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This is not an isolated observation, but a useful principle of design described in this book (self-help). Or haven’t we all spilled a drinking glass on a table that teeters side-to-side on two legs, resting now on the third, now on the fourth? Four-legged tables are fundamentally flawed and represent another design principle described in this book (over-constraint). Agreed, these cases are obvious enough.
But in this book they become principles and guidelines, explicitly stated, to be applied to other design problems—where they may be rather less obvious. This book is not about engineering science. Established books exist for machine design, structural analysis, and kinematics. Neither is it about design for manufacturability nor about the design process; excellent books have been written, especially in recent years, for these subjects as well. Nonetheless, despite an increased emphasis on design and manufacturing in both university curricula and practical literature, existing books have little about mechanical design as practiced by experienced designers.
Good designers often understand and use the ideas in this book whether or not they recognize them as distinct principles. Many designers, myself included, learned them either by trial and error or by exploiting colleagues’ experience. Perhaps everything here would be part of all designers’ practice were they to design long enough. But all too often designers are unaware of or do not fully grasp ideas that experts use to great advantage. Therefore, in this book I explicitly state design principles and practices: 1) so beginning designers do not have to discover them on their own as I had to, and 2) so all designers can apply them as fundamental concepts throughout their designs.
Although nothing in this book is new, its narrow focus on basic, detaillevel mechanical design is unique. Use it as a primer, and a refresher, on good mechanical design. You have heard this often, usually as “Keep it simple!” But for good designers, just keeping it simple is not enough. If you only just keep things simple, you will still have complicated designs. You must simplify, simplify, simplify! So what makes a design simple? Can your intuition alone judge simplicity? Will you know it when you see it? The less thought and the less knowledge a device requires, the simpler it is. This applies equally to its production, testing, and use.
Use these criteria—how much thought, how much knowledge—to judge your designs. Judge best by comparing one solution to another. Of course, it may take lots of thought and knowledge to get to a design requiring little of either; that is design. But some devices are elaborate affairs, and you may have to design one. How can it possibly be simple amid great complexity? What a simple design means is that everyone involved with its production and use sees nothing that looks complicated from his or her own perspective or convention. Complexity is buried and invisible. In other words, there is a hierarchy for knowledge and thought.
Each hierarchical level may be intrinsically complex, yet the device remains simple if the complexity resides only within its own level. Screw threads are a perfect example. Despite their abundant scientific and manufacturing complexity, you and I specify screw threads and threaded fasteners with the click of a mouse. Whatever it takes to make them is largely invisible to designers. We just say, “¼-20.” Another, very contemporary example is part geometry. Complex geometry no longer implies a complicated design. Computerized methods can control tooling and manufacturing processes without human interpretation.