I can comment that in my career (I'm now 50) we always used SI units, in
High School and College, (Ill Institute of Technology, Cleveland State
Univ., Univ. of Akron) Although I never used them in industry.
I personally would like to see us eliminate all use of imperial units
from 'professional' services such as Engineering and manufacturing,
leaving only Imperial units for some consumer goods such as food,
gasoline, building materials, etc... I think that this is happening. I
understand that in England a wooden wall stud is still called a 4x2,
even though it is not manufactured in inches. ( it's probably more
difficult to call it a 100x50, this doesn't quite roll off of the
tongue, maybe 10x5 (in cm) would be better).
My children are all being taught SI units (cm, mm, and km for length,
and kg and gm for weight) in their day to day studies in primary school.
One of the first uses I find for going to SI units is describing
mechanical components, with the concepts of fractional measurements
being the most troublesome.
For example to 'accurately' show a dimension of 3/16", the decimal value
is .1875". When modeling this in Pro-e you need all 4 decimal places,
the system does not use fractions directly. When showing the dimension
on a drawing, showing .1875 +/-.0300 is frowned upon (at my workplace)
because of all of the trailing zeros in the tolerance. I have the
option of showing the dimension as 3/16+/-1/32, but this is messy in
pro-E. I could just round up/down to 2 or three decimal places, but for
'design intent' I do really want the nominal dimension to be 3/16", not
.19".
One of our customers uses imperial units but shows all dimensions to 4
decimal places, trailing zeros be damned. They also comment that the
number of decimal places does not imply a tolerance, and explicitly show
all dimension tolerances that differ from the 'default' tolerance.
Happily this will all go by the wayside when digital product definition
becomes more mainstream.
Note that you do not have this duality of fractional vs. decimal
dimensioning in SI units. I find also that more people are willing to
use 'preferred numbers' when using decimal dimensions.
I anticipate that because a majority of machinists are taught in
imperial units, and most shops have most of their measuring tools
defined in imperial units, that they will be around for some time. Two
things can change this, CMM inspection to digital product definition
models, and the requirement to report inspection results in the native
units. (no measuring the features using imperial gages, and then
converting and reporting the result in SI units).
Christopher F. Gosnell
FPD Company
124 Hidden Valley Road
McMurray, PA 15317
