The theory behind the metric system is that ten's are somehow "natural." Were that the case, harmonious scales would be based on 10 rather than 12 or 16. Just try using a base-10 system to analyze fluid flows, Earthquake propagation, or Kuanda-amplification of aerodynamics! Mind you, I grew up (until about age 10) using metric and "converted" to American inch/pounds/seconds back when the standard of American measurement was 1/128th's of an inch.
The original definition of the metre was to have been 10,000,000 metres from the equator to the North Pole. They screwed up THAT calculation. The kilogram was to be 1000 cubic-centimeters of water. They screwed up THAT calculation. The reason temperature is measured in Celsius degrees today rather than Centigrade degrees is that they screwed up the definition of ZERO and did not correct THAT until the early-1970's. In short, the metric system is a SET of screw-ups.
In most instances I care not whether metric measure or Imperial or American inch-based measure is used. My specific
gripes have to do with the units derived from Pascals (N/m²). The "mesh" is
wrong! The
units provide no "feel" for the measurement being made.
I was the chief mechanical engineer on the program that developed the modern (late-1980's/early-1990's) automotive airbag restraint systems. [This sounds really impressive until you realize that (A) airbags are
chemical rather than mechanical, and (C) I spent nearly three years working on a project that was
designed to blow up in my face!] I worked with Japanese, French, and German engineers (among others) who had never used anything
but metric measures.
They would screw up their pressure/stress calculations
because even
they would get "confused" by the size of their answers. It's kind of like trying to report football (real or American) in
inches -- the numbers easily became meaningless and someone exploded their unit (or failed to inflate the bag) as a result. In 1998, we lost a multi-billion dollar Mars Lander system because a French engineer lost track of the powers-of-ten in their answer for an airbag pressurization!
I have fought with metric screwthreads since the late-1960's. Until 2001, there were
five "standard systems" of metric screwthreads (meaning major diameter and pitch) -- French ISO, British Standard, DIN, JIC, and American ISO. You have no idea how much "fun" I had installing RADAR domes for NATO in the early-1970's. Even today with "standardized" metric screwthread major diameters and pitches, there are
still five different set of
tolerances and allowances in use. If you buy a (say) M6 bolt from a German supplier and a M6 nut from an Asian (JIC-using) supplier, there is a 43% probability that the nut will
not mate properly with the bolt!
Making matters worse from
my knothole, changing from a "standard" metric screwthread to a "fine" metric screwthread of the same major diameter only gains me
3% on the strength/bending properties of that connection. The (currently) Unified National thread series (which derives from the WWI vintage "American-British-Canadian" standardization) gains me
15% in connection properties when I change from
coarse to
fine or from
fine to
extra-fine. [This was actually a
requirement set out for the "ABC" committee by no less than Herbert Hoover!] Further, the "standard" (and "fine") pitches chosen for metric screwthreads are
too fine for direct application in low-shear-strength materials. You
have to use a "thread insert" whenever you face that proposition. [UNC threads work well in
most low-shear-strength materials.]
Metric screw users like to claim that their system is simpler in that you only have to subtract the pitch from the major diameter to find the correct tap drill. (A) You can do the same thing using inch-based threads -- just just have to do the division to turn TPI into pitch. (B) The "answer" you get will only give you 62% (on average) of the maximum strength you
can get with a properly calculated tap drill! [I have many documents about screwthread strength and tap drill calculation posted at
http://www.scribd.com/Lew Merrick if anyone is interested.] Yes, this works well in
most instances, but it does
not work when you get into strength/mass critical applications.
Otherwise obsolete measures such as the league, furlong, and stone are important to understand. A league was the distance a Roman Legion would march in an hour. A furlong was the distance a Roman Legion was expected to run in five minutes. A stone was the mass an underfed peasant was expected to be able to "pick and place" all day long without keeling over and dying. Knowing about how these measures came into being gives a designer an understanding that is
missing from more "modern" units of measure.
What "killed" the metric system here in the U.S. was the advent of the pocket calculator. Dividing by 386.35 (in/sec² for 1g) is now no harder than dividing by 10.
