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Supporting Gd&T Practices Through 3 D Modeling Activities

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1999 Annual Conference


Charlotte, North Carolina

Publication Date

June 20, 1999

Start Date

June 20, 1999

End Date

June 23, 1999



Page Count


Page Numbers

4.477.1 - 4.477.8

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Eric N. Wiebe

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Theodore Branoff

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NOTE: The first page of text has been automatically extracted and included below in lieu of an abstract

Session 2238

Supporting GD&T Practices Through 3-D Modeling Activities

Eric N. Wiebe Ted Branoff North Carolina State University

Abstract Geometric dimensioning and tolerancing (GD&T) has been recognized as an increasingly integral part of engineering design practice. Unfortunately, GD&T is a very difficult concept to teach and for students to apply to their activities in school. Part of the reason for this is the difficulty in representing the concepts using 2-D CAD tools. Whereas the 2-D CAD tools may have robust tools for notating drawings, they lack direct methods of capturing concepts represented in reading materials, lectures, and physical models. The new generation of constraint-based modelers provides a dynamic, 3-D environment where both size and geometric form constraints can be demonstrated and explored directly. This paper will present examples of exercises that can be used in an engineering design graphics course to demonstrate both the concepts of GD&T and good design practice. The examples include: 1) The relationship of datums in the 3-D modeling environment to the theoretical datums used in GD&T. 2) The transformation of implicit feature constraints in the 3-D model to explicit GD&T constraints. 3) How constraint modification in the model can be used to explore maximum and least material conditions. 4) Representation and constraint of symmetry in a model and its relation to GD&T controls.

I. Introduction Geometric dimensioning and tolerancing (GD&T) is a difficult concept to teach students. This difficulty in grasping the basic concepts of GD&T becomes apparent when students are asked to apply these concepts to real world problems of mechanical design. What are the roots of these conceptual difficulties? Some of the problems lie in the fact that students have to be able to understand the differences between theoretical geometric elements and the actual geometry of the part. In addition, they have to have a firm grasp of the representation of geometry in both two and three-dimensional space. Finally, students have to appreciate how the form and size of a feature might change as dimensions on a part dynamically change or as parts in an assembly dynamically change position or orientation.

The conceptual difficulties of understanding GD&T can be coupled with the conceptual difficulties of understanding constraint-based, 3-D modeling software. How can students develop a mastery of applying GD&T to the design of mechanical parts using 3-D modeling software? What follows are three examples of how constraint-based, 3-D modeling software can be used to help support GD&T instruction while also helping to demonstrate the concepts of the how the modeling software functions.

II. Definition and use of datum planes The concept of theoretical datums is central to both GD&T and 3-D modeling. Datum points, axes, and planes are theoretical geometric elements used to locate geometric features on the part. In GD&T, primary, secondary, and tertiary datum planes are established as a basis for both dimensioning the design and for setting up inspection tools (ASME, 1995; Neumann, 1995). These datums are established relative to each other and located relative to the part based on factors such as the functionality of the part and the ease of use in

Wiebe, E. N., & Branoff, T. (1999, June), Supporting Gd&T Practices Through 3 D Modeling Activities Paper presented at 1999 Annual Conference, Charlotte, North Carolina.

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