New Directions for CAD
Kurt Swanson, Boeing

Every day, around the world, a designer sits down to create a physical product utilizing the most complex and sophisticated computer software systems ever created. Yet the forms and shapes that are created are instantly recognizable as being manmade and use a set of primitives that are readily distinguishable from the very fingers by which the task is undertaken. This dichotomy between the manmade and the natural is rooted in the basis of our understanding of the physical world. Our mathematical concepts of Euclidian space predispose our creative capabilities to creating a planar faceted environment. How do we move beyond this limit and toward a new direction using a more full language for design by CAD?

Teleology, the study of evidences of design in nature, is an often a maligned field of study, especially by design engineers. With the advent of new fields such as Computer Aided Tissue Engineering (CATE), teleology cannot be ignored or cast aside. A central purpose of CATE is to create articles that look and function like the organic structures they are intended to replace. These man-made structures seek to mimic the differentiated structures of both soft and hard tissue. (It is not sufficient to imitate skin since skin consists of glands, hair follicles etc, all these are essential to the proper function, yet skin grafting does not include these differentiated features.) Feature sizes for biologics are on the order of 10-40 microns. Cancellous (spongy) bone has features that are well beyond the current capability of CAD based NURBS or solids to represent.

Standard CAD processes fail to fully represent common organic structures, yet in the biological growth of a bone, there are no blue prints, no computer codes, no NURBS and no N/C machines. The driving forces behind these common structures are based in the seeming universal capability of every organism to lay down deposits of calcite and aragonite crystals on a microscopic level. And in addition, is their ability to cause these compounds to go into and come out of solution.

The overriding mechanism that determines the placement of such materials appears to be membrane forces or (more simply) fluids membranes acting under pressure forces. Mathematicians are not new to this problem, which was first formulated by Laplace and later demonstrated by Plateau. But mathematics has not provided the CAD tools necessary to elicit solutions for manmade structures that are common and ordinary in our everyday biologic world. NURBS does not provide it, nor do any of the common solid prismatics such as spheres cylinders and cubes. What is required is a new set of primitives that encompass the existing CAD tools and extend those tools beyond current limitations. These primitives are based on the relation between pressure and curvature of a surface, or rather, surfaces having constant mean curvature.

Subsequent articles will address the history and current status of this dichotomy; will argue the value of teleological awareness; will suggest changes that are needed to improve CAD, and will look at the special relation of pressure and curvature. This is a relationship that brings to our attention additional surfaces beyond the plane, cylinder, sphere and revolutes and that will facilitate a new set of CAD primitives.

About the author: Kurt Swanson is a Senior Engineer (Boeing, Philadelphia, kurt.w.swanson@boeing.com). His interest is to expand the set of CAD surfacing tools currently utilized for design and to provide affordable, quality aerospace products.

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