| The traditional design of jigs and fixtures is performed primarily in a heuristic fashion utilizing the experience of skilled craftsmen. Years of experience and training are required to design and build fixtures in modern high technology, industries. While it is the state-of-the-art, it is not the most efficient methodology for designing new fixtures, often relying on trial and error. Fixturing is a time intensive and costly manufacturing process. In the manufacture of turbine blades at Pratt & Whitney Aircraft (PWA), this is especially true, due to part and process requirements such as precision, accuracy, and material selection. Additionally, due to the complex geometry of the turbine blades, their fixtures are much more difficult to design than the fixtures for prismatic parts. The determination of the optimal position of the locating and clamping elements in nesting fixture design is challenging at best, for experienced human designers.; The Motion Stop Method developed at M.I.T. by Dr. John Bausch has been augmented and applied to the design of these fixtures. The Motion Stop Method has been improved using Everett's method for solving for the reciprocal screw. A correlation was made between the Cartesian-coordinate-based contact Jacobian used by Asada and Pluckers line coordinates, used in screw theory. The Genetic Algorithm was used to decrease the time needed to find a significantly improved fixture layout for the turbine blades. An improved near optimal fixture design, based on the Motion Stop theory, was then built. This fixture was compared using a CMM with a current PWA fixture. The accuracy and precision of the fixtures were evaluated using a methodology based on a statistical analysis. The improved Motion Stop Method was integrated within the Unigraphics CAD modeling package. The augmentation of the design of fixtures has been shown to provide quality fixtures for the production of turbine blades and vanes. |
