Datum Features Specification: Considerations Regarding Capabilities of Manufacturing and Assembly Processes

Mechanical assemblies are subject to dimensional and functional constraints in order that they fit and function properly at higher level assemblies. Their final tolerated dimensions, which must comply with the cited constraints, are composed by the chain dimensioning of the tolerated dimensions of their components. Therefore, the tolerances of each individual component play an important role on the fit and functional requirements in a product and must be properly specified for this intent. However, the origin for each part dimensioning - the datum referencing - is of vital importance for the correct tolerance accumulation of the assembly components. The Standard ASME Y14.5-2009 - Dimensioning and Tolerancing - brought significant improvements over its former edition regarding this subject but, due to the generic approach that a comprehensive standard must keep, it does not deal with some important details, like the one regarding the manufacturing processes previewed in the design phase of a part. The aim of the present work is to attempt supplying designers with some concepts on datum features referencing, generally not taken into consideration due to the lack of information regarding this matter. This is particularly important when features of counterparts fabricated by different processes are assembled. An example considering the design of a part illustrates the differences on the elaboration of detail drawings when distinct manufacturing processes are considered.