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In carpentry, an alternative, more common name is "face side" and "face edge". The artisan nominates two straight edges on a workpiece as the "datum edges", and they are marked accordingly. One convention is to mark the first datum edge with a single slanted line (/) and the second with double lines (//). For most work, the datum references of the workpiece need to be square. If necessary they may be cut, planed or filed to make them so. In subsequent marking out, all measurements are then taken from either of the two datum references.
In GD&T, datum reference frames are typically 3D. Datum reference frames are used as part of the feature control frame to show where the measurement is taken from. A typical datum reference frame is made up of three planes. For example, the three planes could be one "face side" and two "datum edges". These three planes are marked A, B and C, where A is the face side, B is the first datum edge, and C is the second datum edge. In this case, the datum reference frame is A/B/C. A/B/C is shown at the end of feature control frame to show from where the measurement is taken. (See the ASME standard Y14.5M-1994 for more examples and material modifiers.)
The engineer selects A/B/C based on the dimensional function of the part. The datums should be functional per the ASME standard. Typically, a part is required to fit with other parts. So, the functional datums are chosen based on how the part attaches. Note: Typically, the functional datums are not used to manufacture the part. The manufacturing datums are typically different from the functional datums to save cost, improve process speed, and repeatability. A tolerance analysis may be needed in many cases to convert between the functional datums and the manufacturing datums. Computer software can be purchased for dimensional analysis. A trained engineer is required to run the software.
There are typically 6 degrees of freedom that need to be considered by the engineer before choosing which feature is A, B,or C. For this example, A is the primary datum, B is the secondary, and C is the tertiary datum. The primary datum controls the most degrees of freedom. The tertiary datum controls the least degrees of freedom. For this example, of a block of wood, Datum A controls 3 degrees of freedom, B controls 2 degrees of freedom, and C controls 1 degree of freedom. 3+2+1 = 6, all 6 degrees of freedom are considered.
The 6 degrees of freedom in this example are 3 translation and 3 rotation about the 3D coordinate system. Datum A controls 3: translation along the Z axis, rotation about the x axis, and rotation about the y axis. Datum B controls 2: translation along the y axis and rotation about the z axis. Finally, Datum C controls 1 degree of freedom, namely the translation along the x axis.