuts, bolts, taps and dies in
the ‘old’ imperial system - all that the suppliers stock routinely
is a ‘standard’ range of metric parts from 3mm up to 12mm in
diameter ( 3, 4, 5, 6, 8, 10 & 12 ). However, older equipment is
still very much in use, employing fasteners of the older systems and
repairing it has become a nightmare - no longer can you find a 4BA
screw to replace one in an electrical terminal - the nearest one is
a 3.5mm but this is ‘non-standard’ and so can only be found as
screws to fix a light switch plate to the wall and, since these go
into factory produced fittings, no one stocks taps to cut a new
thread. Depending on where your car was made, its engine may use AF,
UNC or UNF threads but you can’t just go and buy a new nut to replace
a stripped one… The change to the metric system was driven
largely, I think, by shopkeepers who wanted to reduce the amount of
inventory they needed to stock and little thought was given to its
effect on the real world of engineering.
Another problem is in the way it is taught. In the ‘old days’ we were
taught that an inch was about the distance from the first knuckle to
the tip of the thumb, that 4 inches was the span across the hand,
that a yard was the distance from the nose to the tip of an
outstretched arm etc. etc.- all measurements we carry around with us
and which we can relate to; but how do you teach a child what a
millimetre looks like, or how to estimate in metres? The answer is,
you don’t - firstly you forget the millimetre altogether until the
young person has to encounter it in science and, instead, you use the
non-standard unit (as far as SSI units are concerned) of a
centimetre. This, then gives the child something visible but which it
has no real bodily reference to ( generally the width of the
forefinger is used but, of course, this varies a lot with age and,
particularly, with body mass ). The metre is taught as the length of
a stride but, of course, these vary as the height of the child
increases! They are also taught about the kilometre but, due to what
some percieve as stubbornness but which is related to all sorts of
other international reasons such as air traffic control, shipping
etc., distance is still routinely measured and displayed on road
signs etc in miles. As far as the supply of many goods are concerned,
most measurements are still simply converted from the imperial - so,
a brick is still generally 9" x 41/2" x 3" but it sold as 229x114x75
and 2" x 2" planed timber is sold as 44x44mm in 2.4metre lengths (6
foot!) - manufactureres don’t want to have to change all their
tooling until it is worn out and a lot of finished goods are imported
from the many countries which still use variations of the Imperial
sytem (we didn’t colonise half the world without leaving them some
lasting legacies!!) - all very confusing!
The various systems of measurement were mostly developed for a
particular purpose - the Whitworth thread was designed to be the
most efficient for use in iron castings etc. while the BA system was
designed to give the strongest connection in brass fittings. The
British Standard Gas thread was a tapered thread to enable gas-tight
connections in small copper pipes and brass fittings while the Steam
Pipe thread did the same for larger steel pipes and iron fittings.
Each thread had its own pitch angle, thread profile and overall size
to maximise the holding torque in a particular range of materials
whilst still allowing joints to be undone easily if needed. As far as
I am aware, the metric system doesn’t have a tapered thread and it
uses an ‘average’ thread form and pitches which are less than ideal
in any situation. Even though I use the metric system routinely now
and have done since well before ‘metrication’ in my watchmaking
endeavours as it has always been the basis of the Swiss watchmaking
industry measurement (although still with various ‘sub standards’
within the metric system) I still hate it and the most cherished part
of my tool collection is the assemblage of ‘old standards’ taps and
dies and the tins of old nuts and bolts without which I couldn’t
function!
Best wishes,
Ian
Ian W. Wright
Sheffield UK