Screw theory is then used to propagate the effect of

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Unformatted text preview: ance zone in ways that the tolerance specification allows. Screw Theory is then used to propagate the effect of this variation onto the assembly to see the effects on the Key Characteristics. Only properly constrained assemblies can be correctly analyzed for the effects of variation at the feature or part level. In the case of over-constraint, a stress analysis is needed. Without taking stress and strain into account, a unique Datum Flow Chain does not exist. In the case of under-constraint, there is no particular nominal condition, requiring the addition of an artificial constraint. Designers often make constraint mistakes. [8] CAD systems do not offer much help in this regard. CAD systems check for geometric compatibility but do not detect situations 1 J.K. Davidson (ed.), Models for Computer Aided Tolerancing in Design and Manufacturing, 1 – 2. © 2007 Springer. 2 D. E. Whitney where locked-in stress could exist under conditions of variation. Screw Theory permits us to fill this gap. Current design practice does not make a clear distinction between creation of a competent nominal design (that is, one that is as close as practical to properly constrained or one in which the designer deliberately inserts desired over-constraint and takes it into account) and performance of...
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