Matthew West wrote: (01)
>> This is a slight generalization of Matt's characterization, in that it
>> recognizes the existence of parts with subfunctions, without requiring
>> substitutability
>
> MW: Can you give an example of a system with parts that were not
> substitutable? (02)
I mentioned in fact that good systems engineering gives rise to
subsitutability by encapsulating the mechanisms used by a subsystem and
defining only their functions, interfaces, and constraints. (03)
There are many examples, particularly in civil engineering, of systems
with parts that are not substitutable, precisely because the engineered
system was conceived as "one-off". The analog is 16th-18th century
European manufacturing, in which all parts were hand-made and
hand-fitted. (I think Eli Whitney is credited with developing the
'standard parts' concept for his machines.) The problem with all such
systems is that they were ultimately designed incrementally, from a
preliminary conceptualization of the system, rather than a system design. (04)
I should also note that certain types of "design optimization", that
combine part designs to perform multiple functions in one physical part,
often create parts that are not 'substitutable' in the sense that any
significantly different design could now perform the functions of that
part in the system. (05)
-Ed (06)
--
Edward J. Barkmeyer Email: edbark@xxxxxxxx
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8263 Tel: +1 301-975-3528
Gaithersburg, MD 20899-8263 FAX: +1 301-975-4694 (07)
"The opinions expressed above do not reflect consensus of NIST,
and have not been reviewed by any Government authority." (08)
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