LINKING CAD AND METROLOGY TO EXPLAIN, DEMONSTRATE, AND TEACH GD&T

Geometric dimensioning and tolerancing, or GD& T, is a symbolic language that permits design engineers, manufacturing personnel, and quality inspectors to communicate in an efficient and effective manner. This communication focuses on providing a clear definition of geometric features (e.g., surfaces, holes) and the allowable variation that each feature may contain. Unfortunately, owing in part to its complex rule based system, GD& T is also difficult to teach and learn. To address this difficulty, a technique has been developed that allows students to visualize geometric tolerances and tolerance zones, and to directly see when a given data point is in or out of tolerance. The technique employs a portable coordinate measuring machine (CMM) interfaced with parametric solid modeling software, a 3D printer, and a granite surface table to accomplish this. A set of engineering drawings is created, and a 3D printer is used to produce imperfect parts. These imperfections are intended to represent significant manufacturing variation. Then using a portable CMM and the surface table, data points are taken to visually map this manufacturing variation to a 3D parametric modeler. Within this software a perfect part is also modeled. Once the inspection data is taken, datum features on the perfect part are used to form the boundaries of the geometric tolerance zones. Through this process, students interactively learn the meaning of datum references, as well as how the various tolerances create different zones. Finally, students use the parametric modeling software to measure the inspection data points to visually see how in or out of specification a given feature is. Having developed a basic working understanding of GD& T, a second module is used to convey design intent through the use of GD& T. Using a simple assembly, students are charged with providing a fully toleranced drawing for one component of this assembly. Students are given a fully dimensioned drawing with basic dimensions and a list of fit, form, and functional requirements. From these resources, students must choose a datum scheme, tolerance part features, and explain which requirements drive their decisions. In summary, the goal of these educational modules is to illustrate the complex topics of geometric dimensioning and tolerancing through practical application.