Geometric Demensioning and Tolerances

Geometric Demensioning and Tolerances, GD&T, is a universal design engineering language that is being used to faithfully capture and transmit the designer's intent through all activities in the product cycle. The language has been adopted by the international Organization for Standardization, (ISO) and the American National Standards Institute, (ANSI). GD&T has been theorized to consititute continious improvement for manufacturing businesses. GD&T language consists of a well-defined set of symbols, rules, definitions, and conventions that can be used to describe the size, form, orientation, and location tolerances of part features.
GD&T was created through a need to overcome some of the problems created in the conventional "plus or minus" tolerancing. With conventional tolerancing, the machininst would often have to guess the designer's intent. For a simple example, consider a triangular part specified with two legs of equal length and no angle. The machinist has no evidence of the importance on the angle. Often times, the general feel is that if something not specified in such a way that the machinist feels correct, then the implication is that the geometry is not very important. The side effect is that the second guessing can lead to scrap parts and increased manufacturing time.
GD&T currently incorporates a universal language in which the designer can clearly state the intentions to the manufacturer. Baselines and Datum points are established, eliminating guesswork. A more powerful feature of GD&T is its use of a Coordinate Measuring Machine, (CMM). A complete setup would consist of a Computer Integrated Manufacturing, CIM, programs in correlation with a CMM to analyze manufactured parts. With a set up like this, the automated analysis would provide feedback to the different stages of manufacturing. This allows for the manufacturing process to constantly readjust the necessary parameters if the part does not meet specifications.

If you are interested in testing your GD&T knowledge, you can take the free GD&T Skills Survey at http://etinews.com/skills/index.html.