GD&T: A Practical Approach to Dimension and Tolerance

Posted by Al Timm on Nov 22, 2013 8:30:00 AM

blank.pngGeometric dimensioning and tolerancing (GD&T) is a symbolic language that is used on engineering drawings and computer-generated models to communicate geometric dimensions and allowable tolerance for various parts. Not only is this a useful exercise for product design, it’s also helpful on the manufacturing floor because engineers and operators can quickly see the degree of tolerance that is required for each part.

GD&T is a basic yet valuable approach that focuses attention on the functionality of the product, and how that defines dimension and tolerance. GD&T is essentially a universally accepted graphic language, with both national and international standards, that improves product design and quality by emphasizing function.

This method is increasingly being used across a wide range of manufacturing industries. It adds details to a part print or drawing, using simple X and Y dimensions, which are based on functional use versus traditional specifications.

Before engineers can apply GD&T they must understand a part’s application—how and where it will be used. By thoroughly understanding the designer’s intent for the part or product, the engineers can then better applyfunctional specifications to test the design and possibly improve on it. This is done by using datums and datum systems to define dimensional requirements, especially with respect to part interfaces. GD&T will show dimensions and related tolerances based on functional relationships and help determine how a part should be shaped to achieve its highest function. These results can also be used to control tooling and assembly interfaces. GD&T also eliminates any confusion regarding nomenclature by applying universally accepted symbols and syntax, which can be especially helpful when communicating and exchanging data with overseas operations.

Datum structure set-up and construction does impact the dimensional results—and therefore product development—when done correctly. Improper construction of datums in the coordinate measuring machine will yield inconsistent, inaccurate results.

Engineers must be certain datums are repeatable and reliable before evaluating the dimensional results. When creating an axis, it is important to have enough surface area to accurately represent the axis; if the area is too small, it may not be possible to create a datum that is repeatable. For example, trying to establish a datum axis for a short cylinder (under .080”) can easily result in non-realistic data points and non-repeatable results—simply bad data that cannot be used.This analysis is especially critical before making steel adjustments.

The option to use “bonus” tolerance can make the manufacturing process easier because tolerance doesn’t have to be as tight. Bonus tolerance is applied in GD&T where the designer can use maximum material condition (MMC) or least material condition (LMC), which gives the manufacturer “bonus” tolerance based on the feature’s size. So a bolt hole of .250”  +/- .005”, which has a true position call-out applied of .005” with a MMC modifier, allows for an actual part with a hole size of .245 to gain .010” of bonus tolerance on top of the .005” true position tolerance. 

From a development standpoint, GD&T does not provide enough information to actually adjust final dimensions. GD&T will let the designer/engineer know if the part meets the print specification; however there is not enough data for the manufacturer to adjust the tool to bring the part into tolerance/specification. The GD&T data should be included in the development inspection phase; for example, capability studies on X-Y will be required prior to adjusting steel.

Another example is profile call-outs—GD&T will only let an engineer know if the part meets the specification or not. In order to analyze this call-out, the actual position results (X-Y-Z) are required, in addition to the feature’s flatness and perpendicularity to the datums. Again, this additional data needs to be included in the development inspection phase.

GD&T does create additional work in the development phase—however, once developed, it provides a simplified inspection plan for production that fully supports the product’s functionality.

Topics: Plastic Part Design