This information provided by Bryan R. Fischer, of TDP360 LLC.
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Terms and definitions | ||
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Datum feature symbol | A symbol used on a drawing or annotated model to identify a datum feature (may use all letters except I, O, & Q). |
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Datum target symbol | A symbol used on a drawing or annotated model to identify a point, line, or area on a surface as a datum target. |
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A datum feature | A datum feature is identified by a datum feature symbol. Datum features on a manufactured part are imperfect (they have imperfect geometry). |
Datum features and datum targets are used to build datum reference frames, which are used to establish geometric relationships like orientation and location on actual parts.
True geometric counterpart (TGC) | A theoretically perfect shape used to relate a datum to a datum feature. |
Datum feature simulator (DFS) | A substitute for the TGC. A substitute is needed because we cannot create a perfect physical TGC (e.g. we use a surface plate instead of perfect plane). |
Datum | A point, straight line, flat plane, or combination of these. |
Datum reference frame | A coordinate system that is fit to an actual imperfect part. Degrees of freedom are constrained between the part and TGCs, which constrains degrees of freedom between the datum reference frame and the part. |
 Datum reference frame |  Datum reference frame fit to actual part |
Degrees of freedom | There are six degrees of freedom that can be constrained between a part and a datum reference frame. There are three translational degrees of freedom and three rotational degrees of freedom. |
The role of datum reference frames Datum reference frames are the most important part of GD&T. They are the foundation of GD&T. Datum reference frames are used to define the context (setup) for geometric tolerances. By defining a datum reference frame, we can establish a coordinate system on an actual manufactured part, and we can relate geometric tolerance zones to the coordinate system. |
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Simulating datum features
Datum features are referenced in a specific order in a feature control frame, which defines a datum reference frame. The datum reference frame is simulated according to the rules of GD&T. First the primary datum feature is simulated. If a secondary datum feature is referenced, then the secondary datum feature is simulated. If a tertiary datum feature is referenced, then the tertiary datum feature is simulated. Datum features are fit to true geometric counterparts (or datum feature simulators), which are the same shape as a perfect datum feature. For a flat datum feature, the datum feature simulator is a perfectly flat plane. For a cylindrical datum feature, the datum feature simulator is a perfect cylinder.
Degrees of freedom
Degrees of freedom are constrained between datum features and their TGCs during datum feature simulation. Simulating datum features relates a datum reference frame to part and constrains degrees of freedom between the datum reference frame and the part.