One basic principle in reviewing a casting design is Geometric Dimensioning & Tolerancing (GD&T). This system defines and communicates engineering tolerances through symbolic language on engineering drawings. It explicitly describes the part geometry and allowable variations.

Using GD&T helps the customer’s design engineer, the supplier’s manufacturing engineer, and quality inspectors 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. Parts can be brought back from the field and checked against the datum structure to determine wear and other 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. They are an “anchor” for the entire part from which where the other features are referenced.

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 relation to each other to allow other mating parts to be assembled to allow the final product to function as designed.

Using this coordinate system, the designer can apply tolerances to features like:

  • Flatness
  • Straightness
  • Perpendicularity
  • Parallelism
  • Position
  • Concentricity
  • Profile

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.

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.

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