Don't Complicate Control
of Your Datum Features.
Datum
features are just that-features. Since they are features, they
need to be controlled. It seems that some people do not apply
controls to datum features because they think datums are
perfect, which is true. However, the features that establish
the theoretical datums must be controlled. Other folks tend to
over control the datum features. Start with the primary datum
feature. Since everything is located and oriented from the
datum established by this datum feature, in this case A, the
only control required is form. Flatness is the appropriate
control here. The secondary datum feature B is nominally at
right angles to the first datum plane so it should be
controlled with perpendicularity. Datum feature C, in this
example, must be located relative to the origin of the datum
reference frame established by A and B. The only geometric
tolerance that locates surfaces is profile of a surface, which
is the required control on datum feature C. Once these datum
features are controlled, other features must be located
relative to the datum reference framework. Use profile for the
surfaces and position for the features of size.
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