Casting Design for Structural Components - Time-Saving Path to Successful Metalcasting Structures 2002-01-0915
Metalcastings are very important in the structural design engineer's arsenal, and significant examples are found in structural applications of all kinds. While significant examples are readily recognizable, the know how to generate them is scarce. The general expertise of costeffective casting design is rare; the specific ability to integrate structural capability in metalcasting geometry is rarer still.
Development of computer modeling and simulation tools has advanced structural design significantly, but the power of these tools encourages leaps to analysis of structures before the manufacturability of the structure is fully developed. This is especially true for metalcastings. Zeal to investigate the stress and deflections of a structure leads to many, many castings being designed poorly or redesigned. Further, many structures that should be castings for the sake of cost effectiveness, weight reduction, or durability wind up being fabricated because of the efficiency of today's modeling and FEA analysis techniques; inadvertently, the conclusion gets ahead of the design process.
This paper suggests an alternative design method based on engineering sketching, for the following reasons. First, dimensioned engineered sketches are probably the most efficient starting point for construction of solid models. Second, sketching allows integration of the four disciplines of good casting geometry: a) geometry of castability; b) geometry of structure; c) process geometry; and d) geometry to minimize/reduce cost of machining and assembly. Third, metalcasting structural geometry can be quickly sketched using the designer's sense of scale, proportion, and load-carrying capability of shape and material. Lastly, reality checks of sections where stress and/or deflection are a concern can be quickly calculated using classic stress analysis. The method of sections, vector algebra, the principle of superposition, and methods of transforming stress and checking allowable stress are all highly graphical techniques, as well. Nothing in stress analysis is more valuable than a simple sketch of the loads and reactions expressed in a free-body diagram. This methodology as a precursor, coupled with today's model building and simulation technology is the most effective pathway to efficient structural metalcasting designs.
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