Dear JHG
I fully agree with your statement:
“None of what we have discussed around here gets us around the need for competent design and drafting. To apply useful tolerances onto a manufacturing drawing, the designer must understand the manufacturing process as well as the final requirements, and the drafting standard.”
However, “Huston, we have a problem.” The problem is how to achieve this. The standard is does not answer a number of vitally important questions (and it actually should not). There should be a number of good design books with examples of datum selection, re-calculating tolerances between different datums (including manufacturing rough and final datums), etc. Otherwise, it is next to impossible to teach engineering students what the design is all about.
Life is very good even without knowing and understanding the datum and principles of locating. Moreover, modern CAD programs, can do the job in just a few seconds.
In my opinion, engineering students have been taught to rely far too completely on computer models, and their lack of old-fashioned, direct, hands-on experience can be disastrous. By the 1980th, engineering curricula had shifted to analytical approaches, and visual and other sensual knowledge of the world seemed much less relevant. Computer programs spewed out wonderfully rapid and precise solutions of obviously complicated problems, making it possible for students and faculties to believe that civilization had at last reached a state in which all technical problems were readily solvable.
As faculties dropped engineering drawing and shop practice from their curricula and deemed plant visits unnecessary, students had no reason to believe that curiosity about the physical meaning of the subjects they were studying was necessary.
Despite the enormous effort and money that have been poured into creating analytical tools to add precision to the design of complex systems, a paradox remains. There have been a harrowing succession of flawed designs with fatal results (For example one famous automotive company could not calculate the location of the center of gravity on its popular SUV so many life were lost due to this “innocent” misfortune). Those failures exude a strong scent of inexperience or bad habits or both and reflect an apparent ignorance of, or disregard for, the limits of stress in materials and people under chaotic conditions. Successful design still requires expert tacit knowledge and an intuitive “feel” based on experience; it requires engineers steeped in an understanding of existing engineering systems as well as in the new systems being designed.
An excellent outlook on design failures may be found in a book titled “Engineer Is Human: The Role of Failure in Successful Design” written by Professor Henry Petrovski. (I strongly recommend that you to read this book). Petrovski uses the 1978 collapse of the modern “space-frame” roof of the Hartford Civic Center under a snow load as an example of the limitations of computerized design. The roof failed a few hours after a basketball game that had been attended by several thousand people, and, providentially, nobody was hurt in the collapse. Petrovski explains the complexity of the space frame, which suggested mammoth Tinkertoys, with long, straight steel roads arranged vertically, horizontally, and diagonally. To design a space frame using a slide rule or a mechanical calculator was a laborious process with too many uncertainties for nearly any engineer, so space frames were seldom built before computer programs became available. With a computer model, however, analysis can be made quickly. The computer’s apparent precision, says Petrovski - six or more significant figures - can give engineers “an unwarranted confidence in the validity of the resulting numbers.”
Who made the computer model of a proposed structure is of more than passing interest. If the model is worked out on a commercially available analytical program, the designer will have no easy way of discovering all the assumptions made by the programmer. Consequently, the designer must either accept on faith the program’s results or check the results - experimentally, graphically, or numerically - in sufficient depth to be satisfied that the programmer did not make dangerous assumptions or omit critical factors and that the program reflects fully the subtleties of the designer’s own unique problem.
To understand the hazards of using a program written by somebody else, Petrovski quotes a Canadian structural engineer on the use of commercial software: “Because structural analysis and detailing programs are complex, the profession as a whole will use programs written by a few. These few will come from the ranks of structural ‘analysis’ and not from the structural ‘designers.’ Generally speaking, their design and construction-site experience and background will tend to be limited. It is difficult to envision a mechanism for ensuring that the products of such a person will display experience and intuition of a competent designer. More than ever before, the challenge to the profession and to educators is to develop designers who will be able to stand up to and reject or modify the results of a computer-aided analysis and design.”
The engineers who can “stand up to” a computer will be those who understand that software incorporates many assumptions that cannot be easily detected by its users, but that affect the validity of the results. There are a thousand points of doubt in every complex computer program. Successful computer-aided design requires vigilance and the same visual knowledge and intuitive sense of fitness that successful designers have always depended on when making critical design decisions. If we are to avoid calamitous design errors, it is necessary for engineers to understand that such errors are not errors of mathematics or calculation, but errors of engineering judgment, a judgment that is not reducible to engineering science or mathematics.
Here, indeed, is the crux of all arguments about the nature of the education an engineer requires. Necessary as the analytical tools of science and mathematics most certainly are, most important is the development in students and neophyte engineers of sound judgment and an intuitive sense of fitness and adequacy.
No matter now vigorously a “science” and computerization of design may be pushed, the successful design of real things in a contingent world will always be based more on art than on science. Unquantifiable judgments and choices are the elements that determine the way a design comes together. Engineering design is simply that kind of process. It always has been. It always will be.
Viktor
