![]() As a result, with only one exception in the 3-D world, the most important feature of a part is always the one that constrains pitch and yaw during the assembly process. As made clear by our Rule of Degrees of Constraint Precedence found in the Rules of Natural DRF Establishment quoted in our August column, the most important degrees of freedom are pitch and yaw. The Datum Feature Sequencing Process: Once the “mating” or “reliable” features have been designated as Datum Features, the order of their importance must be researched. In this case “reliable” takes the place of “mating” as the selection criterion. In the examples that follow, this will be the first order of business.Įxceptions: If all the mating parts for a large part are small, no mating feature on the large part can be an adequate Datum Feature to manage its geometry. Namely at the end of that process, the objective is for the coordinate systems of two mating parts to coincide in order to guarantee functional relationships between their remaining features. The Datum Feature Selection Process: Because Datum Features serve to constrain degrees of freedom and establish coordinate systems relative to which the remaining features of a part are to be oriented and located, they ought in general (but with some exceptions) to be those features which actually constrain it relative to the mating part during an assembly process. ![]() Whether GD&T is functional or merely decorative heavily depends on the validity of these choices. ![]() Failure to get it right in this regard makes even syntactically correct GD&T useless, if not dangerous. Like wearing a mask in public.Given that GD&T is a symbolic language with which to research, refine and ultimately “encode” the functions of each feature of a machine part, and having provided precise definitions of Datums, Datum Features, Datum Feature Simulators, and the Datum Reference Frame establishment process in earlier columns, it’s time to address best practices for selecting Datum Features and for specifying the order in which they are listed in Feature Control Frames. Generally if someone in manufacturing with more experience asks for something that doesn't immediately make sense to you and you don't have a strong argument against it, there's probably a good reason for it. No extra math for them required, less room for error. I imagine surface 'A' would be on an inspection plate, a CMM or indicator would be set to measure the feature surface depth nominally 10 mm from the reference face and measure parallel. The geometry of the face you're making the cut in question (in terms of flatness, parallelism, etc) is not defined (at least not in the image you have shared), so measuring off that surface is less precise than measuring off Datum A.įor inspection purposes it is also much easier to have the distance from 'A' explicitly stated as this is the reference surface for the feature parallelism. They know what they need to see to make sure the part ultimately meets the print.Īlso, since your first datum is on the opposing face this implies it is getting machined first, hence why it is a reference datum. If the request is coming from whoever will be machining or inspecting the part, I would side with them.
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