Calculating Dimensional Drift in Precision Cast Aluminum Tooling Assemblies
Total assembly drift equals the vector sum of residual stress release, thermal creep, and joint slip, demanding empirical metrology validation after thermal soak.
True Position
Meaning
Geometric tolerances define the exact allowable deviation of a feature from its theoretically perfect location. Using true position instead of traditional coordinate tolerances allows for a circular or cylindrical zone of acceptable error. The method provides more functional parts because it accounts for the relationship between the X and Y axes.
Tolerance Zone
Calculations for this limit usually involve a diameter rather than a square area. A true position value represents the distance from the actual center of a hole to its intended location, multiplied by two. This approach shows how parts actually fit together in an assembly.
Datum Reference
Measurements are always taken relative to established baseline surfaces on the part. The true position of a hole might be tied to the outer edges of the housing or to other critical internal features. Without a clear datum, the measurement has no meaning in a manufacturing context.
Inspectors use coordinate measuring machines to verify that every feature falls within the specified boundary. Validation requires fixed references.
Maximum Material
Bonus tolerances are sometimes granted when the size of a hole deviates from its nominal dimension. This application of true position ensures that parts still assemble even if the hole is slightly larger than the minimum.