One of the basic principles used in reviewing a casting design is Geometric Dimensioning & Tolerancing (GD&T). This is a system for defining and communicating engineering tolerances through the use a symbolic language on engineering drawings. It explicitly describes the part geometry and its allowable variations.
Using GD&T is one of the best ways for the customer’s design engineer, the supplier’s manufacturing engineer and the quality inspectors from both organizations to reach consensus on a part’s design needs. If properly implemented, the datum structure will become a key element of the part’s technical information for its entire life. That means that parts can be brought back from the field and checked against the datum structure to determine wear and other beneficial performance data.
The datum reference frame is the starting point for GD&T. A datum reference is a theoretical exact plane, axis or point location to which dimensional tolerances are referenced. You can think of them as an “anchor” for the entire part from which where the other features are referenced. In a simpler form, think of a datum reference as plotting points on a X-Y coordinate system with a third axis, Z, coming out of the paper.
A datum feature is usually an important functional feature (a hole, plane or surface) that needs to be controlled during measurement. The features are controlled in relationship to each other to allow other mating parts to be assembled together to allow the final product to function as designed.
Using this coordinate system, the designer can apply tolerances to features like:
With this knowledge, the tooling and casting engineer can design the pattern and casting process to ensure that the resulting castings meet the customer’s design needs.
If no datum or GD&T is used on the initial design, the casting engineer can work with the design engineer to determine key features that will define the casting process. This is not always an efficient process. More often than not, it takes several conversations, meetings and mark-ups of the design before the design intent is fully understood by the supplier. The resulting agreements will improve the final outcome of the casting.
No part is ever produced perfectly. By implementing a common language for dimensions and tolerances, a properly implemented GD&T can improve the quality and reduce the time and cost of the final product. If it’s not improperly implemented, it could drive up your costs and cause quality concerns.
To learn more about GD&T and how it can help to improve your casting outcomes, please contact The C.A. Lawton Co. and ask about attending our Iron 101 class.